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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).

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.

The dynamics of hydrogen atoms in the hydrogen bonds of benzoic acid dimers have been studied as a function of hydrostatic pressure to pressures in excess of 4 kbar. This paper is primarily concerned with results up to 3.3 kbar. The temperature dependence of the correlation time for the motion at a series of pressures has been investigated using measurements of the proton spin–lattice relaxation time. Strong non‐Arrhenius behavior is exhibited and the data are in good agreement with a model which invokes phonon assisted tunneling at low temperature and thermally activated Arrhenius dynamics at high temperature. The parameters in the model include the asymmetry of the double minimum potential experienced by the hydrogen atoms and dynamical variables relating to the tunneling and hopping processes. The rate of phonon assisted tunneling is observed to increase exponentially with increasing pressure and this is attributed to the increase in the tunneling matrix element which occurs as the distance between the potential wells is decreased and the overlap of the localized eigenstates beneath the barrier increases. We also observe a decrease in the asymmetry of the potential with increasing pressure which is attributable to modifications to the interdimer contributions to the potential. There is evidence in the nuclear magnetic resonance (NMR) data for two phase transitions below 4 kbar and one of these may be correlated with the reduction in asymmetry of the potential.

The populations of the tunneling states of ${\mathrm{CH}}_{3}$ are manipulated by rf irradiation of weakly allowed sideband transitions within the manifold of tunneling-magnetic levels. Substantial positive and negative ${\mathrm{CH}}_{3}$ tunneling polarizations are observed, providing a quantum rotor analogue of dynamic nuclear polarization and the solid effect in NMR. The field-cycling NMR technique used in the experiments employs level crossings between tunneling and Zeeman systems to report on the tunneling polarization. The tunneling lifetimes are measured and the field dependence investigated.

The principles of field-cycling (FC) NMR are described together with a discussion of the practical implementations of the technique and an outline of some selected scientific applications. Keywords: field-cycling NMR; spin-lattice relaxation; relaxometry; quadrupole dips; cross-relaxation; molecular dynamics; relaxation dispersion

The dynamics of hydrogen atoms in the hydrogen bonds of molecular dimers in dodecanoic acid (c phase) have been studied by quasi-elastic neutron scattering and pulsed nuclear magnetic resonance. Q-dependence measurements of the intensity of the quasi-elastic peak have established that the hydrogen atoms move along a line connecting the two oxygen atoms in the hydrogen bond. The correlation time for this motion has been studied by temperature dependence measurements of the width of the quasi-elastic line and of the proton spin-lattice relaxation time,T 1. These studies reveal the quantum mechanical nature of the dynamics in the low temperature region. The dynamical parameters which characterise the motion have been determined by fitting the data to a model which invokes phonon assisted tunnelling. The frequency dependence ofT 1 at low temperature is anomalous because the gradient of the ln(T 1) vs 1/T curve is dependent on the applied magnetic field.

The wavefunction of a quantum system driven by a thermal environment localises when environmentally induced motion overtakes oscillations of the undriven system. The slowing down of the oscillations with increasing temperature is an important aspect of the transition from wave-like to particle-like behaviour. It is clearly displayed in the temperature-dependent motional spectrum of hindered methyl groups. Three resolved peaks at low temperature converge and broaden with increasing temperature. A one-parameter model of the transition extends the high-temperature hopping model into the quantum regime and agrees well with scattering measurements.

Approximate rotational quantisation is associated with imperfectly defined wavenumber and imperfect angular momentum conservation of a rotor interacting with its environment. Standard theories of hindered molecular groups like CH3 allow only perfect quantisation. We show that this is because they attribute two incompatible sets of properties to the nuclei of hydrogen atoms in a false version of the Pauli principle and are thereby restricted to a closed space inconsistent with environmental interaction. They conclude that environmental dynamics is constrained to preserve the perfect quantisation. The true Pauli principle is consistent with an open space and leads to approximate quantisation without symmetry constraint and to similarity with other transport theories.

