September 2004

cis Methyl Formate

Methyl formate (HCOOCH3) was first observed in the dense molecular cloud Sgr B2 in 1975 by Brown et al. and Churchwell & Winnewisser via a pair of rotational lines associated with the A and E torsional states arising from the internal rotation of the methyl group. Various later studies have found numerous other lines. Methyl formate was also identified in comet Hale-Bopp.

Presently, known gas phase formation pathways to interstellar methyl formate cannot account for the observed column densities. It is quite possible that grain surface reactions play an important role in the formation of methyl formate.

In addition to the low energy (~300 cm–1) barrier for rotation of the methyl group, methyl formate has two distinct conformational isomers that are very close in energy but are separated by a barrier that is significant at interstellar cloud temperatures. The figure shows a computed energy diagram for the structures where the carbonyl O is on the same and opposite side as the methyl group.

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