SETCA 2008

A Brief History of the Southeastern Theoretical Chemistry Association

The Southeastern Theoretical Chemistry Association (SETCA) is a loose organization of theoretical and computational chemists from across the Southeastern United States. Since its inception in 1970 by Prof. Bruno Linder of Florida State University, its primary function has been an annual conference to give faculty, students, and postdoctoral associates an opportunity to present their most recent research results. In its first few years, Prof. Linder hosted the conference at Florida State University and limited participation to scientists from Florida, Louisiana, and Georgia. According to Prof. Linder, ``I think that I may have gotten the idea of a SETCA organization in Sanibel (the real Sanibel) in 1969. I spoke to Yngve Öhrn, Bill Rhodes and others and all were enthusiastic about the idea. The idea was to form a group from faculty at Universities that are within driving distance from each other. A faculty member can load up his (her) car with graduate students and postdocs and drive down to the meeting place at an absolute minimum [cost].''

Since its auspicious beginning, SETCA participation has gradually extended to include essentially all of the Southeastern states, and the honor of hosting the event has been passed to more than a dozen major Universities. The known locations and organizers of past, present, and future meetings of SETCA are given below.

SETCA 2008 Sponsors

The University of Alabama

James River Technology

Alabama Supercomputer Authority

Program: SETCA 2008

Friday May 16, 2008

8:00 – 8:30       *T. Daniel Crawford, Virginia Tech, Optical Activity and Chiral Molecular Structure

8:30 – 9:00       *Vince Ortiz, Auburn, Propagators, Experiments and Insights

9:00 – 9:20       Tracy Hamilton, University of Alabama-Birmingham, Conformational Analysis and Steric Contributions of 9-cis-Retinoic Acid Analogs

9:20 – 9:40       Hyun Joo, University of the Pacific, Environmental Effect on Molecular Conformation: Bicalutamide Analogues

9:40 –10:00      Myrna H. Matus, The University of Alabama, Computational Studies on Regeneration of Boron-Nitrogen Compounds for Hydrogen Fuel Cells

10:00 – 10:20   Break

10:20 – 10:50   *Patrick Charbonneau, Duke University, Thermodynamics of Cluster Freezing: a study of the phase behavior of multiple-occupancy crystals through simulation

10:50 – 11:20   *Wesley Allen, University of Georgia, Methodological Advances in State-Specific Multireference Coupled Cluster Theory

11:20 – 11:50   Benjamin Shepler, Emory University, Photodissociation Dynamics of Acetaldehyde

11:50 – 12:20   Vitaly Rassolov, University of South Carolina, Stable  long-time  semiclassical description of zero-point energy in  high-dimensional molecular systems

12:20 – 1:30     Lunch

1:30 – 2:00       *Lawrence Pratt, Tulane, What is special about water as a matrix of life?

2:00 – 2:30       *Mike McKee, Auburn, Hydrogen Exchange Reactions in C₆₀

2:30 – 3:00       C. David Sherrill, Georgia Institute of Technology, Computational approaches for non-bonded interactions

3:00 – 3:20       Break

3:20 – 3:50       *Angela Wilson, University of North Texas, Quantitative Modeling across the Periodic Table

3:50 – 4:10       Steven Wheeler, UCLA, Unraveling the Origin of Substituent Effects in the Benzene Dimer

4:10 – 4:30       James Baird, University of Alabama-Huntsville, Reduced Capillary Length Scale in the Application of Ostwald Ripening Theory to the Coarsening of Charged Colloidal Crystals in Electrolyte Solutions

4:45 – 6:30       Posters & Reception

6:45                 Dinner

Saturday, May 17, 2008

8:00 – 8:30       *Steven Gwaltney, Mississippi State, A Molecular Mechanism for Decreased Activity in Mutant pnb CE Enzymes

8:30 – 9:00       *Diego Troya, Virginia Tech, Potential-energy surfaces for scattering simulations of large chemical reactions

9:00 – 9:30       *Orlando Acevedo, Auburn, Advances in potentials of mean force methodology for organic and biological simulations

9:30 – 10:00     Dave Magers, Mississippi College, Are Conventional Strain Energies in Bicyclic Compounds Additive?

10:00 – 10:20   Break

10:20 – 10:40   Gabor Czako, Emory University, High-Accuracy ab-initio Rovibrational Spectroscopy

10:40 – 11:00   Lucas Speakman, University of Georgia, The SiC₂ Saga Continues: Revised Barrier to Linearity, Equilibrium Structures, Fundamental Frequencies, and Enthalpy of Formation

11:00 – 11:20   Matthew Wilson, University of Tennessee, The Behavior of Time Step Error in Quantum Monte Carlo Studies of Model Rigid Rotor Systems

11:20 – 11:40   Shenggang Li, The University of Alabama, Elucidating Electronic Structure of Transition Metal Oxide Clusters from Photoelectron Spectroscopy and Electron Structural Calculations

11:40 – 12:10   SETCA annual meeting

* Invited Speaker

Advances in Potentials of Mean Force Methodology for Organic and Biological Simulations

Orlando Acevedo, Department of Chemistry, Auburn University

Combined quantum and molecular mechanics (QM/MM) simulations have been used to study reaction mechanisms and the origin of enzymatic rate accelerations and selectivity. Speed and accuracy demands have led to the development of enhanced algorithms and a novel potentials of mean force (PMF) method for analytically reproducing free-energy profiles and surfaces with full sampling of solute and solvent coordinates. The methodology has provided excellent results for free-energies of activation for many reactions and has reduced average computation times from ca. 6 months to 3 weeks. The presentation will focus on the development of these methods and will feature recent examples including fatty acid amide hydrolase (FAAH), antibody 4B2, and a singlet oxygen ene reaction.

Oral

Methodological Advances in State-Specific Multireference Coupled Cluster Theory

Wesley Allen, Center for Computational Chemistry, Chemistry Department, The University of Georgia

The first production level code (PSIMRCC) for state-specific and rigorously size-extensive Mukherjee multireference coupled cluster singles and doubles (Mk-MRCCSD) computations has been developed.  This breakthrough was aided by our derivation of closed-form expressions for the terms coupling different references in the amplitude equations. Moreover, a hierarchy of Mk-MRCCSDT-n (n = 1a, 1b, 2, 3) methods for the iterative inclusion of connected triple excitations has been formulated and implemented for the first time. The effectiveness of our Mk-MRCC methods is established by extensive computations on benchmark problems, including the dissociation of molecular fluorine and the ozone graveyard for theory.  In chemical applications of Mk-MRCCSD theory with the cc-pVTZ basis set, outstanding results have been obtained for the optimum geometric structures, vibrational frequencies, and adiabatic excitation energies of ortho-, meta-, and para-benzyne, as well the automerization barriers of cyclobutadiene, cyclooctatetraene, and other antiaromatic systems.  Finally, we report Mk-MRCC predictions of UV/Vis spectra of novel carbenes that have led to the identification of these species in matrix isolation experiments.

