Measurement and simulation of nuclear inelastic-scattering spectra of molecular crystals

A procedure is presented that allows us to simulate from first principles t he normalized spectra of nuclear inelastic scattering (MS) of synchrotron r adiation by molecular crystals containing a Mossbauer isotope. Neglecting i ntermolecular vibrations the NIS spectrum is derived from the normal modes of the free molecule, that are calculated with the density-functional metho d B3LYP. At low temperatures the inelastic part of the calculated MS spectr um is a superposition of peaks that correspond to the individual vibrationa l modes of the molecule. The area of each peak is proportional to that part of the mean-square displacement of the Mossbauer isotope that is due to th e corresponding vibrational mode. Angular-dependent NIS spectra have-been r ecorded for a guanidinium nitroprusside single crystal and temperature-depe ndent NIS spectra for the spin-crossover system [Fe(tpa)(NCS)(2)] [tpa=tris (2-pyridylmethyl)amine]. Qualitative agreement is achieved between measured and simulated spectra for different crystal orientations of guanidinium ni troprusside. A remarkable increase of the iron-ligand bond stretching upon spin crossover has unambiguously been identified by comparing the measured MS spectra of[Fe(tpa)(NCS)(2)] with the theoretical simulations. [S0163-182 9(99)03102-1].