L. Guibé

14N Nuclear Quadrupole Resonance in Aminopyridines

The nitrogen‐14 nuclear quadrupole resonance (NQR) spectra of the three isomeric aminopyrides and the five isomeric 2‐aminomethylpyrides have been observed. The data is interpreted in terms of the electronic populations of the ring‐nitrogen pπ and σ orbitals. The results show that the amino group is a strong π‐electron donor and, in addition, behaves as a weak σ‐electron donor. A model for the amino group which is consistent with the NQR data is presented.

Phase changes and molecular motion in the thiourea‐cyclohexane inclusion compound

Several experimental results and discussions have previously been reported concerning the thiourea‐cyclohexane inclusion compound (TCIC). They were obtained using nitrogen‐14 NQR, proton wide‐line NMR, relaxation time measurements, differential thermal analysis, and x ray diffraction methods. In the present paper new results of wide‐line proton NMR on partially deuterated samples (thiourea‐d4‐cyclohexane) and a detailed interpretation of x‐ray diffraction patterns are presented; the discussion leads to a better description of the behavior of the host lattice and guest molecules as the temperature of the sample is increased from −196°C to room temperature. The proposed model is also compared to that obtained for urea‐trioxane, a similar inclusion compound described by the authors in an other paper.

Temperature dependence of the 14N quadrupole coupling constants in pyrazine

The frequency of the 14N nuclear quadrupole resonances ν+ and ν− in pyrazine has been measured at several temperatures between 4.2 and 324 K. Below 300 K, only one ν+ line and one ν− line are seen, indicating the existence of a unique sort of crystalline site for nitrogen atoms in the crystal of pyrazine. At 27° and 35°C crystalline phase transitions are observed through a splitting of the lines and a discontinuity of the frequency of resonance. An interpretation of the temperature dependence of the resonance frequencies, ν+ and ν−, between 4.2 and 300 K is proposed using Bayers theory and the molecular motion frequencies reported by Ito and Shigeoka; however, the assignment of these frequencies to specific rotations about the different axes of symmetry of the molecule, which fits nuclear quadrupole resonance data, is different from that given by Ito and Shigeoka.

Molecular motions in the thiourea-d4-ferrocene inclusion compound

Abstract The proton magnetic resonance spectrum of the thiourea- d 4 -ferrocene inclusion compound has been studied as a function of temperature. The five-fold axes of the ferrocene molecules are frozen in a number of non-equivalent orientations at low temperature. The onset of the reorientational motion of these axes is associated with a phase transition at 162 K.

Study of urea and thiourea inclusion compounds by NQR and NMR

Abstract Nitrogen-14 NQR was observed in several urea and thiourea inclusion compounds; the study of the temperature dependence of the NQR spectrum in thiourea-cyclohexane revealed several changes which are discussed in view of the results of DTA, X-ray diffraction, and NMR measurements. A similar study of urea-trioxane inclusion compound is also reported.

14N Nuclear quadrupole resonance in tetrasulfur tetranitride

The 14N nuclear pure quadrupole resonance has been observed in solid S4N4. The spectrum consists of 12 lines assigned to the resonance of nitrogen atoms located at four inequivalent crystallographic sites. The measured quadrupole coupling constants are discussed in terms of the electronic populations of the valence orbitals on the nitrogen atoms. As compared, on a simple sp2 hybridization model, to standard pyridinic compounds, π type orbitals appear to be much more populated in S4N4. It is also shown that the temperature dependence of the ν+ line frequencies is caused by both lattice and intramolecular motions; the deduced frequencies are consistent with the results of ir and Raman spectra.

Temperature dependence of relaxation times and nuclear quadrupole resonance in p-chloroaniline

Abstract The temperature dependence of nitrogen-14 and proton relaxation times is reported together with the temperature dependence of nitrogen-14 quadrupole coupling constants. Discussion shows that relaxation of nitrogen-14 is due to the motion of amino protons via magnetic dipolar coupling and the corresponding energy of activation is 9.6 kcal/mole. From Bayers theory, it appears that only motions of high frequency contribute to the temperature dependence of the nitrogen quadrupole coupling constants.

Temperature dependence of NQR frequencies in CuBr2 and MgBr2·2Et2O

The temperature dependence of NQR frequencies is studied in CuBr2 and MgBr2·Et2O. In CuBr2 the resonance disappears as the temperature is decreased between 77 and 105 K. Since this may be related to a paramagnetic-antiferromagnetic phase transition, the corresponding Neel temperature is much lower than the value 193 K reported in the literature. In MgBr2·Et2O a phase transition associated with a change in the conformation of the ether molecule is clearly seen in the NQR frequency vs temperature diagram.

Study of nitrogen‐14 pure nuclear quadrupole resonance in 1,3,4‐oxadiazole

The nitrogen‐14 nuclear quadrupole resonance has been investigated in 1,3,4‐oxadiazole. The observed quadrupole coupling constants are compared to the values from microwave spectroscopy (MW) and discussed in terms of valence orbital populations on the nitrogen atom using principal directions of the electric field gradient tensors as defined by the MW results. The temperature dependence of the resonance frequencies ν(T) show no phase transition between 4.2 and 246°K where the lines begin to disappear. Discussion of the ν(T) curves reveals the contribution of crystal thermal motions at about 85 cm−1 and of intramolecular motions at about 590 cm−1 which is compared with the value of 625 cm−1 obtained by ir spectroscopy. Deuteration of the sample brings no appreciable change of the resonance frequencies.

Study of the temperature dependence of chlorine‐35 nuclear quadrupole resonance in ClF5

The 35Cl nuclear quadrupole resonance frequency νq has been studied as a function of temperature between 4.2 and 135 °K. No break is seen on either the νq(T) curve or its derivative in spite of a phase change observed in the fluorine NMR at 117 °K. The quadrupole coupling constant observed is discussed in terms of the occupancy of the 3p orbitals on chlorine as determined by molecular orbital calculations and compared to the value found by microwave spectroscopy. The temperature dependence of νq(T) is interpreted in the light of Bayer’s theory and a value of the lowest libration frequency is obtained (νl?30 cm−1) which is also found consistent with the value of the relaxation time, T1, of 35Cl nuclei.