Methyl Tunnelling Research Articles

ChemInform Abstract: Methyl Group Rotation and Tunnelling in Crystals of 5‐Membered Ring Molecules

ChemInformVolume 19, Issue 18 Physical Organic Chemistry ChemInform Abstract: Methyl Group Rotation and Tunnelling in Crystals of 5-Membered Ring Molecules A.-S. MONTJOIE, A.-S. MONTJOIE Inst. Phys. Chem., Westfael. Wilhelms-Univ., D-4400 MuensterSearch for more papers by this authorW. + MUELLER-WARMUTH, W. + MUELLER-WARMUTH Inst. Phys. Chem., Westfael. Wilhelms-Univ., D-4400 MuensterSearch for more papers by this authorH. STILLER, H. STILLER Inst. Phys. Chem., Westfael. Wilhelms-Univ., D-4400 MuensterSearch for more papers by this authorJ. STANISLAWSKI, J. STANISLAWSKI Inst. Phys. Chem., Westfael. Wilhelms-Univ., D-4400 MuensterSearch for more papers by this author A.-S. MONTJOIE, A.-S. MONTJOIE Inst. Phys. Chem., Westfael. Wilhelms-Univ., D-4400 MuensterSearch for more papers by this authorW. + MUELLER-WARMUTH, W. + MUELLER-WARMUTH Inst. Phys. Chem., Westfael. Wilhelms-Univ., D-4400 MuensterSearch for more papers by this authorH. STILLER, H. STILLER Inst. Phys. Chem., Westfael. Wilhelms-Univ., D-4400 MuensterSearch for more papers by this authorJ. STANISLAWSKI, J. STANISLAWSKI Inst. Phys. Chem., Westfael. Wilhelms-Univ., D-4400 MuensterSearch for more papers by this author First published: May 3, 1988 https://doi.org/10.1002/chin.198818053Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume19, Issue18May 3, 1988 RelatedInformation

Methyl tunnelling in acetylacetone

Measurements of the temperature dependence of the proton spin-lattice relaxation time in solid acetylacetone demonstrate that the existence of two physically or chemically distinguishable methyl groups in the unit cell. One of these has a particularly large tunnelling splitting and is the subject of this investigation. Its tunnelling splitting has been measured by inelastic neutron scattering on the IRIS spectrometer and has the value 42 mu eV. The authors hence deduce the threefold barrier height to be 225 K (1.87 kJ mol-1). Using this value, the rate of thermally activated hopping has been calculated as a function of temperature and compared with experimental values.

Rotational tunnelling of methyl groups in bis(trimethyltin) sulfate dihydrate

AbstractRotational tunnelling and librations of methyl groups in bis(trimethyltin) sulfate dideuterate, [(CH3)3Sn)]2SO4·2D2O, have been studied by inelastic neutron scattering (INS) technique. Tunnel splittings of 11. 36 μeV and 1.10 μeV and librational peaks at 9.98 meV and 14.8 meV were observed. The corresponding activation energies are calculated to be 4. 4 kJmol−1 and 2.6 kJmol−1, respectively. The INS measurements showed the existence of two types of inequivalent methyl groups of the occurrence ratio 1:2. The crystal structure of bis(trimethyltin) sulfate dihydrate was determined by single crystal X‐ray diffraction. [(CH3)3Sn)]2SO4·2H2O crystallizes orthorhombic, space group D142h‐Pbcn with 4 molecules in the unit cell. The lattice constants are a = 1124.4(2) pm, b = 1269.3(2) pm and c = 1080.9(2) pm. The sulfur atom lies on a twofold axis and therefore the two “(CH3)3Sn(H2O)” units in the molecule are crystallographically equivalent. The crystal structure analysis shows the three methyl groups bond to one tin atom to be inequivalent. This is not in agreement with the INS results. The discrepancy is discussed.

