Showing papers on "Relaxation (NMR) published in 1968"
TL;DR: In this paper, the equations for the relaxation time of a nuclear spin in a molecule undergoing diffusion reorientation have been extended to include fully anisotropic reorientations for a number of relaxation mechanisms.
Abstract: The equations for the relaxation time of a nuclear spin in a molecule undergoing diffusion reorientation have been extended to include fully anisotropic reorientation for a number of relaxation mechanisms. The full rotational diffusion tensor—both principal diffusion constants and orientation of the principal rotation axes—can be found by the measurement of the NMR relation times of an appropriate set of nuclei in the molecule.
311 citations
TL;DR: In this article, the fluctuation of the order parameter in a superconductor in the temperature slightly above the transition temperature is studied in the framework of the current microscopic theory, and it is shown that the elec- trical resistivity, the nuclear spin lattice relaxation time and the ultrasonic attenuation co-efficient decrease like 1-ai1li- 1I2 and 1-a'i1li − 1I3 in the classical and in the critical region. respectively, where 1l= (TITe) -1 and a and a' are constants of
Abstract: The fluctuation of the order parameter in a superconductor in the temperature slightly above the transition temperature is studied in the framework of the current microscopic theory. It is convenient to divide the temperature region where the fluctuation of the order parameter becomes important into two regions; the classical and the' critical region. In the classical region the spatial correlation of the fluctuation is of the Ornstein-Zernike type, while in the critical region it depends on the relative distance like r-3/2. We show that the elec trical resistivity, the nuclear spin lattice relaxation time and the ultrasonic attenuation co efficient decrease like 1-ai1li- 1I2 and 1-a'i1li- 1I3 in the classical and in the critical region . respectively, where 1l= (TITe) -1 and a and a' are constants of the order (Telp.) 1/2 (lPo)-3/2 and (Tel p.) 1/3 (lpo) -1 respectively. Here p. is the chemical potential, l is the electronic mean free path and Po is the fermi momentum. It is also shown that the thermal conductivity has no singular term at the transition temperature.
255 citations
TL;DR: In this article, the theory of nuclear spin-lattice relaxation via paramagnetic centers in diamagnetic crystals is investigated in terms of both the singlerelaxation-center and the multirelaxation center models.
Abstract: The theory of nuclear spin-lattice relaxation via paramagnetic centers in diamagnetic crystals is investigated in terms of both the single-relaxation-center and the multirelaxation-center models. In this theory, the distances between centers are allowed to be finite. A new case is found for which the theory predicts a new dependence of the spin-lattice relaxation time upon the applied magnetic field, the concentration of the paramagnetic centers, and the magnitude of the diffusion constant. An adaptation of the theory to the rotating reference frame shows that under certain conditions the spin-lattice relaxation time in the rotating frame can be larger than in the laboratory frame.
156 citations
TL;DR: In this article, the electron-electron double resonance was applied to free radicals in solution, where one part of the EPR spectrum of a paramagnetic sample is irradiated with an intense microwave field and the effect of this intense field on other parts of the spectrum is determined utilizing a second weak microwave field.
Abstract: The technique of electron–electron double resonance, in which one part of the EPR spectrum of a paramagnetic sample is irradiated with an intense microwave field and the effect of this intense field on other parts of the spectrum is determined utilizing a second weak microwave field, has been applied to free radicals in solution. EPR signals detected by the weak microwave source are reduced in intensity when the two frequencies are separated by an integral number of hyperfine intervals. In some cases this reduction is as much as 40%. A bimodal cavity of novel design capable of supporting the two resonant microwave modes is described. A nitroxide radical has been investigated in greatest detail. Two mechanisms have been identified: rapid nuclear relaxation induced by electron–nuclear dipolar (END) interaction, which is dominant at low concentrations and temperatures, and Heisenberg exchange (HE), dominant at high concentrations and temperatures. A theoretical analysis of the relaxation processes is present...
