Molecular motions in chloramphenicol studied by35Cl nuclear quadrupole and1H nuclear magnetic resonance spectroscopy

The³⁵Cl nuclear quadrupole resonance (NQR) frequency (v_Q), nuclear quadrupole spinlattice relaxation time (T_1Q),¹H nuclear magnetic resonance second moment (M₂) and nuclear magnetic spin-lattice relaxation timeT ₁) were measured for polycrystalline chloramphenicol (drug) as a function of temperature. Hindered rotation of the CHC1₂ group and the phenyl ring was detected, the relevant activation energies were determined. The rotations are discussed in detail.     

Studies of molecular dynamics of thiazides by<Superscript>35</Superscript>Cl NQR spectroscopy and DFT calculation

Molecular dynamics of three derivatives of 1,2,4-benzothiadiazine-1,1-dioxide, hydrochlorothiazide (HCTZ), althiazide (ATZ) and chlorothiazide (CTZ), was studied by³⁵Cl nuclear quadrupole resonance (NQR) spectroscopy. The temperature dependence of the resonance frequency was analyzed within the 6 known standard models. The activation energies estimated from the temperature dependence of the³⁵Cl NQR frequency assuming the Bayer model were 1.07, 2.35 and 2.76 kJ/mol for HCTZ, ATZ and CTZ respectively, which confirms that HCTZ is less rigid than CTZ and ATZ is much more rigid than HCTZ, and suggests that the mechanism of relaxation is based on small amplitude librations. The characteristic temperatures estimated from the Bayer model, with that for CTZ (332.5 K) being much higher than for HCTZ (132.1 K), mean that the intermolecular interactions in CTZ are much stronger than in HCTZ, as suggested by the melting point of CTZ being higher than that for HCTZ. For ATZ the characteristic temperature (288 K) takes an intermediate value, which suggests that the intermolecular interactions in this compound are stronger than in HCTZ and weaker than in CTZ. A significant narrowing of the resonance³⁵Cl NQR line observed for all these compounds at room temperature, relative to that at the liquid nitrogen temperature, suggests an averaging of dipolar interactions as a result of fast rotation of nonquadrupole nuclei in the vicinity of the quadrupole nuclei, when 2πν_Qτ_c ≫ 1 (a rotation of the −NH₂ group in the direct neighborhood of the chlorine nuclei) or a change in the gradient orientation with its value preserved (which is equivalent to rotation of the quadrupole nucleus Cl). The influence of the rotations of the −NH₂ and −CH₂SCH₂CH=CH₂ groups (ATZ) or −CHCl₂ group (TCTZ) on the³⁵Cl NQR frequency was modelled by the B3LYP/6-31G^* method. The frequencies of the libration vibrations calculated from the temperature dependence of the NQR resonance frequency were compared with experimental ones and those implied by the density functional theory, infrared and Raman spectra. For HCTZ the anomalies in the temperature dependence of the³⁵Cl NQR frequency, the lack of hysteresis and small but notable changes in the slope and the jump in the frequency observed at 253 K which does not exceed 0.05 MHz suggest a second-order phase transition at 253 K.     

Anomalous H/D isotope effect on 35Cl NQR frequencies in piperidinium p-chlorobenzoate

Anomalous isotope effects were detected in the ³⁵Cl nuclear quadrupole resonance (NQR) frequency of piperidinium p-chlrobenzoate (C₅H₁₀NH· ClC₆H₄COOH) by deuteration of hydrogen atoms. The atoms were determined to form two kinds of N–H···O type H-bonds in the crystal structure. Large frequency shifts of the ³⁵Cl resonance lines reaching 288 kHz at 77 K and 278 kHz at room temperature were caused upon deuteration, in spite of the fact that the Cl atoms in the molecule do not form hydrogen bonds in the crystal. Results of single crystal X-ray diffraction measurements and density-functional-theorem calculations suggest that a dihedral-angle change of 1.8° between benzene and the piperidine ring contributes to ³⁵Cl NQR anomalous frequency shifts.     