Oral

Substituent Effects in Parallel-Displaced pi–pi Interactions

Stephen Arnstein and C. David Sherrill, Department of Chemistry, Georgia Institute of Technology

High-quality quantum-mechanical methods are used to examine how substituents tune pi–pi interactions between monosubstituted benzene dimers in parallel-displaced geometries. The present study focuses on the effect of the substituent across entire potential energy curves. Substituent effects are examined in terms of the fundamental components of the interaction (electrostatics, exchange-repulsion, dispersion and induction) through the use of symmetry-adapted perturbation theory. Both second-order Møller–Plesset perturbation theory (MP2) with a truncated aug-cc-pVDZ basis and spin-component-scaled MP2 (SCS-MP2) with the aug-cc-pVTZ basis are found to mimic closely estimates of coupled-cluster with perturbative triples [CCSD(T)] in an aug-cc-pVTZ basis. Substituents can have a significant effect on the electronic structure of the  cloud of an aromatic ring, leading to marked changes in the pi–pi interaction. Moreover, there can also be significant direct interactions between a substituent on one ring and the pi-cloud of the other ring.

Poster 1

Multireference Coupled-Cluster Investigation of the Trimethylene Diradical

RAFAL A. BACHORZ^a AND WESLEY D. ALLEN^b

^a INSTITUT FUER PHYSIKALISCHE CHEMIE, UNIVERSITY OF KARLSRUHE (TH), D-76131 (GERMANY)

^b CENTER FOR COMPUTATIONAL CHEMISTRY, UNIVERSITY OF GEORGIA, ATHENS, GEORGIA 30602 (USA)

Recently implemented state-specifc multireference coupled-cluster (Mk-MRCC) theory[1] has been applied to the trimethylene system, which plays an important role in the thermal isomerization of cyclopropane to propene. Trimethylene is a diradical system requiring multireference treatment for reliable electronic structure predictions. The electronic states of trimethylene arise from the placement of two electrons into two near-degenerate orbitals, yielding three multideterminantal singlets and one triplet. We have performed geometry optimizations at the Mk-PT2 and Mk-CCSD levels of theory with the cc-pVTZ basis set. This led to structures which exhibit various point-group symmetries. We have found stationary points for C2v, Cs and C2 symmetries, and their character was investigated by computing vibrational frequencies at the Mk-CCSD level of theory with the cc-pVDZ basis set. The imaginary frequency modes provide key information about the conrotatory and disrotatory pathways of the ring closure reaction. All the electronic structure computations were performed with the PSI3 package [2].

References

[1]        F.A. Evangelista, W.D. Allen, and H.F. Schaefer, J. Chem. Phys. 127, 024102 (2007).

[2]        T.D. Crawford, C.D. Sherrill, E.F. Valeev, J.T. Fermann, R.A. King, M.L. Leininger,   S.T. Brown, C.L. Janssen, E.T. Seidl, J.P. Kenny, and W.D. Allen, J. Comp. Chem. 28,    1610 (2007).

Poster 2

Reduced Capillary Length Scale in the Application of Ostwald Ripening Theory to the Coarsening of Charged Colloidal Crystals in Electrolyte Solutions

James Baird, Department Chemistry, University of Alabama in Huntsville

A colloidal crystal suspended in an electrolyte solution will ordinarily exchange ions with the surrounding solution and develop a net surface charge density.  The interfacial tension of the charged surface has contributions arising from: (1) background surface tension of the uncharged surface, (2) the chemical energy associated with the adsorption (or desorption) of ions from the surface, and (3) the polarizing effect of the electrostatic field within the double layer.  The chemical and polarization effects are negative and serve to reduce the interfacial tension below the value to be expected for the uncharged surface. The diminished interfacial tension leads to a reduced capillary length scale.  According to the Ostwald ripening theory of particle coarsening, the reduced capillary length will cause the solute supersaturation to decay more rapidly and the colloidal particles to be smaller in size and greater in number than in the absence of the double layer.  Although the length scale for coarsening should is little affected in the case of inorganic colloids, such as AgI, it should be greatly reduced in the case of suspensions of protein crystals, such as apoferritin, catalase, and thaumatin.  The small capillary length scale predicted for protein crystals explains why these crystals not only grow slowly but also achieve only a relatively modest size at equilibrium.  Adjustments can be made to the growth solution pH to increase the capillary length and increase the equilibrium size of protein crystals.  The increased size has the effect of reducing the intensity of the X-ray beams required for the crystallographic determination of protein molecular structure.   

Oral

4-Substituted Pyrolidine Butanamides as Antiepileptic Drugs - A QSAR Study

Manikanthan Bhavaraju, Department of Chemistry, Mississippi State University

A series of 4-substituted pyrolidine butanamides (PB1-PB19), which show inhibitory activity towards epilepsy, were subjected to quantitative structural relationship (QSAR) studies. Multilinear regression analysis was carried out to correlate the physicochemical parameters like electronic parameter σp and molar refractivity (MR) to predict the inhibitory activity of PB. A modeled equation (Eq. 1) was generated which best fits the activity data.

Activity = −0.0297(0.001) * MR + 0.094(0.031) * σp + 0.00026 * MR²                                    (1)

An optimization of the PB analogues for maximum activity suggests a PB analogue with MR value of 57.1 to be best possible pharmacophore, which may act as one of the best possible antiepileptic drug.

Reference:

[1]        Manikanthan B.H.N.V., Saritha P, and Uma Vuruputuri; J.T.R. Chem, 13(1), 2006, p. 59-      65.

Poster 3

Density Functional Calculation of X-ray Absorption Spectra of Water Clusters within the Core-Hole Approximation

William Carlen and Robert J. Harrison, Department of Chemistry, University of Tennessee

Density functional theory is used to calculate the excitation spectra of water clusters.  Specifically, the core-hole approximation is used.  In this scenario, the excitation energies of core electrons are calculated using the approximation that the core energy level be constrained to be unoccupied throughout the calculation.  This allows a more accurate determination of the resulting x-ray spectra.

