Conformational stability, vibrational assignments, and normal coordinate analysis from variable temperature FT-IR spectra of xenon solutions and ab initio calculations of epibromohydrin

Infrared spectra (3500-400 cm(-1)) of epibromohydrin (bromomethyloxirane), c-OC2H3C(Br)H-2, dissolved in liquid xenon were recorded at several tempera tures from -55 degrees C to -100 degrees C. In addition, the Raman spectrum of the liquid was obtained from + 8 degrees C to -111 degrees C. The spect ra are consistent with three stable conformers, existing in both fluid phas es at ambient temperature. These data indicate that the gauche-2 conformer is the most stable form and the gauche-l rotamer(most polar) the second mos t stable form in the xenon solution. However, the gauche-l conformer is the most stable form and the gauche-2 conformer the second most stable form in the liquid. Utilizing the well separated triplets of three fundamentals as a result of the three conformers, the enthalpy differences were determined to be 83 +/- 9 cm(-1) (238 +/- 25 cal/mol) (gauche-2 to gauche-l) and 465 +/- 20 cm (1330 +/- 60 cal/mol) (gauche-2 to cis) in the xenon solution and 254 +/- 15 cm(-1) (725 +/- 44 cal/mol) (gauche-1 to gauche-2) and 433 +/- 15 cm(-1) (1240 +/- 40 cal/mol) (gauche-l to cis) in the liquid. The confor mational stabilities, structural parameters, dipole moments, and vibrationa l frequencies were determined by ab initio calculations with two basis sets (6-31G* and/or 6-311 + + G**) with full electron correlation by the pertur bation method at the MP2 level. Vibrational assignments for the 24 normal m odes, for both the gauche-2 and gauche-1 conformers, were proposed with sev eral of the fundamentals assigned for the cis conformer. Utilizing the ab i nitio predictions and previously reported rotational constants, adjusted r( o) structural parameters were proposed for the gi and gz conformers. (C) 19 99 Elsevier Science B.V. All rights reserved.