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A procedure is presented that allows us to simulate from first principles the normalized spectra of nuclear inelastic scattering ͑NIS͒ 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 ͓Fe(tpa)(NCS) 2 ͔ ͓tpaϭtris͑2-pyridylmethyl͒amine͔. 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.

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Nuclear inelastic scattering (NIS) measurements were performed on a guanidium nitroprusside ((CN(3)H(6))(2)[Fe(CN)(5)NO], GNP) monocrystal at 77 K after the sample was illuminated with blue light (450 nm) at 50 K to populate the two metastable states, MS(1) and MS(2), of the nitroprusside anion. A second measurement was performed at 77 K after warming up the illuminated crystal to 250 K where the metastable states decay to the groundstate. The measured spectra were compared with simulated NIS spectra that were calculated by using density functional methods. Comparison of measured and simulated spectra provides strong evidence for the isonitrosyl structure of the metastable MS(1) state proposed by Carducci et al. (Carducci, M. D.; Pressprich, M. R.; Coppens, P. J. Am. Chem. Soc. 1997, 119, 2669-2678).

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Sn I Mössbauer effect / Electron microscopy / a 2 -Sn / ß 2 SaAbstract. a 2 -Sn, ß 2 -Sn and the solid-state transition of a 2 -Sn to ß 2 -Sn observed in tin grown by reduction of aqueous solutions of SnCl 2 with Mg are investigated by transmission electron microscopy (TEM) with selected area electron diffraction, Mössbauer spectroscopy and X-ray diffraction. Some data on the structure of two tin dioxides grown as additional products during the reduction of SnCl 2 with Mg and Al are included. The low sensitivity of the X-ray diffraction studies towards the a 2 -Sn and its derivates, and towards the unusual Sn0 2 structures is also discussed.

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