Poster 4

Thermodynamics of Cluster Freezing: a Study of the Phase Behaviour of Multiple-Occupancy Crystals through Simulation

Patrick Charbonneau, Department of Chemistry, Duke University

 

Clusters crystals have been predicted to form in materials, such as in certain dendrimers, with steep and bounded repulsive interparticle interactions. The resulting solids tile space with a periodic lattice, where each site is multiply occupied. This "superlattice" presents novel theoretical and simulation challenge. In order to obtain the equilibrium behaviour of cluster crystals it is essential to treat the number of lattice sites as a constraining thermodynamic variable. The resulting free-energy calculations thus differ considerably from schemes used for single-occupancy lattices. Using this approach, the phase diagram and the elastic properties of a cluster crystal are obtained. Surprising mechanical properties emerge, which in turn shed new light on the behaviour of other clustering systems.

Oral
 

Isomerization Reaction between the trans- and cis-BNNO Molecules

Qianyi Cheng and Henry F. Schaefer III, Center for Computational Chemistry, The University of Georgia

Isomerization reaction between the trans-BNNO and cis-BNNO molecules has been investigated using ab initio SCF, coupled cluster singles and doubles (CCSD), CCSD with perturbative triple excitations [CCSD(T)], complete active space self-consistent field (CASSCF), and multi-reference configuration interaction (MRCI) levels of theories utilizing Dunning’s correlation consistent polarized valence basis sets (cc-pVXZ and aug-cc-pVXZ where X = D, T, Q). Both of the isomers are predicted to be minima on the 2A' potential energy surface and have Cs point group symmetry. The bond distance of N-N in BNNO is longer than the N-N triple bond distance in N₂O. Computed harmonic vibrational frequencies of the trans- isomer agree well with the experimental infrared spectroscopic observation. The transition state between the trans- and cis- structures is located on the ²A' surface. The N-N-O angle is 180°, and the B-N-N angle is 129°. The trans- isomer is found to be 11.3 kcal/mol lower than cis- isomer in energy at the CCSD level of theory. The energy barrier for the isomerization reaction is predicted to be 33.8 kcal/mol for the trans- isomer and 22.5 kcal/mol for the cis- isomer.

Poster 5

Benchmark Calculations of the Heats of Formation and Electron Affinities of Transition Metal Fluorides and Hydrides

Raluca Craciun, Shenggang Li, Andrew Vincent, Rebecca Long and David A.

Dixon, Department of Chemistry, University of Alabama

Heats of formation for the 1^st, 2^nd, and 3^rd row transition metal hydrides MH_n and fluorides MF_n, as well as for the radicals MH_n-1 and MF_n-1 were predicted with density functional theory and molecular orbital methods. Electron affinities of the 2^nd and 3^rd row transition metal fluorides MF₆ were also calculated. Calculations were done up to the CCSD(T)/complete basis set limit with additional corrections. The predicted heats of formation were used to calculate the first and the average M-H and M-F bond energies and are compared with the available experimental data. The performance of a wide range of DFT exchange-correlation functionals was benchmarked by comparison to the accurate CCSD(T) results. The predicted electron affinities show interesting periodic behavior.

Poster 6

Optical Activity and Chiral Molecular Structure

T. Daniel Crawford, Department of Chemistry, Virginia Tech, Blacksburg, Virginia

Although the optical activity of chiral structures has been known for nearly two centuries, our chemical intuition for properties such as optical rotation remains woefully immature.  While organic chemists have devised empirical protocols (e.g. the venerated octant rule) to predict optical activity, such rules quickly fail outside the limited spectrum of simple molecules for which they were originally designed.  However, the emergence of advanced quantum chemical models for the first-principles calculation of molecular properties offers an exciting opportunity to examine closely the relationship between intrinsic molecular/electronic structure and chiroptical response.  This talk will provide an overview of the current state of ab initio models of optical rotation and their potential to yield the foundation for a chemically intuitive understanding of optical activity.

Oral

High-Accuracy ab initio Rovibrational Spectroscopy

Gábor Czakó,^a Edit Mátyus,^b Attila G. Császár,^b Bastiaan J. Braams,^a and Joel M. Bowman^a

^a Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, GA 30322

^b Laboratory of Molecular Spectroscopy, Institute of Chemistry, Eötvös University, P. O. Box 32, H-1518 Budapest 112, Hungary

The computation of high-accuracy and/or complete rotational-vibrational spectra of small molecules requires the use of the variational techniques for solving the Schrödinger equation of the nuclei. Strategies are introduced which in principle allow the determination of the complete rotational-vibrational spectrum of triatomic molecules [1,2,3,4]. The newly developed program packages are described. These packages have been employed for computing (ro)vibrational energy levels and effective molecular properties, e.g. temperature-dependent effective structures and vibrationally averaged rotational constants, for several molecules, such as H₂O, CO₂, N₂O, CH₂, CCl₂, CHCl, and H₃⁺.

Variational vibrational calculations can also be performed for molecules having more than 3 atoms. General program packages MULTIMODE and the recently developed DEWE [5], discrete variable representation (DVR) of the Eckart–Watson (EW) Hamiltonian with exact inclusion (E) of an arbitrary potential, are introduced. Results obtained by DEWE for linear and nonlinear test cases are presented [5]. Our recent MULTIMODE study on the F^––CH₄ anion complex is also introduced [6]. The vibrational spectrum of F^––CH₄ has been computed employing newly developed global potential energy and dipole moment surfaces.

References:

[1]     G. Czakó, T. Furtenbacher, A. G. Császár, and V. Szalay, Mol. Phys. 102, 2411 (2004).

[2]     G. Czakó, V. Szalay, A. G. Császár, and T. Furtenbacher, J. Chem. Phys. 122, 024101             (2005).

[3]     G. Czakó, V. Szalay, A. G. Császár, J. Chem. Phys. 124, 014110 (2006).

[4]     G. Czakó, T. Furtenbacher, P. Barletta, A. G. Császár, V. Szalay, and B. T. Sutcliffe,      Phys. Chem. Chem. Phys. 9, 3407 (2007).

[5]     E. Mátyus, G. Czakó, B. T. Sutcliffe, and A. G. Császár, J. Chem. Phys. 127, 084102   (2007).

[6]     G. Czakó, B. J. Braams, and J. M. Bowman, J. Phys. Chem. A submitted (2008).

Oral

Are Cycloboronenes Aromatic?

