The effect of relaxation on the symmetry of N.M.R. spectra

The effect of nuclear relaxation on the symmetry properties of N.M.R. spectra is examined in terms of the properties of the relaxation matrix elements R_ijkl . It is found that the symmetry relations R_ijkl = R_jilk = R_klij = Rlkij obtain for one or more dominant relaxation mechanisms under ‘extreme narrowing’ conditions. Further, the symmetry relation R_ijkl = R_{λi, λj, λk, λl} , where λ is a spin inversion operator, is shown to obtain for (a) only one dominant relaxation mechanism or (b) two dominant relaxation mechanisms which have the same commutation properties with λ. This latter symmetry relation does not obtain for two dominant relaxation mechanisms with the same order of spatial dependence and different commutation properties with λ. The effect of nuclear relaxation can therefore yield unsymmetrical first-order spectra; the application of this analysis to the determination of the absolute sign of the spin-spin coupling constants is proposed.     

N.M.R. relaxation study of crystalline quinuclidine

Transient signals in the fluorescence intensity induced by microwaves can be used to acquire quantitative information on the molecular rates of populating and depopulating of the lowest triplet state of organic molecules. This method seems especially promising when the molecule emits no detectable phosphorescence.

Part of this paper concerns the introduction of an appropriate mathematical model for describing the fluorescence transients. The model is formulated in conjunction with a specific series of experiments from which one may evaluate the molecular quantities in a systematic way. The kinetics of the lowest triplet state of free base porphin (H₂P) are determined explicitly by this method.

In the discussion the results for H₂P are compared with those for Zn porphin. A satisfactory qualitative explanation based on the theory of radiationless transitions can be given for the great difference in behaviour of these two porphyrin molecules.

Short attention is given to effects due to the photochemical shift of the two H atoms in the centre of H₂P, which is proven to occur even at a temperature as low as 1·3 K.     

Librational effects on carbon-13 N.M.R. spin-lattice relaxation times of tricyclic condensed molecules

A general method is described for calculating the effect of internal librational motion on the correlation times in tricyclic molecules having a folded structure about the central hetero-ring. The relaxation equations have been derived for isotropic as well as anisotropic models of overall molecular re-orientation. In the case of the isotropic model of motion, the parameter which determines the effective relaxation times is the librational amplitude; for the anisotropic model a somewhat more complicated correlation function is found. The present treatment provides a tool for interpreting in a quantitative way the spin-lattice relaxation times of hydrogen bearing carbons in pharmacologically important tricyclic systems where internal conformational flexibility could be exclusively monitored by using such a non conventional N.M.R. spectroscopic approach. Application of the method and its limitations are discussed briefly.     

N.M.R. relaxation time studies in liquid methyl iodide

The spin-lattice relaxation times of the various nuclei in methyl iodide, methyl iodide-d ₃, and carbon-13 methyl iodide (¹³C, ¹H, ²D) were measured between 210 and 350 K. The separation of the proton-proton intermolecular relaxation was accomplished by a dilution study in methyl iodide-d ₃; the resulting intermolecular contribution agreed well with the existing theories for this mechanism. It was found that the spin-rotation interaction contributed significantly to the intramolecular relaxation of both the protons and the carbon-13. For both nuclei the separation of the spin-rotation interaction from the intramolecular dipole-dipole interaction was accomplished without making any assumptions about the temperature dependence of the spin-rotation relaxation time. The resulting spin-rotation relaxation times for both carbon-13 and protons offer evidence that the large spin-rotation effects are due to the methyl group reorientation.     

35Cl N.M.R. relaxation in aqueous sodium perchlorate solutions

The relaxation time, T _1ρ, for ³⁵Cl in perchlorate ion in aqueous solutions is found to be 0·25 s. T _1ρ is insensitive to the presence of most first row transition metals; however, a weak complex with manganous ion is demonstrated, and the perchlorate ion-manganous exchange rate is shown to be between 3 × 10⁴ s^-1 and 3·6 × 10⁷ s^-1. In the absence of manganous ion, the relaxation is dominated by the nuclear electric quadrupole interaction; however, a dipolar contribution is observed with manganous ion present.     

