The SiC₂ Saga Continues: Revised Barrier to Linearity, Equilibrium Structures, Fundamental Frequencies, and Enthalpy of Formation.

Lucas Speakman and Henry F. Schaefer, III, Department of Chemistry, University of Georgia

 Silicon dicarbide, SiC₂, remains one of the most demanding challenges for computational methods to achieve converged predictions for the topography, energetics, and vibrational dynamics of the ground state surface.  The silicon dicarbide system exhibits a mercurial surface for the circumnavigation of Si⁺ about C₂^- in that almost all conceivable variations with level of theory are observed.  While the minimum is a “T-shaped” C_2v structure at the highest levels of theory, the barrier to linearity ranges from –5.09 to 6.59 kcal mol-1 depending on the choice of basis set and correlation treatment.  To address basis set incompleteness and to provide a high-order correlation treatment necessary for accurate predictions, we have developed a composite approach consisting of a complete basis set coupled-cluster with singles and doubles method augmented by a perturbative triples term [CBS CCSD(T)], plus a coupled-cluster scheme with a full triples correction (aug-cc-pVTZ CCSDT), a mass-velocity and Darwin relativistic term [aug-cc-pVTZ CCSD(T)], and a core-valence electron correlation adjustment [aug-cc-pCVQZ CCSD(T)].  Although higher-order coupled-cluster treatments affect the barrier to linearity, –1.73 (SCF), 2.17 (MP2), 2.04 (CCSD), 0.37 [CCSD(T)], 0.26 (CCSDT), 0.11 [CCSDT(Q)], 0.06 (CCSDTQ), and 0.23 [CCSDTQ(P)] kcal mol-1, we limit the composite approach to full triples because of its excellent approximation to CCSDTQ(P).  Our composite method (c-CBS CCSDT) is systematically applied to the ground state structures, the quartic force field, and the global potential energy surface of SiC₂.  The c-CBS CCSDT method yields a barrier to linearity of 5.44 kcal mol-1, bond distances of 1.8305 (1.6875) Å and 1.2686 (1.2822) Å for rSi-C and rC-C of the “T-shaped” (linear) structures, fundamental vibration frequencies for the “T-shaped” ground state of 1756.5, 843.3, and 176.2 cm^-1, and ΔH_f° (SiC₂) of 154.52 kcal mol^-1.

Oral