VIBRATIONAL CONFORMATIONAL-ANALYSIS OF ETHYL FORMATE

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. Addi tionally, qualitative depolarization ratios were obtained from the Ram an spectra of the liquids. Ab initio calculations were carried out usi ng a 4-21G basis set and show that the compound can occur as four poss ible conformers, s-cis,trans, s-cis,gauche, s-trans,trans and s-trans, gauche. Their optimized structural parameters, their force field and t he vibrational wavenumbers and potential energy distributions were cal culated for each conformer. Additionally, the potential function gover ning internal rotation from the s-trans,trans to the s-cis,trans confo rmation was calculated. Both the s-cis conformers were identified in t he 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 Ra man spectrum of the fluid phases is due to one of the s-trans conforme rs. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between the s-cis,trans and s-cisgauche conformers was determined to be 1.4 +/- 0.4 kJ mol-1 with the trans conformer be ing the more stable rotamer. Vibrational spectra of the crystalline so lid, which consists only of the s-cis,trans conformer, showed that eth yl formate can crystallize in two different forms. A complete assignme nt of the vibrational fundamentals from the infrared and Raman spectra is proposed.