The vibrational spectra including matrix isolation, conformations and ab initio calculations of 4-azidobut-1-yne

Abstract

4-Azidobut-1-yne can exist in five distinct conformations denoted GG, GG′, GA, AG and AA according to the gauche and anti conformation around C–C and the C–N bonds, respectively. Fully optimised ab initio geometries for all five conformers have been calculated at the MP2(full)/TZ2P//MP2(full)/TZ2P level. The calculated conformational energies were GG (0), AG (+1.24 kJ mol −1), AA (+1.50 kJ mol −1), GG′ (+3.35 kJ mol −1) and GA (+3.75 kJ mol −1). Additional single point calculations were also performed at the MP4(DSQ)/TZ2P//MP2(full)/TZ2P level for all five conformers. IR spectra of the molecule were obtained in the vapour and liquid states at room temperature and as amorphous and crystalline solids at liquid nitrogen temperature. Additional IR spectra of argon and nitrogen matrices were recorded, applying the hot nozzle technique with temperatures between 313 and 600 K. Raman spectra of the liquid in the temperature range 232–295 K and of a crystalline solid at 148 K were also measured. A number of strong bands disappeared in the crystal phase. The same bands increased their relative intensity upon annealing of the low temperature matrices suggesting that the most stable conformer in the vapour and in the liquid is not the one present in the crystal phase. Apparently, all five conformers are present in the liquid at room temperature and in the argon matrix at 12 K, whereas only three conformers (GG, AG and AA) were observed in the nitrogen matrix. The liquid phase Raman spectra and the hot nozzle IR matrix isolation spectra showed very small temperature variations and it was not possible to estimate the energy differences between conformers. To assist in the spectral assignment, the ab initio force fields for all conformers were scaled using six scale factors. The results suggest that the conformer present in the crystal phase is AG and the most stable conformer in the liquid as well as in the low temperature matrices is GG.