Methyl tunnelling spectroscopy in acetophenone

Abstract

Nuclear magnetic resonance (NMR) investigations of methyl tunnelling in acetophenone in the solid state are reported. Measurements of the proton spin-lattice relaxation time and of the methyl tunnelling frequency ( nu t=1.435 MHz) from dipole-dipole-driven low-field NMR demonstrate that the methyl group in acetophenone has an anomalously high potential barrier. A comparison with similar measurements on trans-4-phenyl-3-buten-2-one, along with a theoretical calculation of the respective barrier contributions, leads the authors to conclude that the origin of the barrier lies predominantly in the interaction between hydrogen atoms on the methyl and phenyl groups. The tunnelling sideband measurements reported extend the window of low-field NMR into the megahertz region and illustrate the existence of a NMR sideband intensity enhancement when the system is close to the level crossing between Zeeman states of the E and A manifolds, respectively.

Crystal structure and low-temperature methyl-group dynamics of cobalt and nickel acetates

“…Several efforts have been made to correlate the potentials extracted from tunneling spectra with structural data, in order to extract dynamical information. This has been achieved via ab initio and molecular-mechanics calculations with various pair potentials ͑nitromethane, 2,3 acetophenone, 4 acetamide, 5 p-xylene, 6 diacetyl, 7 halogenomesitylenes, 8 Hofmann clathrates, 9 aspirin, 10 acetic acid, 11 a series of organic molecules, 12 2,6 dimethylpyrazine, 13,14 lithium acetate 15 ͒. The most recent investigations [10][11][12][13][14][15] show clearly that good agreement can often be achieved, but accurate structural data must be available for the crystal at the same temperature as the tunneling measurements.…”

Quantitative Atom-Atom Potentials from Rotational Tunneling: Their Extraction and Their Use

“…In order to have a large number of systems characterized for numerical work, our experimental program has included as many new diffraction measurements as tunneling measurements. Prior to our involvement in modeling weak interactions, only six crystal structures of tunneling systems were known at reasonably low temperature and they were used to calculate rotational barriers from atom-atom potentials (34)(35)(36)(37)(38)(39). The systems were generally studied separately by several authors.…”

Quantum and classical dynamics of a methyl group with tunneling frequency 3.4 MHz studied by low field NMR

“…This measurement 12 considerably extends the range of low field NMR spectroscopy, the previous upper limit being 1.5 MHz. 8 The spectra in Figs. 2 b-e, obtained at various fields between 36 mT and 85 mT, indicate that low field spectroscopy gives well resolved spectra up to relatively high fields compared to the strength of the dipolar interaction.…”

“…5͒ but these are observable only over a narrow range of tunneling frequency. However, low field NMR spectroscopy, 6,7,8 has been developed for the range 1.5 MHz у t у 30 kHz where the transitions which involve changes in tunneling state become more strongly allowed when the dipolar field is of comparable magnitude to the applied magnetic field. A similar effect is exploited in the rotating frame experiments of Pintar and co-workers.…”