The techniques of pulsed NMR spectroscopy and inelastic neutron scattering (INS) have been employed to study the coordinated motion of hydrogen atoms in the hydrogen bonds of carboxylic acid dimers. The proton spin-lattice relaxation time T1 has been measured at two frequencies in diglycolic acid and suberic acid and the reorientation rate in diglycolic acid has been measured directly by quasi-elastic neutron scattering. Furthermore T1 data have been collected on benzoic acid, terephthalic acid and malonic acid at different spectrometer frequencies to those in earlier published data. The NMR and INS data have been successfully fitted to a new model proposed by Skinner and Trommsdorff (1988) in which the low temperature dynamics, under the influence of an asymmetric double-minimum potential, are dominated by phonon-assisted tunnelling while that at high temperature follows an Arrhenius rate law. Fitting the T1 data at two different frequencies simultaneously with the INS data restricts the possible values of the parameters in the theory to a narrow range. The values of the parameters, particularly the barrier heights and asymmetry, are discussed in terms of the known molecular structures. Further discussion centres upon some new NMR data of the proton dipolar relaxation time T1D in terephthalic acid and benzoic acid.
AktracL lhe prssure dependence of tunnelling of mupled pairs of methyl group in lithium acetate dihydrate has been investigated using the wutmn scattering spectrometer IRIS. Methyl tunnelling spctra. recorded at a temperature of 5 K and p u r e s up to 9.5 kbar, rweal SUUcture which has been atlribuled to mupling. The data, which m e n d an earlier invcstigalion by Heidemann d al, have been fitted to a lime-independent Hamiltonian, and the fit parametem include the potential barrien d &e individual methyl groups and the mupling potential between them. The magnitudes of both parameters increase with increasing pressure and their pressure dependence has b x n analyzed to provide information on the inter-molecular foras.the tunnelling spectra could be studied and interpreted as changes were made to both the single-particle and coupling potentials. The experiment was performed as a function of pressure up to 9.5 kbar on the IRIS neutron scattering spectrometer at the pulsed neutron source ISIS, Rutherford-Appleton Laboratory (RAL).
The low-field dipole-dipole driven NMR spectra of materials that contain tunnelling molecular groups have been recorded in the frequency domain. The methyl groups in thioanisole and 2-butanone have tunnelling frequencies of 557+or-3 kHz and 491+or-3 kHz respectively. In addition to the sidebands of Delta m=+or-1 and Delta m=+or-2, tunnelling sidebands of Delta m=0 are clearly observed at the tunnel frequency and are reported for the first time. The intensities of all the sidebands are of similar magnitude. The sidebands of Delta m=0 result from transitions induced by the RF field in which the methyl group undergoes a change in tunnelling state without a change in overall spin. Despite this, the sidebands are observed in a plot of proton magnetisation versus frequency. A thermodynamic model is proposed to explain how the change in magnetisation is generated; level crossings encountered as part of the field cycling sequence are responsible for transferring disorder in the tunnel reservoir into disorder in the Zeeman reservoir. Discussion also centres upon the mechanism responsible for driving the transitions revealed in the low-field spectra.
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