Fingerprints of Damped Quantum Rotation Observed in Solid‐State Proton NMR Spectra

Gutsche, Peter; Schmitt, H.; Haeberlen, Ulrich; Ratajczyk, Tomasz; Szymański, Sławomir

doi:10.1002/cphc.200500572

Cited by 19 publications

(32 citation statements)

References 23 publications

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“…Moreover, the temperature trends of k t , k k , and ∆ determined from the spectra could be reproduced theoretically using the already mentioned simplified version of the DQR formalism, limited to the coherences between torsional sublevels. 18 Hence, results of similar calculations using the improved version of the formalism, to be discussed below, appear trustworthy.…”

Section: Discussionmentioning

confidence: 86%

“…26,27 The departures from unity of their ratio can therefore be used as the mentioned measure of nonclassicallity. The highest values of this parameter reported so far exceed 5, 18 which means that the problem of a breakdown of the jump model is by no means marginal. This finding prompted one of the present authors to readdress the considered relaxation problem in terms of the DQR model.…”

Section: Introductionmentioning

confidence: 94%

“…As has already been mentioned, the values of this ratio exceeding 5 were determined from fits of the discussed equation to the experimental spectra of a hindered CH 3 group. 18 With properly modified spin hamiltonian, Eq. (10) is also valid for methyl groups in molecules freely tumbling in fluids.…”

Section: Overview Of Dqr Modelmentioning

confidence: 99%

See 2 more Smart Citations

A quantum mechanical alternative to the Arrhenius equation in the interpretation of proton spin–lattice relaxation data for the methyl groups in solids

Bernatowicz

Shkurenko

Osior

et al. 2015

Phys. Chem. Chem. Phys.

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CitationQuantum mechanical alternative to Arrhenius equation in the interpretation of proton spin-lattice relaxation data for the methyl groups in solids 2015 Phys. Chem. Chem. Phys. Theory of nuclear spin-lattice relaxation in methyl groups in solids has been a recurring problem in nuclear magnetic resonance (NMR) spectroscopy. The current view is that, except for extreme cases of low torsional barriers where special quantum effects are at stake, the relaxation behaviour of the nuclear spins in methyl groups is controlled by thermally activated classical jumps of the methyl group between its three orientations. The temperature effects on the relaxation rates can be modelled by Arrhenius behaviour of the correlation time of the jump process. The entire variety of relaxation effects in protonated methyl groups has recently been given a consistently quantum mechanical explanation not invoking the jump model regardless of the temperature range. It exploits the damped quantum rotation (DQR) theory originally developed to describe NMR line shape effects for hindered methyl groups. In the DQR model, the incoherent dynamics of the methyl group include two quantum rate, i.e., coherence-damping processes. For proton relaxation only one of these processes is relevant. In this paper, temperature-dependent proton spin-lattice relaxation data for the methyl groups in polycrystalline methyltriphenyl silane and methyltriphenyl germanium, both deuterated in aromatic positions, are reported and interpreted in terms of the DQR model. A comparison with the conventional approach exploiting the phenomenological Arrhenius equation is made. The present observations provide further indications that incoherent motions of molecular moieties in condensed phase can retain quantum character over much broader temperature range than is commonly thought. Eprint version

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Section: Discussionmentioning

confidence: 86%

Section: Introductionmentioning

confidence: 94%

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A quantum mechanical alternative to the Arrhenius equation in the interpretation of proton spin–lattice relaxation data for the methyl groups in solids

Bernatowicz

Shkurenko

Osior

et al. 2015

Phys. Chem. Chem. Phys.

