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キーワード:
internal rotation; microwave spectroscopy; chirped-pulse spectroscopy; quadrupole coupling
要旨:
The rotational spectra of three p-halotoluenes (chloro-, bromo- and iodo-) are reported in the frequency range 2–8.5 GHz, obtained with broadband Fourier-transform microwave spectroscopy. The recorded spectra are highly complicated due to low-barrier V6 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 V6 internal rotation barrier of the methyl group was found to be 145 GHz 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 π-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.