141 citations
TL;DR: The results give first experimental evidence for three effects of major significance: the system exhibits dielectric relaxation due to a chemical rate process, there is a considerable electric field effect of the helix–coil transition and conformation changes in biological systems could be potentially caused by direct action of an electric field.
Abstract: Dielectric relaxation of poly(γ-benzyl L-glutamate) in solution has been studied in the 5 kcps-10 Mcps range for various values of the helix content. The results give first experimental evidence for three effects of major significance. (1) The system exhibits dielectric relaxation due to a chemical rate process (namely helix formation). This confirms recent theoretical predictions. (2) The mean relaxation time τ* of the helix–coil transition could be evaluated as a function of the degree of transition. The results are in excellent agreement with a previously developed theory. At the midpoint of transition it is found τ*max = 5 × 10−7 sec. The elementary process of helical growth turns out to be practically diffusion-controlled (with a rate constant of hydrogen bond formation of 1.3 × 1010 sec−1). (3) There is a considerable electric field effect of the helix–coil transition. This indicates that conformation changes in biological systems could be potentially caused by direct action of an electric field.
133 citations
TL;DR: In this paper, the phase shift of infrared emission from the asymmetric stretch and from the infrared active bend ν4 has been measured as a function of chopping frequency and gas pressure.
Abstract: The asymmetric stretching vibration ν3 of methane has been excited by a chopped He–Ne laser. The phase shift of infrared emission from the asymmetric stretch and from the infrared‐active bend ν4 has been measured as a function of chopping frequency and gas pressure. Vibrational relaxation times have been found for the removal of energy from the asymmetric stretch (pτ = 7.0 ± 1 nsec·atm), for the appearance of this energy in the ν4 bend (pτ = 5 + 3, −2 nsec·atm), and for the relaxation of vibrational energy into translational and rotational energy (pτ = 1.90 ± 0.10 μsec·atm). The mechanism and rates of V → V energy transfer are compared to calculations for a repulsive intermolecular potential. The general applicability of the laser‐excited vibrational fluorescence method and the interpretation of phase‐shift data are discussed.
131 citations
TL;DR: In this paper, the vibrational relaxation of nitrogen in the presence of oxygen atoms generated by the decomposition of ozone has been studied behind incident shock waves in the temperature range 3000°-4500°K.
Abstract: The vibrational relaxation of nitrogen in the presence of oxygen atoms generated by the decomposition of ozone has been studied behind incident shock waves in the temperature range 3000°–4500°K. Relaxation of the nitrogen was followed by measurement of the postshock density gradient using a laser‐beam deflection technique. Analysis of the data to yield PτN2–O, the relaxation time for nitrogen dilute in 1 atm of oxygen atoms, requires consideration of the simultaneous effect of molecular oxygen produced by the ozone decomposition The values obtained for PτN2–O are subject to some uncertainty, but clearly lie below those for pure nitrogen by nearly two orders of magnitude. The results are interpreted as evidence of a chemical effect.
118 citations
113 citations
TL;DR: In this paper, it was shown that the dip observed in the Q branch of the pressure-induced vibrational spectra at high densities is an interference effect due to the correlations existing between the dipole moments induced in successive binary collisions.
Abstract: The dip observed in the Q branch of the pressure-induced vibrational spectra at high densities is shown to be an interference effect due to the correlations existing between the dipole moments induced in successive binary collisions. A similar dip is predicted to exist in the translational spectra of inertgas mixtures at zero frequency. This intercollisional interference effect has the same origin as the dip in the spectral density of the intermolecular force, discussed by Purcell in connection with nuclear electric dipole relaxation. The effect does not occur for the anisotropic part of the induced dipole moments, and this explains the observed absence of any splitting of the S lines and of the QQ component of the Q branch of the induced infrared spectra.
105 citations
TL;DR: In this article, a systematic investigation of orientational relaxation in liquids has been initiated employing the technique of Rayleigh scattering, and the experimental results are interpreted based on an approximate theory which is developed and justified.
