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 Mössbauer 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 Mössbauer 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 $\left[\mathrm{Fe}\right(\mathrm{tpa}\left)\right(\mathrm{NCS}{)}_2]$ [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 $\left[\mathrm{Fe}\right(\mathrm{tpa}\left)\right(\mathrm{NCS}{)}_2]$ with the theoretical simulations.

• Received 24 July 1998

DOI:https://doi.org/10.1103/PhysRevB.59.975