The Conformation of Pentanoates in the Solid and in the Gas Phase

Abstract

Suitable derivatives of the four isomeric pentanoates have been structurally characterized in the solid and the gas phase. For the latter, the volatile ethyl esters of valeric, isovaleric, methylbutyric, and pivalic acid were investigated by a combination of molecular beam Fourier transform microwave (MB-FTMW) spectroscopy and theoretical calculations. Crystalline salts rather than esters were formed by reaction between the carboxylic acids and trans-1,2-diaminocyclohexane. For both gaseous and crystalline methylbutyrates, an essentially perpendicular arrangement of carboxylate and methyl group was observed; earlier structure determinations documented in the data base agree with this result. Two competing conformers of favourable energy were relevant for the corresponding isovalerates: They were associated with torsion angles around 20° and 50° between the carboxylate and the alkyl chain. Good agreements in conformation have also been achieved for our experimentally observed unbranched valerate derivatives and fully branched pivalates in solid and gas phase. Despite the apparent simplicity of the pentanoates, the identification of their lowest energy conformers represents a challenge for different methods and levels of theory.