Anisotropies in carbon-13 chemical shift tensor,H-C-H bond angles and the signs of the coupling constants in 13CH3I, 13CH3Br, 13CH3Cl

A pulsed fast Fourier transform N.M.R. spectrometer is used to investigate the temperature dependence of the ¹³C chemical shifts in the nematic liquid crystal solutions of the following halides: ¹³CH₃I, ¹³CH₃Br, ¹³CH₃Cl. The H-C-H bond angles are accurately measured in this investigation from proton magnetic resonance studies. The signs of the coupling constants D _13CH , D _HH, J _13CH and the ordering parameter have been assigned to be all positive. The ¹³C magnetic shielding anisotropy is found to become more positive with the increasing electronegativity of the halogen substituents. The values of the anisotropies are: Δσ = σ _∥ - σ _⊥ = - 101 ± 15 p.p.m. (¹³CH₃I), - 10 ± 5 p.p.m. (¹³CH₃Br), 22 ± 5 p.p.m. (¹³CH₃Cl). Results for Δσ obtained from the slope of σ_n versus S ₃₃ agree with those obtained from the nematic-isotropic phase subtraction method.     

Effect of solvents on carbon-13 spin-lattice relaxation in 1-phenyladamantane

Rotational diffusion of 1-phenyladamantane was studied by means of carbon-13 spin-lattice relaxation. Seven different solvents were used. Rotational correlation times showed a linear dependence on solvent viscosity, except for chloroform and 1,1,2,2-tetrachloroethane which retarded motion of the phenyl moiety through specific interactions.     

Spin-lattice relaxation in 13CH3 compounds: application to 13C enriched aspirin

Spin-lattice relaxation processes in ¹³CH₃ groups in methyl compounds are studied both theoretically and experimentally. The four spin-½ nuclei in such methyl groups give rise to 16 spin-rotational states, which are split by rotational tunnelling. From the corresponding populations (15 independent) five long lived combinations are formed: the ¹³C magnetization M _C, proton magnetization M _H, tunnelling energy TE, rotational polarization RP and dipolar energy DE. Their spin-lattice relaxation via the transitions induced by the ¹³C-proton dipolar interaction is studied in detail. Direct relaxation rates and coupling terms between these combinations are derived. Predictions are compared with experimental data for ¹³C spin-lattice relaxation at 75.4 MHz in 99% enriched (only methyl carbons enriched) single crystal of aspirin. Above 40 K, the M _C recovery is exponential and describable in terms of the direct relaxation transitions without couplings. The same is true for the initial relaxation in the region of non-exponential relaxation between 30 K and 40 K. The orientation dependence of the initial relaxation rate agrees with the theoretical calculations. The non-exponentiality is related to resonant level-crossing transitions with ω_t, + ω_C = ω_H, where the angular frequencies represent rotational tunnelling and carbon and proton resonances, respectively. The resonant transitions produce couplings between M _C, M _H and TE that are described quite accurately by the present model.     

Characterization of molecular motion in the solid state by carbon-13 spin-lattice relaxation times

Relaxation calculations for rapidly spinning samples show that spin-lattice relaxation time (T(1Z)) anisotropy varies with the angle between the rotor spinning axis and the external field. When the rate of molecular motion is in the extreme narrowing limit, the measurement of T(1Z) anisotropies for two different values of the spinning angle allows the determination of two linear combinations of the three static spectral densities, J(0)(0), J(1)(0), and J(2)(0). These functions are sensitive to molecular geometry and the rate and trajectory of motion. The utility of these linear combinations in the investigation of molecular dynamics in solids has been demonstrated with natural abundance (13)C NMR experiments on ferrocene. In an isolated (13)C-(1,2)H group, the dipole-dipole interaction has the same orientational dependence as the quadrupole interaction. Thus, the spectral densities that are responsible for dipolar relaxation of (13)C are the same as those responsible for deuteron quadrupolar relaxation. For ferrocene-d(10), deuteron T(1Z) and T(1Q) anisotropies and the relaxation time of the (13)C magic angle spinning peak provide sufficient information to determine the orientation dependence of all three individual spectral densities.     

Precise value for the three-bond 13C-1H coupling constant in acetone

A method based on the utilization of continuous space-filling curves is applied to a two-dimensional elastic scattering problem involving a central Yukawa potential. The Lippmann-Schwinger equation is brought into the form of a one-dimensional inhomogeneous integral equation and solved numerically. The results agree to within about 5 per cent with those obtained by a recent direct numerical solution based on two-dimensional quadratures.     

Proton-phonon coupling in a hydrogen bonded system

The Hamiltonian for the dynamics of a structural defect in a linear hydrogen bonded system (e.g., one analogous to an excess proton in ice) is developed, including explicit expressions for the coupling between the defect and phonons. The nature of these couplings are discussed and the results of a minimal basis set SCF calculation on a model system presented. The coupling to the longitudinal phonons is found to be very strong.

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Zusammenfassung Der Hamiltonoperator für die Dynamik eines Strukturdefektes in einem linearen System von Wasserstoffbrückenbindungen (etwa analog dem Excessproton in Eis) wird entwickelt samt expliziten Ausdrücken für die Kopplungen zwischen dem Defekt und den Phononen. Die Natur dieser Kopplungen wird diskutiert und die Ergebnisse einer SCF-Rechnung mit minimaler Basis an einem Modell-System dargelegt. Die Kopplung zu den longitudinalen Phononen stellt sich als sehr stark heraus.

