35Cl Quadrupole resonance in 1,3,5-trichlorobenzene at high pressure

Abstract

High pressure isotherms of the three ³⁵Cl quadrupole resonance frequencies in 1,3,5-trichlorobenzene have been obtained up to 1600 MPa. A careful analysis of the pressure data with an assumed equation of state leads to the conclusion that the pressure dependence of the ³⁵Cl frequencies and their differences cannot be explained wholly in terms of a first-order electric field polarization of the C—Cl bond, nor can the shifts be correlated with the known Stark coefficients. It is proposed that in these planar aromatic systems, the frequency differences and the negative pressure coefficient of the frequency observed at high pressures originate mainly in a polarization of the chlroine 3p_π electrons due largely to anisotropy in the intermolecular potential in the crystal. On the basis of a simple distortion model in which the 3p_π orbitals are mixed with 3d_π orbitals of the appropriate symmetry, it is shown that polarization of the chlorine 3p_y orbital in the molecular plane is largely responsible for the observed crystal effects in 1,3,5-trichlorobenzene.