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.     

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.     

Interpair dipolar interactions in solids containing spin-½ pairs

Recent experimental work in powdered crystalline hydrates has shown that the interpair second moment of the proton N.M.R. absorption spectrum is ? of the value given by the van Vleck expression. Using a model of two planar spin-½ pairs, this reduction factor of 5/6 is shown to originate in the quenching of non-secular terms in the interpair dipolar hamiltonian which give rise to weak, unobservable resonances far into the wings of the spectrum. This effect is analogous in origin to the well-known reduction factor of 4/9 in the second moment due to the interaction between resonant and non-resonant spins.     

N.M.R. spectra of oriented molecules

By analysis of the high resolution N.M.R. spectrum of 1,2,2,3-tetrachloro-propane partially oriented in a liquid-crystal solvent, it has been shown that this molecule takes up predominantly the G′G′ type of conformation.     

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.     

N.M.R. Studies of Lanthanide(III) Complexes. High Field Shifts in Complexes of 1,10-Phenanthroline

In continuation of our studies on the complexes of lanthanide(III) ions with N-donor ligands^1,2, especially those of 1,10-phenanthroline (phen)^3,4, we wish to report the proton N.M.R. spectra of several lanthanide(III) complexes of phen in aqueous (D₂O) solution at 330 K. Table 1 contains the observed chemical shifts of the four types of protons in phen relative to the methyl resonance of the internal standard 2-methyl-2-propanol. For comparison, the spectrum of an authentic sample of Nd(phen)₂Cl₃.2H₂O complex prepared by the method of Sinha⁵ was recorded and the spectrum found to be virtually indistinguishable from that obtained by mixing the individual components in the proper ratio.     

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.     

A study of the molecular structure and of the barrier to methyl rotation in o-chlorotoluene partially oriented in the nematic phase

The N.M.R. spectrum of o-chlorotoluene, dissolved in the nematic phase of 4-methoxybenzylidene-4-amino-α-methylcinnamic acid n-propylester is analysed and the shape of the proton skeleton determined. The position of lowest energy of the potential barrier to methyl rotation is found to be that in which the chlorine atom is staggered to the methyl group, and the height of the barrier is of the order of 5·0 kJ mol^-1 (1200 cal/mole).     

Alignment of non-polar molecules—electric field effects in the 2H N.M.R. spectrum of perdeuterobenzene in solutions with nitrobenzene

Electric field experiments on the ²H N.M.R. spectrum of perdeuterobenzene, dissolved in nitrobenzene, are reported. The alignment of the polar molecules, induced by a strong electric field, is transferred to the non-polar ones and as a result their N.M.R. spectrum is modified.

Local field models, e.g. those of Lorentz and Onsager, treat the environment of a molecule as a continuum and are unable to explain the alignment of the non-polar molecules. Intermolecular interactions are essential. In an expression for the alignment these interactions are described in terms of correlation functions for the mixture.

N.M.R. and Kerr data of nitrobenzene-benzene solutions have been compared. The Kerr constant of a binary mixture is shown to depend on the alignments of both the polar and non-polar component. N.M.R. electric field experiments may be useful to separate these two contributions.     

13C Dipolar spectroscopy of nitromethane

The ¹³C dipolar N.M.R. spectrum of ¹³CH₃ ¹⁴NO₂ has been recorded at 25 K. The principal values of the ¹³C shielding tensor, obtained by a computer fit of the experimental lineshape, are similar to those found in other ¹³CH₃ groups. ¹⁴N quadrupolar parameters and direct dipolar couplings are also reported. In fitting the spectrum we find that an effective trace must be added to the dipolar interaction to account for librational motion.     

Proton magnetic shielding anisotropy in acetylene

A high resolution N.M.R. study of acetylene in different liquid crystals is reported. The anisotropy in proton chemical shift has been measured accurately. The present result differs considerably from the values previously reported and is found to be independent of liquid crystal medium, concentration or density and reference of chemical shift measurement. Additional chemical shift data in gaseous acetylene and methane-tetramethylsilane (TMS) relative shifts in nematic and isotropic phases of the liquid crystals are also given.     

