A theoretical study of the SiCNH isomers

We have studied the lowest-lying species of the SiCNH system, which is of astrophysical and general chemical interest. We have characterized 12 singlet and triplet species, established an energy ordering and computed the enthalpies of formation using accurate ab initio methods that include basis set and wavefunction extrapolations. The lowest-lying form is NCSiH(1A′). CNSiH(1A′) and c-SiNCH(1A′) are only 2.2 and 3.8 kcal/mol higher in energy, whereas SiNCH(1A′), SiCNH(1A′) and c-SiCNH(1A′) lie at 29.3, 30.3 and 33.1 kcal/mol, respectively. The most stable triplets are NCSiH(3A″) (26.5 kcal/mol), SiCNH(3A″) (29.1 kcal/mol) and SiNCH(3A″) (29.3 kcal/mol); the last two species are very close in energy to the corresponding singlets, SiCNH(1A′) and SiNCH(1A′). The electronic structure has been rationalized through the natural bond orbital method. The vertical electronic spectrum has been computed by multi-reference configuration interaction methods. All the singlet species present 11A″ as the lowest-lying excited singlet. The corresponding transition from the ground state has a moderate intensity in the case of NCSiH(1A′) and CNSiH(1A′). There is, at least, one intense valence transition in each of the singlet forms, although they tend to be of relatively high energy. SiNCH(1A′) and SiCNH(1A′) have very small 11A″-X1A′ vertical gaps (15.7 and 9.22 kcal/mol), respectively. Among the triplet species, SiNCH(3A″) and SiCNH(3A″) present rather intense valence transitions.