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

The structures of α-antimony tribromide and its molecular complex with benzene 2SbBr3·C6H6 were investigated by studying the Zeeman effect on the nuclear quadrupole resonance (NQR) due to 81Br at room temperature. The bond angles, ∠Br-Sb-Br, of α-antimony tribromide in its solid state were found to be 94°14±4′, 94°31′±8′, and 98°27′±7′. The polar angles specifing the directions of the Sb-Br bonds with respect to the orthorhombic crystal axes are (60°0′±9′, 62°13′±7′), (59°10′±7′, −54°35′±8′), and (57°39′±17′, 180°0′±3′). These values are consistent with those of the X-ray analysis by Gushen et al. within about 1°. The asymmetry parameters at the three bromine atoms were 0.068±0.002, 0.049±0.002, and 0.080±0.004. On the other hand, the bond angles, ∠Br-Sb-Br, of the SbBr3 molecule in the complex 2SbBr3·C6H6 were found to be 92°24′±11′, 97°7′±14′, and 97°40′±6′. The crystal has a monoclinic symmetry, and the three Sb-Br bonds have directions making angles of 29°4′±21′, 76°26′±9′ and 63°33′±11′ relative to t...

Structural Studies of Antimony Tribromide and Its Molecular Complex with Benzene by Means of the 81Br Nuclear Quadrupole Resonance

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.

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

Determination and comparison of association constants for weak organic complexes by thermodynamic, resonance, and optical methods

35Cl nuclear quadrupole resonance frequencies were measured in electron donor-acceptor complexes between chloranil (tetrachloro-p-benzoquinone) and such donors as aromatic hydrocarbons, polyaromatics and amines. Some correlations between the resonance frequencies and infrared spectra were found and discussed in a qualitative way in terms of the π-electron transfer between donors and the chloranil molecule as acceptor. A Zeeman effect experiment was carried out on a single crystal of the 1:2 chloranil-8-hydroxyquinoline complex. The direction of one of the two non-equivalent C-Cl bonds obtained slightly differs from that determined by an X-ray diffraction study. Possible effects of the charge transfer on the resonance frequency and the asymmetry parameter are discussed according to the Mulliken theory of ’weak’ charge-transfer complex. No simple relations could be found between the experimental resonance frequencies and the ionization potential of the donor. This suggests that crystal field effects are rat...

Nuclear Quadrupole Resonance Study of Charge Transfer Complexes: Chloranil Complexes

The 81Br nuclear quadrupole resonance spectra have been examined for a series of complexes of CBr4 with methyl‐substituted benzenes and pyridines. The p‐xylene−CBr4 complex gives two lines as expected from the known structure, and the benzene–CBr4 spectrum is the same, with a lower splitting. The spectra of a number of other complexes of unknown structure do not correlate simply with these. Estimates of the direct crystal‐field gradients due to the charge distribution in the aromatics suggest that the direct field gradient effects are much too low to account for observed shifts in quadrupole frequencies. The temperature dependencies of the resonances in the p‐xylene and durene complexes have been studied. The p‐xylene complex follows the Bayer equation, but the durene complex requires the inclusion of a temperature dependence of the torsional frequency. Both complexes show a fading of the resonances at about 50° below the melting points.

C. T. O'Konski

Papers

Nuclear Quadrupole Resonance Studies of Charge‐Transfer Complexes of Carbon Tetrabromide with Methyl‐Substituted Benzenes and Pyridines