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

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 [Fe(tpa)(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 [Fe(tpa)(NCS)₂] with the theoretical simulations.