Temperature and pressure variation of the electric field gradient in arsenic metal

The variation of the NQR frequency ⁷⁵v_Q for arsenic metal has been measured at 195 and 295 K for pressures up to 2 kbar and at atmospheric pressure between 77 and 525 K. The data are compared with those for other metallic elements.     

NQR study of spin-freezing in superconducting La2−x Sr x CuO4: the example ofx = 0.06

Nuclear quadrupole resonance (NQR)¹³⁹La and⁶³Cu spin-lattice relaxation rateT ₁ ^t-1 measurements in a La_1.94Sr_0.06CuO₄ single crystal are described. Slowing-down of Cu²⁺ spin fluctuations is evidenced through a dramatic increase of¹³⁹ T ₁ ^?1 on cooling. While the onset of diamagnetism occurs atT _c = 8 K,¹³⁹ T ₁ ^?1 has a peak atT _g ? 5 K, when the characteristic frequency of magnetic fluctuations reaches the NQR frequencyv _Q ? 9 MHz. In agreement with a number of previous studies, these results show that the so-called “cluster spin-glass” phase persists in the superconducting regime. Issues concerning the coexistence of the two phases are discussed.     

Cl35 NQR study of the structural phase transition in (CH3NH3)HgCl3

The Cl³⁵ nuclear quadrupole resonance spectra of (CH₃NH₃)HgCl₃ have been measured between -150°C and + 100°C. The spectra clearly show that a structural phase transition of first order takes place around T_c? 60°C. The transition may be related to a disordering of the CH₃NH₃ groups which are reorienting both above and below T_c. The positive temperature coefficient of the Cl NQR frequency, dv/dT may be also explained by the CH₃NH₃ motion.     

Pressure dependence of phase transitions in K2SnCl6 from NQR frequency measurements

The influence of hydrostatic pressure 0 ? p ? 4 kbar on the ³⁵Cl NQR in K₂SnCl₆ was studied in the temperature range 238 K ? T ? 300 K. The phase transition temperatures T_C1 and T_C2 were determined from changes in the NQR line pattern.The phase boundaries in the p-T diagram are straight lines in the region studied. The pressure coefficients are given by dT_C1/dP = 1.35 (10) K kbar^?1 and dT_C2/dP=?1.25 (20) K kbar^?1.     

Magnetic resonance investigations of phase transitions and modulation wave motions in incommensurate solids

We measured⁸⁷Rb nuclear magnetic resonance (NMR) and³⁵Cl nuclear quadrupole resonance (NQR) Hahn spin-echo magnetization decays in the incommensurate (I) phase of Rb₂ZnCl₄ and, in each case, obtained a Hahn echo decay that was shorter than the Carr-Purcell-Meiboom-Gill decay and one which decayed with a time constant proportional to the cube of the echo time. From these measurements we obtained from both the⁸⁷Rb NMR and³⁵Cl NQR measurements values for the diffusion coefficients that are comparable in magnitude, a fact that strongly supports the existence of slow modulation wave diffusionlike motions in the I phase, since such motions should affect both Rb and Cl ions similarly. In addition, we used⁸⁷Rb two-dimensional exchange-difference NMR to study atomic motions in the incommensurate (I) and paraelectric (P) phases to elucidate the nature of the I-P transition. We measured as a function of the mixing time the frequency shifts of the cross peaks from the main diagonal and observed a gradual increase towards an asymptotic value in the I phase but a sudden jump to the final value in the P phase. We interpreted the motions observed in the P phase as normal modes arising from simultaneous reorientations of ZnCl₄ tetrahedra and corresponding Rb ions displacements between two sites. These normal modes freeze out in the I phase and change to the diffusionlike motion of the modulation wave. We also performed³⁵Cl NQR lineshape andT ₁ measurements in K₂ZnCl₄ and obtained conclusive evidence for the presence of a narrow 1q (singly modulated) I phase between 146 and 149 K.     

DTA and NQR studies of polymorphism in p-chlorofluorobenzene

Differential thermal analysis, and the ³⁵Cl Nuclear Quadrupole Resonance frequency (ν_Q) and spin-lattice relaxation time (T₁) were measured as a function of temperature in p-chlorofluorobenzene. Three different phases were found according to the thermal history of the sample and only one was stable up to the melting point. In one of the phases, the presence of molecular reorientations can be inferred from T₁ data with activation energy of 3.7 kcal/mol. A value of 13.9 kJ/mol for the fusion heat of this compound has been obtained.     