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“…2 For both the CH 3 and CD 3 groups in crystalline solids, investigated at temperatures below 100 K, the validity of the DQR theory was already confirmed, 4,5 with a rather spectacular outcome in the perprotio case. 5 Unquestionable occurrence of the DQR effects was also detected in liquidphase NMR spectra of the methyl protons in molecules bearing extremely hindered methyl groups. [6][7][8] It is thus natural to ask whether such effects can also be observed for "sizable" molecular rotors.…”

Section: Introductionmentioning

confidence: 93%

Theory of damped quantum rotation in nuclear magnetic resonance spectra. II. Numerical simulations for the benzene rotor

Ratajczyk

Szymański

2007

The Journal of Chemical Physics

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In Part I of this series of papers, the damped quantum rotation (DQR) theory, formulated originally for hindered threefold molecular rotors in solids, was generalized to the N-fold case. The stochastic dynamics of such objects, evidenced in NMR line shapes, was shown to be more complicated than in the standard model of classical jumps between the wells of the N-fold torsional potential. Actually, it comprises certain quantum rate (i.e., coherence-damping) processes subject to the requirements of the Pauli principle. The jump picture is recovered only when the quantum rates fit specific patterns. In this work, one of the ways of approaching such a classical limit is identified for the benzene rotor. This is inferred from a quantum mechanical model whose validity was earlier confirmed for a methyl group. Based on that model, theoretical calculations for the benzene ring dynamics in a clathrate crystal, 1-(9-anthryloxy)anthraquinone/benzene-d6, confronted with the pertinent literature data, point to possible deviations from the classical limit. However, the predicted DQR effects are too small to be observed in solid echo 2H NMR spectra of the C6D6 isotopomer. The chances of detecting the effects are improved when Carr-Purcell echo 1H spectra of a single crystal of the isotopomer including C6H6 as a guest are considered. The substantial differences in the sensitivity to the DQR effects of the spectra of protonated and deuterated benzene are concerned with different magnitudes of the intramolecular dipolar spin couplings. The dynamic isotope effect (C6D6 vs C6H6), which is small in this case, is only of secondary importance. Legitimacy of the use of the jump model in 2H NMR line shape studies of benzene-d6 is fully confirmed by the present considerations. However, the physical significance of the dynamic parameters extracted from such studies is shown from a new perspective.

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“…2-4, 10, 11, 13 We do note an interesting 1 H spectroscopy study in a single crystal of partially deuterated methylmalonic acid [CDCH 3 (COOD) 2 ] where, for spectra in the range 69-104 K, using a quantum-mechanical model for methyl group rotation gave a better fit of the data than the classical hopping model. 21 In the end, the tests of the validity of a model are (1) whether or not it is consistent with a wide variety of experimental results and (2) that it has a reasonably small number of adjustable parameters, all of which can be determined uniquely by the experiment.…”

Section: Introductionmentioning

confidence: 99%

Methyl group rotation, 1H spin-lattice relaxation in an organic solid, and the analysis of nonexponential relaxation

Beckmann

Schneider

2012

The Journal of Chemical Physics

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We report 1 H spin-lattice relaxation measurements in polycrystalline 4,4 -dimethoxybiphenyl at temperatures between 80 and 300 K at NMR frequencies of ω 0 /2π = 8.50, 22.5, and 53.0 MHz. The data are interpreted in terms of the simplest possible Bloch-Wangsness-Redfield methyl group hopping model. Different solid states are observed at low temperatures. The 1 H spin-lattice relaxation is nonexponential at higher temperatures where a stretched-exponential function fits the data very well, but this approach is phenomenological and not amenable to theoretical interpretation. (We provide a brief literature review of the stretched-exponential function.) The Bloch-Wangsness-Redfield model applies only to the relaxation rate that characterizes the initial 1 H magnetization decay in a high-temperature nonexponential 1 H spin-lattice relaxation measurement. A detailed procedure for determining this initial relaxation rate is described since large systematic errors can result if this is not done carefully.

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Fingerprints of Damped Quantum Rotation Observed in Solid‐State Proton NMR Spectra

Cited by 19 publications

References 23 publications

A quantum mechanical alternative to the Arrhenius equation in the interpretation of proton spin–lattice relaxation data for the methyl groups in solids

A quantum mechanical alternative to the Arrhenius equation in the interpretation of proton spin–lattice relaxation data for the methyl groups in solids

Theory of damped quantum rotation in nuclear magnetic resonance spectra. II. Numerical simulations for the benzene rotor

Methyl group rotation, 1H spin-lattice relaxation in an organic solid, and the analysis of nonexponential relaxation

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