The reorientation of CF 3 rotors in solid hexafluoroacetylacetone has been studied using pulsed NMR techniques. The temperature dependence of both fluorine ( 19 F) and hydrogen ( 1 H) nuclear spin-lattice relaxation has been measured using saturation-recovery pulse sequences. Following saturation of one of the spin reservoirs, the evolution with time of the longitudinal magnetisations associated with both nuclei are observed to be strongly bi-exponential. A theory is presented for the spin-lattice relaxation which is mediated by the heteronuclear dipole-dipole interaction between 19 F and 1 H nuclei. The experimental results are in good quantitative agreement with this relaxation theory. The spectral density functions have been evaluated using the known molecular structure and they are consistent with the experimental data. From this analysis we conclude that the relaxation is dominated by an intramolecular 19 F&.z.sbnd; 1 H dipolar interaction. The spectral density function J (0) ( ω H - ω F ) makes an important contribution to the relaxation theory with the result that the motional spectrum is conveniently sampled at the difference frequency between 1 H and 19 F Larmor precessions. Two physically distinguishable CF 3 rotors have been identified in this investigation, their dynamics are characterised by activation energies of 7.5 ± 0.4 and 13.3 ± 0.8 kJ mol −1 , respectively.

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.

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.

Experimental investigations of molecular tunnelling in symmetrical molecular groups using the techniques of neutron scattering are reviewed. Particular emphasis is laid upon the tunnelling spectra of methyl groups. The relationship between the dynamics of the rotors and their “static” molecular environment is discussed with examples from published work.

We report an inelastic neutron-scattering (INS) investigation of coupled quantum translation and rotation of hydrogen molecules trapped inside the closed isotropic cages of ${\text{C}}_{60}$. The low-lying states that characterize the translation-rotation manifold of the hydrogen molecules are accurately determined in our study of the INS peak energies. A comparison between the spectra of ${\text{H}}_{2}$ and HD isotopomers provides quantitative insight into the coupling between rotational and translational angular momentum with HD exhibiting the strongest effects due to mixing of the rotational eigenstates.

Using low temperature dynamic nuclear polarisation (DNP) in conjunction with dissolution makes it possible to generate highly polarised nuclear spin systems for liquid state applications of nuclear magnetic resonance spectroscopy. However, in its current implementation, which requires the transfer of the solute between two different magnets, the hyperpolarisation strategy is limited to spin systems with relatively long longitudinal relaxation time constants. Here we describe the design and construction of a dedicated spectrometer for DNP applications that is based on a magnet with two isocentres. DNP enhancement is carried out in the upper compartment of this magnet in a low temperature environment at 3.35 T, while a 9.4 T isocentre in the lower compartment is used for high resolution NMR spectroscopy. The close proximity (85 cm) of the two isocentres makes it possible to transfer the sample in the solid state with very little loss of spin polarisation. In first performance tests this novel experimental set-up proved to be superior to the strategy involving two separate magnets.

Experimental investigations of molecular tunnelling in symmetrical molecular groups using the techniques of neutron scattering are reviewed. Particular emphasis is laid upon the tunnelling spectra of methyl groups. The relationship between the dynamics of the rotors and their “static” molecular environment is discussed with examples from published work.

The techniques of dipole-dipole driven low-field NMR spectroscopy have been employed to measure the tunnelling frequency, νt, of methyl groups in solid diacetyl (2,3-butanedione) at 4 K. The value of νt 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).

Proton tunnelling in the hydrogen bonds of two fluorine substituted benzoic acid dimers has been investigated using field-cycling NMR relaxometry. The close proximity of the (19)F nuclei to the hydrogen bond protons introduces heteronuclear (19)F-(1)H dipolar interactions into the spin-lattice relaxation processes. This renders the (1)H magnetisation-recovery biexponential and introduces multiple spectral density components into the relaxation matrix characterised by frequencies that are sums and differences of the (19)F and (1)H Larmor frequencies. Using field-cycling NMR pulse sequences that measure the spin-lattice relaxation and cross-relaxation rates we demonstrate how some of these multiple spectral density components can be separately resolved. This leads to an accurate determination of the correlation times that characterise the proton tunnelling motion. A broad spectrum of relaxation behaviour is illustrated and explored in the chosen samples and the investigation is used to explore the theory and practise of field-cycling NMR relaxometry in cases where heteronuclear interactions are significant.