NM R T1-Analysis of Methyl Tunnelling in Molecular Crystals at Intermediate Barriers

The frequency and temperature dependences of the 1H NMR relaxation rates T1 -1for rotating CH3 groups in solids look quite anomalous at intermediate hindering potential barriers. In order to explain the experimental behaviour and to extract parameters from the NMR T1 experiments that account for tunnelling at low temperatures and random reorientation at elevated temperatures, we have calculated T1 -1vs. T-1 curves to be expected under various conditions. New experimental results for methyl iodide, methyl bromide, 3-methylthiophene, p-xylene, and methyl isocyanate have been fitted by this procedure and all the parameters that describe the motional behaviour have been derived. The single particle rotational potential has been determined using the first two terms of the Fourier expansion.

Methyl tunnelling in α-crystallised toluene by inelastic neutron scattering: temperature and pressure effects

The methyl tunnelling in alpha -crystallised toluenes C6H5CH3 and C6D5CH3 has been studied by high-resolution inelastic neutron scattering. Two tunnel peaks are measured, corresponding to the two inequivalent methyl groups of this crystalline structure. Their frequencies change when deuterating the benzene ring. They are found to exhibit different temperature and pressure (up to 4.6 kbar) effects. All the collected data lead to the determination of two potential models for the methyl group rotation.

Intermolecular interaction in Sn(CH3)4: An inelastic neutron scattering study of matrix isolated molecules

Rotational tunneling of methyl groups in Sn(CH3)4 molecules isolated in solid argon has been observed by inelastic neutron scattering at ℏωt =72 μeV. In contrast to the bulk material all CH3 groups are equivalent, and the hindrance potentials is strongly reduced. The tunneling lines show an inhomogeneous broadening which decreases with decreasing concentration of the guest molecules. A discontinuity in the temperature dependence of the tunnel splitting could only be interpreted as due to a spontaneous rearrangement of the guest molecules in the argon lattice. Transitions at 2.1 and 20.7 meV are assigned to a methyl torsion on the basis of the temperature dependence of the tunneling lines and to a scelettal bending mode, respectively. A mode at 11.0 meV (88 cm−1) had not been found by other techniques. It is tentatively assigned to a libration of the Sn(CH3)4 against it cage. An alternative interpretation in a model of coupled methyl groups is also possible. The spectra become similar to the bulk material if the sample is annealed at T∼45 K indicating a clustering of the Sn(CH3)4 molecules to a not completely ordered phase.

A study of methyl group dynamics and barrier heights in a homologous series of unbranched ketones

The temperature dependence of the nuclear (proton) spin-lattice relaxation time, T 1, has been measured in the range 10–300 K for the following series of unbranched ketones; 2-butanone to 2-nonanone, 3-pentanone to 3-octanone and 4-heptanone. This data has been analysed to provide estimates for the magnitudes of the three-fold potential barriers to reorientation of all methyl groups in these materials. The corresponding methyl tunnel splittings have also been predicted. Measurements of six tunnel splittings in four of the samples encompassing an energy range of four orders of magnitude confirm these predictions to be accurate and provide refined values for the barrier heights. The tunnelling spectroscopy was performed using the techniques of high-resolution inelastic neutron scattering, field-cycling level-crossing N.M.R. spectroscopy and double sideband irradiation N.M.R. spectroscopy. The observed trends in barrier height within the series of materials have been rationalized in terms of the known molecu...

Proton NMR and Tunneling of Methyl Groups in γ Phase of Tetramethylsilane

The Larmor frequency-dependence of the slowly-relaxing component of spin-lattice relaxation due to methyl rotation in solid γ-Si(CH 3 ) 4 is consistent with the tunneling frequencies of 15.8 and 24.0 MHz in the limit or uncoupled rotation. Inconsistencies among the published results and interpretations are discussed.

Proton Spin-Lattice Relaxation from Tunneling Methyl Groups in Rhombohedral Acetamide

Proton Spin-Lattice Relaxation from Tunneling Methyl Groups in Rhombohedral Acetamide

N.M.R. Proton Lineshape in Single Crystals (TMTSF)2 x and Antiferromagnetic Satellites

Abstract Beetwen 1.2K and 4.2K in the metallic state of relaxed (TMTSF)2 CL04, the tunneling of methyl groups gives a central line and four symmetrical satellites in proton N.M.R. In quenched (TMTSF)2CL04 and (TMTSF)2PF6 new satellites give the orientation and amplitude of the local fields (15 G in CL04 and 27 G in PF6) corresponding to an S.D.W. amplitude of 137. and 25% respectively. The S.D.W. seems incommensurate to the lattice.

