THERMOCHEMISTRY OF CHN, SIHN (N = 0-4), AND THE CATIONS SIH+, SIH2+, AND SIH3+ - A CONVERGED QUANTUM-MECHANICAL APPROACH

We have determined 0 K heats of formation of CH(n) and SiH(n) (n=0-4) as well as the cations SiH+, SiH2+, and SiH3+ using large atomic natural orbital basis sets and coupled cluster methods including all single, double, and (perturbatively) triple excitations [CCSD(T)]. Core-correlation effects on the bond dissociation energies have been explicitly evaluated. For the intermediate hydrides CH(n) and SiH(n) (n=1-3), heats of formation are determined from theoretical bond dissociation energies in two ways: using experimental heats of formation of the H and C (or Si) atoms; and using experimental heats of formation of the H atom and the parent hydrides CH4 (or SiH4). In principle, this procedure allows us to place rigorous upper and lower bounds on the heats of formation of the intermediate hydrides. Because our theoretically predicted atomization energies are already of high quality, estimation of remaining deficiencies in the one-particle basis sets can be obtained from extrapolation of observed trends in atomization energies upon basis set expansion. These extrapolated results are in outstanding agreement with experimental values where they are known to high accuracy. For the SiH(n) compounds, a serious problem occurs: our predicted atomization energy of SiH4 is larger than that obtained from experimental heats of formation for the silicon atom and silane. Thus either relativistic effects on the atomization energy of SiH4 are large, or the experimental heats of formation of Si and SiH4 are incompatible. Excepting the atomization energy of SiH4, and thus the heats of formation of Si and SiH4, none of our other SiH(n) thermochemical predictions (properly interpreted) are clearly incompatible with experiment. Furthermore, our theoretical predictions are again in outstanding agreement with experimental determinations that are most certain.