Hindered ammonium ions and level crossing NMR are used to study thermally activated rotation. Hindered rotation is a quantum trajectory occurring with a symmetry breaking deformation of the ion which creates rotational coordinates with an associated vector potential. The coordinates label the protons and modify the proton spin symmetry rules which apply to the wavefunctions of an ion in a static hindering potential. The degeneracy of the Kramers doublets is lifted and twelve orientations of the tetrahedron are identified instead of the six distinguished by the proton spin in the static case. Level crossing NMR spectra of ammonium persulphate between 4 and 20 K demonstrate the splitting through the large difference in the thermal broadening of transitions involving doublet and singlet states. New discrete structure which appears in the tunnelling spectrum with increasing temperature may be due to frequency modulation due to precession of the rotation axis about the crystal field symmetry axis. The symmetry breaking makes the quantum and classical descriptions of hindered rotation compatible.

We report an inelastic neutron scattering investigation of the quantum dynamics of hydrogen molecules trapped inside anisotropic fullerene cages. Transitions among the manifold of quantized rotational and translational states are directly observed. The spectra recorded as a function of energy and momentum transfer are interpreted in terms of the rotational potential and the cage dimensions. The thermodynamics of orthohydrogen and parahydrogen are investigated through temperature dependence measurements.

The low-temperature structure and dynamics of guest molecules of p-xylene incorporated in the isopropyl-calix[4] arene(2:1) p-xylene complex have been investigated by solid state nuclear magnetic resonance (NMR). Using one-dimensional 1H-decoupled 13C cross-polarization magic-angle-spinning (MAS) NMR and two-dimensional 1H-13C correlation spectroscopy, a full assignment of the 13C and 1H chemical shifts has been made. Using 1H NMR relaxometry, the effects of thermal history on the structure of the system have been investigated. Rapidly cooled samples have 1H spin-lattice relaxation times T1, which at low temperature (T60 K) are typically two orders of magnitude faster than those observed in annealed samples which have been cooled slowly over many hours. In both forms, the low-temperature relaxation is driven by the dynamics of the weakly hindered methyl rotors of the p-xylene guest. The substantial difference in T1 is attributed in the rapidly cooled sample to disorder in the structure of the complex leading to a wide distribution of correlation times and methyl barrier heights. A comparison of the linewidths and splittings in the high resolution 13C MAS spectra of the two forms provides structural insight into the nature of the disorder. Using 1H field-cycling NMR relaxometry, the methyl dynamics of the p-xylene guest in the annealed sample have been fully characterized. The B-field dependence of the 1H T1 maps out the spectral density from which the correlation times are directly measured. The methyl barrier heights are determined from an analysis of the temperature dependence.

Correction for ‘Low-field thermal mixing in [1-13C] pyruvic acid for brute-force hyperpolarization’ by David T. Peat et al., Phys. Chem. Chem. Phys., 2016, 18, 19173–19182.

The transfer of protons involved in hydrogen bonding is fundamental to many chemical and biological processes. Quantum tunnelling can play an important role in this process1,2. It manifests itself in strong isotope effects3,4 and has been observed directly in the solid state5. The tunnelling behaviour seen in such studies usually displays the characteristics of a particle confined in a double-well potential. But proton tunnelling can also occur in a coordinated fashion that involves many hydrogen bonds simultaneously. Such a process may significantly affect the properties of linear and circular networks of hydrogen bonds, which occur in ice and in macromolecules containing hydroxyl groups6,7. Here we report the direct observation by NMR relaxometry of coordinated proton tunnelling in a cyclic array of four hydrogen bonds in solid p-tert-butyl calix[4]arene at low temperature. We are able to quantify the parameters that describe this phenomenon and find good agreement with theoretical predictions for phonon-assisted tunnelling8.

