N.M.R. studies of liquid boron trifluoride

Previous ¹⁹F N.M.R. studies of liquid BF₃ have been extended by obtaining the ¹¹B spectrum of BF₃ and the ¹⁹F spectrum of ¹⁰BF₃ at various temperatures. Values for the B-F coupling constant and the boron relaxation times have been obtained by computer matching and visual matching of observed and calculated spectra. Relaxation times have the Arrhenius temperature dependence found previously. The temperature dependence of the B-F coupling constant is discussed. Some of the possible advantages of ‘high spin spectra’ are discussed.     

75As N.Q.R. and 19F N.M.R. study of solid AsF3

The temperature dependence of the ⁷⁵As pure quadrupole resonance frequency in AsF₃ has been studied from 77 K to a temperature very close to the melting point. It is found that the effective electric field gradient at the As site does not shift much from the gas-phase value and that the temperature dependence of the ⁷⁵As N.Q.R. frequency seems to follow the Bayer-Kushida model. The occurrence of a line-width transition in the ¹⁹F N.M.R. spectrum between 200 and 150 K demonstrates the presence of low-frequency, large-amplitude molecular motions which are too slow to average out the ⁷⁵As quadrupole coupling.     

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.     

N.M.R. spectra of molecules containing quadrupolar nuclei

The general expression giving the N.M.R. band-shape of a spin-1/2 nucleus coupled to a spin-1 nucleus is considered in detail for the ‘fast-exchange’ limit, where the quadrupolar-induced relaxation rates of the spin-1 spin states are much greater than the coupling constant between the nuclei. The problem of obtaining the coupling constant from measurements on the band-shape of the spin-1/2 resonance is discussed, and it is concluded that this is only possible without further information when departures from lorentzian shape are observed. Possible sources of extra data are mentioned. The case of 3,4,5-trichloro-2,6-difluoropyridine is examined. The (N, F) coupling constant is obtained from the ¹⁵N satellites in ¹⁹F resonance. Hence values of the spin-lattice relaxation time for ¹⁴N are derived as a function of temperature.     

N.M.R. study of self-diffusion in solid N2

From the measurement of the N.M.R. relaxation times T₂ in ¹⁵N₂ and in ¹⁴N₂ containing Ar impurities we extract the self diffusion coefficient D $\text{i}$n h.c.p. N₂. We find that self-diffusion is a thermally activated process with an activation enthalpy δH¹/k = 1030 ± 25 K. We discuss the diffusion process and conclude that it can be related to the motion of thermally activated vacancies.     

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.     

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.     

1H and 2D N.M.R. studies of the radical anions of biphenyl,fluorenone and phenanthrene

The proton N.M.R. spectra of biphenyl^-, fluorenone^- and phenanthrene^- and the deuterium N.M.R. spectra of biphenyl-d10^- and phenanthrene-d10^- have been measured in ethereal solutions at room temperature. Sign and magnitude of the hyperfine splitting constants derived from the measured contact shifts are reported and compared with E.S.R. data and predictions from theory. From the measured proton and deuterium relaxation times values for the electron spin and the rotational correlation times have been obtained.     

Co2+ spin-lattice relaxation narrowing of 19F NMR in KCoF3

The temperature dependence of the ¹⁹F NMR linewidth ΔH in KCoF₃ has been measured over the entire paramagnetic solid state region. The dramatic decrease in the hyperfine-broadened, exchange narrowed ΔH that occurs above 200 K is interpreted as arising from fast Co²⁺ single-ion, spin-lattice relaxation. A model theory of the temperature dependence of ΔH is given which incorporates the interplay of exchange and spin lattice relaxation effects on the decay of the spin autocorrelation function.     

An N.M.R. study of molecular motion in solid trimethylaminegallane

Continuous wave and pulsed ¹H N.M.R. data have been obtained for solid H₃GaN(CH₃)₃ over the temperature range 63–300 K. A theoretical expression for the relaxation behaviour of a methyl group in a trimethylamine moeity undergoing various motions has been obtained to aid analysis of the data. We find the activation energy to rotation of the -GaH₃ group to be 3·6 ± 0·3 kJ/mole (0·86 ± 0·07 kcal/mole), and to a different motion in the molecule to be 21 ± 2 kJ/mole (5·0 ± 0·5 kcal/mole). In the continuous wave spectra effects due to motion of the -CH₃ groups and the whole -NMe₃ moeity may be distinguished.     

