Methyl rotors in the gas phase: A study of o- and m-toluidine by chirped-pulse Fourier transform microwave spectroscopy

Microwave spectra of o- and m-toluidine were recorded in a pulsed supersonic jet using chirped pulse techniques. The spectra show both torsional and 14N quadrupole splittings at high resolution. From the torsional splittings, barrier heights were determined of 531 cm−1 and 2.0 cm−1 respectively. Using the quadrupole splittings, orbital occupancy numbers of the amino-nitrogen atoms were calculated. An apparent relationship between these values and the barriers to internal rotation of the methyl groups is discussed.

Graphical abstractThe microwave spectra of ortho- (left) and meta-toluidine (right) display vastly different tunneling splittings based on the position of the methyl rotor with respect to the amide group.Download full-size image.Highlights► CP-FTMW spectra of o- and m-toluidine have been collected from 6-18 GHz. ► Spectra show tunneling splittings produced by hindered rotation of a methyl group. ► Spectra also exhibit N-14 quadrupole couplings from attached nitrogen. ► Spectra fit to obtain rotational constants, barrier heights, and quadrupole constants. ► Analysis shows a correlation between the orbital occupation numbers of the lone pair electrons on nitrogen and the barrier heights.