High pressure NQR study of some monohydrogen arsenates with positive temperature coefficients

Existing studies of the NQR frequency of ⁷⁵ As nuclei in mono- and dihydrogen arsenates under high hydrostatic pressure have provided highly relevant information concerning the lattice dynamics of these compounds. Recent measurements in such crystals prove that totally different molecular mechanisms are responsible fo the temperature and pressure dependence of the NQR frequency in arsenates with positive and negative ?v/?T.     

Electronic structure and lattice dynamics of domeykite Cu<Subscript>3</Subscript>As according to nuclear quadrupolar resonance of<Superscript>75</Superscript>As and<Superscript>63,65</Superscript>Cu

The temperature dependences of nuclear quadrupole resonance (NQR) frequencies, the line width and nuclear relaxation of⁷⁵As and^63,65Cu, as well as the electrical resistivity in domeykite Cu₃As are studied in the temperature range of 4.2-300 K. The comparison of the calculated with the measured lattice contribution to the NQR frequencies points at a substantial role played by the conduction electrons in creating the electric field gradient at the nuclei sites. The temperature dependence of the copper and arsenic nuclear spin-lattice relaxation linear at 4.2<T<200 K and that of the electric resistivity (30<T<200 K) prove the metallic character of the conductivity of domeykite. The enhancement of nuclear relaxation, the narrowing of copper and arsenic NQR line widths are considered as arising due to the ionic movement starting beyond 200 K. This movement influences the electric resistivity, most likely due to the inreasing density of states at the Fermi surface.     

Quadrupole interaction of117In in Zn metal

We have used the TDPAC method to measure the temperature dependence of the quadrupole interaction frequencyv _Q of¹¹⁷In in Zn lattice. The observedv _Q reduces from 187.4 (20) MHz at 77°K to 155.3 (20) MHz at 673°K. The variation ofv _Q is seen to be proportional toT ^3/2.     

Doppler-limited spectroscopy of the AΠ1 ← XΣ system of ICl in the 0.8 μm region using a Ti:sapphire ring laser

Doppler-limited vib-rotational absorption spectra of the A ← X electronic transition of I^35/37Cl are measured in the range 11,352-13,507 cm⁻¹ using a Ti:sapphire ring laser. The P-, Q-, and R-branch lines belonging to the v ← v″ = (0-7) ← (0-7) transition in I³⁵Cl and the v ← v″ = (0-6) ← (2-6) transition in I³⁷Cl are assigned. Under Doppler-limited conditions, the P- and R-branch lines are split into doublets by the nuclear quadrupole coupling effect of the I atom. The unperturbed positions of these lines are correctly calculated, whereas splitting in the Q-branch lines was not observed. The mass-reduced Dunham expansion coefficients U_l,m of the A and X states and the spectroscopic constants , and H_v^′ of the A state are determined using a global least-squares fitting procedure.     

Charge symmetrical pairing correlations for Even-Even nuclei withN=Z

The temperature dependence of the critical heat currentQ _c in He II has been measured in the temperature region 3 · 10^?5 (?K)<T _λ?T<1.2 · 10^?2 (?K). The result Q_c∫ (T _λ?T)^1.07±0.01is consistent with a divergent mutual friction nearT _λ proposed recently byAhlers.     

Nuclear magnetic relaxation by self-diffusion in solid lithium:T 1-frequency dependence

