Molecular Structure of Cyclopropyl (Isocyanato) Silane: A Combined Microwave Spectral and Theoretical Study

The molecular equilibrium structures of two conformers (cis and gauche) of C₃H₅–SiH₂–NCO have been deduced by a combination of microwave (MW) spectra at natural abundance including data from ¹³C and ^29,30Si isotopomers and ab initio calculations. The MW rotational constants (RCs) for the most abundant isotopes are cis: A = 4216.3617(64), B = 1225.76654(91), and C = 1037.31468(77) MHz and gauche: A = 4955.55(79), B = 1094.9276(81), and C = 942.7031(80) MHz. The symmetric quartic centrifugal distortion constants have been evaluated for the cis conformer, using the I^r representation for <i>C</i>_<i>S</i> symmetry. Only partial substitution structures (PSSs) could be derived from the spectra after inclusion of the above isotopic combinations at each center. Using the PSSs, the full structures were determined by ab initio calculation of the equilibrium structures using coupled-cluster singles and doubles with selected triples configuration calculations (CCSD­(T)); the two conformers have an energy difference of 228 cm^–1 (cis lower than gauche). The similarity of the calculated and MW RC results confirms the identities of the two compounds. The more interesting cis conformer has bond lengths C₂–Si₃, 1.9072(73), C₂–C₉ 1.464(22), and C₉–C₁₀ 1.4944(33) Å and angles Si₃–C₂–C₁₀ 119.4(12)° and C₉–C₂–C₁₀ 57.1(12)°, with similar results for the gauche conformer. The Si₃N₄C₅ angle is wide in the cis conformer (145°) and nearly linear in the gauche conformer (179°). New physical insights into the bonding of cis conformers of this type have led the identification of an attractive force between the relatively crowded cyclopropyl and isocyanato groups in the cis conformation. This is demonstrated by three methods: Comparing electronic charges (both AIMALL and Mulliken analyses) in the pair of conformers shows a relative shift of density between these groups in the cis compound. Comparison of the highest occupied molecular orbitals (HOMOs) shows major mixing of density, exemplified by HOMO-1 in these structural units for the cis conformer but which is absent for the gauche conformer. Finally, the nearly linear isocyanate moiety (and the molecular dipole moment) of the cis conformer points closely toward the connected C atom of the cyclopropyl ring, while the gauche conformer dipole moment is significantly different in direction and points toward the midpoint of the C₂Si₃ bond. Both the HCSiN torsional and Si–NC bending surfaces connecting these conformers were explored at the Møller–Plesset second-order perturbation theory level (MP2), which led to the exclusion of other conformers. The bending surface shows a very high amount of quartic potential function