Andreas Detken

Reorientational tunneling of partially deuterated methyl groups: A single-crystal deuteron NMR study of aspirin-CH2D

Partially deuterated methyl groups in single crystals of aspirin are investigated by deuteron NMR at temperatures between 8 K and room temperature. The CH2D groups perform reorientations which are governed by a rotational potential with three wells, two of which are almost equally deep whereas the third is significantly deeper. At temperatures below 20 K, a so far unobserved type of incoherent tunneling process is identified. This process consists in reorientations between the two upper potential wells which are fast on the time scale of the quadrupolar interaction, whereas transitions into the deeper well are slow on this time scale. At temperatures above 35 K, the methyl groups perform thermally activated stochastic reorientations between all three potential wells. By determining the relative populations of the three wells as a function of temperature, the energy difference between the lower and the two upper wells is found to be 3.3 meV. This amounts to almost 8% of the average barrier height, which is...

Rotational tunneling of methyl groups in the hydroquinone/acetonitrile clathrate: A combined deuteron NMR, INS, and computational study

Abstract The methyl groups (CH 3 and CD 3 ) of the acetonitrile molecules in the clathrate of hydroquinone are investigated by single-crystal deuteron NMR and inelastic neutron scattering (INS). The NMR spectra below 40 K exhibit the characteristics of rotational tunneling with tunnel frequencies on the same scale as the quadrupole coupling constant of the deuterons. By comparing the spectra with simulations, the tunnel frequencies of the three crystallographically inequivalent CD 3 groups are determined with high precision. By INS, the energies of librational excitations of the CH 3 groups are measured. These two pieces of information allow for an accurate characterization of the rotational potentials of the methyl groups. The results are contrasted to force-field calculations based on a newly determined low-temperature neutron structure. Incoherent reorientations dominate the NMR spectra above 40 K. Their correlation times are determined in a wide temperature range and compared to theoretical predictions.

C-13 NMR of tunnelling methyl groups

The dipolar interactions between the protons and the central 13C nucleus of a 13CH3 group are used to study rotational tunnelling and incoherent dynamics of such groups in molecular solids. Single-crystal 13C NMR spectra are derived for arbitrary values of the tunnel frequency νt. Similarities to ESR and 2H NMR are pointed out. The method is applied to three different materials. In the hydroquinone/acetonitrile clathrate, the unique features in the 13C NMR spectra which arise from tunnelling with a tunnel frequency that is much larger than the dipolar coupling between the methyl protons and the 13C nucleus are demonstrated, and the effects of incoherent dynamics are studied. The broadening of the 13C resonances is related to the width of the quasi-elastic line in neutron scattering. Selective magnetization transfer experiments for studying slow incoherent dynamics are proposed. For the strongly hindered methyl groups of L-alanine, an upper limit for νt is derived from the 13C NMR spectrum. In aspirin™ (ac...

Geometry and tunneling dynamics of CHD2 groups in aspirin: A single-crystal deuteron nuclear magnetic resonance study

CHD2 groups in aspirin are studied by single-crystal deuteron NMR at temperatures between 7 and 290 K. They perform stochastic reorientations which are governed by a rotational potential possessing three wells, two of which are almost equally deep whereas the third is significantly higher. The three minima of the rotational potential are separated by angles substantially different from 120°. Still, at least in the two lower wells, the geometry of the CHD2 groups is close to that of a symmetric rotor. By selective magnetization transfer experiments, the incoherent tunneling rate between the two lower potential wells is determined. At temperatures below 15 K, Raman processes are the dominating rotor-phonon coupling mechanism, while at higher temperatures, thermally activated processes with activation energies close to the total potential height prevail.

Deuteron NMR of methyl groups in the tunneling regime. A single crystal study of aspirin-CD3

We report the first single crystal deuteron NMR spectra of CD3 groups which display the socalled ±ß, ±(|α| ± ß) and ±(2|α| ± ß) lines characteristic of rotational tunneling in a sufficiently clear manner to allow a quantitative comparison with the respective theory developed in 1988 by the group of W. Müller-Warmuth. The molecular system we study is aspirin-CD3. We recorded spectra for differently oriented single crystals and measured spin-lattice relaxation times T1 in a wide temperature range. At 12.5 K we exploit the dependence of the ±(|α| ± ß) and ±(2|α| ± ß) lines on the orientation of the applied field B0 for determining the equilibrium orientation of the CD3 group in the crystal lattice. The spectra display features which allow, by comparison with simulated spectra, a measurement of the tunnel frequency vt. Its low temperature limit is (2.7 ± 0.1) MHz. It allows to infer the height V3 of the potential V(φ) in which the CD3 group moves, provided that this potential is purely threefold. We get V3 = (47.2 ± 0.5) meV. The transition from the tunneling to the classical, fast reorienting regime occurs in the 15 K ≲ T ≲ 35 K temperature range. In this range we observe a broadening, merging and eventually narrowing of the ± |α| and ±2|α| lines in very much the way predicted by Heuer. His theory, however, must be extended by taking into account all librational levels. The behaviour of the ± ß lines in the transition temperature range signalizes a reduction of the observable tunnel frequency with increasing temperature. This reduction allows an independent measurement of the potential height and represents a test of the assumption of a purely threefold potential. From the T1 -data we derive the temperature dependence of the correlation time Ƭc of the reorientational jumps. The plot of log Ƭc vs. 1 /T follows a straight line for more than five decades. From its slope we get yet another independent number for the potential height. It agrees well with the other ones, which confirms the assumption of the essentially threefold potential V(φ) in aspirin-CD3.