Estimation of the Vibrational Contribution to the Entropy Change Associated with the Low- to High-Spin Transition in Fe(phen)2(NCS)2 Complexes:  Results Obtained by IR and Raman Spectroscopy and DFT Calculations

IR and Raman (λex = 785 and 1064 nm) spectra of Fe(phen)2(NCS)2 were recorded at T = 298 and 100 K, and the observed vibrations were assigned by comparison with the results obtained by DFT/BP86 calculations. The latter resulted, in accordance with crystal data, in an equilibrium geometry with C 2 symmetry for the low-spin state. In the high-spin state, two closely lying extrema were found on the BP86 energy hypersurface: a saddle point (C 2 symmetry, one imaginary vibrational frequency) and, ca. 9.6 kJ/mol lower in energy, a minimum with C 1 symmetry. The differences in the geometrical parameters of the low-spin and high-spin states are in good agreement with the changes observed experimentally by X-ray crystallography. The calculated wavenumbers of the (NCS) vibration differed from the experimentally determined ones by more than 50 cm-1. Since it could be shown that anharmonicity is not the only cause for this discrepancy, two pyridins at optimized distances were included to model the interaction in the crystalline state. We find a correct wavenumber shift of this solid-state model versus the isolated molecule, corroborating the prominent role of intermolecular interactions, which are considered to be responsible for the sharp transition from the low-spin to the high-spin state. The partition function was calculated for both spin states by considering all calculated vibrational wavenumbers. The vibration-related entropy change connected with the low- to high-spin transition is determined via well-known thermodynamic relations. For the title compound, we found ΔS vib ≈ 19.5 J/(mol K), or approximately 40% of the experimentally determined total entropy change of 49 J/(mol K).