Wigner crystal model for quasi one-dimensional systems based on TCNQ

The value of the electron transfer, ρ, from donors (Rb, NMP) to acceptors (TCNQ) is studied by calculating the electrostatic energy of the Rb-TCNQ and NMP-TCNQ crystals as a function of ρ and using the spin susceptibility data for the NMS-TCNQ crystal. It is shown that for the Rb-TCNQ crystal ρ = 1 and for the NMP-TCNQ crystal $\text{ρ =}\text{1}\text{8}$. The Wigner crystal model for highly conductive TCNQ salts is discussed.     

Nonlinear refraction in photo-induced anisotropic states of azo-dye containing materials

The optical anisotropic states in the azo-dye Disperse Red 13 doped poly(methyl methacrylate) polymeric film are induced by polarized light and the nonlinear refractive index in these states are studied by the Z-scan technique. It is found that the n₂ of the sample can be controlled by changing both the polarization direction and the intensity of the 514 nm light. Photo-induced isomerization and reorientation of azo chromophores in polymer matrices are used to explain the results.     

Proton magnetic relaxation and molecular motion in polycrystalline amino acids

A proton magnetic relaxation study has been carried out on a further seven polycrystalline amino acids from 130 to 500 K at 60·2 MHz, supplemented by measurements of the spectrum down to 4 K on five of them. These amino acids all have one or two methyl groups in their side chain, and exhibit two relaxation minima. Clear evidence is given that the relaxation minima at lower temperatures are attributable to reorientation of the methyl groups, while those at higher temperatures are attributable to reorientation of the -NH₃ groups in the zwitterion configuration of the molecules. Values of the relaxation constants, activation energies and time factors which best characterize the kinetics of both motions are tabulated. Effects of methyl group tunnelling are found for methionine and valine.     

Proton relaxation NMR study of polycrystalline progesterone.

Polycrystalline progesterone (4-pregnene-3,20-dione, C21H30O2) has been investigated by proton NMR methods between 80 and 350 K. A reduction in dipolar second moment is ascribed to methyl group reorientation. Minima in the spin-lattice relaxation time found in measurements at five frequencies from 7 to 200 MHz are attributed to reorientation of two of the three methyl groups in each molecule, characterized by activation energy Ea = 10.9 +/- 0.8 kJ/mol and tau o = (2.3 +/- 0.2) x 10(-13) s. Additional relaxation at lower temperatures is attributed to reorientation of the third methyl group with Ea approximately 3.4 kJ/mol. Measurements were also made of relaxation in the rotating frame.     

Molecular dynamics in comb-like ionene as studied by NMR

Proton and fluorine second moment and spin-lattice relaxation timesT ₁ andT _lρ have been employed to study molecular dynamics in the comb-like I-6,6-16-Me-BF₄ ionene in the temperature range from 110 up to 300 K. The existence of motions of methyl groups, trans-gauche isomerization, and/or rotation of the main- and side-chain méthylene groups, as well as isotropic reorientation of tetrafluoroborate ions were established. The observed relaxation behaviors are explained by motional models which assume Davidson-Cole asymmetrical distribution of correlation times. The best-fit motional parameters are given.     

Molecular dynamics of n-dodecylammonium chloride studied by nuclear magnetic resonance

The relation between molecular dynamics and phase properties of the bilayered compound C₁₂H₂₅NH₃C1 is studied by differential scanning calorimetry, proton second moment, and spin-lattice relaxation times. In the low-temperature phase I of the compound methyl and ammonium groups execute a classical threefold reorientation, while the alkylammonium chains are rigid on the nuclear magnetic resonance time scale. In the intermediate-temperature phase δ a trans-gauche isomerization of the alkyl chains is observed. In the high-temperature phase α the reorientation of the whole chains about their long axes, which are parallel to the normal to the ionic layer is evidenced. In the metastable ε phase the dynamics involves classical rotation of methyl and ammonium groups and CH₂ groups motion of the trans-gauche isomerization type.     

