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, CH 3 -O-(C=O)-C 5 H 11 , 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 γ-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 CH 3 -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 CH 3 -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 .