Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate

The gas-phase structures of the fruit ester methyl hexanoate, CH3-O-(C=O)-C5H11, have been determined using a combination of molecular jet Fourier-transform microwave spectroscopy and quantum chemistry. The microwave spectrum was measured in the frequency range of 3 to 23 GHz. Two conformers were assigned, one with C-s symmetry and the other with C-1 symmetry where the gamma-carbon atom of the hexyl chain is in a gauche orientation in relation to the carbonyl bond. Splittings of all rotational lines into doublets were observed due to internal rotation of the methoxy methyl group CH3-O, from which torsional barriers of 417 cm(-1) and 415 cm(-1), respectively, could be deduced. Rotational constants obtained from geometry optimizations at various levels of theory were compared to the experimental values, confirming the soft degree of freedom of the (C=O)-C bond observed for the C-1 conformer of shorter methyl alkynoates like methyl butyrate and methyl valerate. Comparison of the barriers to methyl internal rotation of methyl hexanoate to those of other CH3-O-(C=O)-R molecules leads to the conclusion that though the barrier height is relatively constant at about 420 cm(-1), it decreases in molecules with longer R.

Automated summary

The gas-phase structures of the fruit ester methyl hexanoate have been determined using molecular jet Fourier-transform microwave spectroscopy and quantum chemistry. Two conformers were assigned, with internal rotation observed in the methoxy methyl group CH3-O. Comparison of barriers to methyl internal rotation in different molecules revealed a relatively constant barrier height at about 420 cm(-1) that decreases in molecules with longer R groups.