Impact of Spin State Transition on Vibrations of [Fe–(PM–BiA)2(NCS)2] and [Fe–(PM–PEA)2(NCS)2] Spin Crossover Compounds: Experimental and Theoretical Far IR and Raman Study

Far IR and Raman spectroscopy of [Fe–(PM-BiA)2 (NCS)2] and [Fe –(PM-PEA)2 (NCS)2] are performed to record the vibration modes of these complexes and their evolutions during spin state transition. THz spectroscopy is used to measure the far IR absorption of these compounds down to 20 cm-1. Using density functional theory calculations we are able to retrieve and to visualize these vibrations in both high spin and low spin states. In both compounds, Raman spectra recorded in the 800 cm-1 2250 cm-1 wavenumber range are well accounted for by our computations. For far IR vibration modes, most of the experimental absorption peaks above 100 cm-1 are well reproduced. However, below 100 cm-1 , marked differences are recorded. For [Fe–(PM-BiA)2 (NCS)2] we demonstrate that the interactions with surrounding molecules impacts non-negligibly the computed far IR spectra and can partly correct for this difference.