Experiments and DFT calculations,show that normal mode frequency values of the molecule of the spin conversion (SQ compound [Fe(phen)(2)(NCS)(2)] vary when the Fe(II) ion passes from its fundamental level, low spin, to its first excited one, high spin. That reveals a coupling between the iron 3d electrons and the molecular vibrations. This coupling exists in others (SC) compounds molecules. In this articles we take into account this coupling in the calculation of the entropy and heat capacity related to the (SC) phenomenon. We also study the coupling term and the thermal variations of the high spin fraction. (C) 2007 Elsevier B.V. All rights reserved.