A. Péneau

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.

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.

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.

Pyrrole: an Example of an Incommensurate Phase in a Molecular Crystal Studied by 14N NQR

Nitrogen-14 NQR experimental data previously obtained in molecular polycrystalline pyrrole are reanalysed in terms of an incommensurate structure.

Thiourea and Thiourea Inclusion Compounds. A14N NQR Investigation

A reinvestigation of the nitrogen-14 NQR in thiourea has permitted to record, apparently for the first time, the resonances above 169 K. The first order and second order of the transitions at 169 K and 202 K, respectively, appear clearly from the temperature dependence of the resonance frequencies; the modulated structure, incommensurate or commensurate, is also seen on the 14N NQR spectrum. This study is the first step of a new investigation of thiourea and its inclusion compounds.

Chlorine Substituted Acetic Acids and Salts. Effect of Salification on Chlorine-35 NQR

Abstract The NQR of a quadrupolar probe nucleus is often used to investigate the effect of substituent in molecules. The inductive effect, based on a partial charge migration along the molecular skeleton is the only one present in saturated aliphatics, the conjugative effect appearing in conjugated molecules, especially aromatics. As the stepwise charge migration mechanism, formerly used to explain the inductive effect, is now believed obsolete, we have wanted to reexamined the case of chlorine substituted acetic acids and salts. The data in literature was extended by observing reso-nances and determining NQR frequencies in several acids and salts. The present analysis of the salification of mono-, di-and tri-chloroacetic acids, which is equivalent to a deprotonation or the substitution of the acid hydrogen by a negative unit charge, shows that a model based on the polarization of the chlorine atom(s) by the carboxyle group is consistent with experimental results: the polarization energy appears to be proportional to the NQR frequency shifts; experimental data show a correlation between the NQR frequency shifts accompanying salification and the variations of the intrinsic acidity measured in the gas phase; this, in turn shows that there is a proportionality between the polarization energy and the variations in the acid free enthalpy of dissociation. From the comparison between fluorine, chlorine, bromine and iodine, it also appears that an alternative mechanism, the polarization of the carboxyl group by the halogen, would be important only in the case of the fluoroacetic acid.