Methyl Tunnelling Research Articles

Low field NMR investigations of methyl tunnelling

Low-field dipole-dipole-driven NMR has been used to measure the ground state methyl tunnelling frequencies in dimethylmalonic acid ( ν t = 337 ± 3 kHz), 3-hexanone ( ν t = 210 ± 10 kHz) and 4-heptanone ( ν t = 495 ± 3 kHz).

Methyl group tunnelling studies in calixarenes

Inelastic neutron scattering has been used to study the tunnelling of methyl groups belonging to several guest molecules (toluene, p-xylene, γ-picoline) incarcerated in a host calixarene matrix. In all the cases investigated, the low temperature neutron spectra show a number of bands which may be interpreted as being due to transitions between tunnel-split librational states of the guest methyl groups. The main line occurs near 0.63meV, very close to the CH 3 quantum free rotor limit. In the toluence complex, the quantum regime persists at least up to 60 K. Effects of coupling between rotational and vibrational modes are discussed. Very subtle structural changes not revealed by diffraction measurements are suggested by the neutron spectroscopy results.

Inelastic neutron-scattering study of methyl tunnelling and the quantum sine-Gordon breather mode in isotopic mixtures of 2,6-dimethyl-pyridine at low temperature

The INS spectra of 2,6-lutidine at low temperature reveal bands at 11.7, 15, 23, 30, 53 and 190μeV. The bands at 15 and 23μeV are broad and asymmetric. Both can be decomposed into two components at 14.3 and 16.0μeV, 22.8 and 24.7μeV, respectively. In addition, the band at 190μeV can be decomposed into three components at 182, 190 and 202μeV. At higher energy transfer, four broad bands can be identified at 3.64, 6.14, 7.30 and 9.67meV. INS spectra in the 100–200μeV region of isotopic mixtures containing 5, 15, 30, 40, 60, 73 and 85wt% (weight per cent) of 2,6-lutidine-h 9 into 2,6-lutidine-d 9 give a continuous frequency shift for the main feature from 190μeV (100wt% 2,6-lutidine-h 9) to 119μeV (5wt% 2,6-lutidine-h 9). These bands show upward frequency shifts with temperature above 10 K. For pure 2,6-lutidine-h 9 the frequency variation is consistent with a Boltzmann law and an activation energy ∼ 4.6meV. The bands below 50μeV show no significant temperature effect. These observations reveal collective rotation of the methyl groups. The quantum sine-Gordon theory is used to describe the dynamics in infinite chains of coupled rotors. The observed transitions are interpreted as travelling-states of the breather mode in a periodic potential (self-pinning).

On the Apparent Methyl Internal-Rotation Barrier Decrease in Weakly Bound Methanol Complexes

The microwave spectra of the van der Waals complexes acetone-20Ne and acetone-22Ne were measured using a cavity-based supersonic jet Fourier-transform microwave spectrometer in the region from 5 to 18 GHz. For these two isotopologues, both c- and weaker a-type transitions were observed. The transitions are split into multiplets due to the internal rotation of the two methyl groups in acetone. Initial electronic structure calculations were performed at the MP2/6-311++g (2d, p) level of theory and the internal rotation barrier height of the methyl groups was calculated to be ∼2.8 kJ/mol. The ab initio rotational constants were the basis for the spectroscopic searches, but the multiplet structures and floppiness of the complex made the quantum number assignment very difficult. The assignment was finally achieved with the aid of constructing closed frequency loops and predicting internal rotation splittings using the XIAM internal rotation program. The acetone methyl group tunneling barrier height was determined experimentally to be 3.10(6) kJ mol−1 [259(5) cm−1] in the acetone-Ne complex, which is lower than in the acetone monomer but comparable to the acetone-Ar complex (Kang et al., 2002). Experimental data and high-level CCSD(T)/aug-cc-pVTZ calculations suggest that the Ne atom lies directly above the plane formed by the carbonyl group and the two carbon-carbon bonds, which is different than the slightly offset position found previously in the acetone-Ar complex. Additionally, ab initio calculations and Quantum Theory of Atoms in Molecules analyses were used to analyze the methyl internal rotation motions in acetone and acetone-Ne.

