THE VIBRATIONAL-SPECTRA AND AB-INITIO CALCULATIONS OF TRANS-1,3-DIBROMO-1,3-DIMETHYLCYCLOBUTANE AND CIS-1,3-DIBROMO-1,3-DIMETHYLCYCLOBUTANE

Infrared spectra of the title compounds were recorded as vapours, as s olutes in various solvents, and as solids at ambient temperature and a t 80 K. Raman spectra of the solids were obtained at various temperatu res between ambient and 80 K. While trans-1,3-dibromo-1,3-dimethylcycl obutane exists in one puckered conformer only, the cis compound can be present as a mixture of diequatorial and diaxial bromine conformers. In the trans compound a large majority of the IR and Raman bands of th e crystal did not coincide, suggesting C-2h molecular symmetry. Accord ingly, this cyclobutane ring appears to be planar or very close to pla nar in the crystal, as observed for certain cyclobutanes. However, no significant changes between the IR spectra of the vapour, melt and cry stal were detected. No additional IR or Raman bands of the cis compoun d were detected in the vapour phase, in the melt or in saturated solut ions compared to the solid state spectra, suggesting the presence of n egligible amounts of the diaxial bromine conformer. The IR and Raman s pectra of the trans and cis compounds were assigned and compared with the results of ab initio quantum chemical calculations employing the b asis sets STO-3G, HF/3-21G and HF/6-31G*. (C) 1997 Elsevier Science B .V. (C) 1997 Elsevier Science B.V.