A CCSD(T) and ccCA study of mixed silicon hydrides and halides: Structures and thermochemistry

A set of 24 mixed silicon hydrides and halides (SiHnXm−n, where X = F, Cl; m = 1-4; and n = 0-m) has been investigated using; (1) CCSD(T) with the correlation consistent basis sets of double-through quintuple-ζ quality; (2) CCSD(T) with the augmented tight d form of the correlation consistent basis sets for the second row atoms (Si and Cl); and (3) the recently developed correlation consistent composite approach (ccCA). Optimized geometries and harmonic vibrational frequencies are reported, as well as enthalpies of formation, free energies of formation, and dissociation reaction enthalpies. Relativistic effects, including atomic spin-orbit coupling and scalar relativistic effects, and core-valence correlation, have been accounted for via the ccCA formulism. As demonstrated in previous studies, the tight d basis sets accelerate the convergence, as compared to the standard correlation consistent basis sets, of CCSD(T)-computed geometries and energetics of molecular species containing second row, main group atoms. The ccCA enthalpies of formation are shown to be similar and, on average, better than the corresponding CCSD(T) enthalpies of formation for the silicon species. Further, large DKH-CCSD(T) computations with explicit inclusion of core-valence and scalar relativistic effects have been performed to predict the enthalpies of formation of SiHF, SiHCl, SiH2F, SiHF2, SiH2Cl, and SiHCl2.