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.     

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.     

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.     

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.     

Dielectric relaxation studies of alkanols solubilized by sodium dodecyl sulphate aqueous solutions

Considerable attention has been paid in recent years to the influence of alcohols on ionic micellar structures, alcohol co-surfactants most commonly employed in the preparation of microemulsions. Dielectric relaxation spectroscopy (DRS) is a versatile tool to monitor the dynamic process of such micellar systems. We report here the changes in the dielectric constant of the medium and the relaxation time of micellar solutions of sodium dodecyl sulphate (SDS) in water in the presence and absence of alkanols—hexan-1-ol, octan-l-ol, decan-l-ol and 1,2-ethanediol. The time domain dielectric data were obtained in the reflection mode in the frequency range of 10 MHz to 20 GHz using a HP54750A sampling oscilloscope and HP 54754A TDR plug-in-module. The sample is held at 303 K in a SMA cell with an effective pin length of 1.35 mm. We have determined the relaxation time (τ) using the Cole–Cole method. The relative viscosity ηr on the micellar solutions were also determined. The result shows that the dielectric constant increases linearly with SDS concentration in pure aqueous solutions up to 400 mM and decreases thereafter. The relaxation times are of the order of 15 ps to 25 ps and are far greater than that of Debye rotational relaxation of the monomer species. It is assigned to the dispersion step to water molecules surrounding to hydrophobic particles or the “bound water”. The addition of alcohols results in a linear increase in relaxation time in all the systems. As the concentration of SDS increases τ/ηr attains a near constant value showing that solubilization becomes more difficult at these concentrations. Our results show that the alcohol molecules are solubilized at the surface of the SDS micelle (i.e.) the micelle–water interface.     

Dielectric relaxation in aqueous solutions and suspensions

A method is proposed for measuring the relaxational dielectric losses of liquids at radio frequencies on a background of large conductivity losses. The absorption of the electromagnetic field energy is explained by structural relaxation, i.e., by the processes of formation and destruction of clusters.Translated from Isvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 81–85, February, 1975.     

Proton spin-lattice relaxation in aqueous ionic solutions

The proton spin-lattice relaxation time, T ₁, has been measured for aqueous solutions of sodium, potassium, rubidium and magnesium chlorides up to a concentration of about 4N at 25°c. The sodium and the magnesium chloride solutions were also measured at 60°c and 100°c. The variation of 1/T ₁ is not linear with concentration, at least above about ¼N, in contrast with previous reports [1, 2]. The behaviour depends markedly on the nature of the salt and on the temperature. It is shown that almost the whole of the variation of T ₁ as compared with that of water can be directly and simply related to the corresponding changes in the shear viscosity of the solutions. It is noted that the viscosity correction is better the higher the temperature of the solution.     

Chemical relaxation spectrometry in aqueous surfactant solutions

Chemical Relaxation Spectrometry is the term coined by Eigen to describe the range of experimental techniques which enable the study of the kinetics of fast reactions in solution in the time range 1–10⁻¹⁰s. This paper reviews the progress in the application of some of these techniques as well as the interpretation of the data in various surfactant solutions since the pioneering work in the early 1970's.     

The effects of morphology and exchange on proton N.M.R. relaxation in agarose gels

The effects of morphology and exchange on N.M.R. relaxation times in agarose gels are interpreted within a unified theoretical framework based on the generalized Bloch equations. By acknowledging the spacial dependence of the N.M.R. parameters it is shown how the relaxation behaviour depends on the distance scale characterizing the heterogeneity of the gel. If this distance scale is sufficiently small to allow complete diffusive averaging we recover the traditional results based on the Bloch-McConnell equations describing relaxation in a homogeneous system. This is the case for fresh agarose gels which show monoexponential relaxation and has been widely interpreted in terms of the rapid exchange of protons between populations of ‘free’ and ‘bound’ water. Conversely, if the distance scale characterizing the heterogeneity is sufficiently large to prevent complete diffusive averaging our model predicts multiexponential relaxation. This is the case with the transverse magnetization in agarose gels that have been slowly frozen then thawed. These results show how it is possible to probe the degree of microheterogeneity in gel samples using N.M.R. For the purpose of deriving simple analytical expressions for the N.M.R. relaxation times we only consider one-dimensional solutions to our model. More realistic morphologies can be treated using numerical methods.     

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.     

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.     

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.     

Dielectric relaxation in aqueous solutions and suspensions I. Solutions and suspensions

The dielectric constants and dielectric loss factors of aqueous solutions and suspensions are measured as a function of the specific electrical conductivity, temperature, and electric field frequency. The dispersion of the dielectric constant and the dielectric loss factor in solutions is explained by the formation of a electric double layer at the electrodes and the redistribution of the electric field between the region adjoining the electrodes and the remaining volume of the measuring cell. An equivalent-circuit calculations for suspensions displays acceptable agreement with experiment.     

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.     

N.M.R. study of solid perfluorocyclobutane

spin-lattice relaxation times and linewidth measurements for fluorine nuclei in solid perfluorocyclobutane are presented. The results are discussed in terms of D _2d molecular species performing fast internal motion. The relaxation measurements corroborate the existance of four solid-solid phase transitions and give some insight into their nature. The activation energies for molecular reorientation and self-diffusion processes are found to be 28·0 and 32·2 kJ mol^-1, respectively.     

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.