Electron diffraction study of the molecular structures of monomethylsilyl isocyanate and monomethylsilyl isothiocyanate in the gas phase

Abstract

The molecular structures of the methylsilyl pseudohalides SiH₂MeNCO and SiH₂MeNCS have been determined by electron diffraction in the gas phase. The molecules appear bent at nitrogen, but it is shown that this is probably due to a shrinkage effect caused by a low-frequency, large amplitude bend at nitrogen, as found for the simple silyl analogues SiH₃NCY (Y = O or S). The major structural parameters (r_a, bond lengths in pm, angles in degrees) are: SiH₂MeNCO, r(Si–N) 171.8(2), r(Si–C) 185.3(3), r(NC) 121.3(23), r(CO) 116.9(18), Si–N–C 151.7(16), N–Si–C 104.5(21);SiH₂MeNCS, r(Si–N) 172.4(6), r(Si–C) 184.8(8), r(NC) 119.5(4), r(CS) 157.8(3), Si–N–C 156.4(16), N–Si–C 108.5(14). In each case the torsion angle between the plane containing the SiC and SiN bonds and that containing the SiN and NC bonds is close to 90°. Comparisons are drawn with other pseudohalides MH_nMe_3-nNCY, where M = Si or C and n= 0–3.