Abstract: A systematic investigation of orientational relaxation in liquids has been initiated employing the technique of Rayleigh scattering. The experimental results are interpreted based on an approximate theory which is developed and justified. Scattering spectra from 15 different liquids, including the homologous alkyl bromides and polyethylene glycols as well as glycerol, 1,3‐butanediol, and n‐octyl alcohol, were obtained as a function of temperature using a 6328‐A Ne–He laser source and a Fabry–Perot interferometer. All spectra were found to have approximately Lorentzian form symmetric about the source frequency, which implies molecular reorientation is a relaxational process. Also, the width of these Lorentzian lines (which is reciprocally related to the orientational relaxation time) increased with increasing temperature in such a way that an effective orientational activation enthalpy was determined. In both the alkyl bromide and polyethylene glycol series, increased molecular size resulted in increased relaxation time, which is explained using both activation‐enthalpy and free‐volume approaches. The measured orientational relaxation times which reflect motion of the induced dipole moments are compared with dielectric relaxation times associated with the permanent moments and mechanical relaxation times determined ultrasonically. This comparison provides additional information about the detailed nature of the reorientation mechanism. For example since orientational relaxation is shown to be governed by rotation of ellipsoidal‐shaped molecules about their minor axes, it is concluded that mechanical motions are more nearly about major axes. In addition, it is possible to determine whether reorientation is a diffusion, jump, or an intermediate process. Experimental results indicate molecules in n‐octyl alcohol and probably in the alkyl bromides reorient by a diffusional process and that glycerol molecules reorient by rather large jumps, while 1,3‐butanediol is an intermediate case. These results are in excellent agreement with predictions based upon the McCuffie–Litovitz explanation for the distribution of dielectric relaxation times.
105 citations
TL;DR: In this article, the authors used the ν3 and ν4 infrared spectra of methane in an argon matrix to determine the rotational energies of a hindered rotor in its solid matrix.
Abstract: The ν3 and ν4 infrared spectra of methane in an argon matrix have been studied. The assignment of vibration‐rotation features enables the methane rotational energies to be determined. The spacings of these levels, which are discussed in terms of crystal‐field effects, suggest that methane is a hindered rotor in its solid matrix. The vibration‐rotation features exhibit time‐dependent absorption changes. These changes which are evidence of triplet →quintet nuclear‐spin conversion that accompanies the J = 1→J = 0 rotational relaxation, follow first‐order kinetics with a half‐life of 90 min. The half‐life is unchanged by addition of up to 1% N2 to the matrix but decreases to 3 min with only 0.2% O2. Two mechanisms are proposed which are in order of magnitude agreement of observed relaxation rates. In the absence of paramagnetic impurities, spin‐spin interaction within the molecule mixes spin states and allows J = 1→J = 0 to relax into the crystal lattice. In the presence of O2 the spin states are mixed by the magnetic‐field gradient which is set up across CH4.
TL;DR: In this article, the temperature-dependent emission spectral shifts of aminophthalimide in propanol were investigated by time resolved emission spectroscopy on a nanosecond time scale.
TL;DR: In this paper, it was shown theoretically and illustrated experimentally that an entirely diffusion-controlled adsorption process leads to a frequency-dependence of an electrode impedance which is analogous to that produced by a dielectric relaxation process with a particular spread of relaxation times.
TL;DR: In this article, the position and linewidth of the main resonance in the spin-wave spectra of 80% Ni−20% Fe evaporated films 150 to 3200 A at frequencies from 0.8 to 4.0 GHz, at room temperature and with the static magnetic field perpendicular to the film plane have been performed.