Rotational quantization of methyl groups in a rotating frame

The rotational tunnel spectrum of pairs of coupled methyl groups in dimethylsulphide was studied at low temperatures using low field NMR. Two tunnel frequencies of 750 and 100 kHz give rise to sidebands of the NMR spectrum. The sidebands change to a broad asymmetric spectrum near internal resonances where nuclear Larmor and methyl tunnel frequencies coincide. An oscillating field at 100 kHz restores the sidebands and removes the asymmetry. It is proposed that the changes are due to methyl rotation induced by the applied oscillating field modulating the dipole-dipole terms, which become secular at the resonances. The mechanism extends to molecular dynamics the concepts of spin thermodynamics, and is also similar to the electromagnetic Aharonov-Bohm effect.

Rotational tunnelling and the rotational potential of methyl groups in the zinc, manganese and cobalt chloride salts of 4-methylpyridine

The inelastic neutron scattering spectra from the zinc, manganese and cobalt chloride salts of 4-methylpyridine are presented in the methyl tunnelling region and the molecular vibrational region. Rotational potentials are suggested for the methyl groups. For the zinc compound, two tunnelling peaks are found which correlate well with the two peaks found in the librational spectrum. This pairing of peaks is in agreement with the two distinct types of methyl group reported for the molecular and crystal structure. A rotational potential is derived which is consistent with previous NMR data. Tunnelling lines are also found in the manganese and cobalt compounds at lower energies. Inelastic spectra out to 160 meV for all three compounds are remarkably similar.

The pressure dependence of methyl tunnelling in acetylacetone

Pressure dependence measurements are reported on methyl tunnelling in acetylacetone and an isotopic derivative in which the hydrogen bond in the ENOL ring is deuterated. The effect of deuteration is to increase the intra-molecular component of the potential which hinders the reorientation of neighbouring methyl groups. Application of pressure, however, results in a decrease in the inter-molecular component of the hindering potential in both samples and the methyl group tunnelling frequency increases with compression. Above 3 kbar the principal tunnel peak no longer shifts with change in pressure but intensity is transferred progressively to new peaks. This structure is typical of coupled methyl groups and two models, coupled pairs and infinite coupled chains, are considered.

Reconsideration of the NMR three-pulse responses of methyl protons in CH3COONa.3D2O

The response of triads of strongly coupled spin-1/2 nuclei to the generalized Goldman-Shen pulse sequence ( alpha )phi - tau 1-( alpha )phi +180 degrees - tau 2-( beta )0 degrees is reconsidered by taking into account the spin diffusion between the equidistant energy-level pairs of the spin-rotor system during the time tau 2. The refined theory gives quantitatively correct model responses and sheds new light on the discrepancy between the previous theory for an ensemble of isolated triads and the analogous theory for pairs of coupled triads, in the limit of no coupling. Theoretical three-pulse spectra are compared with experimental responses of the tunnelling methyl protons in a CH3COONa.3D2O single crystal at 30 K.

Nonadiabatic screening of proton charges in the case of tunneling methyl groups in external magnetic field

The equation of motion corresponding to methyl groups embedded in solid lattices and reorienting around their symmetry axes is considered in the presence of external magnetic field. It is known that the Born-Oppenheimer approximation does not describe the electronic screening of the magnetic field as seen by the nuclei, i.e., the protons interact with the magnetic field with their bare charges. To obtain the screening, one has to include the non-adiabatic terms in the analysis. Using the rotating coordinate system fixed to the methyl group, we have shown that the effective chargeq, describing the interaction of the protons with the magnetic field, satisfiesq>0.24e (−e is the electron charge). The possibility of experimental verification is also discussed.

Resonant broadening of NMR line shapes of tunnelling methyl groups at level anticrossings

At the magnetic field which makes the proton Larmor frequency equal to the rotational tunnelling frequency of hindered methyl groups at low temperatures, terms in the dipole-dipole interactions which are normally non-secular become secular and strongly influence the observed line shape. A study of the low and zero field NMR spectrum is reported for a sample containing groups whose tunnel frequency is 75 kHz. Broadening due to anticrossings occurs at 0·9 and 1·8 mT, in good agreement with calculated spectra. The possible modification of the tunnelling frequency at the resonances is discussed.

