The rα structure of C6F5Br determined by 13C NMR in liquid crystal. Distortions of the C6F6 ring induced by single substitution

The 19F with C13-satellites and the 13C NMR spectra of bromopentafluorobenzene C6F5Br dissolved in the nematic phase ZLI 1695 were analyzed and dipolar couplings were obtained. It was shown that extracting carbon-13–fluorine dipolar couplings from the 13C spectrum is more advantageous than from the 19F with C13-satellites one. Excluding the fluorine–fluorine dipolar couplings from the fitting because of their high pseudo-dipolar contributions and neglecting the anisotropy of the carbon-13–fluorine ones seems to be an acceptable approximation which makes it possible to reach the rα structure of bromopentafluorobenzene. Combining obtained results with those we have previously measured for molecules C6F5X (X: H, Cl, I), distortions of hexafluorobenzene ring induced by single substitution were investigated. Experimental NMR structures being modestly accurate, computations using the quantum chemical methods were needed. Investigations have been conducted at the DFT(M062X)/aug-cc-pVTZ-PP level of theory and in the frame of SCRF/PCM(SMD) solvation model. The largest deformations relatively to the reference value 120° were observed in the immediate vicinity of the substituted carbon and consist of a reduction in ring angle at the ipso position and an increase in the adjacent angles. Structural variations are discussed in relation to the electron-donor/electron-acceptor character of the substituent. Substituent effects on the benzene and hexafluorobenzene molecules are compared and are found to be opposite.