Abstract: Measurements of the position and linewidth of the main resonance in the spin‐wave spectra of 80% Ni‐20% Fe evaporated films 150 to 3200 A thick at frequencies from 0.8 to 4.0 GHz, at room temperature and with the static magnetic field perpendicular to the film plane have been performed. The center field for resonance was independent of thickness and consistent with the Kittel resonance condition for the uniform precession. The linewidth was about 30 Oe (measured as the field separation of inflection points on the absorption curve), independent of film thickness and frequency. The linewidth data have been compared to previous data for resonance with the static field parallel to the film plane and interpreted in terms of relaxation processes for the uniform precession. In general, frequency‐swept linewidths, not field‐swept linewidths, are proportional to the relaxation rate and are the quantities which should be compared. The frequency‐swept linewidth for perpendicular resonance is equal to that obtained for parallel resonance in films thinner than 500 A. This result indicates that two‐magnon scattering between the uniform precession mode and spin‐wave states for which the exchange energy term is large (wavenumber k appreciably different from zero) does not contribute to the perpendicular resonance linewidth and is the origin of the linewidth increase with thickness for parallel resonance. It appears that scattering to states with k≈0 and exchange conductivity broadening are the most reasonable sources for the residual 30 Oe linewidth.
TL;DR: In this article, the free volume concept is used to describe the temperature and the pressure dependence of the relaxation time for the micro-Brownian motion of amorphous chain segments, while the β relaxation due to local-mode motion of the frozen main chain shows weak dependence on temperature and pressure.
Abstract: The dielectric properties and the d. c. conductivity of poly(vinyl chloride), poly(vinyl acetate), polychlorotrifluoroethylene, and poly(ethylene terephthalate) were measured at temperatures above and below the glass transition temperature and at various pressures up to 3000 atm. The α relaxation associated with the micro-Brownian motion of amorphous chain segments depends strongly upon temperatures and pressure, while the β relaxation due to local-mode motion of the frozen main chain shows weak dependence on temperature and pressure. It is found that the free volume concept is valid for description of the temperature and the pressure dependence of the relaxation time for the α process. Activation energy and volume for both relaxation processes are determined from the experimental data. A simple relation between activation energy and volume for the α relaxation and pressure dependence of the glass transition temperature are derived. Temperature and pressure dependence of the d. c. conductivity in the rubbery state are notably different from those in the glassy state. Ionic conduction appears to be dominant in these polymers.
TL;DR: The high temperature phase transition in KH2PO4 has been investigated by differential thermal analysis, thermogravimetric analysis, proton magnetic resonance and relaxation, infrared spectroscopy, and x-ray and quasielastic cold-neutron scattering as mentioned in this paper.
Abstract: The high‐temperature phase transition in KH2PO4 has been investigated by differential thermal analysis, thermogravimetric analysis, proton magnetic resonance and relaxation, infrared spectroscopy, and x‐ray and quasielastic cold‐neutron scattering. The transition is not connected with a static breaking up of the hydrogen‐bond network but rather with the onset of disordered hindered rotation of the H2PO4 groups around all three axes. It seems that rotation of H2PO4 groups is as well rate determining for the proton conductivity of this crystal.
TL;DR: In this paper, the authors measured the line widths and relaxation times of phospholipid molecules in a variety of environments including coarse aqueous dispersions, sonicated annealing, and dissolved in organic solvents.
TL;DR: The complex dielectric constant of Rochelle salt has been measured at frequencies from 25 to 13 GHz and at 155 MHz as mentioned in this paper, and the relaxation time was found to be proportional to the difference between the low-frequency, clamped, differential Dielectric constants and its limiting high-frequency value.
Abstract: The complex dielectric constant of Rochelle salt has been measured at frequencies from 25 to 13 GHz, and at 155 MHz The measurements were made in a temperature range from -50 to 45\ifmmode^\circ\else\textdegree\fi{}C which includes the ferroelectric region and significant parts of both paraelectric regions Measurements were made both with and without dc biases The dielectric constant is found to undergo a relaxation of a strictly Debye character Over the entire temperature range, the relaxation time is found to be proportional to ${T}^{125}$ times the difference between the low-frequency, clamped, differential dielectric constant and its limiting high-frequency value This result is shown to be consistent with a model of the high-frequency dielectric behavior based on the Kubo susceptibility formalism Following Mitsui, it is assumed that the ferroelectric properties of Rochelle salt arise from the rotation of hydroxyl groups within the tartrate molecules The basic relaxation process probably involves phonon-induced transitions between the lowest states of these hydroxyl ions However, the cooperative dipolar interaction between ions reduces the over-all relaxation rate of the coupled system An approximate model Hamiltonian of the coupled hydroxyl system is constructed in terms of the Pauli matrices and is used to calculate an explicit expression for the relaxation rate
TL;DR: In this article, the complex dielectric constant of NaNO 2 crystal was measured in a frequency range between 1 MHz and 24 GHz from room temperature to 210°C, and the measuring methods in the various frequency ranges were described in detail.