We have observed 1H NMR spectra of hydrogen molecules trapped inside modified fullerene cages under cryogenic conditions. Experiments on static samples were performed at sample temperatures down to 4.3 K, while magic-angle-spinning (MAS) experiments were performed at temperatures down to 20 K at spinning frequencies of 15 kHz. Both types of NMR spectra show a large increase in the intramolecular 1H-1H dipolar coupling at temperatures below 50 K, revealing thermal selection of a small number of spatial rotational states. The static and MAS spectra were compared to estimate the degree of sample heating in high-speed cryogenic MAS-NMR experiments. The cryogenic MAS-NMR data show that the site resolution of magic-angle-spinning NMR may be combined with the high signal strength of cryogenic operation and that cryogenic phenomena may be studied with chemical site selectivity.

Concerted double proton transfer in the hydrogen bonds of a carboxylic acid dimer has been studied using 13C field-cycling NMR relaxometry. Heteronuclear 13C-1H dipolar interactions dominate the 13C spin-lattice relaxation which is significantly influenced by the polarisation state of the 1H Zeeman reservoir. The methodology of field-cycling experiments for such heteronuclear spin-coupled systems is studied experimentally and theoretically, including an investigation of various saturation-recovery and polarisation-recovery pulse sequence schemes. A theoretical model of the spin-lattice relaxation of this coupled system is presented which is corroborated by experiment. Spectral density components with frequencies omega(C), omega(C) + omega(H), and omega(C) - omega(H) are mapped out experimentally from the magnetic field dependence of the 13C and 1H spin-lattice relaxation and the proton transfer rate at low temperature is determined from their widths. Any dynamic isotope effect on the proton tunnelling in the hydrogen bond arising from 13C enrichment in the skeletal framework of the dimer is found to be smaller than experimental uncertainties (approximately 5%).

The methyl rotational tunneling spectrum of p-xylene confined in nanoporous zeolite crystals has been measured by inelastic neutron scattering (INS) and proton nuclear magnetic resonance (NMR), and analyzed to extract the rotational potential energy surfaces characteristic of the methyl groups in the host-guest complex. The number and relative intensities of the tunneling peaks observed by INS indicate the presence of methyl-methyl coupling interactions in addition to the methyl-zeolite interactions. The INS tunneling spectra from the crystals (space group P2(1)2(1)2(1) with four crystallographically inequivalent methyl rotors) are quantitatively interpreted as a combination of transitions involving two coupled methyl rotors as well as a transition involving single-particle tunneling of a third inequivalent rotor, in a manner consistent with the observed tunneling energies and relative intensities. Together, the crystal structure and the absence of additional peaks in the INS spectra suggest that the tunneling of the fourth inequivalent rotor is strongly hindered and inaccessible to INS measurements. This is verified by proton NMR measurements of the spin-lattice relaxation time which reveal the tunneling characteristics of the fourth inequivalent rotor.

The isotope effects associated with double proton transfer in the hydrogen bonds of benzoic acid (BA) dimers have been measured using field-cycling (1)H NMR relaxometry and quasielastic neutron scattering. By studying mixed isotope (hydrogen and deuterium) samples, the dynamics of three isotopologues, BA-HH, BA-HD, and BA-DD, have been investigated. Low temperature measurements provide accurate measurements of the incoherent tunneling rate, k(0). This parameter scales accurately with the mass number, m, according to the formula k(0)=(E/m)e(-Fm) providing conclusive evidence that the proton transfer process is a strongly correlated motion of two hydrons. Furthermore, we conclude that the tunneling pathway is the same for the three isotopologue species. Measurements at higher temperatures illuminate the through barrier processes that are mediated via intermediate or excited vibrational states. In parallel with the investigation of proton transfer dynamics, the theoretical and experimental aspects of studying spin-lattice relaxation in single crystals of mixed isotope samples are investigated in depth. Heteronuclear dipolar interactions between (1)H and (2)H isotopes contribute significantly to the overall proton spin-lattice relaxation and it is shown that these must be modeled correctly to obtain accurate values for the proton transfer rates. Since the sample used in the NMR measurements was a single crystal, full account of the orientation dependence of the spin-lattice relaxation with respect to the applied B field was incorporated into the data analysis.

