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

Abstract

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).

Computational approach to the study of thermal spin crossover phenomena

“…Inclusion of these modes accounts for ∼70%-75% of the vibrational part of ?S HL and the remaining contribution was ascribed to intermolecular or lattice vibrations. Similar results and conclusions were found by Brehm et al 40 by combining IR and Raman spectroscopy with Density Functional Theory (DFT) calculations. Recent studies combining various vibrational spectroscopic techniques and DFT calculations on the Fe(phen) 2 (NCS) 2 complex 41,42 allowed the identification of the vibrational modes that contribute most to the entropy variation under spin crossover.…”

Targeting Intermediates of [FeFe]-Hydrogenase by CO and CN Vibrational Signatures

“…It is well known that this combination of a high quality basis set and the BP86 density functional produces accurate molecular structures (see, e.g., Refs. [34,[44][45][46]) as well as harmonic vibrational frequencies (see, e.g., Refs. [34][35][36]).…”

“…[34,[44][45][46]) as well as harmonic vibrational frequencies (see, e.g., Refs. [34][35][36]). The latter is due to a fortunate error compensation [35] that allows us to directly compare harmonic BP86 frequencies with experimental fundamental ones.…”

“…[23,31], and hence the uncertainty of the prediction might be too large. Therefore, we attempt to re-calibrate calculated vibrational frequencies employing a different density functional, namely BP86 [32,33], and a larger triple-zeta basis set, which are known to produce quite accurate vibrational frequencies [34][35][36][37]. Still, it needs to be shown whether the scatter of the calculated frequencies around their experimental references and, in particular, the scatter of the scaled frequencies can be reduced.…”

MOVIPAC: Vibrational spectroscopy with a robust meta‐program for massively parallel standard and inverse calculations

“…[15] Often one observes a very good agreement of calculated harmonic frequencies and measured fundamental ones (see, e.g., Refs. [16][17][18][19]), which is due to a fortunate error compensation of some density-functional approximations and sufficiently large basis sets. [14] If this is not the case, scaling factors may be used; note that the so-called scaled quantum mechanical force fields are one particular ' 'flavor' ' of these techniques.…”