Abstract
A procedure is presented that allows us to simulate from first principles the normalized spectra of nuclear inelastic scattering (NIS) of synchrotron radiation by molecular crystals containing a M\ossbauer isotope. Neglecting intermolecular vibrations the NIS spectrum is derived from the normal modes of the free molecule, that are calculated with the density-functional method B3LYP. At low temperatures the inelastic part of the calculated NIS spectrum is a superposition of peaks that correspond to the individual vibrational modes of the molecule. The area of each peak is proportional to that part of the mean-square displacement of the M\ossbauer isotope that is due to the corresponding vibrational mode. Angular-dependent NIS spectra have been recorded for a guanidinium nitroprusside single crystal and temperature-dependent NIS spectra for the spin-crossover system $[\mathrm{Fe}(\mathrm{tpa})(\mathrm{NCS}{)}_{2}]$ [tpa=tris(2-pyridylmethyl)amine]. Qualitative agreement is achieved between measured and simulated spectra for different crystal orientations of guanidinium nitroprusside. A remarkable increase of the iron-ligand bond stretching upon spin crossover has unambiguously been identified by comparing the measured NIS spectra of $[\mathrm{Fe}(\mathrm{tpa})(\mathrm{NCS}{)}_{2}]$ with the theoretical simulations.
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