Absorptions of methyl and t-butyl halogenobenzoates in the infrared carbonyl region

Solutions of the methyl and t-butyl esters of o-, m-, and p-fluoro-, -chloro-, and -bromo-benzoates have been examined in the i.r. CO region. All but one of the o- and p-esters show doublets whereas the m-isomers give single bands. The doublets of the o-esters arise from rotational isomers, the thermochemically less stable syn-s-trans forms absorbing at higher wavenumbers than the anti-s-trans rotamers; those of the p-isomers are caused by the operation of Fermi resonance. It is probable that the rotamers of the m-esters have similar CO wavenumbers. A convenient, safe procedure for preparing t-butyl esters has been developed.