Abstract: The complex dielectric constant of NaNO 2 crystal was measured in a frequency range between 1 MHz and 24 GHz from room temperature to 210°C The measuring methods in the various frequency ranges were described in detail and especially the measuring accuracies in the complex dielectric constant and the temperature were considered closely It was found that a dielectric dispersion occurs in the ferroelectric b direction and this dispersion obeys nearly the formula of the Debye-type with a single relaxation time above the transition point The relaxation frequency is proportional to ( T - T 0 ) near the transition point, where T is the temperature and T 0 is about 162°C, and deviates from this linear relationship above ca 180°C, bending upward In the a and c directions, any remarkable change in the dielectric constant was not observed up to 24 GHz
TL;DR: In this article, the dielectric properties of four methacrylate polymers (methyl, ethyl, n-butyl and n-octyl) were studied in the frequency range 0.0001 cps-300 kcps at temperatures above and below the glass transition temperature and at various pressures up to 2500 atm.
Abstract: Dielectric properties of four methacrylate polymers (methyl, ethyl, n-butyl and n-octyl) were studied in the frequency range 0.0001 cps–300 kcps at temperatures above and below the glass transition temperature and at various pressures up to 2500 atm. At temperatures well above Tg a single relaxation peak (α′ peak) was observed in the case of the higher n-alkyl methacrylates. However, this peak was split into two peaks, α and β, with decrease in temperature or increase in pressure. The molecular motions corresponding to the α and the β relaxation processes are the micro-Brownian motions of amorphous main chains and of flexible side chains, respectively. From the temperature and the pressure dependence of the average dielectric relaxation time of these polymers the single relaxation process (the α′ process) was attributed to the micro-Brownian motion of the main chain coupled with that of the side chain. The effects of temperature and pressure on the d.c. conductivity of these polymers were also studied.
TL;DR: In this paper, the authors measured the infrared spectra of benzene, cyclohexane, n-heptane, carbon disulfide, carbon tetrachloride, and TCLOethylene.
Abstract: Measurements of dielectric constant and loss at 2.1‐mm wavelength have been carried out upon benzene, cyclohexane, n‐heptane, carbon disulfide, carbon tetrachloride, and tetrachloroethylene, and similar measurements at centimeter wavelengths have been carried out where needed. The infrared absorption spectra of the liquids were measured from 17–170 cm−1. The points for dielectric absorption fit on a smooth curve drawn through the infrared absorption points. Similar measurements have been made upon several binary mixtures of these liquids. Measurements upon liquids to which very small quantities of water have been added show that the effect of water can be corrected for or disregarded. Relaxation times calculated from the dielectric constant and loss measurements are close to 1 × 10−12 sec for all of the liquids, which is the magnitude of the time between the collisions of a molecule in the liquid with its neighbors. Apparent dipole moments calculated for the molecules from these measurements are close to ...
TL;DR: In this article, the Fourier spectrometer was extended down to 2 cm-1 and the results of a study of the absorption and dispersion in the liquid and dissolved states of selected polar molecules (methyl chloroform CH3CCl3; chlorobenzene C6H5Cl; benzonitrile C 6H5CN; tolune C6E5CH3) from this wave number to beyond 100 cm−1 are reported.
