A range of highly correlated ab initio methods is used to predict the geometrical parameters of silicon cyanide (SiCN), silicon isocyanide (SiNC), and two transition states (2A’ and 2A’’) for the isomerization reaction transforming one to the other. Also predicted are dipole moments, rotational constants, and harmonic vibrational frequencies. At all levels of theory, the SiCN and SiNC molecules are predicted to have linear equilibrium structures. The SiNC isomer is found to lie 1.5 kcal/mol above the SiCN species at the coupled cluster (CC) with single, double, and full triple excitations (CCSDT) level of theory with the correlation-consistent polarized valence quadruple zeta (cc-pVQZ) basis set. These theoretical predictions complement the recent laboratory production of SiCN and SiNC and subsequent astronomical detection of SiCN in the envelope of the C Star IRC110216/CW Leo. The theoretical Be values of 5481 MHz (SiCN) and 6316 MHz (SiNC) at the CC with single, double, and iterative partial triple excitations (CCSDT-3) level of theory with the cc-pVQZ basis set are consistent with the experimental B0 values of 5543 MHz (SiCN) and 6397 MHz (SiNC). The transition states for the isomerization reaction SiCN->SiCN are found to proceed through the 2A’ and 2A’’ surfaces, which lie 20.9 and 21.8 kcal/mol above the SiCN minimum at the cc-pVQZ CCSDT-3 level of theory. The ground states of SiCN and SiNC radicals are subject to Renner–Teller interactions. At the CC with single, double, and perturbative triple excitations [CCSD(T)] level of theory, the Renner parameters and the averaged harmonic bending vibrational frequencies are determined to be 0.318 and 249 cm-1 for SiCN and 0.412 and 195 cm-1 for SiNC.
Richardson, Nancy A., "Isomerization of the Newly Discovered Interstellar Molecule SiCN to SiNC Through Two Transition States" (2003). Faculty Publications and Presentations. Paper 70.