Molecular Quadrupole-Moment and Effective-Quadrupole-Centre of Fluorobenzene

Measurements of the infinite-dilution molar field-gradient birefringence constant, Kerr constant and Cotton-Mouton constant of fluorobenzene as a solute in carbon tetrachloride at 25°C and at two visible-spectrum wavelengths (441.6 and 632.8 nm) are reported. Analysis of the observations, in conjunction with other known electric and magnetic properties of fluorobenzene, yields the out-of-plane component of the molecular quadrupole moment relative to the effective-quadrupole-centre, establishes that at both wavelengths the effective-quadrupole-centre is close to the centre-of-ring, and permits the evaluation of a combination of elements of the dipole-qudrupole polarizability, A, and the optical-activity tensor, G′. A complete, although approximate, specification of the centre-of-mass quadrupole moment can be obtained by invoking reasoning due to Shoemaker and Flygare; the result (Θ/10 C m, Θ/10 C m, Θ/10 C m) now found (-1.5 ± 2.9, 22.5 ± 2.7, -21.0 ± 1.0) is at variance with that from the microwave Zeeman effect of this molecule (-6.3 ± 2.7, 17.0 ± 5.0, -10.7 ± 5.0)