Methyl Tunnelling Research Articles

Low-temperature methyl group dynamics of hexamethylbenzene in crystalline and glassy matrices as studied by 2H NMR

2H NMR spectra of hexamethylbenzene (HMB) in protonated crystalline and amorphous matrices at low temperatures are presented. All spectra reveal lineshape changes which can be attributed to methyl group tunnelling. Compared to neat HMB, a drastic increase of the tunnelling frequency is found for all systems. This indicates that the hindering potential originates predominantly from intermolecular forces. We studied the temperature dependence of these spectra and the spin-lattice relaxation in order to exclude a distribution of motional correlation times describing a thermally activated process. In addition, we find a distortion of the methyl tetrahedron.

ChemInform Abstract: Inter‐ and Intramolecular Interactions and Rotational Tunneling of Methyl Groups in Tetramethyltin

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Low-frequency nuclear magnetic resonance and nuclear quadrupole resonance spectrometer based on a dc superconducting quantum interference device

A sensitive spectrometer, based on a dc superconducting quantum interference device, for the direct detection of low-frequency pulsed nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR), is described. The frequency response extends from about 10 to 200 kHz, and the recovery time after the magnetic pulse is removed is typically 50 μs. As examples, NMR spectra are shown from Pt and Cu metal powders in a magnetic field of 6 mT, and NQR spectra are shown from 2D in a tunneling methyl group and 14N in NH4ClO4.

Inter- and intramolecular interactions and rotational tunneling of methyl groups in tetramethyltin

Rotational tunneling of methyl groups in partially deuterated tetramethyltin compounds, (CH3)xSn(CD3)4−x with x=1 and 3, and in various isotopic mixtures, [(CH3)4Sn]x[Sn(CD3)4]1−x, x=0.027, 0.20, and 0.50, and [CH3Sn(CD3)3]x[Sn(CD3)4]1−x, x=0.10, respectively, has been studied using the inelastic neutron-scattering (INS) technique. Compared with the INS spectrum of fully protonated tetramethyltin, (CH3)4Sn, the spectra obtained in the present study show remarkable shifts and broadenings or splittings of tunnel lines. The interpretation of the INS results is based on the assumption that all partially deuterated compounds and isotopically mixed systems have the same crystal structure as pure (CH3)4Sn. Combining the INS results with a detailed discussion of the crystal structure, all features can be explained in terms of both the intra- and intermolecular interactions of methyl groups. The overall decrease of tunnel splittings with deuteration is explained by the reduction of the lattice parameter and the increased octopole moment of the CD3 groups. The interaction between different molecules is mediated by a subset of methyl group pairs only.

The low temperature dependence of the correlation time for the thermally assisted reorientation of slowly tunneling methyl groups

We derived the correlation time τ c for the thermally assisted methyl reorientation from measurements of the nuclear spin-lattice relaxation and the spin conversion in four methyl compounds with resembling small tunnel frequencies. The exact values of τ c at high temperatures coincide but they show some important differences at low temperatures.

Methyl group tunneling in molecular solids at low temperatures: A study via optical holeburning of the electronic state dependence and the relaxation dynamics

We report a new approach to study rotational tunneling in condensed phases using high resolution spectral holeburning as line narrowing technique. The method is demonstrated in mixed crystals of di-methyl-s-tetrazine in durene and allows to determine directly the difference of tunneling splitting in different electronic states. It also opens a way to measure relaxation processes between tunneling levels (spin conversion) over large temperature ranges and time scales.

Optical measurements of methyl-group tunneling and nuclear-spin conversion.

We report a novel approach to measure rotational tunneling in condensed phases. High-resolution optical spectroscopic methods are used to unravel the tunneling level structure and to measure the rate of the nuclear-spin conversion. The method is demonstrated in experiments on the rotational motion of methyl groups in a crystal of durene doped with di-methyl-s-tetrazine but it can readily be transposed to other tunneling systems.

Methyl tunnelling and rotational potentials in solid xylenes and fluorotoluenes

CH3 rotational excitations have been investigated in solid xylenes and fluorotoluenes using inelastic neutron scattering (INS) and nuclear resonance (NMR) techniques. For two of the six materials the temperature dependence of the tunnel line (shift and broadening) has been compared with the density of states and the slopes of the NMR-T, data. The rotational potentials have been derived from tunnel splittings, librational excitations and activation energies. The tunnel energies have been found to occur between 0.026 and 26 mu eV. In one case nonequivalent methyl groups have been identified. As compared with results from isolated molecules, the potentials of ortho isomers are distinguished by a considerable intramolecular contribution, while the rotational potential of the para and meta compounds are governed by intermolecular interactions. The hindering potential calculated for p-xylene on the basis of pair potentials is inconsistent with the experimental results; reasons are discussed.

