RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, VIBRATIONAL ASSIGNMENT AND AB-INITIO CALCULATIONS FOR ETHYL CHLOROFORMATE

Infrared (4000-50 cm(-1)) spectra of gaseous, amorphous and crystalline solid ethyl chloroformate, ClCOOCH2CH3, and mid-infrared spectra (4000-508 cm(-1)) of the chloroformate dissolved in liquefied krypton were recorded. Raman spectra of the liquid and solid phases were investigated from 4000 to 50 cm(-1). MP2 and RHF ab initio calculations were carried out using a 6-31G* basis set. The calculations show that the compound can occur as the s-cis,trans, the s-cis,gauche, the s-trans,trans and the s-trans,gauche conformers. For each conformer the optimized structural parameters, the vibrational wavenumbers, infrared intensities, Raman scattering activities and potential energy distributions were calculated. Both s-cis conformers were identified in the vibrational spectra of all non-crystalline phases. Evidence for the presence of the s-trans conformers is found in the liquid-phase Raman spectre. The enthalpy difference between the s-cis,ganche and s-cis,trans conformers was determined in the pure liquid phase to be 3.2 +/- 0.3 kJ mol(-1). Also in the liquid phase, the enthalpy difference between the observed s-trans and the s-cis,trans conformers was determined to be 14.3 +/- 0.8 kJ mol(-1). Vibrational spectra of the crystalline solid, which contains only the s-cis,trans conformer, showed that ethyl chloroformate can crystallize in two different forms. A complete assignment of the infrared and Raman spectra is proposed.