We observed coherent proton tunneling in the cyclic network of four hydrogen bonds in calix[4]arene. The tunneling frequency of 35 megahertz was revealed by a peak in the magnetic field dependence of the proton spin-lattice relaxation rate measured with field-cycling nuclear magnetic resonance in the solid state at temperatures below 80 kelvin. The amplitude of the coherent tunneling peak grows with temperature according to a Boltzmann law with energy D / k B = (125 ± 10) kelvin (where k B is Boltzmann's constant). The tunneling peak can be interpreted in the context of level crossings in the region where the tunneling frequency matches the proton Larmor frequency. The tunneling spectrum reveals fine structure that we attribute to coupling between the hydrogen bonds in the network. The characteristics of the tunneling peak are interpreted in the context of the potential energy surface experienced by the hydrogen atoms in the network.

The quantum dynamics of a hydrogen molecule encapsulated inside the cage of a C 60 fullerene molecule is investigated using inelastic neutron scattering (INS). The emphasis is on the temperature dependence of the INS spectra which were recorded using time-of-flight spectrometers. The hydrogen endofullerene system is highly quantum mechanical, exhibiting both translational and rotational quantization. The profound influence of the Pauli exclusion principle is revealed through nuclear spin isomerism. INS is shown to be exceptionally able to drive transitions between ortho -hydrogen and para -hydrogen which are spin-forbidden to photon spectroscopies. Spectra in the temperature range 1.6≤ T ≤280 K are presented, and examples are given which demonstrate how the temperature dependence of the INS peak amplitudes can provide an effective tool for assigning the transitions. It is also shown in a preliminary investigation how the temperature dependence may conceivably be used to probe crystal field effects and inter-fullerene interactions.

Over the years, several strategies have been developed for generating highly polarized nuclear spin systems, including dynamic nuclear polarization, optical pumping, and methods exploiting parahydrogen. Here, we present an alternative strategy, using an enhanced 'brute-force' approach (i.e. exposure to low temperatures and high applied magnetic fields). The main problem with this approach is that it may take an excessively long time for the nuclear polarization to approach thermal equilibrium at low temperatures, since nuclear relaxation becomes exceedingly slow due to the loss of molecular motion. We show that low-field thermal mixing can alleviate the problem by increasing the rate at which slowly-relaxing nuclei reach equilibrium. More specifically, we show that polarization can be transferred from a relatively rapidly relaxing (1)H reservoir to more slowly relaxing (13)C and (31)P nuclei. The effects are particularly dramatic for the (31)P nuclei, which in experiments at a temperature of 4.2 K and a field of 2 T show a 75-fold enhancement in their effective rate of approach to equilibrium, and an even greater (150-fold) enhancement in the presence of a relaxation agent. The mixing step is also very effective in terms of the amount of polarization transferred-70-90% of the maximum theoretical value in the experiments reported here. These findings have important implications for brute-force polarization, for the problem becomes one of how to relax the solvent protons rather than individual more slowly-relaxing nuclei of interest. This should be a much more tractable proposition, and offers the additional attraction that a wide range of nuclear species can be polarized simultaneously. We further show that the (1)H reservoir can be tapped repeatedly through a number of consecutive thermal mixing steps, and that this could provide additional sensitivity enhancement in solid-state NMR.

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.

The dynamics of hydrogen atoms in hydrogen bonds have been studied by nmr spectroscopy The data is in excellent agreement with a theory which invokes phonon assisted tunnelling at low temperature.

The techniques of low-field dipole-dipole driven nmr spectroscopy have been applied to the study of the quantum dynamics of molecular groups.

We present an overview of solid-state NMR studies of endohedral H(2)-fullerene complexes, including (1)H and (13)C NMR spectra, (1)H and (13)C spin relaxation studies, and the results of (1)H dipole-dipole recoupling experiments. The available data involves three different endohedral H(2)-fullerene complexes, studied over a wide range of temperatures and applied magnetic fields. The symmetry of the cage influences strongly the motionally-averaged nuclear spin interactions of the endohedral H(2) species, as well as its spin relaxation behaviour. In addition, the non-bonding interactions between fullerene cages are influenced by the presence of endohedral hydrogen molecules. The review also presents several pieces of experimental data which are not yet understood, one example being the structured (1)H NMR lineshapes of endohedral H(2) molecules trapped in highly symmetric cages at cryogenic temperatures. This review demonstrates the richness of NMR phenomena displayed by H(2)-fullerene complexes, especially in the cryogenic regime.