Matthew Duvall and Henry F. Schaefer III, Center for Computational Chemistry, University of Georgia

Geometries for Li, Na, and K cycloboronenes (three-membered boron rings) were computed at the B3LYP DFT level using Dunning's DZP basis sets for each atom.  Na and the unsubstituted parent cycloboronene prefer D3h symmetry.  However, Li and K prefer C1 stuctures as both the D_3h and C_3h structures have two imaginary vibrational frequencies.  NICS calculations reveal the diatropic ring currents for each molecule and show an aromaticity comparable to benzene.

Poster 7

UV Spectra of Interstellar Radicals

Ryan Fortenberry and T. Daniel Crawford, Department of Chemistry, Virginia Tech

In the depths of space, the interstellar medium (ISM) has been sprinkled with molecular species that are not typical of those found on earth.  The seemingly unusual nature of these molecules has been observed and, only through the use of guesswork and systematic spectroscopic analysis, been better understood.  This process can be tedious and difficult, especially in the laboratory.  Our project uses theory in this area to explain and predict electronic (UV absorption) spectra.  Four interstellar linear chain radicals were studied, C₄H, SiC₃H, C₆H, and SiC₅H.  The geometries were optimized and ground states determined using the CCSD(T) level of theory with the cc-pVTZ basis set.  TD-B3LYP, CIS, EOM-CCSD, and CC3 methods with aug-cc-pVDZ and aug-cc-pVTZ basis sets were used to calculate excited states for each molecule.  Simulated UV absorption spectra were subsequently created.  These were analyzed in the hopes of better understanding computational tools, interstellar electronic spectra like the Diffuse Interstellar Bands (DIB's), and the potentials of astrobiology.

Poster 8

Novel Derivatives of Bicyclo[2.2.2]octane and Their Strain Energies

E. Chauncey Garrett III, Edward J. Valente, David H. Magers, Department of Chemistry and Biochemistry, Mississippi College

Recently, a new derivative of bicyclo[2.2.2]octane was synthesized at Mississippi College.  To our knowledge, this ester derivative, 1,8,8-trimethyl-2,6-dioxabicyclo[2.2.2]octane-3,5-dione, has not been previously reported in the chemical literature.  The experimental structure of the 2,6-dioxabicyclo[2.2.2]octane ring system was determined from the crystal structure of 1,8,8-trimethyl-2,6-dioxabicyclo[2.2.2]octane-3,5-dione through the use of 13C-NMR, IR, and X-Ray Crystallography.

In the current study, we investigate the conventional strain energy of this system and other related derivatives of the parent compound to see how substitutions in and on the ring affect the overall strain.  For each compound, the conventional strain energy is determined within the n-homodesmotic reaction model.  The n-homodesmotic reaction model provides a theoretical basis for determining thermochemical data and includes the isodesmic, homodesmotic, and hyperhomodesmotic reaction types.  Optimum equilibrium geometries, harmonic vibrational frequencies, and corresponding electronic energies are computed for all pertinent molecular systems using SCF theory, second-order perturbation theory (MP2), and density functional theory.  The DFT functional employed is Becke's three-parameter hybrid functional using the LYP correlation functional.  Two correlation consistent basis sets are employed: cc-pVDZ and cc-pVTZ. 

We gratefully acknowledge support from the NSF (MRI-0321397) and from the Mississippi College Catalysts.

Poster 9

Computational Studies of the Thermodynamics of PF_xO_y and SF_xO_y Compounds: Bond Energy Relationships

Daniel J. Grant, Jackson R. Switzer, Myrna H. Matus, and David A. Dixon, Department of Chemistry, TheUniversity of Alabama

The energetics for the decomposition of sarin have been predicted using high level electronic structure calculations.  The approach is based on calculating the heats of formation of sarin and its fragments from isodesmic reactions at the G3(MP2) level.  Benchmark studies of simpler phosphorus and sulfur fluorine oxides were done at the CCSD(T) level to validate the G3 results The bond energies of the phosphorus compounds show that the P-F bonds are very strong and that in PF₃O, the P-F bond is stronger than the P-O bond. The S-F bonds are weaker than the P-F bonds. The S-O bond is stronger than the S-F bond in SF₂O but weaker than the S-F bond in SF₂O₂. The bond strengths are used to interpret which are the appropriate Lewis structures to draw for these compounds. A natural bond orbital analysis is also used to aid in interpreting the structures.

Poster 10

A Molecular Mechanism for Decreased Activity in Mutant pnb CE Enzymes

Steven Gwaltney, Department of Chemistry, Mississippi State University

 The human carboxylesterase 1 (hCE1) enzyme is catalyzes the hydrolysis of xenobiotic esters.  Hence, it is one of the body’s primary defenses against ester-containing toxicants.  In addition, hCE1 has been implicated in the hydrolysis of cholesteryl oleate, and thus in the transport and removal of cholesterol from cells.  An eight-fold difference in rates for the hydrolysis of the pyrethroid insecticide transpermetherin were found between eleven human liver samples.

The p-nitrobenzyl esterase enzyme from Bacillus subtilis (pnb CE) is an analogue to hCE1.  To better understand the source of the variability in carboxylesterase activity, we compared a series of mutant pnb CE enzymes to the wild type.  The mutant enzymes were formed by replacing the leucine residue 362 at the “side door” of the wild type pnb CE with either a neutral alanine (L362A), a negatively charged glutamate (L362D), or a positively charged arginine (L362R).  We showed the L362R mutant had the largest decrease in activity.

This talk presents our molecular dynamics studies designed to reveal the mechanism for these observed differences.  We believe that the primary cause of the loss in activity in the L362R mutant is because of a change in the hydrogen bonding pattern relative to the wild type.  Specifically, the Arg362 side chain in L362R reaches across the side door opening and hydrogen bonds to Gln276, thereby reducing conformational flexibility in the enzyme.  None of the other mutants show this behavior.  In addition, we will discuss some preliminary results of docking experiments, which show additional docked structures in the mutants, when compared to the wild type.

The project described was supported by Grant Number P20RR0177661 from the National Center for Research Resources.  The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Center for Research Resources of the National Institutes of Health.

