Ab initio calculation 2-fluoroethanol and 2-chloroethanol with reference to the infrared photorotamerization processes in low-temperature matrices

Abstract

Ab initio STO-3G optimizations have been carried out on the five conformers of 2-fluoro and 2-chloro-ethanol and on ten saddle point conformations of 2-fluoroethanol. The relative order of the calculated conformer energies is found to be Gg′ « Tt < Gt ·~ Tg ≈ Gg for 2-fluoroethanol and Gg' « Tt < Tg < Gt < Gg for 2-chloroethanol, the Tt - Gg′ energy differences being 4.0 and 5.1 kJ mol −1, respectively. Large variations (4–7°) are found for the angles CCO and OCH from conformer to conformer and a general angle rule is presented for primary alcohols. Neglect of this effect is the main source of error in rigid-rotation calculations on alcohols (as compared with fully optimized calculations), of the order of 5 kJ mol −1. The use of calculated OH stretching force constants in assigning νOH band fine structure in IR spectra is discussed. Evidence for a weak intramolecular hydrogen-bonding interaction is found, especially in the case of 2-chloroethanol. On the basis of calculated conformer energies, barriers to rotation and force constants it is deduced that for both of these alcohols the photorotamerization process in matrices is either Gg′ → Tt or Gg′ → Gt.