Heroin hydrochloride monohydrate, the normal crystalline form of this material, has been studied by X-ray crystallography, nuclear quadrupole resonance spectroscopy, and theoretical calculations. The X-ray data at 120 K could be refined in either of the two space groups P41 or P41212 with the latter higher symmetry form being the more likely, but for the NQR spectra at 4.2 K the doublet structure suggests that a slight symmetry breaking has ocurrred, which lowers the symmetry to P41. The crystals have the unusual unit cell dimensions of a = b = 7.5335(2) A, c = 71.976(4) A, with Z = 8 (and Z′ = 2 in P41); the molecules are arranged on a four-fold helix of pitch 72.0 A with its axis parallel to the c axis, requiring a total of eight molecules in order to complete one turn. As in the free base the nitrogen ring system of the N-protonated heroin molecule has a chair conformation; in P41 the N–H hydrogen is bonded to a Cl− with N–H⋯Cl distances of 3.056(15) and 3.095(14) A and each Cl− ion is also hydrogen bonded to two water molecules. 14N and 35Cl quadrupole resonance frequencies have been observed in both the pure material and material from a drugs seizure by two double resonance techniques; in cross-relaxation spectra, a broad 35,37Cl resonance is found near 1.95 MHz at room temperature and an analysis of the line shape predicts an average quadrupole coupling constant for the two ions of 3.817 MHz and an asymmetry parameter of 0.4. In double resonance by level crossing experiments, 14N doublets are observed near 0.965 and 1.035 MHz at 4.2 K, predicting a mean quadrupole coupling constant of 1.333 MHz and an asymmetry parameter of 0.158. A doublet 35Cl signal is also observed at 1.965 MHz. The doublet patterns observed are consistent with a P41 space group at 4.2 K. The similarity of the NQR data from the pure sample and the seizure confirms that the illicit sample is of the helical monohydrate, suggesting that these methods can be used to detect the narcotic in real situations. Theoretical calculations with Gaussian at the HF/6-31+G* level for two molecules at the configurations they adopt in the solid state are in reasonable agreement with these conclusions for 14N and one of the 35Cl ions.

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...

The temperature dependence of the tunneling motions of the methyl groups in Manganese Acetate Tetrahydrate has been studied by high resolution inelastic neutron scattering in a range between 0.06 K and 40 K. The results are in qualitative agreement with models based on rotor-phonon and rotor-rotor coupling valid for temperature regions above and below 5 K respectively. The finite line width of one of the tunneling lines which persists at very low temperatures and the anomalous positive shift of the same line at higher temperatures is not understood.

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.

Structure indicative of a mechanical coupling between pairs of methyl groups is observed in the inelastic neutron scattering spectrum arising from methyl rotation in 4-methylpyridine. The structure has a quite different form from that due to a mutual scalar hindering potential. It agrees closely with the form expected from a dynamic interaction between the angular momenta of two methyl groups giving rise to a mutual gauge or vector potential. Detailed agreement is obtained when both types of interaction, scalar and vector, are assumed to occur. This is the first observation of the dynamic interaction.

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.

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.

Measurements have been made of the low temperature tunnelling frequencies of strongly hindered methyl groups in four samples; dimethyl sulphide, 2-pentanone, 2-hexanone and 2-heptanone; using the technique of double sideband irradiation. The temperature dependence of the proton spin lattice relaxation time for dimethyl sulphide has been measured and found to be anomalous. An explanation in terms of methyl coupling is suggested. A semi-quantitative electrical analogue model of double sideband irradiation is discussed and used to calculate the expected sensitivity of the method.

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.

A method of measuring tunnel frequencies of molecular reorientation in crystals at low temperatures is demonstrated. Magnetic field variation brings the tunnel frequency to equality, first with the Larmor frequency of paramagnetic impurities and then with twice the Larmor frequency of protons whose magnetism has been saturated. In the first step paramagnetic ions cool the tunneling reservoir, and in the second the tunneling reservoir cools the proton Zeeman system, leading to enhanced proton relaxation.