The CH₃tunnelling sidebands of Δm=0 studied by dipole-dipole driven low-field NMR in the frequency domain

Abstract

The low-field dipole-dipole driven NMR spectra of materials that contain tunnelling molecular groups have been recorded in the frequency domain. The methyl groups in thioanisole and 2-butanone have tunnelling frequencies of 557+or-3 kHz and 491+or-3 kHz respectively. In addition to the sidebands of Delta m=+or-1 and Delta m=+or-2, tunnelling sidebands of Delta m=0 are clearly observed at the tunnel frequency and are reported for the first time. The intensities of all the sidebands are of similar magnitude. The sidebands of Delta m=0 result from transitions induced by the RF field in which the methyl group undergoes a change in tunnelling state without a change in overall spin. Despite this, the sidebands are observed in a plot of proton magnetisation versus frequency. A thermodynamic model is proposed to explain how the change in magnetisation is generated; level crossings encountered as part of the field cycling sequence are responsible for transferring disorder in the tunnel reservoir into disorder in the Zeeman reservoir. Discussion also centres upon the mechanism responsible for driving the transitions revealed in the low-field spectra.

Methyl tunnelling sidebands in the low-field NMR spectrum of 3-pentanone: Driving A–E transitions using rf irradiation

“…One transition, labelled 'c', is independent of applied field B lf and its centre identifies the methyl tunnelling frequency, m t ¼ 3:05 AE 0:01 MHz. This is a sideband of the Dm ¼ 0 spectrum [11], and the observation of such a transition that involves no net change in nuclear magnetisation can be attributed to cross-relaxation between the tunnelling and Zeeman systems during the passage through the level-crossings that arises as part of the field-cycling sequence. This will be discussed in further detail in Section 5.…”

“…These sidebands involve simultaneous changes in tunnelling and 1 H spin states and therefore provide the opportunity to study the mechanisms of driving A-E transitions by rf irradiation. The experiment can be conducted either by sweeping the B-field at constant m rf [6,[8][9][10] or by sweeping m rf at constant B-field [11][12][13].…”

A group theoretical study of planar methyl rotation

“…So the techniques which have been employed are, for the most part, quite complicated in nature, and there is then the question of how to interpret them. In many cases this has been done using, as a framework, the 4A and ' E picture, which is probably incorrect (see Wurger (1989), Heidemann et a1 (1989), Vuorimaki and Punkkinnen (1989) and Horsewill and Aibout (1989), which are good reference sources for work using this scheme). We are thus unable to claim that experiment supports the conclusions which follow from our analysis.…”