Infrared studies and vibrational analyses of simple acyclic esters

Abstract

A series of simple saturated and unsaturated esters have been investigated at these spectroscopic laboratories. In the infrared, these esters were examined as vapors from 400-50 cm⁻¹. Liquid samples were studied by the Raman effect from a wavenumber shift of approximately 3200 to the Rayleigh line. These Raman studies included exposure of the sample to parallel and perpendicularly polarized light. A number of esters have been examined for which no previous report could be found. The results of these examinations range from a complete vibrational analysis to a spectral cataloging of the resultant data. Precautions were taken to maintain sample stability and purity at all times. A simple gas chromatographic scheme was developed to monitor sample purity. Whenever spectral interpretation depended on structure, the data could usually be rationalized on the basis of assumed C[subscript s] symmetry. Vibrational analyses were facilitated in most cases by the apparent lack of rotational isomers. Among the torsional rotors which were within the range of observation at these laboratories, were included the methyl group, trifluoro group, vinyl group and ethyl group. The methyl torsion appeared to occur in the range of 170-190 cm⁻¹ when in the acyl position. As part of the methoxy group it was observed in the range 130-140 cm⁻¹. Trifluoro torsions occur at lowest frequencies. In the case of vinyl trifluoroacetate, frequencies of 63 and 72 cm⁻¹ were observed which we assume are due to this specie. Vinyl torsions could be seen in the range of 100-150 cm⁻¹. Two species which characteristically occur together (i.e., superimpose) are the C0C bend and the skeletal torsion. High resolution spectra of this two-species system were taken for methyl formate and ethyl formate. A normal coordinate analysis was performed on methyl formate, the simplest possible ester, with the aid of digital computer programming.