Raman and infrared spectra, conformational stability, ab initio calculations and assignment of fundamentals for 1-bromo-3-fluoropropane

The infrared and Raman spectrum of 1-bromo-3-fluoropropane is reported in the gas, liquid, amorphous solid and annealed polycrystalline states. Only one of the five possible conformers is stable in the crystal, designated the C conformer. The disordered phases show the presence of several other conformers of higher energy, due entirely to conformers designated B and D. Ab initio calculations were performed as rhf/4-31g*/MIDI-4*, rhf/6-31g* and mp2/6-31g* (both frozen core and full electron correlation) for all five conformers. The scaled harmonic force field obtained using the mp2 = full/6-31g* level of the theory is reported for the most stable conformer together with an assignment of fundamentals and potential energy distributions for local symmetry coordinates. Selected computational results are reported for all conformers together with scaled and unscaled wavenumbers and infrared and Raman intensities. The temperature dependent Raman spectrum is reported from room temperature to -100 degrees C. Only three of the five possible conformers can be identified in this spectrum, and there is no evidence of the other two. The energy differences between conformers in the liquid phase were found experimentally to be 132+/-27, 232+/-46 and 106+/-30 cm(-1), respectively between the D and C, B and C and D and B conformers. These differences are substantially less than the differences calculated ab initio at the highest level of the theory used, suggesting that energy differences were decreased by large dipole-dipole interactions present in the liquid but not in the gas.