Methyl Tunnelling Research Articles

13C 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...

Rotational dynamics of methyl groups in durene: A crystallographic, spectroscopic, and molecular mechanics investigation

Neutron powder diffraction measurements of perdeutero durene in the temperature range from 1.5 K to 290 K have been performed. The lowest temperature structure is the starting point for calculations of the methyl group tunneling and librational dynamics. Ab initio methods and atom–atom potentials are used to determine rotational single particle and coupling potentials. Tunneling splittings and librational bands are calculated by numerical solution of Schrödinger’s equation for a system of many coupled methyl groups. High-resolution inelastic neutron scattering measurements of methyl tunneling and molecular vibrations have been repeated, the tunneling results resolving an inconsistency with earlier NMR work. Quantum molecular dynamics provide a stringent test of the numerical methods and the data are ultimately well reproduced. These results are also discussed in the context of optical measurements of dye molecules in a host lattice of durene.

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

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.

Quantum Rotational Tunneling of Methyl Groups in Polymers

Quantum rotational tunneling of methyl groups has directly been observed for the first time in polymers by high resolution inelastic neutron scattering. The experimental results are readily explained and reproduced quantitatively, without any adjustable parameter, by considering the distribution of tunneling lines which is obtained from the distribution of energy barriers for classical hopping of methyl groups at high temperature.

Methyl tunnelling, reorientation and NMR relaxation in solid acetates

Rotational excitations of methyl groups in six solid acetates have been investigated by 1H nuclear magnetic resonance (NMR) relaxation time measurements at 15 MHz and 30 MHz and at temperatures between 10 K and the melting point. Hindering barriers between 1.6 kJ/mol and 3.7 kJ/mol have been found that could be correlated to the tunnelling frequencies observed by inelastic neutron scattering. A consistent description of the relaxation rate dependences from the classical regime at high temperatures to the quantum-mechanical regime at low temperatures is possible by Haupt's equation. The rotational potentials are mainly determined by inter-molecular interaction with an important influence of water of crystallization, if present.

Inelastic neutron scattering study of methyl tunnelling in an oriented single-crystal of 2,6-dimethylpyrazine at low temperature and rotational–potential calculations

Inelastic neutron scattering (INS) on an oriented single-crystal is used to assign directly the tunnelling splittings to the two crystallographically-inequivalent methyl groups in 2,6-dimethylpyrazine (C 6N 2H 8) at 2 K. Their antiparallel conformation found by crystallography and their respective rotational barriers are reproduced by a combination of ab-initio and molecular mechanics calculations. These results are used for a better understanding of the methyl groups dynamics.

The crystal structure determination of dimethylsulphide by high-resolution neutron powder diffraction

The crystal structure of perdeuterated dimethylsulphide, S(CD 3) 2, has been solved ab initio from high-resolution neutron powder diffraction data at low temperature. The structure is triclinic, space group Pl̄; V rmc = 179.31 A ̊ 3 at 5 K and Z = 2. A technique harnessing the anisotropy of the cell expansion with temperature has been used in order to improve the reliability of extracted intensities for the Direct Methods solution, especially in regions of severe peak overlap in the powder pattern. The structural results are discussed in relation to recent NMR experiments investigating the reorientational quantum tunnelling of methyl groups at low temperature. In particular, conclusions are made regarding the degree of coupling between individual methyl rotors.

Rotational tunneling of methyl groups: Probes of intermolecular potentials

Rotational tunneling of methylated molecules (aspirin, 4-methylpyridine, dimethyl-s-tetrazine) is investigated by optical spectroscopy, neutron scattering, and numerical modeling with the objective to gain information about the hindering potentials and to test the force field used in the model.