Site factors in N.M.R. solvent effects

An algorithm was developed enabling implementation of a Nosé—Hoover thermostat within the framework of grand canonical molecular dynamics [Lynch, C. G. and Pettitt, B. M., 1997, J. chem. Phys., 107, 8594]. The proposed algorithm could readily be extended to mixtures of molecular species with different chemical potentials as shown in the paper. This algorithm was first applied to simulate a μVT ensemble of TIP4P water molecules at 298 K by means of a system comprising a number of full particles and a single scaled (fractional) particle, with the scaling factor considered as a dynamic variable in its own right and chemical potential a pre-set parameter. Our finding showed that the scheme with a single fractional particle tended to freeze in metastable states as well as failed to reproduce either the real-life (?24.05 kJmol^?1) or the model-specific chemical potential of water (?23.0kJ mol^?1). In order to overcome the inadequacy of a single fractional particle as a chemical potential ‘probe’ the treatment of Pettitt and co-workers was extended to introduce multiple fractional particles. The extended scheme (with 4 fractional particles) was able to reproduce the actual density of water for the driving chemical potential of -24.0k mo^?1. The actual behaviour of the density as a function of the chemical potential also agreed quite well with both the results of thermo-dynamic integration and the findings of Pettitt and co-workers.     

Elucidation of cross relaxation in liquids by two-dimensional N.M.R. spectroscopy

Two-dimensional N.M.R. spectroscopy is applied to the elucidation of cross relaxation pathways in liquids. The theory underlying two dimensional studies of cross relaxation and of transient nuclear Overhauser effects is developed. The influence of the correlation time of the molecular random process is investigated. It is found that in the limit of short correlation times (extreme narrowing limit) weak negative cross-peaks are observed. However, for long correlation times (spin diffusion limit) strong positive cross-peaks can be obtained. The technique appears particularly promising for the study of cross relaxation in macromolecules. Examples of intra- and intermolecular cross relaxation in the extreme narrowing limit are presented.     

Proton N.M.R. studies of chemical and diffusive exchange in carbohydrate systems

A computer-simulated stochastic model is developed capable of predicting the combined effects of chemical and diffusive exchange on the transverse relaxation of spin-1/2 nuclei in a heterogeneous system. Comparison is first made with previous analytical theories for the special case of two site chemical exchange in a homogeneous system and the experimental data on several homogeneous aqueous carbohydrate systems are analysed. Results show that transverse water proton relaxation in these systems is dominated by proton exchange between water and carbohydrate hydroxyl groups. Analysis of model heterogeneous carbohydrate systems shows that in addition to chemical exchange, diffusion coefficients, particle morphology and local magnetic field gradients all have a role to play in determining the proton relaxation behaviour.     

VI. Preparation of agarose gels as reference substances for NMR relaxation time measurement

A standard method has been developed for the production of reference materials for NMR relaxation time measurement. These are based on agarose gels doped with gadolinium. The reproducibility and temporal stability of the gels have been found to be excellent. Electron microscope studies have shown good homogeneity. The relaxation behavior of such gels has been mathematically modeled with good agreement with experiment.     

Molecular reorientations of 1-bromo- and 1-iodo-adamantanes 1H N.M.R. relaxation study

Second moments and spin-lattice relaxation times, T ₁ and T _1ρ, have been measured from 100 K to 400 K for the protons in powdered 1-bromo and 1-iodo-adamantanes. Analysis of these data have shown that the reorientations are uniaxial in the low temperature phases. In the high temperature disordered phase of bromo-adamantane, the reorientation is endospherical and a slow molecular translational motion also exists. In the high temperature disordered phase of iodo-adamantane the reorientation is 12-fold uniaxial, in agreement with the Incoherent Quasi-elastic Neutron Scattering (I.Q.N.S.) experiments. All the results correspond to the crystallographic structures deduced from X-ray scattering.     

