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The vibrational modes of the low-spin and high-spin isomers of the spin crossover complex [Fe(phen)(2)(NCS)(2)] (phen = 1,10-phenanthroline) have been measured by IR and Raman spectroscopy and by nuclear inelastic scattering. The vibrational frequencies and normal modes and the IR and Raman intensities have been calculated by density functional methods. The vibrational entropy difference between the two isomers, DeltaS(vib), which is--together with the electronic entropy difference DeltaS(el)--the driving force for the spin-transition, has been determined from the measured and from the calculated frequencies. The calculated difference (DeltaS(vib) = 57-70 J mol(-1) K(-1), depending on the method) is in qualitative agreement with experimental values (20-36 J mol(-1) K(-1)). Only the low energy vibrational modes (20% of the 147 modes of the free molecule) contribute to the entropy difference and about three quarters of the vibrational entropy difference are due to the 15 modes of the central FeN(6) octahedron.

Density functional theory (DFT) provides a theoretical framework for efficient and fairly accurate calculations of the electronic structure of molecules and crystals. The main features of density functional theory are described and DFT methods are compared with wavefunction-based methods like the Hartree-Fock approach. Some recent applications of DFT to spin crossover complexes are reviewed, e.g., the calculation of M\"ossbauer parameters, of vibrational modes and of differences of entropy, vibrational energy, and total electronic energy between high-spin and low-spin isomers

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.

Spin‐crossover (SCO) complexes are an ongoing challenge to quantum chemistry due to the delicate balance of their electronic and entropic contributions to the adiabatic enthalpy difference between the high‐ and low‐spin states. This challenge has fuelled an improvement in the existing quantum chemical methods and the development of new ones and will continue to do so. The progress in electronic structure calculations performed on SCO complexes in recent years has made quantum chemical methods valuable tools that may aid the design of new SCO compounds with desirable features. Post‐Hartree–Fock ab initio methods can be used to calculate the adiabatic energy difference between high‐ and low‐spin states with satisfactory accuracy but are currently limited to model systems or smaller molecular SCO complexes. The results obtained by these methods serve as references for other electronic structure calculations that may also be applied to larger systems. The methods of choice for the calculation of geometries and molecular vibrations of isolated SCO complexes and of crystalline compounds are based on density functional theory (DFT). Recent hybrid functionals can be used to calculate the adiabatic energy difference to an accuracy that is in some cases close to that of ab initio calculations, although no unique functional has been identified up to now that is superior to other functionals in all cases. DFT methods can now also be applied to crystalline systems and allow intermolecular effects to be investigated that are important for understanding the cooperativity of spin transitions.