We report on measurements of the⁷Li nuclear spin relaxation timeT ₁ in solid lithium as a function of temperature (?170°C≦T≦+180°C) and Larmor frequency (450kHz≦v _Li≦31.5 MHz). Using a relaxation model developed by Wolf and Cavelius and combining it with Seeger's diffusion formalism, the diffusion parameters for mono-and divacancy migration were evaluated by a least squares fit to the newly obtainedT ₁ data as well as to previousT _1? measurements. The result for the self-diffusion coefficientD ^SD is given byD ^SD=D ₁₀·exp(?Q ₁/RT)·[1+D ₂₁·exp(?Q ₂₁/RT)], withD ₁₀=0.038 cm²s^?1,Q ₁=12.0 kcal mol^?1,D ₂₁=250,Q ₂₁=4 kcal mol^?1 andR=1.98₅·10^?3 kcal mol^?1 degree^?1. Due to the flexibility of Seeger's formula, which contrasts with the standard Arrhenius interpretation of diffusion, discrepancies between earlier high- and low-frequency NMR investigations were eliminated. Furthermore, an excellent agreement with available results from tracer experiments was achieved by taking into account the theoretical predictions of the isotope effect and the vacancy correlation factor.     

Nuclear magnetic resonance of175Hf oriented in iron

Nuclear magnetic resonance of¹⁷⁵Hf oriented at low temperature in iron has been observed with a sample prepared by ion implantation. The centre frequency of the broad resonance line isv _L (B _ext = 0)=138.53(36)MHz. Possible origins of the large inhomogeneous line width of FWHM=11.0(1.1) MHz are discussed. A comparison with model calculations for combined magnetic and electric hyperfine interaction indicates that the centre frequency may be interpreted as the magnetic interaction frequency for¹⁷⁵Hf in unperturbed substitutional sites of the host iron. With theg-factor of¹⁷⁵Hf from literature the magnetic hyperfine field of Hf in Fe is derived asB _hf=?64.9(9.3) T fitting well into systematics.     

Vacancy-induced heat transfer in solid Kr and Ar

We have measured constant-volume thermal conductivities of solid krypton at six molar volumes between 28.84 and 29.54 cm³ mole^?1 and of solid argon at a molar volume of 24.30 cm³ mole^?1 at temperatures of order and above the respective Debye temperatures. An excess thermal conductivity above that following from the T^? law is ascribed to an additional energy flux carried over by vacancies. The estimated activation energy Q_P for self-diffusion derived from the constant-volume values Q_V as determined from the temperature dependences of the “excess” thermal conductivities are in good agreement with the available data of direct diffusion measurements in solid Ar and Kr.     

Relaxation effects in two-dimensional zero-field nutation NQR spectroscopy

The relaxation and off-resonance effects on the NQR (nuclear quadrupole resonance) nutation frequency and on the damping constant of the nutation spectrum for spinsI = 3/2 are considered. Using the solution of dynamic equations for nuclear magnetisation in the rotating frame, we obtained a general expression for determination of the asymmetry parameter η, which includes the relaxation time and frequency offset. Specific requirements for the RF (radio frequency) field allowing the presence of nutation and the sufficient resolution of nutation spectra are determined. It is shown that at low RF field it is necessary to take into account theT ₂ relaxation effect on the location of frequency singularities in the nutation spectrum. The values ofH ₁ andT ₂ must be exactly known in nutation NQR spectroscopy. The 2D NQR spin-echo separation technique was used for determination of transverse relaxation of individual spin packets in case of inhomogeneously broadened NQR lines, so that the asymmetry parameter could then be determined by the nutation experiment with consideration of relaxation effects and frequency offset. The theoretical results are confirmed experimentally for the narrow and inhomogeneously broadened 2D-nutation NQR spectra in several molecular compounds containing the³⁵Cl and⁷⁵As nuclei.     

35Cl NQR and XRD Studies on Cs2[Ag(I) x Au(I)1 − x Cl2][Au(III)Cl4]

³⁵Cl NQR and XRD have been investigated on the solid solution Cs₂[Ag(I) _x Au(I)_1???x Cl₂][Au(III)Cl₄] which yielded NQR lines by annealing. The crystal forms a tetragonal cell in the whole range of x. However, a change of the modification occurs at x?≈?0.85. The increase of c and decrease of a were observed in the range of 0.0?≤?x?≤?0.7.     

Metastable chaotic state in K2ZnCl4

The ³⁵Cl NQR frequencies of K₂ZnCl₄ have been measured close to the incommensurate-commensurate transition. A calculation of the temperature dependence of the soliton density n_s from the NQR data showed that in K₂ZnCl₄ close to T_c exists a metastable chaotic state.