Raman and infrared spectra, conformational stability, barriers to internal rotation and ab initio calculations for 3-methyl-2-butenoyl fluoride

Abstract

The Raman (3200 to 20 cm −1) spectra of liquid and solid and infrared (3200 to 400 cm −1) spectra of gaseous and solid 3-methyl-2-butenoyl fluoride (3,3-dimethylacryloyl fluoride), (CH 3) 2C=C(H)CFO, have been recorded. The far-infrared spectrum of the gas has also been recorded from 360 to 50 cm −1 at a resolution of 0.1 cm −1. The syn (s-cis) conformer is the only stable rotamer (two double bonds oriented cis to each other) present in the fluid at ambient temperature and solid states. The barriers for the syn (s-cis) to anti (s-trans) and anti to syn conformers are predicted to be 2498 and 1931 cm −1, respectively, with an energy difference of 567 cm −1 (1.62 kcal mol −1) from the RHF/6-31G* calculations. The energy difference was only slightly smaller, 487 cm −1 (1.39 kcal mol −1) with electron correlation, i.e. MP2/6-31G*. Barriers to internal rotation of the two non-equivalent methyl rotors with torsional fundamentals at 205 (trans methyl rotor) and 109 cm −1 were determined to be 920 cm −1 (2.63 kcal mol −1) and 304 cm −1 (0.87 kcal mol −1), respectively. A complete assignment of the normal vibrational modes is proposed. The structural parameters, force constants and vibrational frequencies for the syn conformer have been determined from ab initio gradient calculations employing the RHF/3-21G* and RHF/6-31G* basis sets along with electron correlation at the MP2/6-31G* level and the theoretical results are compared to the experimental values when appropriate. These results have been compared with the corresponding ones obtained for some similar molecules.