Inelastic-neutron-scattering study of methyl tunneling and the quantum sine-Gordon breather in isotopic mixtures of 4-methyl-pyridine at low temperature.

Les spectres de diffusion inelastique de neutrons des melanges isotopiques du methyl-4-pyridine completement hydrogene (4MP- h 7) et du methyl-4-pyridine completement deutere (4 MP−d7) montrent la canalisation (effet tunnel) et un deplacement (mouvement) collectif des groupements methyles a basse temperature. La theorie sine-Gordon quantique constitue un outil tres compact et elegant pour decrire la dynamique particuliere de la torsion du groupement methyle dans 4 MP. On remarque que l'equation du mouvement (de depart) depend de deux parametres (nommement le potentiel V o sur le site et le potentiel de deformation Vc) qui sont determines experimentalement a partir des frequences de canalisation (effet tunnel). Cette theorie fournit un spectre d'energies de breather qui s'ecarte de moins de 5% des valeurs experimentales. En outre, cette theorie fournit une comprehension claire de la dilution isotopique et des effets de la temperature en termes de la dynamique du breather dans des boites finies. Cette theorie merite des affinages complementaires

A group theoretical study of planar methyl rotation

Most papers on methyl group tunnelling use a model in which the motion of a rigid rotator is hindered by a potential of threefold symmetry. There are a number of difficulties with such a description and, in this paper, the model is replaced by one which describes the motion of three protons, which have mutual interactions and which are hindered by an external potential. The analysis makes considerable use of symmetry ideas and arrives at conclusions which are substantially different from those of the previous model. A comparison of the two models shows that, in the absence of dipolar interactions, both will have an orbital singlet level with spin of 3/2, but that the level described as 2E in the rigid-rotator model will not occur in the present model and that there will be no corresponding level with fourfold degeneracy. Instead there will be two separated orbital singlets, each with spin 1/2. The inclusion of the dipolar interaction splits the quartet into two doublets, so that finally there are four separated low-lying energy levels, each being doubly degenerate.

Magnetic resonance spectrometer with a dc SQUID detector

We describe a dc superconducting quantum interference device (SQUID) based magnetic resonance spectrometer particularly suited to measurements in the frequency range of 100 kHz to several megahertz. Results are presented for nuclear quadrupole resonance (NQR) of boron-11 in boron nitride at 4.2 K, yielding ωQ =1467±2 kHz. We also present a direct measurement of the methyl group tunneling frequency, ωt =210±10 kHz, of propionic acid in low field, at 4.2 K.

Effect of isotopic dilution in the methyl tunnelling spectra of molecular crystals

The rotational tunnel and torsion frequencies of crystalline nitromethane and toluene have been investigated using inelastic neutron scattering. The effects on these frequencies of complete deuteration, selective deuteration (in the case of toluene) and dilute mixing of the protonated species in a deuterated sample are reported. Small shifts in the tunnel frequencies of the protonated species dissolved in the deuterated matrix can be partially understood on the basis of lattice contraction upon deuteration. More surprising is the magnitude of the increase in the intrinsic energy width of both the tunnel and the torsional energy levels upon dilution.

Methyl tunnelling in tiglic acid and 2-methyl-2-butene

The tunnelling frequencies of methyl groups in tiglic acid and 2-methyl-2-butene have been measured by inelastic neutron scattering and field-cycling NMR spectroscopy at low temperature. The three-fold barrier heights have been deduced accurately from these experiments and their values interpreted in terms of the molecular and electronic structure of the two molecules. The temperature dependence of the proton spin-lattice relaxation time has also been measured and compared with the predictions of current theory.

Rotational tunneling of methyl groups in a strong magnetic field: a path-integral approach

The magnetic-flux splitting of the E levels of the methyl group in a threefold potential is analyzed using path-integral techniques. The results are in agreement with the conclusions obtained previously using the traditional methods of wave mechanics. The influence of the magnetic-flux splitting on the spin-lattice relaxation in single crystal samples is also discussed.

