Abstract
Matrix isolation infrared (IR) studies have been carried out on the vaporisation of the alkaliβmetal azides MN~3~ (M = Na, K, Rb and Cs). The results show that under high vacuum conditions, molecular KN~3~, RbN~3~ and CsN~3~ are present as stable highβtemperature vapour species, together with variable amounts of nitrogen gas and the corresponding metal atoms. The characterisation of these molecular azides is supported by ab initio molecular orbital calculations and density functional theory (DFT) calculations, and for CsN~3~ in particular, by the detection of the isotopomers Cs(^14^N^15^N^14^N) and Cs(^15^N^14^N^14^N). The IR spectra are assigned to a βsideβonβ (C~2__v__~) structure by comparison with the spectral features predicted both by vibrational analysis and calculation. The most intense IR features for KN~3~, RbN~3~ and CsN~3~ isolated in nitrogen matrices lie at 2005, 2004.4 and 2002.2 cm^β1^, respectively, and correspond to the N~3~ asymmetric stretch. The N~3~ bending mode in CsN~3~ is identified at 629 cm^β1^. An additional feature routinely observed in these experiments occurred at approximately 2323 cm^β1^ and is assigned to molecular N~2~, perturbed by the close proximity of an alkaliβmetal atom. The position of this band appeared to show very little cation dependence, but its intensity correlated with the extent of sample thermal decomposition.