Proton spin-lattice relaxation in MBDBP

We have measured the proton spin-lattice relaxation rate R in 4,4′-methylenebis(2,6-di-t-butylphenol) (MBDBP) and MBDBP-OD (deuteriated hydroxyl proton) between 88 K and 380 K. The experimental results are compared with those from a previous study in the closely related but simpler molecule 4-methyl-2,6-di-t-butylphenol (MDBP). The present experiment is analysed in terms of relaxation resulting from intramolecular reorientation modulation of the proton spin-spin interaction. This intramolecular reorientation involves different combinations of superpositions of methyl, t-butyl, aromatic ring and hydroxyl group reorientation and a discussion is presented concerning the effects on the relaxation of superimposing these motions. The MBDBP-OD data is best fitted with the assumption that only methyl and aromatic ring reorientations occur. In MBDBP it is found that, in addition, an OH flip-flop motion is superimposed on the aromatic ring reorientation.     

Ground state reorientation effects in elastic scattering cross sections

Elastic scattering cross sections are shown to be quite sensitive to ground state reorientation amplitudes and cross sections. These effects are dramatically evident when comparing scattering from even-even nuclei with that from high-spin odd-A nuclei, and may affect choices of potentials for odd versus even nuclei. Neutron scattering data from ²³⁵U and ²³⁸U are analyzed via coupled channels methods. The diffraction minima in elastic scattering from ²³⁵U are found to be dominated by reorientation cross sections. The majority of this reorientation is shown to follow from second order virtual nuclear excitations rather than proceeding through amplitudes for direct ground state reorientation.     

Spin reorientations in R2Fe14-xCoxB systems (R = Pr,Nd and Er)

The effect of cobalt substitution on spin reorientation phenomena in R₂Fe_14-xCo_xB systems (R = Pr, Nd, Er) was studied by means of bulk magnetometry in the temperature range 4.2–1100 K. It was established that in the Nd-based system the introduction of cobalt resulted not only in a shift of the low temperature spin reorientation (cone to axis) but also triggered, for x ⩾ 10, an appearance of a second spin reorientation (axis to plane) at high temperature. In the Pr-based system, for x ⩾ 9.5, a high temperature spin reorientation (axis to plane) was also observed. Its dependence on Co content was determined. For the Er-based system, an increase of the spin reorientation temperature (plane to axis) was observed as more cobalt was introduced into that system. It was also established that the tetragonal single-phase materials in this system exist only up to x = 5. Temperature-composition diagrams are presented, indicating types of spin arrangements observed in the investigated systems.     

Cerium influence on the spin reorientation in Er2−xCexFe14B evidenced by Mössbauer and DSC techniques

Polycrystalline Er_2−xCe_xFe₁₄B (x=0.25, 0.5, 1.0) compounds have been studied by ⁵⁷Fe Mössbauer spectroscopy and by differential scanning calorimetry (DSC) in the temperature range 80–470 K. The spin reorientation phenomenon has been studied extensively by narrow step temperature scanning in the vicinity of the spin reorientation temperature. It was found that in the region of transition, each Mössbauer subspectrum splits into two Zeeman sextets, which are characterised by different hyperfine magnetic fields and quadrupole splittings. A consistent way of describing the Mössbauer spectra in the reorientation region, below and above, was proposed. The composition and temperature dependencies of hyperfine interaction parameters and subspectra contributions were derived from experimental spectra.The endothermic DSC peaks were observed for all studied compounds, which correspond to the transition from basal plane to axial easy magnetisation direction on increasing the temperature. The spin reorientation temperatures and the enthalpies of transitions were established from DSC data. The spin arrangement diagram was constructed and the spin reorientation temperatures obtained by the two methods were compared.     

Overall and internal motions of substituted benzenes studied by means of selective proton magnetic longitudinal relaxation time measurements

Selective T ₁ measurements at different temperatures on seven substituted benzenes in CDCl₃ solution show that for these molecules the rotational diffusion model applies. Anisotropic reorientation is important if the molecules contain two polar substituents para to each other. Anomalies in the temperature dependence of the T ₁ values in a 2-methyl substituted phenol are explained by intermolecular hydrogen bonds, which strongly influence the molecular motion. In a 2,6-dimethyl substituted phenol this effect is absent. The correlation times and the energy barriers for methyl rotation of methyl and methoxy groups are determined. The reorientation of methoxy groups around the aryl oxygen bond is slower than the molecular motion. The T ₁ values of ring protons and substituents can in some cases be used for spectral assignment. Expressions are given for the T ₁ value of a ring proton relaxed by an ortho methyl or methoxy group and for the T ₁ value of a reorienting methoxy group in the case of dipolar relaxation and axially symmetric behaviour of the molecule.     

