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

Abstract

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 (13)C and (29,30)Si 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 CS 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.

Molecular Structure of 1-Isocyano-1-silacyclopent-3-ene: A Combined Microwave Spectral and Theoretical Study

“…The SiNCO unit is trans, as regularly found in simple isocyanate structures. Except for the CSiC angles, comparison with cis -cyclopropyl (isocyanato) silane (Figure ,I) is the most appropriate. This conformer has bond lengths (Å) and angles (°) in the CSiNCO unit as C–Si 1.849, Si–N 1.744, N–C 1.209, and C–O 1.175 Å.…”

“…The SiNCO unit is trans, as regularly found in simple isocyanate structures. Except for the CSiC angles, comparison with cis-cyclopropyl (isocyanato) silane (Figure 1,I) 4 We now consider the present results in the light of related compounds. There has been considerable interest in the parent silicon analogue of cyclopentadiene, 19−28 1-silacyclopent-3-ene (C 4 H 6 SiH 2 ), and particularly its SiF 2 analogue.…”

“…Our previous studies − on silyl isocyanates have shown the necessity for use of all-electron basis sets, which are triple zeta in the valence shell exponents; in addition, two or more sets of polarization functions are necessary. This was applied here, where we used the largest of the Pople et al − series of bases, denoted as 6-311++G­(3df,3pd), for all atomic centers.…”

“…Recently, we reported combined microwave spectral (MW) and theoretical studies of some silyl isocyanates where unusual structural features, such as wide SiNC angles, occur. − A similar study of cyclopropylsilyl isocyanate (C 3 H 5 –SiH 2 –NCO) by MW methods showed the presence of cis and gauche conformers, which have extremely wide CNSi angles of 156.2 and 153.0°, respectively. The cis structure (Figure ,I) shows that the classical isocyanato group lies almost above the center of the ring.…”