Vibrational absorption intensities in chemical analysis, part 6. Group vibrations in ethyl ethanoate

Abstract

The optimized structures and energies of ethyl ethanoate are computed at the 3-21G level for three conformations. It is shown that the trans—trans form is about 1 kJ mol −1 less stable than the trans—gauche configuration at this basis level. The cis—trans form is computed to be 44.5 kJ mol −1 less stable. Calculated spectra are reported for the trans—trans form. Experimental intensity trends are well reproduced. In particular it is demonstrated that the CH stretching intensity of the CH 3 of CH 3(CO) is reduced to one tenth of that of ethane, whereas the CH bending modes are enhanced in intensity. By contrast to ethers, where the CH stretching of the CH bonds trans to the lone pair electrons of oxygen are enhanced, the CH stretching of the (CO).OCH 2 group is weakened, and indeed is less than half the strength per CH of the CH stretching bands of ethane. This is ascribed to delocalization of the n electrons into the π bond of the carbonyl. Substantial effects of the ester remain at the CH 3 of the ethyl group for which the intensities per CH are predicted to be still only 2 3 of that of ethane. On the basis of the computed spectra using scaled force constants, two revisions are made to recent literature assignments.