Spiral-like motion of the magnetization direction during reorientation in Ni-films on Cu(0 0 1)

The magnetization reorientation from in-plane to out-of-plane, occurring in Ni-films on Cu(0 0 1) around 9 to 10 ML, was studied in situ during growth by second harmonic generation. s→S and p→S polarization combinations of incident fundamental and reflected second harmonic light were used to show that reorientation proceeds in a spiral-like turn of the magnetization from parallel into the direction perpendicular to the steps, in accordance with reports in the literature that reorientation is driven by magneto-elastic forces.     

Proton spin relaxation in bisphenol-a polycarbonate,butyl rubber,and their composites

Proton spin-lattice relaxation times of bisphenol-A polycarbonate, butyl rubber, and blends of the two polymers were studied at 18 Mc/sec in the temperature range 90°-450°K. The proton spin-lattice relaxation is primarily dipolar in each polymer, due to methyl group reorientation and to reorientation of chain segments. In a blend of bisphenol-A polycarbonate with 7 and 10 wt of butyl, a nonexponential decay of magnetization was observed in the temperature range 280°-380°K. This was explained by the existence of two spin temperatures in these blends, indicating that processes which bring about the equilibrium within the spin system are slow compared to the spin-lattice relaxation times of the two components of the blend.     

Magnetic properties of (NdxY1-x)Co2(1⩾ x ⩾ 0)

The concentration dependence of T_c and T_R (T_c magnetic ordering temperature, T_R spin reorientation temperature) of the pseudobinary system (Nd, Y)Co₂ is reported. Furthermore the influence of an external magnetic field on the spin reorientation and the magnetization is studied. The observed variation of the magnetization in the vicinity of the spin reorientation is compared with theoretical results. For the calculation a Hamiltonian with terms describing a molecular field, a cubic crystal field, and an external field is used.     

Wavelength dependence of fluorescence spectra of liquid coumarin solutions

The excitation wavelength dependence of steady-state fluorescence spectra of low-viscosity dye solutions is reexamined. Measurements are made on some coumarins in methyl alcohol at room temperature. It is found that the fluorescence spectra depend on the excitation energy and their profile changes are due to the excitation wavelength dependence of reorientation processes in the excited state.     

E.S.R. spectra of the radicals produced in γ-irradiated diborane in the solid state

The hopping and Coulomb repulsion integrals of the Hubbard model, as applied to the NMP-TCNQ charge-transfer crystal, are found by an INDO calculation of the neutral and charged monomer and dimer of TCNQ. The ratio of these integrals is in satisfactory agreement with experiment, although the calculated integrals themselves are very different from the effective ones for electronic transport, due to renormalization, caused largely by counter-ion polarizability.     

Raman spectra of conducting TCNQ salts; Estimation of the degree of charge transfer from vibrational frequencies

The linear dependency of TCNQ ν₄ frequencies on the formal charge was observed and was attributed to the motional narrowing of the Raman band by hopping of conduction electrons. The formal charges of TTF-TCNQ (0.59) and NMP-TCNQ (0.62) estimated from this dependency agreed well with those from other experiments.     

<Superscript>1</Superscript>H NMR study of molecular dynamics of acetylcholine chloride

The longitudinal relaxation timeT ₁ and the second momentM ₂ of¹H nuclear magnetic resonance line in a wide temperature range have been measured for acetylcholine chloride. Two different types of the methyl groups reorientation occurred. The first type was the hindered rotation of the methyl group denoted as C(1)H₃ about the threefold symmetry axis. The second type was the reorientation of the trimethyl group-N(CH₃)₃ around the pseudo C₃^′ axis of C(6)-N(7) bond, which accompanied the standard C₃ motion of the methyl group. The Dunn-McDowell model was applied to analyze the dynamics observed.     

Reduced Coulomb repulsion in NMP-TCNQ through intra- and interchain polarization

The “bare” Coulomb repulsion between two delocalized electrons in a one-dimensional poly (TCNQ^?) chain of the organic charge transfer crystal NMP-TCNQ is shown to be substantially reduced by attractive terms arising (1) from the polarization of the localized electronic system of the TCNQ chain itself and (2) from the polarization of the delocalized electrons in neighbouring NMP chains. The resulting effective repulsion is 0.33 eV at the Fermi level.