Proton NMR relaxation studies of solid L-alaninamide

The molecular dynamics of solid L-alaninamide were studied by proton spin lattice relaxation times in the laboratory and rotating frames, and second moment measurements over temperature range of 100 to 350 K. The methyl group tunnelling was observed below 130 K. The rotational motions of both methyl and amino groups were responsible for spin-lattice relaxation at higher temperature. Three reductions of dipolar second moment were observed over the temperature range. The rotations of methyl and amino groups attributed to the first and second reductions of the second moment at 120 and 240 K. The third reduction above 340 K is due to the molecular motion occurred at crystal melting.

Rotational tunneling of methyl groups of Sc(CH 3COO) 3 and Sc(CD 3COO) 3

The rotational tunneling of CH 3 and CD 3 groups of Sc(CH 3COO) 3 and Sc(CD 3COO) 3 were studied. Tunneling splittings around 8 μeV were observed for CH 3 groups of Sc(CH 3COO) 3 with neutron inelastic scattering. Deuteration of the methyl groups reduced the tunneling splitting to the order of ⩾ 0.01 μeV (⩾ 2 MHz). This was evidenced by specific side bands in the deuterium NMR spectrum below 20 K.

Optical probes of proton tunneling in molecular solids

Low-temperature optical spectroscopy of guest-host molecular crystal systems provides a very sensitive tool to study proton tunneling processes. Examples include the light-induced creation and evolution of “proton defects”, translational tunneling along hydrogen bonds as well as rotational tunneling of methyl groups. Absorption spectra at 2 K of γ-picoline single crystals evolve over time scales of days due to the slow spin conversion of the methyl groups which leads to an ordering of the crystal.

A high resolution, inelastic neutron scattering investigation of tunnelling methyl groups in aspirin

The tunnel frequency of protonated methyl groups in aspirin has been measured, by inelastic neutron scattering, at 2 K, to be 1.22 μeV. This result and the temperature dependence up to 42 K are in poor agreement with NMR measurements of deuterated methyl groups which conclude that the rotational potential is purely three-fold symmetric. The discrepancy is attributed to a six-fold contribution in the rotational potential for which justification is provided by a calculation of the rotational potential based on the room temperature crystal structure.

Rotational motion of methyl groups in solids.

The symmetry properties of the X${\mathrm{H}}_{3}$-type molecules are investigated. The possible wave functions of the molecule in the ground electronic state are calculated in the Born-Oppenheimer approximation and the question of the symmetry group of the molecular Hamiltonian (${\mathit{C}}_{3}$ or ${\mathit{C}}_{3\mathit{v}}$ or other) is considered. Further, the concept of quantum coherence is examined for the case of tunneling methyl groups. The results are extended to the high temperature region by introducing a suitable ``pointer basis'' which appears to be appropriate for the description of dynamics of the methyl groups interacting with lattice. Finally, the spin-lattice relaxation time is calculated within this model and is found to agree closely with the semiclassical result. \textcopyright{} 1996 The American Physical Society.

Proton tunneling in molecular solids

Low-temperature optical spectroscopy of guest-host molecular crystal systems gives direct access to proton tunneling processes. Examples include the photo-induced creation and evolution of “proton defects”, as well as translational tunneling along hydrogen bonds and rotational tunneling of methyl groups. The first observation of the effect of rotational tunneling on conventional absorption spectra is reported for γ-picoline single crystals.

Quantum and classical dynamics of a methyl group with tunneling frequency 3.4 MHz studied by low field NMR

Nuclear magnetic resonance (NMR) investigations of methyl tunneling in 2,5-dimethyl-1,3,4-thiadiazole are reported. The tunneling frequency (νt = 3.39 MHz) was obtained using low field NMR spectroscopy. By means of rapid field cycling irradiation and relaxation measurements the probabilities of the absorption transitions, responsible for the spectral lines in the low field NMR spectra, can be quantified. The results obtained agree well with the calculated probabilities of rf induced transitions between the eigenstates of a methyl rotor with νt = 3.39 MHz. Measurements of the temperature dependence of νt, the spin conversion time τcon and an analysis of the proton spin lattice relaxation time T1, the latter two revealing the correlation time τc, enabled the study of the methyl group dynamics over a temperature range encompassing both the quantum mechanical and the classical regimes. The dynamical data can be explained well with a threefold hindering barrier of height V3/kB = 1175 K and are compared with existing theoretical models.

