The infra-red spectra of the Borate esters

The infra-red spectra of a series of alkyl and aryl esters of boric acid have been obtained in the region from 1800 to 670 cm-1. These all show a strong characteristic absorption band at 1340 ± 10 cm-1 which is assigned to the asymmetrical stretching frequency of the BO3 group. Comparison of the infra-red and Raman spectra of trimethyl borate suggests the assignment of the 728 cm-1 Raman band to the symmetrical stretching mode. The non-coincidence of infra-red and Raman bands indicates a fairly high degree of symmetry with restriction of the rotation of the methyl groups round the B-O link. This contrasts with other data on the behaviour in the vapour where such rotation may occur.