Abstract: The use of the Fourier spectrometer has been extended down to 2 cm–1 and the results of a study of the absorption and dispersion in the liquid and dissolved states of selected polar molecules (methyl chloroform CH3CCl3; chlorobenzene C6H5Cl; benzonitrile C6H5CN; tolune C6H5CH3) from this wave number to beyond 100 cm–1 are reported. It is concluded that the available representations (Record, Powles) of the decay of the dipole relaxation (Debye) absorption due to the inertia factor are inadequate. The further distinct absorption of polar liquids predicted by Poley has been observed for each of the liquids studied. Systematic evaluation of the nature of these absorptions has shown intense features of considerable breadth located in the wave number range 30 to 80 cm–1.
Observed data are :[graphic omitted]
This (Poley) absorption is of intensity proportional to the molar concentration and is apparently independent of the solvent on dilution. The wide significance of these data is discussed.
TL;DR: In this article, the authors analyzed the temperature dependence of the shear spectrum in terms of a distribution of activation energies and found that activation energies at low temperatures are smaller than the activation energy in the Arrhenius region.
Abstract: Ultrasonic shear and longitudinal measurements were made in Bz03 from 650° to 1000° C. From these data both the shear and volume relaxation time spectra were determined. The spectra had the same temperature dependence, although the volume spectrum was always broader than the shear spectrum. The shear relaxation process can be represented by a single relaxation time above 800° C in the region where the shear viscosity is Arrhenius. Both processes exhibit an increasingly broad distribution of relaxation times in the non-Arrhenius region. The temperature dependence of the shear spectrum was analyzed in terms of a distribution of activation energies. A surprising conclusion of this study is that activation energies at low temperatures are smaller than the activation energy in the Arrhenius region.
TL;DR: In this paper, a uniform vertical electric field produces an instability in poorly conducting liquid subject to a vertical temperature gradient, and the threshold conditions for the instability are predicted using linear perturbation theory.
Abstract: A uniform vertical electric field produces an instability in a poorly conducting liquid subject to a vertical temperature gradient A gradient in conductivity resulting from the temperature gradient causes free charge to accumulate in the fluid when an electric field is applied For the cases considered, the gradient in dielectric constant can be neglected with the significant electric force being that due to the free charge The threshold conditions for the instability are predicted using linear perturbation theory Approximations are made which allow the equations with space‐varying coefficients to be solved The analysis shows that, for liquids with short or moderate electrical relaxation times, the electric field causes the internal gravity wave propagating downward to become unstable
TL;DR: The path of entry into and exit from the triplet state T 0 of quinoxaline in a durene host was investigated in this article, where it was found that for both perdeutero-and perhydroquinoxaline the lifetime is shortened by a factor of about three when the temperature is lowered from 4·2 to 1·34°k.
Abstract: Experiments have been performed to determine the path of entry into and exit from the phosphorescent triplet state T 0 of quinoxaline in a durene host. First of all the decay of phosphorescence after flash excitation was followed at 4·2 and 1·34°k. It was found that for both perdeutero- and perhydroquinoxaline the lifetime is shortened by a factor of about three when the temperature is lowered from 4·2 to 1·34°k. At 1·34°k relaxation between the spin components (i.e. re-orientation of the triplet spin angular momentum) is slow relative to the decay, and the observed reduction in lifetime indicates that entry into and exit from T 0 are through the same spin component. Similar decay experiments were then carried out at 1·34°k in a 10 kg magnetic field or in a somewhat weaker field so chosen that the effect of microwave saturation of one of the E.S.R. transitions between the components could be observed. From the results it follows that on intersystem crossing the molecules enter the manifold T 0 through the...
TL;DR: In this paper, the nitrogen-14 quadrupole relaxation broadening effect on the proton spectrum of formamide was eliminated by a 1H−{14N} double resonance technique.
Abstract: The nitrogen-14 quadrupole relaxation broadening effect on the proton spectrum of formamide was eliminated by a 1H–{14N} double resonance technique. The completely-decoupled proton spectra of formamide in the neat liquid and in acetone, dioxane, and water solutions were observed at several temperatures. The spectra observed at room temperature were analyzed on the basis of an ABC system, while the spectra at higher temperatures (∼100°C) were analyzed on the basis of an AB2 system. From the changes in the resonance lines of the formyl proton with the temperature, the values of the activation energy for the hindered internal rotation about the C–N bond, Earot, and for the intermolecular proton exchange involving the amine protons, Eaex, were obtained. It has been found that the hydrogen-bond formation on the carbonyl oxygen of formamide increases the Earot value and the magnitude of the vicinal coupling constants, while it decreases the magnitude of the geminal coupling constant.
