Silaacetylene: A possible target for experimental studies

The equilibrium geometries and transition states for interconversion of the CSiH₂ isomers in the singlet electronic ground state are optimized at the MP2 and CCSD(T) levels of theory using a TZ2P basis set. The heats of formation, vibrational frequencies, infrared intensities, and rotational constants are also predicted. There are three energy minima on the CSiH₂ potential energy surface. Energy calculations at CCSD(T)/TZ2P(fd) + ZPE predict that the global energy minimum is silavinylidene (1), which is 34.1 kcal mol^-1 lower in energy than trans-bent silaacetylene (2) and 84.1 kcal mol^-1 more stable than the vinylidene isomer (3). The barrier for rearrangement 2 → 1 is calculated at the same level of theory to be 5.1 kcal mol^-1, while for the rearrangement 3 → 2 a barrier of 2.7 kcal mol^-1 is predicted. The natural bond orbital (NBO) population scheme indicates a clear polarization of the C - Si bonds toward the carbon end. A significant ionic contribution to the C-Si bonds of 1 and 2 is suggested by the NBO analysis. The C - Si bond length of trans-bent silaacetylene (2) is longer than previously calculated [1.665 Å at CCSD(T)/TZ2P)]. The calculated carbon-silicon bond length of 2 is in the middle between the C - Si double bond length of 1 (1.721 Å) and the C - Si triple bond of the linear form HCSiH (4), which is 1.604 Å. Structure 4 is a higher-order saddle point on the potential energy surface.