The structures of the azido-, isocyanato- and isothiocyanato- derivatives of methane and silane and their derivatives. A comparison of ab initio with experimental results

Abstract

The equilibrium structures of various conformers of the azides, isocyanates and isothiocyanates with general type Me 3− n H n W–NXY, together with a small number of their halogenated derivatives, have been determined by Möller–Plessett (MP2) calculations under standard triple zeta valence + polarisation basis set conditions. The structures are compared with each other, with previous and other theoretical studies including CCSD(T) results, and with experimental spectral and diffraction results. The MP2 and CCSD(T) results correlate strongly for a series of these molecules; thus the CCSD(T) bond lengths are larger by about 1.9%. The angles are little different, partly as a result of balancing effects; where some CNN and CNC angles are smaller in CCSD(T) by ∼2–3°, but some CCSD(T) dihedral angles HCNN and HCNC are larger. The MP2 results for the wider group correlate closely with microwave and infrared spectra, and with electron diffraction data. In contrast, the B3LYP method, with the same bases, whilst apparently giving reasonable structures, gives very poor rotation constants, as a result of poor CNC angle determination. The present results suggest that the spectra of ethyl isocyanate, silyl azide, trimethylsilyl azide, and silyl isocyanate require reinvestigation. This is part of a wider study where the central atom W is the series of Group 14 tetrahedral atoms up to Pb; in this study, the central atom W is C and Si, with XY either NN, CO or CS.