Vibrational study of the Fe(phen)2(NCS)2 spin-crossover complex by density-functional calculations

Area of Science:

• Inorganic Chemistry
• Computational Chemistry
• Materials Science

Background:

• Spin-crossover complexes exhibit distinct low- and high-spin states.
• Understanding these states is crucial for molecular switches and sensors.
• Iron(II) complexes are widely studied for their spin-crossover properties.

Purpose of the Study:

• To calculate the geometries and vibrational frequencies of an iron(II) spin-crossover complex, Fe(phen)2(NCS)2, in its low- and high-spin states.
• To define internal coordinates and obtain a valence force field for the complex.
• To analyze the electronic charge distribution changes and vibrational modes.

Main Methods:

• Density Functional Theory (DFT) calculations using the BP86 functional.
• Application of multiple basis sets: 6-311G* + Wachters + f, 6-31G*, and LANL2DZ.
• Definition of a redundancy-free set of internal coordinates and valence force field.
• Utilizing symmetry coordinates and generalized harmonic mode scrambling.

Main Results:

• Calculated geometries and vibrational frequencies for both low- and high-spin states of Fe(phen)2(NCS)2.
• Identified stretching-bending couplings within the FeN6 framework.
• Force constant comparisons revealed changes in electronic charge distribution.
• Demonstrated that ligands act as nearly rigid units in low-lying normal vibrations.

Conclusions:

• The vibrational modes of the iron(II) complex involve ligands acting as rigid units.
• The energies of these modes correlate with interactions in the solid phase.
• DFT calculations provide insights into the electronic and vibrational behavior of spin-crossover complexes.