RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, AND AB-INITIO CALCULATIONS OF BROMOCARBONYL ISOCYANATE

The Raman (3100-10 cm(-1)) and infrared (3100-30 cm(-1)) spectra of bromocarbonyl isocyanate, BrC(O)NCO, have been recorded for the fluid and solid phases. The observed bands are assigned on the basis of the trans conformer (isocyanate group trans to the carbonyl bond) being more stable and the cia conformer being less stable in the fluid phases, and only the trans conformer being present in the solid. Variable temperature studies of the Raman spectrum of the liquid give a Delta H value of 423 +/- 42 cm(-1) (1.21 +/- 0.12 kcal mol(-1)). A complete vibrational assignment is proposed for the trans conformer, based on infrared band contours, group frequencies, relative intensities, and normal coordinate calculations. Additionally, several fundamentals of the cis conformer have been assigned. The experimental conformational stability and fundamental vibrational frequencies are compared with those obtained from ab initio gradient calculations employing the RHF/STO-3G* basis set and with the corresponding quantities obtained for similar molecules.