Pressure dependence of methyl tunnelling in solid diacetyl

Methyl tunnelling has been measured in solid diacetyl at low temperatures as a function of pressure using low field NMR spectroscopy. In addition the associated thermally activated rotation at higher temperatures has been measured as a function of pressure by spin-lattice relaxation time (T 1) analysis. The tunnel frequency decreases exponentially from 76 kHz at atmospheric pressure to 9 kHz at 6·25 kbar. The T 1 data can be correlated with the tunnel frequency according to the methyl thermometer model, extending the model to lower tunnelling frequencies than previously demonstrated. A small sixfold symmetric term in the potential hindering rotation can be deduced from the data. The overall height of the hindering potential and its shape are compared with the potential calculated from the crystal structure.

The pressure dependence of tunnelling of coupled pairs of methyl groups in lithium acetate dihydrate

The pressure dependence of tunnelling of coupled pairs of methyl groups in lithium acetate dihydrate has been investigate using the neutron scattering spectrometer IRSIS. Methyl tunnelling spectra recorded at a temperature of 5 K and pressures up to 9.5 kbar, reveal structure which has been attributed to coupling. The data, which extend an earlier investigation by Heidemann et al. (1987) have been fitted to a time-independent Hamiltonian, and the fit parameters include the potential barriers of the individual methyl groups and the coupling potential between them. The magnitudes of both parameters increase with increasing pressure and their pressure dependence has been analyzed to provide information on the inter-molecular forces.

Rotational frequencies of methyl group tunneling

Methyl tunnel frequencies, measured at 4 K, are found to be 455 ± 8 kHz in methyl malonamide and 496 ± 8 kHz in methyl ethyl ketone. The first is unaffected by deuteration of the amide groups. Measurements of the temperature dependence of the spin lattice relaxation time are also reported for methyl malonamide and a further test is made of a previously reported correlation between tunnel frequency and the temperature of the T 1 minimum. The measurements are in good agreement with the universal correlation curve.

Optical measurements of methyl group tunneling in molecular crystals: Temperature dependence of the nuclear spin conversion rate

The tunneling methyl groups in dimethyl-s-tetrazine (DMST) doped single crystals of durene were investigated by high resolution optical spectroscopy using spectral hole burning. The experiments probe the level structure as well as the relaxation dynamics of the tunneling methyl groups in different electronic states of DMST. The tunneling splitting differs by 1.24 GHz in the ground and the first excited singlet states of DMST. In the ground electronic state, relaxation (spin conversion) between the spin 3/2 (A) and 1/2 (E) tunneling levels was measured between 1.5 and 12 K. The spin conversion time is larger than 100 h at 1.5 K and decreases with Arrhenius-type behavior above 3.5 K. The activation energy of 20 cm−1 also is observed as a phonon sideband in emission, and is, in agreement with theoretical predictions, tentatively assigned to a librational mode of the methyl group.

Spin thermodynamics and methyl tunnelling

Experiments are reported in which the populations of nuclear Zeeman and methyl tunnelling levels are manipulated by driving selected transitions in a sequence. Saturation bottlenecks can be avoided and the intensity of spectral lines enhanced by driving two transitions alternately and repetitively. The results are explained in terms of a two-temperature model. The method allows transitions between tunnelling levels that do not change the magnetization to be observed.

An ENDOR study of the temperature dependence of methyl tunnelling

We have studied the motion of the methyl group in the free radical obtained by γ-irradiation of 4-methyl-2,6-di-t-butylphenol (MDBP). The ENDOR spectra have been measured in the temperature range 4.5 to 200 K. The positions of the methyl proton lines are temperature dependent. Their variation is accounted for by considering the transition from the quantum regime of motion of the methyl group at low temperature to the classic regime at higher temperature. This transition is found to occur in a temperature range narrower than that anticipated by Allen's theory.

