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The dynamics of hydrogens in double well potentials: The transition of the jump rate from the low temperature quantum-mechanical to the high temperature activated regime (1991)

Heuer, Andreas ; Haeberlen, Ulrich

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 95 (1991), S. 4201-4214

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The temperature dependence of the deuteron spin lattice relaxation time T1 in four carboxylic acids, which were deuterated in the carboxyl groups, is presented. These data allow determination of the rate Γ of the hydrogen transfer between the two minima of the double well potential in which the hydrogens move along the hydrogen bonds. The temperature dependence of Γ has clearly distinct low temperature quantum-mechanical and high temperature thermally activated regimes. Evidence is presented that the transfer of hydrogens in dimers of carboxylic acids is strongly influenced by a high-frequency mode which is strongly coupled to the hydrogens. This supports the conclusions recently put forward by the ETH group [J. Chem. Phys. 93, 1502 (1990)]. A model based on the spin–Boson–Hamiltonian with a bath consisting of acoustic phonons plus a unique high frequency mode explains fully the jump rate in the quantum mechanical regime of all four compounds studied. The way in which the unique bath mode influences the hydrogen transfer is clarified.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.461795

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Quantum mechanical behaviour of deuterated methyl groups (1992)

Heuer, Andreas

Springer

The European physical journal 88 (1992), S. 39-51

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ISSN: 1434-6036

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract We discuss the temperature dependence of deuterium NMR spectra and spin-lattice relaxation rates 1/T 1 of deuterated methyl groups. We restrict ourselves to the temperature range where it is sufficient to consider only the two lowest librational levels of the CD3 group. Specifically we derive explicit expressions for 1/T 1 and the widthГ NMR of the so-called tunneling resonances in the high-field NMR spectrum in the limit where the tunneling frequencyΔ 0 is large compared to the quadrupole coupling constant of a methyl group. We demonstrate that the disappearance of the tunneling resonances at elevated temperatures follows the scenario of the NMR of nuclei which exchange classically between two inequivalent sites. We further show that the NMR linewidths and relaxation rates are strongly related to the widths of the quasielastic and inelastic lines in incoherent neutron spectra. It turns out that for NMR spectra the classical limit is usually reached at lower temperatures than for neutron spectra. All calculations are based on the projection operator formalism.