Quantum tunnelling and hopping motion of methyl groups in cadmium acetate dihydrate

Methyl groups in cadmium acetate dihydrate occur in two very different sites, and the rotation of one is much more strongly hindered than the other. The hopping motion of both types has been studied by nuclear spin-lattice relaxation and the tunnel motion of the more weakly hindered type has been very extensively studied by field-cycling tunnel resonance techniques at 4K. The tunnel frequency is 9.9 GHz. Internal resonances between the tunnel frequency and electronic Larmor frequencies of various types of paramagnetic impurities have been detected by observing the field dependence of the nuclear magnetisation recovery after saturation. Examples of impurities interacting with the tunnelling methyl group are Cu2+, Cd+ and free radicals CH2 formed by gamma -irradiation. Attempts to measure the tunnel frequency of the more strongly hindered type of methyl groups by similar methods have not been successful. From the nuclear spin-lattice relaxation data, its tunnel frequency is estimated to be about 500 MHz. Calculations indicate that this more hindered type is the one lying close to the planes of the hydrogen bond network which links the parallel Cd-O helices while the weakly hindered type is identified with the groups lying in the channels between these planes.

Tunneling of methyl groups in toluquinone: Dependence on the electronic state of the molecule

The torsional frequency and the tunneling splitting of the methyl group of crystalline toluquinone are reported for different electronic states of the molecule. Whereas in the ground state both values can be derived from a conventional cos 3φ potential with a barrier height of approximately 8 kJ/mol, the values in the lowest excited triplet state cannot be reconciled with such a simple potential. It is suggested that a chemical interaction between the methyl protons and the adjacent oxygen of the excited carbonyl group is responsible for the modification of the potential. Numerical calculations based on a simple modeling of this chemical interaction support this idea.

ChemInform Abstract: The Impact of the Symmetry Properties of Classical Reorienting and Tunnelling Methyl Groups on the Spin‐Lattice Relaxation in Solids

Chemischer InformationsdienstVolume 15, Issue 36 Reviews ChemInform Abstract: The Impact of the Symmetry Properties of Classical Reorienting and Tunnelling Methyl Groups on the Spin-Lattice Relaxation in Solids R. A. WIND, R. A. WINDSearch for more papers by this authorS. EMID, S. EMIDSearch for more papers by this authorD. J. LIGTHELM, D. J. LIGTHELMSearch for more papers by this authorJ. F. J. M. POURQUIE, J. F. J. M. POURQUIESearch for more papers by this authorJ. SMIDT, J. SMIDTSearch for more papers by this author R. A. WIND, R. A. WINDSearch for more papers by this authorS. EMID, S. EMIDSearch for more papers by this authorD. J. LIGTHELM, D. J. LIGTHELMSearch for more papers by this authorJ. F. J. M. POURQUIE, J. F. J. M. POURQUIESearch for more papers by this authorJ. SMIDT, J. SMIDTSearch for more papers by this author First published: September 4, 1984 https://doi.org/10.1002/chin.198436364Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume15, Issue36September 4, 1984 RelatedInformation

Coupled tunnelling motion of a pair of methyl groups in lithium acetate studied by inelastic neutron scattering

The tunnelling and torsional motions of methyl groups in lithium acetate dihydrate (CH3COOLi·2H2O) have been studied in detail by incoherent inelastic neutron scattering. The results are interpreted by a model of pairs of methyl groups performing a coupled tunnelling motion. The strength of the coupling term is estimated to be about twice as strong as the threefold hindering barrier for the single particle motion.