Oral

Conformational Analysis and Steric Contributions of 9-cis-Retinoic Acid Analogs

Tracy P. Hamilton, Bryan D. Cox, Kenneth F. Nguyen and Donald D. Muccio, Department of Chemistry, University of Alabama-Birmingham

9-cis-UAB30 is a novel retinoid analog of 9-cis-retinoic acid that is approaching clinical trials for cancer prevention.  When retinoic acid binds to the retinoic nuclear X receptor (RXR), there is significant twisting about the C8-C9 bond. The stable conformations of UAB30 and substituted butadienes have been examined using a calculated torsion potential curve along the C8-C9 torsion.  The energy, E, was calculated after a constrained geometry optimization as a function of the torsion angle, φ, at the B3LYP/6-31G* level of theory.  For UAB30 it was found that the most energetically favorable torsion angle was +/-50° (+/- s-gauche) and is dependant on the conformation of the ring system.  Another favorable conformation was found at 180° (s-trans) which is 1.3 kcal/mol greater in energy than the global minimum at s-gauche.  A barrier of 3.0 kcal/mol separates the global minimum at s-gauche from s-trans, and a much larger barrier at 0° separates the (+)-gauche from the (-)-gauche conformations.  Populations of 9-cis-UAB30 were calculated using a normalized Boltzmann distribution.  The populations for s-gauche and s-trans were found to be 82% and 18%, respectively.  Using the methods described by Guo and Karplus, a series of diene models were used to analyze the steric effects that contribute to the torsion potential curve of 9-cis-UAB30.

1.         Muccio D, Brouillette W, Breitman T, Taimi M, Emanuel P, Zhang X, Chen G, Sani B, Venepally P, Reddy L, Alam M, Simpson-Herren L, Hill D.  J. Med. Chem. 1998, 41,            1679-1687.

2.         Guo H, Karplus M.  J. Mol.  Struct. (THEOCHEM) 1992, 250, 347-393.

Oral

Short Intramolecular Hydrogen Bonds: Derivatives of Malonaldehyde with Symmetrical Substituents

Jacqueline Hargis and Henry F. Schaefer III, Center for Computational Chemistry, The University of Georgia

A systematic study of various derivatives of malonaldehyde to explore very short hydrogen bonds (rOO < 2.450Å) has been carried out. Various electron withdrawing groups, including CN, NO₂, and BH₂ have been attached to the unique carbon. To the two equivalent carbons, a strong electron donator and/or steric hindered substituent was used to strengthen the intramolecular hydrogen bond, including but not limited to NH₂, N(CH₃)₂, and C(CH₃)₃. Six molecules were found to have extremely short intramolecular hydrogen bonds.

The chemical systems investigated are intriguing due to the low energetic barrier for the intramolecular proton transfer. Utilizing B3LYP geometries, the energy barrier was computed using highly correlated methods including MP2 and coupled cluster theories in conjunction with a correlation consistent hierarchy of basis sets, cc-pVXZ (X=D, T, Q, 5). A focal point analysis allowed for the barrier to be calculated at the CBS limit including core correlation and zero point vibrational energy corrections.

This study benchmarks B3LYP energies against highly accurate correlated energies for intramolecular hydrogen bonded systems. A linear relationship was drawn between the underestimated B3LYP energetic barriers and the predicted barrier at the CBS limit.  By relating these two quantities, barrier heights for larger systems are predicted accurately on systems that possess too many atoms to perform rigorous computations using correlated electronic structure methods. 

Poster 11

Improvement of the Coupled-Cluster Singles and Doubles Method via Scaling Same- and Opposite-Spin Components of the Double Excitation Correlation Energy

Edward Hohenstein, Tait Takatani, and C. David Sherrill, Department of Chemistry, Georgia Institute of Technology

There has been much interest in cost free improvements to second-order Møller-Plesset perturbation theory (MP2) by scaling the same-spin and opposite-spin components of the correlation energy (spin-component scaled MP2). We apply this procedure to coupled-cluster with single and double excitations (CCSD) with similar success. Scaling factors for the same-spin (1.13) and opposite-spin (1.27) components were optimized for a set of 48 reaction energies. The spin-component scaled CCSD (SCS-CCSD) method was applied to several van der Wals complexes. SCS-CCSD outperformed all MP2 type methods considered for describing the potential energy surface of these complexes. For methane dimer, benzene dimer, benzene-pyridine, and pyridine dimer SCS-CCSD shows excellent agreement with CCSD(T) at O(N⁶) scaling instead of O(N⁷). SCS-CCSD was used to compute the equilibrium geometry of 19 molecules and their vibrational frequencies. SCS-CCSD outperforms MP2, SCS-MP2, and CCSD and shows excellent agreement with the experimental values.

Poster 12

A Comparative Density Functional Theoretical Investigation of Metal Doped Gold Nano-Clusters M@Au₉ (M = W, Co and Ir)

Delwar Hossain and  Dr. Steven Gwaltney, Department of Chemistry, Center for Environmental Health Sciences, and HPC2 Center for Computational Sciences, Mississippi State University

A major interest of cluster science is to discover highly stable clusters which may be used as building blocks for novel materials.  Gold nanoclusters have attracted considerable interest over the past two decades due to their catalytic properties,1-3 applications as sensors¹, and the use in molecular electronics. They have been employed as a bioconjugate probes for amplification tags in gene analysis², antibody or antigen detection³, DNA sequencing and gene mapping^4,5. The existence of the endohedral gold clusters, M@Au₁₂ confirmed both theoretically and experimentally.^6,7 In addition, element-centered ligated gold cluster compounds, such as the octahedral [{C@Au₆}(Ph₃P)₆X₂] or the icosahedral [{Pd@Au₁₂}(Ph₃P)₈C_l4]₈ clusters, are known.

 

Despite the tremendous research progress on endohedral gold M@Au_n (n = 9 to 12) clusters, studies of metal-encapsulated gold (M@Au_n) clusters containing less than 12 gold atoms are still in their preliminary stages. There are many questions still remained to be address. For example, can a cage with a transition metal encapsulated within a cluster of less than 12 gold atoms exist?  What types of bonding and electronic interactions involved between the transition metal and the gold atoms? In this presentation, we will discuss the relative stabilities, highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gaps, and vertical ionization potentials of metal doped gold clusters, M@Au₉ (M = W, Co, and Ir), investigated by using the different density functional theories (DFT).