Methyl rotation in acetamide: the transition from quantum mechanical tunneling to classical reorientation studied by inelastic neutron scattering

Quasielastic and inelastic incoherent neutron scattering has been used to study in detail the transition from quantum mechanical tunneling motion to classical reorientation of the methyl groups in rhombohedral acetamide CH3CONH2. The temperature dependence of the low temperature quasielastic and inelastic scattering due toE a E b and AE transitions of the tunneling methyl groups has been investigated with μeV resolution and — together with the higher temperature quasielastic scattering-compared with theoretical predictions. Microscopic theories are capable to describe most of the experimental observations at low temperatures. A heuristic theoretical approach accounts well for the high temperature results.

Crystal Structure of Bis(trimethyltin)Selenate Dihydrateand Rotational Tunnelling of the Methyl Groups

Abstract Rotational tunnelling and librations of methyl groups in bis(trimethyltin) selenate dihydrate, [(CH3)3Sn]2SeO4 · 2 H2O, have been studied at low temperature using inelastic neutron scattering (INS) technique. Three tunnel transitions at 1.31 μeV, 3.47 μeV, and 21.0 μeV of equal intensity were recorded. In the energy range of phonons a broad, slightly structured intensity distribution is observed. An attempt is made to identify and assign librational modes. The crystal structure of the title compound was determined by single crystal X-ray diffraction. [(CH3)3Sn]2SeO4 · 2 H2O crystallizes orthorhombic, space group D14 2h-Pbcn with 4 molecules in the unit cell. The lattice constants at room temperature are a = 1142.9(3) pm, b = 1304.6(4) pm, and c = 1084.6(3) pm. According to the molecular symmetry, the two "(CH3)3Sn(H2O)" units in molecule are crystallographically equivalent, but the three methyl groups bound to one Sn-atom are inequivalent. This result coincides well with the observation of three tunnel transitions in INS. A comparison with the related compound, bis(trimethyltin) sulfate dihydrate, is presented. The existence of hydrogen bonding and the role of intermolecular interactions on rotational tunnelling are discussed.

Methyl tunnelling spectroscopy in acetophenone

Nuclear magnetic resonance (NMR) investigations of methyl tunnelling in acetophenone in the solid state are reported. Measurements of the proton spin-lattice relaxation time and of the methyl tunnelling frequency ( nu t=1.435 MHz) from dipole-dipole-driven low-field NMR demonstrate that the methyl group in acetophenone has an anomalously high potential barrier. A comparison with similar measurements on trans-4-phenyl-3-buten-2-one, along with a theoretical calculation of the respective barrier contributions, leads the authors to conclude that the origin of the barrier lies predominantly in the interaction between hydrogen atoms on the methyl and phenyl groups. The tunnelling sideband measurements reported extend the window of low-field NMR into the megahertz region and illustrate the existence of a NMR sideband intensity enhancement when the system is close to the level crossing between Zeeman states of the E and A manifolds, respectively.

Coupled quantum rotors in dimethyl-tin-dichloride: The isotope effect

The isotope effect of tunneling and liberational states of methyl groups in dimethyl-tin-dichloride is explained on the basis of coupled pairs of quantum rotors. The temperature dependence of the tunnel splitting shows the scaling behaviour expected from theory.

Thermally driven hindered internal rotation

The study of the coupling between the overall rotation of a molecule and its hindered internal rotation supports the concept of asymmetric driven tunnelling of methyl groups in solids at low temperatures as an explanation of the experimental observations.

Methyl tunnelling rotation in a series of ketones

The frequencies are reported of tunnelling rotation of the terminal methyl groups of n-alkyl chains in polycrystalline samples of ten members of the series n-alkane-2-one. The dependence of the frequency on chain length exhibits a sharp change. Alkyl chains longer than seven carbon atoms have methyl tunnel frequencies close to 300 kHz, as found earlier for long n-alkanes. Shorter chains have frequencies less than 160 kHz, except for ethyl, which has a frequency of 495 kHz. All of the ketone series give a single tunnel frequency per sample for the terminal methyl group of the alkyl chain, though up to four tunnel frequencies per sample have been observed for the much less hindered CO·CH 3 methyl group, reflecting distinguishable molecular sites in the unit cell. It is concluded that the molecular packing adapts to equalise the tunnel frequencies of strongly hindered groups and that the 300 kHz observed for longer chains may indicate alkane-like molecular packing.