On entropy driven spin crossover in pristine and fluorinated Fe(phenanthroline)2(NCS)2: Insight from DFT

An ab initio total energy calculation of the Fe(phenanthroline)2(NCS)2 (Fephen) molecule adsorbed on a copper surface revealed that substituting hydrogen with fluorine in the spin-crossover Fephen molecule drastically alters its electronic structure, leading to an inversion of its magnetic state. To understand the temperature-dependent physics of spin crossover, various vibrational modes were computed within the quasi-harmonic approximation using a finite-difference method. The analysis showed that the primary frequency differences between the high-spin (HS) and low-spin states are concentrated in the N-C-S and Fe-ligand stretching modes for both the pristine and fluorinated molecules. This observation aligns well with expectations, as these specific bonds are directly involved and most impacted by the spin-crossover phenomenon. Free energy calculations indicated that the pristine Fephen molecule undergoes a spin crossover at 167 K, which closely matches the experimental value of 175 K. In contrast, the fluorinated Fephen molecule does not exhibit any spin crossover, with its HS state remaining the most stable across all temperatures. These calculated frequencies were also instrumental in determining the evolution of the HS fraction with respect to temperature in the pristine molecule, utilizing the Slichter-Drickamer model. The results from this model showed good agreement with experimental observations, further validating this approach.