Vibrational conformational analysis of ethyl formate

Abstract

Infrared (4000-50 cm-1) spectra of gaseous, amorphous and crystalline ethyl formate, HCOOCH2CH3, were recorded. Raman spectra of the gaseous, liquid and solid phases were investigated from 3500 to 25 cm-1. Additionally, qualitative depolarization ratios were obtained from the Raman spectra of the liquids. Ab initio calculations were carried out using a 4-21G basis set and show that the compound can occur as four possible conformers, s-cis trans, s-cis, gauche, s-trans trans and s-trans, gauche. Their optimized structural parameters, their force field and the vibrational wavenumbers and potential energy distributions were calculated for each conformer. Additionally, the potential function governing internal rotation from the s-trans, trans to the s-cis trans conformation was calculated. Both the s-cis conformers were identified in the vibrational spectra of the gaseous, amorphous and fluid phases, and there are strong indications that a weak band at 623.5 cm-1 in the Raman spectrum of the fluid phases is due to one of the s-trans conformers. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between the cis, trans and s-cis gauche conformers was determined to be 1.4 ± 0.4 kJ mol-1 with the trans conformer being the more stable rotamer. Vibrational spectra of the crystalline solid, which consists only of the s-cis, trans conformer, showed that ethyl formate can crystallize in two different forms. A complete assignment of the vibrational fundamentals from the infrared and Raman spectra is proposed.