Orientation distribution function of aromatic molecules in frozen liquid crystals from their triplet E.S.R.-spectra

The motional narrowing of ¹⁹F N.M.R. resonance of the halothane tri-o-thymotide clathrate was studied as a function of temperature. It was shown that the CF₃ group exhibits free rotation about its C₃ axis at temperatures as low as 108 K. Other motional narrowings were observed as the temperature was raised and finally at a temperature of about 25 K below the melting point of the clathrate diffusion of the halothane molecules through the lattice reduced the second moment to practically zero.     

Proton N.M.R. study of the dynamics of the ammonium ion in ferroelectric langbeinite, (NH4)2Cd2(SO4)3

The proton N.M.R. lineshape of polycrystalline Langbeinite, (NH₄)₂Cd₂(SO₄)₃, has been studied in the temperature range 300 K to 1·8 K. The resonance line is motionally narrowed over the entire temperature range, and the low temperature proton line shows clear evidence for tunnelling motion of the ammonium ion between spin-symmetry states. From a computer simulation of the lineshape, we obtain an estimate for the tunnelling splitting parameter, J, of the torsional ground state of the ammonium ion, as 375 ± 125 gauss. For an undistorted tetrahedral crystal field this corresponds to a tunnelling splitting Δ = 4J = 6·3 ± 2·1 MHz.

Pulsed proton N.M.R. studies have also been carried out on the above compound at 30·8 MHz and 48·2 MHz and the spin-lattice relaxation time (T ₁) has been measured by the π - t - π/2 pulse sequence as a function of temperature down to 77 K. At 30·8 MHz, a T ₁ minimum of 13 ms occurs at 105·8 K, and is ascribed to random reorientations of the NH₄ ⁺ ion. An activational energy barrier of 0·74 ± 0·1 kcal/mole and an associated pre-exponential factor of 8·0 × 10^-13 s are calculated for the above motional process, and the value of the activation energy is correlated with the tunnelling splitting of the torsional ground state.

An anomaly in T ₁ has been observed at the ferroelectric Curie point (95 K), indicating the order-disorder nature of the transition. This is the first experimental evidence relating to the nature of the transition in Langbeinite.     

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.     

NMR proton relaxation and chemical exchange in the system H16 2O/H17 2O-[2H6]dimethylsulphoxide

NMR proton relaxation rates of normal and ¹⁷O enriched water in a mixture of 68 mol% water and 32 mol% [²H₆]dimethylsulphoxide were measured for temperatures between 298 K and 183 K. In the range between 240 K and 204 K the limit of fast proton-proton exchange between H¹⁶ ₂O and H¹⁷ ₂O is not obeyed, and relaxation curves deviate from mono-exponential behaviour. By fitting the relaxation curves to a model of NMR two-phase relaxation the proton-proton exchange rate within the aqueous component could be obtained. With decreasing temperature, proton-proton exchange slows down and a residence time of about 125 ms at 215 K is found, but it becomes faster again for still lower temperatures. From the phase-averaged relaxation rates of water in the ¹⁷O enriched mixtures, the ¹⁷O induced proton relaxation rate was derived as a function of temperature. This yields the rotational correlation times of the water molecule in the mixture and the dipolar spin-lattice coupling parameter. The latter is considerably lower than the one predicted from the geometry of water.     

