Atomic thermal motions studied by variable-temperature X-ray diffraction and related to non-linear optical properties of crystalline meta-dinitrobenzene.

The meta-dinitrobenzene crystal structure has been determined at five temperatures in the 100-300 K temperature range. The thermal expansion coefficients have been calculated from the temperature variation of the lattice parameters. Rigid-body motion analysis with allowance for large-amplitude internal motions provided the T, L and S tensors' values at the temperatures studied and was used to characterize the torsional motion of two nitro groups in the molecule. Frequencies of the translational and librational modes and of the torsional modes of the nitro groups have been compared with the wave numbers at the maximum of bands in the low-frequency Raman and IR spectra. Ab initio calculations were performed in order to assess the contribution from large-amplitude internal motions to the static first-order hyperpolarizability of the m-dinitrobenzene molecule.