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Investigation of the low‐spin to high‐spin transition in a novel [Fe(pmea)(NCS) 2 ] complex by IR and Raman spectroscopy and DFT calculations
Author(s) -
Brehm Georg,
Reiher Markus,
Le Guennic Boris,
Leibold Michael,
Schindler Siegfried,
Heinemann Frank W.,
Schneider Siegfried
Publication year - 2006
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.1437
Subject(s) - raman spectroscopy , spin crossover , chemistry , molecule , singlet state , spin states , molecular vibration , spin transition , spin (aerodynamics) , crystallography , atomic physics , thermodynamics , excited state , physics , inorganic chemistry , organic chemistry , optics
The X‐ray structures of the novel spin crossover complex [Fe(pmea)(NCS) 2 ] with pmea = (bis[(2‐pyridyl)methyl]‐2‐(2‐pyridyl)ethylamin) were determined in its low‐spin (singlet, LS) and high‐spin state (quintet, HS) and compared with the optimized geometries produced by BP86 calculations. IR and Raman (λ ex = 514.5, 785 and 1064 nm) spectra were recorded at different temperatures between T = 298 K and T = 30 K. Most of the experimentally observed bands could be assigned unambiguously and discussed by comparison with the normal modes calculated for both spin states. The detailed analysis of the temperature dependence of the band contours of specific modes provides strong evidence that significant changes of the inter‐molecular distances occur for all molecules when a certain fraction of the individual molecules has undergone the low‐spin (LS) to high‐spin (HS) transition. The partition function for intra‐molecular vibrations was calculated for both spin states by considering all vibrational wavenumbers that were calculated. The vibration‐related entropy change connected with the LS to HS transition is determined via well‐established thermodynamic relations. For the title compound we found ΔS vib ≈ 40.3 J/(mol·K), which is about twice that determined for [Fe(phen) 2 (NCS) 2 ]. Consequently, the sum of the purely electronic contribution (∼13.4 J/(mol·K)) and that of the intra‐molecular vibrations equals the average entropic driving force determined for a series of spin‐crossover complexes (∼50 J/(mol·K)). It is therefore most interesting to experimentally determine the entropy change for the complex investigated here and to find out whether this complex exhibits an unusually large entropy change upon the LS to HS transition. Copyright © 2006 John Wiley & Sons, Ltd.