References:

1          Y. Kim, R. C. Johnson and J. T. Hupp, Nano Lett., 2001, 1, 165-167.

2          F. Patolsky, Y. Weizmann, O. Lioubashevski and I. Willner, Angew. Chemie, 2002, 41,           2323-2327.

3          J. F. Hainfeld and F. R. Furuya, J. Histochem. and Cytochem., 1992, 40, 177-184.

4          Y. C. Cao, R. Jin and C. A. Mirkin, Science, 2002, 297, 1536-1540.

5          T. M. Herne and M. J. Tarlov, J.  Am.  Chem. Soc., 1997, 119, 8916-8920.

6          P. Pyykko and N. Runeberg, Angew. Chem., 2002, 41, 2174-2176

7          X. Li, B. Kiran, J. Li, H.-J. Zhai and L.-S. Wang, Angew. Chem., 2002, 41, 4786-4789.

8          M. Laupp and J. Strahle, Angew.  Chem., 1994, 33, 207.

Poster 13

Geochemistry and Biogeochemistry: Carbon-isotope Fractionation Factors, Actinyls, and LPS Acidities

Virgil E. Jackson,^a James R. Rustad,^b Sierra L. Nelmes,^b Raluca Craciun,^a and David A. Dixon,^a

^a Department of Chemistry, The University of Alabama

^b Department of Geology, University of California-Davis

^12,13C isotopic fractionation between gaseous CO₂(g), the aqueous carbonate species [CO₂(aq), HCO₃^-(aq), CO₃^2-(aq)], and the common carbonate minerals (calcite, dolomite, and aragonite) is fundamental to a variety of geochemical processes involving the carbon cycle.  Quantum chemical calculations on large supermolecular carbonate-water and carbonate mineral clusters are used to predict equilibrium constants for ^13,12C isotope-exchange reactions between CO₂(g), aqueous carbonate species, and the common carbonate minerals.  For the aqueous species, we evaluate the influence of the size and conformational variability of the solvation shell, the exchange-correlation functional, and the basis set. There is a mixing of the modes of the ion with the solvent leading to more than the minimal number of vibrational modes in the ion being important in determining the isotope fractionation factor. Carbon-isotope fractionation factors for gas, aqueous and mineral phases can be integrated into a single theoretical/computational framework.

Poster 14

Excited Electronic States of Xenon Difluoride

 

Heather Jaeger and Henry F. Schaefer III, Center for Computational Chemistry, The University of Georgia

The properties of the electronic states of xenon fluoride facilitate population inversion for use in high-powered excimer lasers. Excited electronic states arise from a net transfer of an electron from xenon to fluorine resulting in a strong ionic bond. Xenon fluoride in the ground state can be viewed as a weakly interacting Van Der Waals complex. Vacuum-UV photodissociation of XeF2 produces XeF in the metastable ionic state providing a suitable lasing medium.  Investigation of xenon difluoride dissociation via theoretical methods reveals the mechanisms by which all of the non-Rydberg electronic states of XeF are produced. In order to ensure that the ground and excited state wavefunctions are treated equally, CASSCF in conjunction with MRCI calculations were carried out in an active space spanning molecular orbitals derived from all valence-type atomic orbitals. Xenon difluoride in the ground state has a bond order of one. Exciting an electron into the only available molecular orbital leads to dissociation for all but the ³∑⁺_u electronic state.

Poster 15

 Environmental Effect on Molecular Conformation: Bicalutamide Analogues

Hyun Joo, Elfi Kraka, and Dieter Cremer, Department of Chemistry, University of the Pacific

Two Bicalutamide analogues (N-[4-nitro-3(trifluoromethyl)phenyl]-3-(4-fluorophenyl)sulfinyl-2-hydroxy-2-methyl-propanamide 2 and its 4-cyano derivative 3) with an R-configured asymmetric carbon atom and a chiral sulfoxide group are described quantum chemically to determine their properties in dependence of their conformation and their (R,S)-configuration at the sulfoxide S atom. Compounds 2 and 3 are known to be novel androgen receptor antagonists with biological activities that depend significantly on the configuration of their stereogenic centers.  For the purpose of a rapid differentiation between active and less active diastereomers of 2 and 3, relative energies, conformational preferences in different mediums, NMR chemical shift values, vibrational spectra, and vibrational circular dichroism (VCD) spectra are calculated for up to 12 different conformers.  It is demonstrated that both 2 and 3 prefer strongly different conformations in dependence of the surrounding medium and as a consequence of the change from intra- to intermolecular H-bonding. The different diastereomers can be easily distinguished by specific NMR chemical shifts, infrared bands, or VCD rotational strengths.

Oral

Hydrogen Generation Mechanism from Lithium Amidoboranes; an ab initio Study

Tae Bum Lee and Michael L. McKee, Department of Chemistry & Biochemistry, Auburn University

Ammonia borane, BH₃NH₃, has received significant attention as a promising hydrogen storage material due to its high content of hydrogen, 12wt%. However, the activation barrier and kinetics for releasing hydrogen is still a bottleneck preventing practical applications. Here, we are motivated by some recent studies on the catalytic effect of LiH for hydrogen generation in BH₃NH₃. The stepwise mechanism and kinetics in the LiH/BH₃NH₃ system is studied at the CCSD(T)/6-311++G(3d,2p)//MP2/6-311++G(2d,p) level. Addition of LiH lowers the activation barriers compared with pure BH₃NH₃. Our stepwise mechanism coincides well with the experimental observations that show a gradual increase of hydrogen generation over time.

Poster 16

Elucidating Electronic Structure of Transition Metal Oxide Clusters from Photoelectron Spectroscopy and Electron Structural Calculations

Shenggang Li and David A. Dixon, Department of Chemistry, The University of Alabama

Transition metal oxides (TMOs) form an important class of materials widely employed as industrial catalysts and catalyst supports.  Due to the presence of low-lying electronic states, accurate determination of the ground electronic states of these clusters poses a significant challenge.  Using the CCSD(T) method, we have studied the electronic structure of the neutral and anionic clusters of the group IVB and VIB TMOs in various oxidation states.  Our predicted electron affinities, electron excitation energies, clustering energies, atomization energies, and heats of formation are compared with data from recent anion photoelectron spectroscopic measurement and other experiments.  Simulated photoelectron spectra from accurate Franck-Condon calculations are also compared with experiment.  Such comparison proves essential in accurate determination of electronic structure and energetic properties for these clusters.  We have also benchmarked a larger number of density functional methods for calculating these properties, which will be crucial in applying density functional theory to the study of catalytic reaction mechanism involving these TMOs.

Oral

 

Are Conventional Strain Energies in Bicyclic Compounds Additive?

 

David Magers, Department of Chemistry, Mississippi College

Conventional strain energies are computed for bicyclo[1.1.0]butane, spiro[2.2]pentane, bicyclo[2.2.0]hexane, and spiro[3.3]heptane within the isodesmic, homodesmotic, and hyperhomodesmotic models.  In addition, the conventional strain for the individual rings in each bicyclic system is computed with the use of different hyperhomodesmotic equations to see if conventional strain energies might be additive.  The electronic energies and corresponding zero-point-energy corrections are computed for all relevant molecular systems at the levels of SCF theory, DFT, and second-order perturbation theory with both the cc-pVDZ and cc-pVTZ basis sets.  

