Communication through the phenyl ring: internal rotation and nuclear quadrupole splitting in p-halotoluenes

The rotational spectra of three p-halotoluenes (chloro-, bromo- and iodo-) are reported in the frequency range 2-8.5GHz, obtained with broadband Fourier-transform microwave spectroscopy. The recorded spectra are highly complicated due to low-barrier V-6 internal rotation of the methyl group as well as strong nuclear quadrupole coupling of the halogen atoms. However, these additional effects allow us, in a comparative manner, to study potential crosstalk of the two substituents via the phenyl ring. The rotational constants and other molecular parameters are reported and compared with quantum chemical calculations. The V-6 internal rotation barrier of the methyl group was found to be 145GHz for both p-chlorotoluene species. We found that the magnitudes of the quadrupole coupling constants are increased in the halotoluenes compared to the halobenzenes. This increase is due to the +I inductive effect of the methyl group that injects additional electron density into the phenyl -cloud, thus giving more electron density for the halogen atom to extract. This additional extraction makes the halogen-carbon bond more ionic than in the halobenzenes.