A high hydrostatic pressure cell for optical experiments at low temperatures

A pressure cell for high resolution spectroscopic experiments at low temperatures is described. The use of helium as pressure medium allows convenient and precise pressure adjustment and assures that minimal strain is experienced by the sample, while the optical access by quartz fibers makes it possible to keep the dimensions of the cell minimal. Hole burning experiments on methyl group tunneling in organic molecular crystals are used to demonstrate potential applications of this cell.

High pressure NMR study of methyl group tunnelling in dimethyl sulphide

The rotational tunnel splittings of pairs of methyl groups in dimethyl sulphide have been determined by low field proton NMR as a function of hydrostatic pressure up to 4·5 kBar. Two principal splittings are clearly observed, one being equal to 750 kHz for all pressures, the other decreasing exponentially from 100 kHz at atmospheric pressure to 40 kHz at 4·0 kBar. For the first time, tunnelling sidebands of a sideband in the NMR spectrum have been seen. The temperature dependence of the spin-lattice relaxation time has also been measured as a function of pressure and has revealed double minima at higher pressure. The results have been analysed using both weak and strong methyl coupling models.

The Rotational Spectrum of Tunnelling Methyl Groups

Abstract The quantum-classical transition of methyl rotation highlights conflicting assumptions of quan­tum and classical mechanics, the first assuming that identical fermions are indistinguishable while the second recognises they are distinguishable through their histories. When this is introduced into quantum mechanics, the two are compatible and there is a smooth transition of the methyl rotational spectrum with increasing temperature through the merger of three broadening peaks. The changing character of methyl dynamics through this transition is discussed.

Driven tunnelling: New methods in NMR spectroscopy

Experiments on tunnelling methyl groups at 4K create localised wavepacket excitations which are either stationary or have a characteristic frequency v t which is determined by the height of the hindering barrier through which the methyl group tunnels. These are free tunnelling wavepackets. In contrast to most experimental techniques low field, dipole-dipole driven NMR enables the methyl groups to be accelerated and the rotation frequencies of the wavepackets to be changed. This is driven tunnelling. The flexibility of the experiment enables the wavepacket dynamics to be manipulated in a number of different ways. Spectra with broad, asymmetric peaks for both single and coupled methyl groups are presented as evidence of driven tunnelling.

Methyl tunnelling in pure and matrix isolated halogenomesitylenes

The trihalogenomesitylenes, C 6(CH 3) 3X 3, where X = Cl, Br, I crystallise in the triclinic P 1 space- group, such that the three methyl groups in one molecule see different environments. Indeed, we have detected three tunnelling and three torsional transitions in the pure compounds. In the case of a 3.7% mixture of protonated tribromomesitylene in a matrix of hexabromobenzene, we have detected only one tunnelling transition at 102μeV; from this, we conclude that the three methyl groups of the TBM molecule in the dilute mixture are equivalent in what is a relatively symmetrical environment and as a consequence also in the isolated molecule. A solid solution of 6% fully protonated trichloromesitylene in a matrix of fully deuterated TCM gives three transitions similar to those in the pure compound, indicating that there is no strong coupling between the different methyl groups. We also present Raman and FIR spectra of pure TBM giving the phonon frequencies at the Γ point. Some IR absorption maxima fall near maxima of the neutron spectra, indicating coupling between some phonons and the torsional transitions. So as to give us a model of the tribromomesitylene system we have optimised atom-atom Buckingham-type coefficients so as to reproduce the experimental phonon frequencies. The coefficients were used to calculate the intermolecular contribution V ex(θ) to V s(θ), the potential hindering the methyl groups. The intramolecular contribution V in(θ) was found using the quantum chemistry program AM1. The total hindering potential V s(θ) so obtained allowed us to calculate the principal dynamic features of the methyl rotors in reasonable agreement with the experiments.