Correlations between 35Cl Nuclear Quadrupole Resonance (NQR) Frequencies of Chlorobenzene Derivatives and Characteristic Reaction Parameters of the Substituents

Abstract

The 35Cl NQR frequencies (at 77°K) of a number of chlorobenzene derivatives have been measured. The frequencies (including additional data from the literature) have been correlated with characteristic parameters, such as Taft's σI and σR. It is found that the Bray–Barnes relation can be somewhat improved by introducing a parameter κ: ν = ν0 + ∑ iκı. The following relations are found: (a) the effects of more than one substituent on the benzene are additive; (b) steric effects (o-substituents) are not important for the NQR frequencies; (c) a correlation between the NQR frequency ν and Taft's σI and σR (correlation coefficient r = 0.976) shows the importance of the inductive effect on NQR. The mesomeric effect plays only a minor role in 35Cl NQR in benzene derivatives. Unknown σI values may be gained from NQR measurements; for the evaluation of σR, the determination of the chemical shift of 19F NMR seems to be the better method.