Far-Infrared Spectrum, Conformational Stability, Barriers to Internal Rotation, Normal Coordinate Calculations, and Vibrational Assignment for Chloroacetaldehyde

The far infrared spectrum (350 to 25 cm(-1)) of gaseous chloroacetaldehyde, ClCH2CHO, has been recorded at a resolution of 0.10 cm(-1) The first excited-state transition of the asymmetric torsion of the more stable near s-cis (chlorine atom s-cis to the aldehyde hydrogen atom) conformer has been observed at 26.9 cm(-1), with seven additional upper state transitions falling to higher frequency. Additionally, the fundamental torsional transition of the s-trans conformer has been observed at 58.9 cm(-1) with two excited states also falling to higher frequency. From these data, the asymmetric torsional potential coefficients have been determined to be: V-1 = 414 +/- 11; V-2 = 191 +/- 3; V-3 = -203 +/- 5; V-4 = 44 +/- 1 and V-6 = -26 +/- 1 cm(-1). The s-cis to s-trans barrier is 500 +/- 5 cm(-1) (1.43 +/- 0.01 kcal mol(-1)) with the s-cis conformer being more stable by 267 +/- 19 cm(-1) (0.76 +/- 0.05 kcal mol(-1)) than the s-trans form. The Raman (4000 to 100 cm(-1)) and infrared (4000 to 400 cm(-1)) spectra of the gas have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values obtained. Complete vibrational assignments are proposed for both conformers based on band contours, depolarization values, and group frequencies. The assignments are supported by ab initio Hartree-Fock gradient calculations employing the 3-21 G* basis set to obtain the frequencies and the potential energy distributions for the normal vibrations for both rotamers. Additional ab initio calculations at the MP4/6-31 G* level have been carried out to determine the structural parameters for both conformers. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.