The Rotational Zeeman Effect in Fluorobenzene and the Molecular Quadrupole Moment in Benzene

The rotational Zeeman effect in fluorobenzene is reinvestigated with a resolution improved by a factor of almost five to give more accurate g-tensor elements, magnetic susceptibility anisotropics and molecular electric quadrupole moments. The results fit into the pictures of a linear dependence of the out of plane molecular electric quadrupole moment, Q , on the number of fluorine substituents and of a linear correlation between the nonlocal (ring current) susceptibility, Χ , and the CNDO/2-π-electron density alternation. They lead to a gasphase molecular electric quadrupole moment in benzene, Q = - (28.4 ± 4.7) · 10 Cm which is slightly less negative than the value deduced from electric field-gradient birefringence experiments on dilute benzene solutions with carbon tetrachloride as solvent. A detailed description of the high resolution microwave spectrometer is given in the appendix.