Computational study on the energies and structures of the [H, Si, N, C, S] isomers

Computational chemistry methods have been used to study the lowest lying [H, Si, N, C, S] isomers which are of astrochemical interest. A number of [H, Si, N, C, S] isomers have been investigated at the B3LYP/aug-cc-pVTZ level of theory. Of these, the seven lowest isomers have been further investigated using different levels of theory, including MP2 and QCISD(T). It has been found that at all levels of theory the lowest energy isomer is HSi(S)CN. For the seven lowest isomers, rotational constants and vibrational frequencies have been evaluated. In addition, the enthalpies of formation of the seven lowest isomers have been evaluated using the G3MP2 multilevel method and shows that the isomers are relatively thermodynamically stable compared to other silicon-containing species that have been detected in space. The structures and energies of the lowest lying excited singlet states of these isomers have also been investigated. For the isomers HSi(S)CN, HSi(S)NC, HS–SiCN and HS-SiNC, the lowest lying excited state is predicted to be non-planar, while for HSiNCS the lowest lying excited state is predicted to be planar with similar structural changes occurring on excitation as seen in HSiNCO and HSiNC.