CPC, Haeberlen, 1H NMR investigation of non-classical motion of methyl groups - i.e., tunnelling

ChemPhysChem
Volume 7, Issue 4 , Pages 886 - 893
Fingerprints of Damped Quantum Rotation Observed in Solid-State Proton NMR Spectra
Peter Gutsche, Dr. 1, Heike Schmitt 1, Ulrich Haeberlen, Prof. Dr. 1 *, Tomasz Ratajczyk 2, Slawomir Szymanski, Prof. Dr. 2 *
1Max-Planck Institute for Medical Research, 69120 Heidelberg, Germany, Fax: (+) 49 6221 4856351
2Institute of Organic Chemistry, Polish Academy of Sciences, 01224 Warsaw, Poland, Fax: (+) 48 2263 26681

Keywords
methyl group • NMR spectroscopy • quantum rotation • rate processes • tunnelling

Abstract
1H NMR spectra of the methyl group in an oriented crystal sample of methylmalonic acid with all three non-methyl protons replaced by deuterons are interpreted in terms of the damped quantum rotation (DQR) theory of NMR line shapes. The DQR approach offers a perfect theoretical reproduction of the observed spectra while the conventional Alexander-Binsch line-shape model shows evident defects in the present case. The temperature trends of the quantities characterizing the coherent and incoherent dynamics of the methyl group in the DQR approach (the effective tunnelling frequency and two coherence-damping rates) derived from the spectra are fairly reproduced using a model reported previously. The present findings provide further evidence of limitations to the validity of the common belief that molecular rate processes in condensed phases are necessarily classical.