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

Abstract

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 54 Fe– 57 Fe and 14 N– 15 N 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 (~10%) of the calculated frequencies lead to a relatively large error (~50%) of the total entropy change. .