The photochemical behaviour of full-aromatic β-carbolines (nor-harmane, harmane and harmine) in chloroform medium and the corresponding UV absorption and fluorescence emission spectra are discussed. Irreversible electron transfer from the singlet excited β-carboline molecule to the chloroform molecule in the transient excited charge transfer complex has been proposed as the primary photochemical process initiating the mechanism of secondary reactions in this system. The results obtained are interpreted in terms of diffusional quenching mechanism and are compared with those obtained using different halomethanes.

Photochemical reaction of full-aromatic β-carbolines in halomethanes 2. CHCl3: Electronic spectra and kinetics

Nuclear Quadrupole resonance of 14N in potassium tetracy anoquinodimethanide

A quantitative investigation of the crystal field effect is quite difficult. From the large number of NQR measurements available, the following qualitative conclusions can be drawn.

a)

The crystal field effect is of the order of δv (35Cl) = ±500 kHz in Cl atoms bound to carbon.




b)

With increasing ionic character of a bond Cl-X, the crystal field effects become more pronounced, because the multipole moments of the molecules in the lattice and the quadrupole polarizability of the core electrons of the 35Cl nucleus increase. This second point is equivalent to an increasing antishielding factor γ∞ for Cl.




c)

Quantitative calculations of the charge transfer in molecular compounds or of the ionic character induced within a molecule through the formation of the complex are only possible if the crystal structure is known and single-crystal NQR data (including η) are available. From NQR powder data, only qualitative conclusions can be drawn.




d)

The temperature dependence of NQR frequencies should be investigated carefully to correct for the crystal field effect as accurately as possible.




e)

For other nuclei besides 35Cl, and 79,81Br, such as 127I, 14N,..., more experimental evidence is needed to clarify the connection between NQR and the specific interactions between molecules in the solid state and in molecular addition compounds.

Crystal field effects in nuclear quadrupole resonance

Gd(III) complexes as paramagnetic tags: Evaluation of the spin delocalization over the nuclei of the ligand.

In the present days of high resolution NMR spectroscopy, broad spectral lines are often considered as a mere manifestation of life’s everyday embarassments. One may, however, also take up the converse opinion and consider linewidths as a new piece of information about a property of the system studied, the nuclear relaxation times, which together with chemical shifts and coupling constants† make up the main information content of a high resolution NMR spectrum. Nuclear relaxation times are characteristic time constants representing the rate of change of the longitudinal (longitudinal or spin-lattice relaxation time T 1) or of the transverse (transverse or spin-spin relaxation time T 2) components of the macrosopic nuclear magnetization. They are determined by the interactions existing within the spin-system and between the spins and the lattice.

Nitrogen-14 Nuclear Quadrupole Effects

We have studied nitrogen and chlorine pure quadrupole resonance spectra in some charge transfer complexes with a chloroform molecule as acceptor, the donors being some amino and heterocyclic compounds. A discussion of the present results and consideration of previous data confirm the charge transfer in these complexes. From the observed frequency shifts with respect to the uncomplexed compounds, the charge transfer can be estimated to a few per cent of an electron charge. Nous avons etudie la resonance quadripolaire pure de l'azote et du chlore dans quelques complexes a transfert de charge. L'accepteur y est le chloroforme, tandis que les donneurs y sont des amines ou des heterocycles. La discussion des resultats obtenus, compte tenu de donnees anterieures, semble confirmer le transfert de charge dans ces complexes. L'estimation du transfert, a partir des deplacements de frequence mesures, fournit une valeur egale a quelques centiemes de la charge electronique.

L. Guibé

Papers

Observation de la résonance quadripolaire pure de l'azote et du chlore dans quelques complexes à transfert de charge