N.M.R. study of internal motion in hexahydrated acid fluorides of cobalt and nickel

A linear relationship between the viscosity B-coefficient of the Jones-Dole equation for aqueous solutions of certain alkali metal salts and the enthalpy of hydration of the gaseous monatomic constituent ions has been established. The assumption that a similar rectilinear law applies to ammonium halides appears justified and the enthalpies of solution of NH₄ ⁺(g)+X^-(g) have been estimated and used to obtain magnitudes for the lattice energies of NH₄X(c) [X=F, Cl, Br, I]. In conjunction with experimental thermochemical data, the latter yield consistent results for the proton affinity of ammonia ΔH ₁ ^?=860·5±2·0 kJ mol^-1 (298·15 K). The lattice energies of the salts are, (in kJ mol^-1) 834 (NH₄F), 708 (NH₄Cl), 682 (NH₄Br) and 637(NH₄I).     

Experimental and theoretical N.M.R. studies of the coupling constants in seven isotopic ethyl fluorides

The N.M.R. spin-spin coupling constants and chemical shifts are reported for seven isotopically substituted ethyl fluorides. ¹H, ²H and ¹⁹F spectra have been observed and a consistent set of data obtained. Isotope effects are reported in ¹H and ¹⁹F spectra due to replacement of ¹H by ²H and ¹²C by ¹³C.

Using an accurate microwave structure of ethyl fluoride, CNDO and INDO calculations have been carried out, including dipolar and orbital terms. As independent options, configuration interaction between all single-excited states, variable radial electron distribution <r ^-3> and variable electron density at the site of nuclei, <s(0)²>, are included. Calculated values for all combinations of the options are given and a discussion of the results presented.     

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.     

A Raman and NMR study of KSbF6

The ¹⁹F spin-lattice relaxation results in the laboratory frame show that upon cooling from the tetragonal phase, KSbF₆(I), the transition to the cubic phase, KSbF₆(II), occurs over a wide temperature range (~38 K) in which the two phases co-exist. The Raman results using powdered samples agree with this observation but co-existence of phases has of course not been observed in single crystal Raman measurements. Upon rapid cooling of the powdered samples in the Raman studies the tetragonal phase could be super-cooled. Upon heating from the cubic phase, the transition was observed at 302 ± 2 K in all the measurements. The Raman spectra of KSbF₆(I) give no evidence of a non-centrosymmetric structure but it is shown that this is so because the Sb-atoms are only very slightly displaced from centro-symmetrical positions. ¹⁹F second moment results are in agreement with a model in which the SbF^?₆-octahedra are stationary below 180 K and reorient isotropically above 260 K. The importance of scalar spin-spin coupling between fluorine and antimony nuclei is reflected by the T_1p results in the vicinity of the T₁ minimum. The Raman spectra of the cubic phase at higher and lower temperatures are different and the polarized spectra of single crystals are used to assign the bands in terms of a C₃-site group symmetry for the SbF^?₆-groups and a T unit cell group symmetry.     

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.     

PMR studies of molecular motions,phase transitions and quantum tunnelling in NH4SnCl3 and N(CH3)4SnCl3

Molecular reorientation and low temperature relaxation effects of NH⁺ ₄ ion and the effect of CH₃ substitution (in place of H) are investigated by proton spin lattice relaxation time (T₁) measurements at 10 MHz in NH₄SnCl₃ and N(CH₃)₄SnCl₃ in the temperature range 4.2 K upto the melting points of the compounds (? 440 K). Phase transitions around 360 K in NH₄SnCl₃ and around 361 and 116K in N(CH₃)₄SnCl₃ have been observed. In NH₄SnCl₃, the high temperature minimum at 330.5 K is attributed to the translational diffusion of the NH⁺ ₄ ions, while the other T₁, minima at 103.5, 60 and 50 K are ascribed to the reorientations of the NH⁺ ₄ ion about the C₂ and C₃ axes. The low temperature minimum at 13.5 K is attributed to rotational tunnelling of the NH⁺ ₄ ions. In N(CH₃)₄SnCl₃, in addition to the high temperature minima at 212.2 and 182.6 K due to N(CH₃)₄ tumbling and CH₃ reorientation, a temperature independent T₁ behaviour between 83 and 31 K is observed, below which T₁ decreases and tends to go through a minimum around 5 K. This low temperature minimum is attributed to rotational tunnelling of the CH₃ groups. The motional parameters and tunnel frequencies are estimated.