Methyl tunnelling in solid diacetyl

The techniques of dipole-dipole driven low-field NMR spectroscopy have been employed to measure the tunnelling frequency, ν t, of methyl groups in solid diacetyl (2,3-butanedione) at 4 K. The value of ν t is 74±1 kHz from which the height of the barrier to reorientation (assumed to have three-fold symmetry) is deduced to be 1845 K (15.3 kJ mol −1). The intermolecular contribution to the barrier has been calculated by evaluating the lattice sum of pairwise interatomic potentials as a function of methyl group orientation. This analysis reveals that the predominant contribution to the barrier arises from the methyl group of a neighbouring molecule. Temperature dependence measurements of the spin-lattice relaxation time have also been employed to study the dynamics of methyl reorientation in diacetyl and in the related molecule acetonylacetone (2,5-hexanedione).

NMR Spin-Lattice Relaxation and Tunnelling of Partially Deuterated Methyl Groups

Abstract Proton spin lattice relaxation rates have been measured at 15 and 30 MHz and down to 5 K for the partially deuterated molecular crystals 4-F-toluene, 4-Cl-toluene, and 2,6-Cl2-toluene. The behaviour of these materials is governed by methyl group tunnelling. As compared with the undeuterated compounds, the low temperature relaxation is enhanced and the details depend on the removal of the symmetry coupling between rotator and spin states. The hindering barriers remain unchanged, the A to E conversion rates are faster, and relaxation is dominated by spectral density contributions J(2ωo) and J(2ω0). In one case an additional influence of level-crossing energy transfer on relaxation is observed. Field-cycling spectroscopy reveals steps rather than peaks if the proton spin Zeeman and tunnelling splittings match

Rotational Excitations and Tunneling of Nonequivalent Methyl Groups in Tetramethylstibonium Iodide as Studied by Nuclear Magnetic Resonance and Inelastic Neutron Scattering

Abstract Nuclear magnetic resonance (NMR) and inelastic neutron scattering techniques (INS) have been applied to study the rotational motions and methyl group tunneling in tetramethylstibonium iodide, [Sb(CH3)4 ] I, over er a wide temperature range. Parameters describing the [Sb(CH3)4]+ cation tumbling and the methyl group reorientation at high temperatures and quantum mechanical tunneling of the methyl groups at low temperatures were determined. The results for INS and NMR experiments at low temperatures can be explained in terms of two crystallographically inequivalent methyl groups CH3(1) and CH3(2), which were established earlier by the crystal structure determination. In the INS spectra two tunneling lines at 22.0 μeV for CH3(1) and 1.05 μeV for CH3(2) with inelastic intensities in the ratio 3:1 were observed at T = 4 K. The activation energies derived from proton NMR spin-lattice relaxation time measurements for the thermally activated methyl group rotation are 1.50 kJ/mol for CH3(1) and 3.81 kJ/mol for CH3(2). They are in accordance with the activation energies obtained from neutron fixed-window measurements. The activation energy for [Sb(CH3)4]+ cation tumbling is 50.9 kJ/mol as determined from the high temperature spin-lattice relaxation behaviour. Rotational potentials for the methyl groups are derived. For both kinds of inequivalent methyl groups the threefold potential terms dominate; three- and sixfold potential contributions are shifted by 180°

Methyl torsional fine structure in the high-resolution S→T(nπ*) excitation spectrum of acetophenone and its three methyl-deuterated isotopomers

The cold-beam excitation spectrum of the S0→T(nπ*) origin band of acetophenone shows two maxima separated by about 1.2 cm−1, interpreted as due to methyl tunneling. Consecutive deuteration of the methyl group leads to three maxima in acetophenone-d1, two maxima in acetophenone-d2 and a single broad maximum in acetophenone-d3, with maximum separations of about 7 cm−1 in -d1 and about 3.5 cm−1 in -d2. These separations are due to the occurrence of two rotamers for isotopomers without threefold permutation symmetry. The spectra are analyzed in terms of hindered rotor potentials. It is shown that S0 and T(nπ*) share the same torsional equilibrium configuration, relative to which the S1(nπ*) equilibrium configuration is rotated by 60°. However, both nπ* states have very shallow torsional potentials with barrier heights of 70 and 90 cm−1 for T and S, respectively, compared with a barrier of about 800 cm−1 in the ground state. Hence, relative to S0 the two potentials are quite similar despite the shift. The potential terms entering upon partial deuteration are quite similar for S0 and T(nπ*) and hence depend only weakly on the barrier height. They are ascribed to kinetic coupling of the asymmetric rotor of the CH2D or CHD2 group with other modes in the molecule.