Precursor effects of the orthorhombic to monoclinic phase transition in benzocaine form (II) revealed by X-ray diffuse scattering

We described the development of a Monte Carlo computer model for the room-temperature form (II) polymorph of benzocaine that incorporates, on a local scale, structural features derived from the low-temperature form (III) polymorph. The introduction of this extra information convincingly reproduces those observed diffraction features that an earlier harmonic model was unable to achieve. In both form (I) and form (II) the hydrogen-bonded chains of molecules that extend along the respective a axes tend to slide backward and forward along their lengths. While in form (I) the motion is well modelled by a harmonic potential in form (II) there is a degree of anharmonicity that means that some intermolecular contact vectors, which are identical in the average structure, are distributed bimodally with either longer or shorter vectors being preferred to the average. Moreover there is a tendency for these deviations from average to be correlated to give short-range ordered domains that are the precursors of the two twinned variants of the long-range ordered low-temperature form (III) structure.