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Résumé On a développé l’Hamiltonien de la dynamique d’un défaut structurel dans un système linéaire de liaisons hydrogène (par exemple, un défaut produit dans de la glace par un proton excessif). On a tenu compte explicitement des couplages entre le défaut et les phonons. On a discuté de la nature de ces couplages et on a présenté, pour un système modèle les résultats d’un calcul opérant sur un “self consistent field” (SCF) à ensemble minimal de bases vectorielles. Il s’est révélé que le couplage aux phonons longitudinaux est particulièrement fort.

     

Cross-correlated quadrupolar spin relaxation and carbon-13 lineshapes in the (13)CD(2) spin grouping.

Lineshape calculations are reported for a spin system consisting of a spin-12 nucleus scalar-coupled to two magnetically equivalent spin-1 nuclei, e.g., a CD(2) group. It is found that, because of the differential line broadening, the peak height ratio of the five spin-12 nucleus transitions is expected to deviate from the integrated intensity ratio of 1:2:3:2:1. The deviation is dependent on the extent of cross-correlation between the two quadrupolar interactions. The theoretical predictions for peak height ratios and for bandshapes are tested for the carbon-13 spectrum of perdeuterated ethylene glycol, and iterative fitting is used to obtain an estimate of the extent of the correlation between the quadrupolar interactions for the two deuterons.     

Odderon with a running coupling constant

The running coupling is introduced into the equation for the odderon via the bootstrap relation. It is shown that the previously found odderon state with a maximal intercept, which is constructed from an antisymmetric pomeron wave function, continues to exist in the running coupling case. Its intercept is found to remain equal to unity independent of the behaviour assumed for the running coupling at low momenta.     

Observation of nitrogen-14, carbon-13 indirect spin-spin coupling in high-resolution 13C CP/MAS spectra of solids.

The 13C CP/MAS NMR spectrum of [(n-C3H7)4N][Cd(SCN)3], 1, indicates the presence of three non-equivalent thiocyanate ligands, in agreement with the results of a recent single-crystal X-ray diffraction study. Examination of the 13C MAS line shapes allows direct measurement of the indirect spin-spin coupling constants, 1J(14N, 13C) = 16 +/- 1 Hz and 2J(111/113Cd, 13C) = 75 +/- 5 Hz, for the unique N-bonded thiocyanate ligand. This is the first reported measurement of 1J(14N, 13C) and 2J(111/113Cd, 13C) in the solid state. Possible reasons for the failure to observe 1J(14N, 13C) values in previous high-resolution 13C CP/MAS NMR studies are summarized.     

Deuteron quadrupole coupling constants and relaxation times for the isomeric contiguously 13C 2H-labeled cyclohexenes

The deuteron quadrupole coupling constants (Q_D's) have been determined for a mixture of isomeric monodeuterocyclohexenes (containing a low level of ¹³C enrichment in the CD bonds) from the ²H NMR spectrum in nematic phase. The measured Q_D's are: 213.6± 2.5, 179.9± 2.9, and 178.8±E 3.0 kHz for the vinyl, allyl, and homoallyl deuterium-substituted species, respectively. Employing these Q_D values and spin-lattice relaxation times (T₁'s), it is concluded that reorientation of cyclohexene perpendicular to the molecular (pseudo)-plane is ca. twice as rapid as out-of-plane reorientation.     

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.     

Semi-phenomenological bounds for the pion-pion coupling constant

Upper and lower bounds on the Chew-Mandelstam ππ coupling constant λ are derived as functions of the D-wave scattering length a₂. The only information used is the axiomatic analyticity domain of the ππ scattering amplitude, and the crossing and unitarity properties. For a₂ = 7 × 10^?4, one obtains ?0.164<λ<0.162. This is compared with previous bounds.     

On the linear relaxation of a one-dimensional bonded fluid model

The linear relaxation of a one-dimensional bonded fluid model is studied in the dynamical mean field approximation at the pair level. The results are compared with simulation data. Relaxation at low temperatures shows a cooperative character that can be understood in terms of a simple diffusion picture.     

A systematic study of QCD coupling constant from deep-inelastic measurements

Physics of Atomic Nuclei - We reanalyze deep-inelastic scattering data of the BCDMS Collaboration by including proper cuts of ranges with large systematic errors. We also perform the fits of...     

Quark-pion coupling constant in a chiral quark model

Incorporating chiral-symmetry to the potential model of quarks with confining potentialU(r)=1/2 (1 +γ°)ar ² with m _q =10 MeV anda=2.273 fm⁻³ that gives a reasonable quark-core contribution to μ _p , 〈r ² 〉 _p ^1/2 andg _A , the quark-pion coupling constant for quarks in a nucleon is estimated.G _qqπ ² /4π obtained between 0.4 and 0.5 is consistent with those extracted from experimental vector meson decay-width ratios by Suzuki and Bhaduri. The nucleon-pion coupling constantG _NNπ ² /4π comes out to be of the order of 13.1 in reasonable agreement with the experimental value.