We gratefully acknowledge support from the NSF (MRI-0321397) and the Mississippi College Catalysts.

Oral

Consequences of High-Dimensionality on Quantum Monte Carlo Simulations

Brent Magnusson, Department of Chemistry, University of Tennessee

Because the Schrödinger equation cannot be solved analytically for systems larger than hydrogen, approximate solution methods must be applied. Quantum Monte Carlo is one of these methods and necessitates the use of one of two imaginary time propagators, 2nd or 4th order. The choice of the propagator implemented depends on a balance between accuracy and efficiency. Obtaining accurate solutions is the primary objective; once this is accomplished, finding ways to do it efficiently is the next intention. With a model system of a multidimensional harmonic oscillator, this study intends to compare and contrast both 2nd and 4th order propagators' diminished accuracy as the dimensionality of the system is increased. We start a simulation by placing walkers at the origin, moving them forward one time step in imaginary time, creating normalized histograms of their positions and comparing these with the known result for the imaginary time propagation of the delta function d(x=0).

Poster 17

Potentials for Intermolecular Interactions in Molecular Dynamics: Rare Gas Atoms

Michael S. Marshall, William B. March, Ashley L. Ringer, C. David Sherrill, Department of Chemistry, Georgia Institute of Technology

The development of fast dual-tree algorithms could offer significant speedup to large scale molecular dynamics (MD) simulations.  Such simulations require a potential form which accurately captures the two-body and even three-body intermolecular interactions between particles.  Various two-body potentials, such as the Lennard-Jones (LJ) and the so called HFD potentials, have been fit to interaction energies for argon dimers computed using highly-correlated electronic structure methods in conjunction with very large basis sets.  Additional functional forms were selected through a functional space search of even-power terms beyond LJ and fit to the theoretically determined interaction energies.  Three-body effects may be included via the Axilrod-Teller (AT) potential.  The parameters for the AT term have been obtained by fitting to CCSD(T)/aug-cc-pV5Z interaction energies for argon trimers.

Poster 18

Computational Studies on Regeneration of Boron-Nitrogen Compounds for Hydrogen Fuel Cells

Myrna H. Matus^a, Daniel J. Grant^a, Jackson R. Switzer^a, Benjamin L. Davis^b, Frances H. Stephens^b, and David A. Dixon^a

^a Department of Chemistry, The University of Alabama

^b Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545

Chemical hydrogen storage is an alternative storage approach which eliminates issues such as high pressure and low temperature, as hydrogen is stored in a compound and delivered via a chemical reaction. We have found that hydrogen elimination from boron-nitrogen compounds, particularly ammonia borane, is a feasible process that can be achieved with the help of a catalyst. In order to reach the Department of Energy's goals for on-board transportation systems, these hydrogen depleted materials must be able to undergo a regeneration process with low or non environmental impact. We have used computational chemistry methods to predict the energetic of a range of reactions using density functional theory and molecular orbital theory. These reactions include digestion reactions, for example with thiols, the subsequent reduction with tin hydrides as well as other digestion, reduction, and redistribution reactions.

Oral

Rotational and Vibrational Energy Levels of the H-/H2 Dimer Using Coupled Cluster Calculations in Preparation for Quantum Monte Carlo Methods

Patrick Moehlen, Department of Chemistry, University of Tennessee

Wang and Andrews [J. Phys. Chem. A, 108, 1103 (2004)] recently observed a feature in the infrared absorption spectra of metal doped solid hydrogen matricies, substantially red-shifted from the H₂ gas phase vibrational frequency, that they attributed to H₂ molecules near H- atomic anions.  The V=0 and V=1 energy levels for H^-/H₂ were calculated from high-level first-principle calculations.  We investigate the effect of the polarizability and hyperpolarizability of H^- and H₂ on the dipole moment function for this interaction.  Accurate fitting techniques are used to model the interaction in preparation for employing high dimensional Quantum Monte Carlo calculations.

Poster 19

Hydrogen Exchange Reactions in C₆₀

Michael McKee, Department of Chemistry, Auburn University

The interactions of several H₂ molecules (H₂)_n n=1-5 within C₆₀, C₇₀, and C₈₂ have been studies with several density functional methods as well as with MP2 and SCS-MP2.  As expected B3LYP, significantly underestimates dispersion interactions while the M05-2X and M06-2X methods are in much better agreement with SCS-MP2 results.  Degenerate exchange reactions were calculated for 3H₂; 3H₂ inside C₆₀, C₇₀ and C₈₂.  The free energy barrier at 298 K is reduced from 88.8 to 36.2 kcal/mol within C₆₀.  Steric compression, dispersion, and a favorable entropy contribution contribute similar increments to the barrier reduction.  At 298K k_cat/k_uncat = 10³⁶.

Oral
 

Propagators, Spectra and Bonding Concepts

J. Vincent Ortiz, Department of Chemistry, Auburn University

Propagator methods enable efficient calculation of transition energies and probabilities and also provide a succinct, pictorial description of the corresponding differences in electronic structure.  New methods that are based on transition operator reference ensembles, quasiparticle virtual orbitals and alternative schemes for the evaluation of electron repulsion integrals have extended the scope of propagator methods for the determination of electron binding energies.  New, perturbative self-energy expressions have proven successful for two-electron propagator calculations of double electron binding energies and polarization propagator calculations of excitation energies. The interpretive power of these methods is illustrated by applications which enable assignment of spectra on reactive, anionic clusters, molecules of biochemical importance and species with unusual, diffusely-bound electrons.

Oral

Capture of Elusive Hydroxymethylene and its Fast Disappearance through Tunneling

Peter R. Schreiner^a, Hans Peter Reisenauer^a, Frank C. Pickard IV^b, Andrew C. Simmonett^b, Wesley D. Allen^b, Edit Mátyus^c & Attila G. Császár^c

^a Institut für Organische Chemie der Justus-Liebig-Universität, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany.

^b Center for Computational Chemistry, The University of Georgia, Athens, GA 30602, USA.

^c Laboratory of Molecular Spectroscopy, Institute of Chemistry, Eötvös University, H-1518 Budapest 112, P.O. Box 32, Hungary.