The proton N.M.R. spectrum of partially oriented norbornadiene. Molecular structure and the effect of vibrational averaging

The proton N.M.R. spectrum of norbornadiene partially oriented in the nematic solvent, Merck Phase (IV), has been measured and compared with an earlier study using EBBA as solvent. It is shown that the structure of the proton framework is significantly different in the two nematogens. Vibrational averaging of the dipolar couplings is carried out in order that effective r_z structures can be compared.     

The influence of chemical and diffusive exchange on water proton transverse relaxation in plant tissues

Transverse water proton relaxation in parenchyma tissue of courgette, onion and apple shows a dependence on CPMG pulse spacing characteristic of each tissue. An analysis of this dependence suggests that transverse relaxation in these tissues is caused by various combinations of fast proton exchange between water and cell biopolymers (or solutes) and diffusion through internally generated magnetic field gradients. Diffusion between intra- and extracellular water compartments also averages the water proton signal to an extent that depends on cell morphology and membrane permeability and this is calculated using a two-compartment model. No recourse need be made to popular concepts such as exchange between free and "bound" water. The implications of our results for NMR image contrast are discussed.     

Calculation of proton N.M.R. chemical shifts by means of a finite perturbation method

The magnetic shielding effect due to neighbouring carbon atoms through the diamagnetic and paramagnetic terms is calculated by means of a finite perturbation method in cooperation with a semi-empirical INDO molecular orbital method. The behaviour of the components of the shielding tensor arising from 2s and 2p orbitals centred on a neighbouring carbon is shown as a function of the distance, R, between the proton considered and the carbon. It is pointed out that the treatment by Pople and McConnell in the calculation of the proton chemical shift must be used carefully in unsaturated hydrocarbons, because some of the contributions from a neighbouring carbon do not show an R ^-3 dependence but an R ^-2 dependence. It is also shown that the gross trend of the observed proton chemical shifts in hydrocarbons is reproduced fairly well in the present calculation.     

Multiple field modulation in N.M.R.

The Bloch equations are solved in the case of multiple field modulation. The result is applied to the problem of homonuclear stabilization of the magnetic field. It is found that the stability of the field/frequency lock may be degraded by disturbance signals. A particular phase setting can be chosen to avoid disturbances.     

19F N.M.R. and 127I N.M.R. and N.Q.R. study of the molecular motions and deformations of iodine heptafluoride in its solid phases

In solid IF₇, ¹⁹F and ¹²⁷I N.M.R. and ¹²⁷I N.Q.R. absorption line shapes, frequencies and relaxation times have been studied from the melting temperature down to 56 K. Two new solid-solid transitions have been found at 180 K and 96 K in addition to the one already known at 153 K. The two high temperature phases are shown to be plastic. The fast molecular rotations and the slow molecular diffusion are studied from ¹⁹F and ¹²⁷I relaxation times. In the ordered phases, an analysis of the ¹⁹F second moments and longitudinal relaxation times shows the existence of a fast intramolecular exchange between axial and equatorial fluorine atoms, together with a much faster reorientation about the D ₅ axis. The characteristic times of these two motions are obtained and a mechanism for the exchange process is proposed. The chemical shift values of ¹²⁷I, the temperature dependence of the N.Q.R. frequencies and of the N.Q.R. relaxation times of ¹²⁷I, as well as the dipolar energy relaxation times have been measured and are discussed.     

N.M.R. spectra of oriented molecules

The phosphorescence spectra of anthracene-h₁₀ and -d₁₀ have been studied in a number of glasses and crystalline matrices. The line widths are very matrix dependent and they range from about 150 cm^-1 to 1 cm^-1 in n-heptane. There is a correlation between the line width and the wavenumber position of the origin. A study of the relative intensities of fluorescence and phosphorescence from various sites in Shpolskii matrices has shown that the intersystem crossing rate is site dependent. It is postulated that this involves the relative energies of the singlet state and a triplet state, nearly degenerate with it, which are site dependent.