Raman and infrared spectra, conformational stability, vibrational assignment and ab initio calculations for ethyl chloroformate

Abstract

AbstractInfrared (4000‐50 cm−1) spectra of gaseous, amorphous and crystalline solid ethyl chloroformate, ClCOOCH2CH3, and mid‐infrared spectra (4000‐500 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 spectra. The enthalpy difference between the s‐cis,gauche 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.