Raman spectroscopic study of fourteen positional isomers of carboxylic acid esters with seventeen skeletal atoms

The Raman spectra of 14 positional isomers of carboxylic acids esters with 17 skeletal atoms, CH3(CH2)m−1COO(CH2)n−1CH3(CmCOOCn) with m + n = 15 and m = 1—14, have been measured in the crystalline state at liquid nitrogen temperature. The relationship between the observed wavenumbers of the characteristic bands and the positions of the —C(=O)—O— group in the molecular chain was examined. All of the positional isomers of CmCOOCn with m + n = 15 exhibit a strong LAM-1 band in the region between 147 and 136 cm−1. The LAM-1 wavenumbers for the isomers which have the —C(=O)—O— group almost in the middle of the molecular chain are slightly higher (147—143 cm−1) than those for the isomers which have the same group almost at the end of the chain. In order to elucidate the observed LAM-1 wavenumbers, the Raman spectra of two homologous series of esters, C5COOCn and C6COOCn, were examined. All isomers of CmCOOCn give a strong band at about 1135 cm−1 assignable to the k = 1 mode of the C—C stretching vibrations. Some of the isomers give an additional k = 1 band at about 1125 cm−1. The isomers of CmCOOCn, which exhibit the two k = 1 bands, have the —C(=O)—O— group almost in the middle of the molecular chain, and appear to give the LAM-1 band at the higher wavenumbers. In the crystalline state all of the esters examined take the all-trans conformation with the —C(=O)—O— group part incorporated.