Although singlet carbenes incorporating divalent carbon (R–C–R) have grown from laboratory curiosities to common reagents in the growing field of stable singlet carbene chemistry,1 several parent systems still await preparation and characterization. Among them, hydroxymethylene (H–C–OH, hydroxycarbene) may be the most important because it is implicated in the photochemistry of its proton tautomer formaldehyde (H₂CO)₂, and was called the “activated formaldehyde” in formation of simple carbohydrates as early as 1913. Hydroxymethylene is the parent of alkoxycarbenes that lie at the heart of transition-metal carbene chemistry, but all attempts to observe alkoxycarbenes have failed, yielding only aldehydes4. Nonetheless, electronic structure theory indicates that hydroxymethylene should be isolable5, due to its position in a relatively deep well on the potential energy surface. Here we report the first synthesis and spectroscopic characterization of this elusive species from an extraordinary confluence of experimental data and state-of-the-art quantum mechanical computations. Hydroxycarbene was prepared by high-vacuum flash pyrolysis from glyoxyclic acid by thermal extrusion of CO₂. Remarkably, H–C–OH rearranges with a half-life of only 2 h to formaldehyde in an argon matrix at 11 K by hydrogen-tunneling through a barrier of over 30 kcal mol–1; in contrast, H–C–OD, prepared from the deuterated precursor, is stable under these conditions. The rapid tunneling of H–C–OH through a large barrier was unanticipated and bears consequences for hydrogen-transfer reactions.

1.         Bourissou, D., Guerret, O., Gabbadï, F. P. & Bertrand, G. Chem. Rev. 100, 39-91 (2000).

2.         Kemper, M. J. H., Vandijk, J. M. F. & Buck, H. M. J. Am. Chem. Soc. 100, 7841-7846       (1978).

3.         Baly, E. C. C., Heilbron, I. M. & Barker, W. F. J. Chem. Soc., Trans. 119, 1025-1035          (1921).

4.         Sierra, M. A. Chem. Rev. 100, 3591-3637 (2000).

5.         Pau, C. F. & Hehre, W. J. J. Phys. Chem. 86, 1252-1253 (1982).

Poster 20

What is Special about Water as a Matrix of Life?

Lawrence Pratt, Department of Chemical & Biomolecular Engineering, Tulane University

A striking feature of known biomolecular structures is that competing hydrophilic and hydrophobic interactions are exploited to achieve spatial organization.  It is natural then to expect that an effective alternative matrix of life should support both solvophobic and solvophilic interactions.  In addition, the availability of solvophobic and solvophilic microphases expands the chemistry that may operate.  Recent progress in understanding hydrophobic effects clarifies how they extend the temperature range for stable function of nanoscopic structures in water, e.g. micelles, membranes, and globular proteins, to encompass the temperature range of observed life, roughly (-20°C, 120°C).  We discuss that recent progress in understanding hydrophobic effects, then on that basis survey liquids and liquid mixtures that might provide alternative media for life.   Particularly identified are polyalcohols (glycerol is a common laboratory example), amino-alcohols such as 2-amino ethanol, and amides such as formamide. These are also solvents with substantial dielectric constants and known electrolyte solution chemistry. Formation of surfactant micelles has been observed in all these solvents, and conventional studies of membranes and globular proteins are clearly possible too.  These are liquids and liquid mixtures that crystallize only with difficulty; this possibility for avoiding crystallization damage also would be an advantage for a non-terrestrial matrix of life.

Oral

 Stable  Long-Time  Semiclassical Description of Zero-Point Energy in  High-Dimensional Molecular  Systems

Vitaly Rassolov, Department of Chemistry, University of South Carolina

Classical   molecular dynamics provides a reasonable general picture of chemical reaction dynamics in most systems of practical interest.  However, comparison of typical reaction energies and zero-point energy show that quantum mechanical effects play an important role in many systems, particularly for reactions of proton transfer. The adequate theoretical description of quantum effects is proved to be a very challenging task, to large extend due to lack of theoretical method that works for anharmonic high dimensional systems for a time longer than few oscillation periods.  We present such a method. It is based on a numerically cheap linearized quantum force approach;  stabilizing  terms compensating  for the linearization errors are added into the time evolution equations for the classical and nonclassical components of the momentum operator. The wavefunction normalization and energy are rigorously conserved.  Numerical tests are performed for model systems of up to 40 anharmonic degrees of freedom.

Oral

Photodissociation Dynamics of Acetaldehyde

Benjamin Shepler, Joel Bowman and Bastiaan Braams, Department of Chemistry, Emory University

 Recently, two mechanisms have been described for the photodissociation of formaldehyde to form molecular products H₂ + CO.  The dominant pathway involves a conventional transition state (TS) saddle point that separates reactants from products.  The minor pathway involves an intramolecular hydrogen abstraction and has been dubbed a “roaming” mechanism.  Acetaldehyde has been proposed as a second molecule that may exhibit a so-called roaming mechanism that bypasses the conventional TS.  To simulate the photodissociation of acetaldehyde several independent full-dimensional potential energy surfaces (PES) have been constructed based on data sets of 130, 000 and 230, 000 ab initio data points.  Details of the construction of the PES and comparison to accurate benchmarks will be presented.  Quasiclassical trajectory calculations have been run on these PES to simulate the 308 nm photodissociation of acetaldehyde.  Trajectories were initiated at both the acetaldehyde equilibrium geometry and the conventional transition state dividing acetaldehyde from the molecular products CH₄ + CO.  The results of these trajectory calculations will be compared with recent experimental reports of the CO rotational distribution and CH₄ vibrational distribution produced from the photodissociation of acetaldehyde.

Oral

Computational Approaches for Non-Bonded Interactions

C. David Sherrill, Department of Chemistry, Georgia Institute of Technology

Non-bonded interactions govern molecular recognition, self-assembly, biomolecular structure, and drug docking.  However, most standard computational methods fail to accurately describe these interactions due to the difficulty in modeling weak dispersion (van der Waals) terms.  Large-basis coupled-cluster computations have allowed the generation of high-quality results for prototype systems featuring non-bonded interactions.  Not only do these computations shed light on the fundamental nature of pi-pi, CH/pi, SH/pi, interactions, etc, but they also allow us to test approximate methods suitable for modeling much larger systems.  Various ab initio, empirical, and semi-empirical approximations have been assessed for their reliability in computing non-bonded interactions.  Of particular interest is the spin-component-scaled coupled-cluster (SCS-CCSD) approach recently introduced by our group.

Oral

Enthalpies of Formation of TNT Derivatives by Homodesmotic Reactions

Amika Sood, Department of Chemistry & Biochemistry, Mississippi College