TL;DR: In this article, a model of rotameric transitions and the molecular geometry calculated from the known crystal structure of polyethylene oxide was used to analyze the relaxation mechanism of the C-C bond.
Abstract: Proton magnetic relaxation times have been measured in solutions of polyethylene oxide. Molecular weight, temperature, and solution concentration have been systematically varied. Several low‐molecular‐weight oligomers have been studied as well as polymers which vary in molecular weight from 4 × 103 to 4 × 106. The results show that in dilute solution both T1 and T1 are independent of molecular weight. T1 decreases with increasing concentration for concentrations greater than 0.2 g per ml of polymer, but is independent of concentration for more dilute solutions. T2 is independent of concentration in the limit of low concentration, but decreases rapidly with increasing concentration. The concentration dependence of T2 is a sensitive function of molecular weight. The relaxation mechanism has been analyzed using a model of rotameric transitions and the molecular geometry calculated from the known crystal structure of the polymer. The most important term is that arising from a conformational change about the C‐C bond, in which the oxygen atoms move from a gauche to a gauche′ position. The relaxation times (T1) of water in aqueous polyethylene oxide solutions decrease with increasing polymer concentration, and are independent of the molecular weight of the polymer.
TL;DR: The origin of the broadening of the N.M.R. signal of the proton(s) in the α position to the nitrogen atom in nitrogen-containing heterocycles is examined in this article.
Abstract: The origin of the broadening of the N.M.R. signal of the proton(s) in the α position to the nitrogen atom in nitrogen-containing heterocycles (pyridine, thiazole, isothiazole, isoxazole) is examined. The contributions of the various nuclear relaxation mechanisms to the spin-lattice T 1 and spin-spin T 2 relaxation times of Hα are considered. It is concluded that the observed broadening arises from incomplete washing out of an N-Hα spin-spin coupling by 14N quadrupolar relaxation. The calculated line-broadenings agree well with the observed line-widths. The effect of temperature changes, solvents, protonation and quaternization is discussed in terms of the 14N quadrupolar relaxation time. Various 15N-H couplings over two or three bonds are reported.
TL;DR: In this article, the authors investigated the hindered rotations of methyl groups in the solid state of a series of methylbenzenes and found that the reorientation of the methyl groups persisted down to 2°K.
Abstract: The hindered rotations of methyl groups have been investigated in the solid state of a series of methyl‐benzenes. The materials studied were the three tetramethylbenzenes and pentamethylbenzene. Their proton spin‐lattice relaxation times T1 and second moments M2 were measured from helium temperatures to the melting points of the respective samples. The reorientation of the methyl groups is found to persist down to 2°K, and in the three tetramethylbenzenes this is the only motion which is well established in the solid state at a rate greater than about 30 kc/sec. However, the T1 results from the low‐temperature phase of each hydrocarbon do not always exhibit a single minimum. Furthermore, there exists a correlation between the number of nonequivalent methyl sites in these compounds and the number of minima displayed in their T1 data. The identification of a particular site with a particular minimum is not unambiguous and only tentative suggestions can be made of the relative magnitude of the constraints on...
TL;DR: In this paper, the spin-lattice time of gaseous mixtures of H2 and He has been measured as a function of composition and temperature in the range 77°-300°K at pressures such that T1 is proportional to the density.
Abstract: The spin–lattice time T1 of gaseous mixtures of H2 and He has been measured as a function of composition and of temperature in the range 77°–300°K at pressures such that T1 is proportional to the density. The relationship between the experimental T1 and cross sections for molecular reorientation of H2 is given. From comparison of computed cross sections to the data, experimental values are obtained for parameters in an approximate form for the He–H2 anisotropic intermolecular potential.