Isotope effects on the vibrational spectra of the Fe(Phen)2(NCS)2 spin-crossover complex studied by density functional calculations

A geometry optimization and vibrational frequency calculation has been carried out on the Fe(phen)(2)(NCS)(2) (phen = 1, 10 phenantroline) molecule in both spin states (S = 0 and S = 2) using the density functional theory (DFT) method with the B3LYP functional and the 6-31G(d) basis set. The frequency shifts upon Fe-54-Fe-57 and N-14-N-15 isotope substitutions have been calculated and compared with the experimental frequency shifts as well as with calculated data (DFT/BP86) published in the literature. In the low frequency region (100-600 cm(-1)), both DFT methods reproduce correctly the measured isotope shifts. The accuracy of the calculated vibrational entropy change upon the spin crossover (SCO) is lower: reasonable errors (similar to 10%) of the calculated frequencies lead to a relatively large error (similar to 50%) of the total entropy change. (c) 2005 Academie des sciences. Published by Elsevier SAS. All rights reserved.