Methyl tunnelling spectroscopy and level crossing phenomena in solid acetone

Reports the observation in acetone at low temperature of a number of new phenomena connected with methyl tunnelling motion and field-cycling nuclear magnetic resonance spectroscopy. The authors observe three strong anomalies in the field-cycling nuclear magnetic resonance spectroscopy. The authors observe three strong anomalies in the field dependence of proton spin-lattice relaxation at 4K for fields B0, B0/3 and B0/3 and B0/3 and B0/3 with B0=2.25T. They are due to internal resonances (or level crossings) occurring for n nu 0= nu t, (n=1, 2, 3) where nu 0 is the field-dependent nuclear Larmor frequency and nu t is the methyl tunnel frequency. This structure is a consequence of the field-cycling technique which involves rapid switches of the magnetic field to enable all measurements of proton magnetisation to be made at a single field 0.6 T. A simple thermodynamic model is described in which there is very rapid thermal equilibration between the nuclear Zeeman and methyl tunnel reservoirs at the fields corresponding to the n=1 and 2 anomalies and the nuclear magnetisation is otherwise unaffected by the rapid field changes. This model successfully simulates the observed spectrum and a variety of other field-cycling experiments.

Coupled relaxation of tunnelling methyl groups

The coupled relaxation of the Zeeman and tunnel systems of methyl groups has been studied by N.M.R. spectroscopy in powdered pentamethyl-benzene at temperatures below 15 K, in the magnetic field range 0-2 T. The observed rapidity, saturation and recovery of the resonance confirm theoretical predictions of Nijman, Sprik and Trappeniers [12] for (X-H4) substances, and the equivalence of their theory adapted by us to methyl groups using the simplified relaxation model of Emid and Wind [4] is established. Reasons for the semi-quantitative agreement between theory and experiment are discussed.

Pseudo-classical narrowing of the proton NMR absorption line at low temperature in the tunneling methyl system

The lineshape of the NMR absorption of a methyl group was measured down to 4.2 K for trichloromethylsilane, tetramethylsilane, and tetramethylpyrazine. A small reduction of the linewidth was observed at 30–50 K, which was attributed to averaging of the contribution of intermethyl dipole interactions by tunnel rotation of methyl groups. The mechanism of narrowing is discussed in relation to the magnitude of tunnel splitting and correlation frequency of fluctuation of the dipole interaction.

Methyl Tunnelling and Reorientation in the Solid Tetramethyl Compounds of Silicon, Germanium Tin and Lead, and in Methyl Lithium

Quantum-mechanical tunnelling and thermally activated reorientation of methyl groups have been studied in the series compounds X(CH3)4 with X = Si, Ge, Sn, Pb and LiCH3 using NMR and. if possible, neutron scattering (INS and QNS) techniques. The temperature and frequency dependence of the relaxation rates 1/T1 and 1/T1Q mirror the transition from small to large tunnel splittings as the barrier to rotation decreases in the succession X = Si. LiCH3, X = Ge, Sn, Pb. The various relaxation maxima, correlation times and apparent activation energies have been discussed in terms of current theories. Using the NMR, INS, QNS results and available literature data, relationships between tunnelling and temperature, activation, molecular structure and potential functions have been derived. Potential barriers decrease from about 7 to 0.2 kJ/mol and the symmetry is not far from threefold.

Methyl Zeeman-tunnel resonance and nuclear spin relaxation in copper acetate

The methyl tunnel spectrum of copper acetate is studied at low temperatures by means of nuclear magnetic resonance level-crossing spectroscopy employing a field-cycling technique. The recovery of the proton magnetisation following saturation is measured as a function of magnetic field. Changing the magnetic field causes energy levels to cross when the methyl tunnel frequency is equal to one or two times the proton Larmor frequency, and abrupt changes in the recovered magnetisation, showing up as peaks or dips in a smooth background, reveal the positions of the level crossings. A variety of possible field cycles and sequences of cycles leads to a series of different experimental procedures which highlight different aspects of the thermodynamic properties of the system. In particular the dependence of the spectra on scanning schemes and on the thermodynamic preparation conditions is investigated. The experiments are understood in a semi-quantitative way in terms of a simple mathematical model for the thermal equilibration of three energy reservoirs, of which two are resonantly coupled at the level crossings. This simulation shows that, while the dominant processes have been clearly identified and incorporated in the model, some important factors remain to be explained. The methyl tunnel frequency in copper acetate is found to be (64+or-5) MHz.

The correlation of methyl tunnelling and thermally activated reorientation: II

The correlation of methyl tunnelling and thermally activated reorientation: II