The proton magnetic resonance spectrum of β-propiolactone in two thermotropic nematic solvents

The molecule β-propiolactone has been studied in two thermotropic nematic solvents at 30·5°C. The proton magnetic resonance spectrum shows that the four protons are arranged in a plane with C _2v rather than D _2h symmetry. The interproton distance ratios are in reasonable agreement with the structure previously reported from a study of the rotational spectrum. These ratios are the same within experimental error in both thermotropic solvents reported here. Differences in line width and spectral quality are noted between the two solvents and this suggests that studies in more than one solvent are desirable to obtain the best analysis. The N.M.R. method is especially sensitive to extremely small changes in geometry reflected by the symmetry change D _2h to C _2v .     

N.M.R. study of the 125Te shielding anisotropy of tellurophene dissolved in various nematic liquid crystals

The ¹²⁵Te N.M.R. spectrum of tellurophene in three liquid crystals was recorded and analysed. The different orientational behaviour of tellurophene in the liquid crystals enabled the anisotropy of the tellurium-125 shielding tensor, σ _zz - 1/2(σ _xx + σ _yy ), and the difference σ _xx - σ _yy of the tensor elements to be determined. The resulting values are -1569 ± 21 ppm and 307 ± 26 ppm, respectively. The analysis of the ¹²⁵Te N.M.R. spectra is also discussed.     

Un exemple d'utilisation spécifique de la RMN du carbone-13: les transferts protoniques sur centre carbone

A new method is described to study proton transfers on a carbon centre by means of carbon-13 N.M.R., using non-enriched samples, and without proton decoupling. As an example, the proton exchange rate k between dimethylsulphoxide and its conjugate base, the dimsyl ion, is measured at 25°C : k = 22·1 M^-1 s^-1.     

Nitroxide radicals

The proton hyperfine constants have been measured, using N.M.R., for a series of sterically hindered alkyl aryl nitroxides. Computer reconstructions of the electron resonance spectra show that the coupling constants obtained from the pure radical are very similar to those obtained from the electron resonance spectra of dilute solutions. Comparison of methyl and proton splittings in identical positions suggests that the radicals are σ in character although the molecular-orbital calculations are unable to distinguish between σ or π radicals.     

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.     

On the theory of pseudocontact N.M.R. shifts due to lanthanide complexes

The pseudocontact N.M.R. shifts due to complexes of lanthanide ions are considered in a crystal field theoretical framework including all the states J, M of an L-S manifold and an arbitrary set of crystal field parameters. Deviations from Bleaney's earlier theory are shown to be of the order of 20 per cent when the higher crystal field parameters are added.     

Through-bond and through-space effects on proton chemical shifts. The magnetic anisotropy effects of freely rotating -XY3 groups upon distant protons

A general expression is presented for calculating the contributions of the magnetically anisotropic X-Y bonds of freely rotating -XY₃ groups to the shielding of protons of a freely rotating methyl group elsewhere in the same molecule. The expression can also be modified to calculate (a) the anisotropy contribution of a freely rotating -XY₃ group to the shielding of a fixed proton and (b) the contribution of a fixed anisotropic X-Y bond to the shielding of a freely rotating methyl group. Application is made to calculate the contribution of magnetic anisotropy changes, when a hydrogen atom is replaced by a methyl group, to the aryl and methyl proton chemical shifts of the methylbenzenes relative to benzene and toluene, respectively. Although the contribution of magnetic anisotropy is not negligible it is insufficient to account for the observed shifts. This conclusion applies also to the acetylenic proton chemical shifts of some methyl derivatives of acetylene. Some comments are made on the values of magnetic anisotropy obtained by the N.M.R. method.