Quantum mechanical investigation of the charge transfer molecular complexes of methimazole with I2, IBr and ICl

The structural, frequency and energy characteristics and the nature of bonding of the molecular complexes between N-methyl imidazoline-2-thione (methimazole) and the related saturated analog, N-methyl imidazolidine-2-thione and the dihalogens IY (Y = Cl, Br, I) are investigated using quantum mechanical computational techniques. Two types of stereoisomeric energy minima have been studied for each parent thioamide system. Both present a collinear S-I-Y geometry and are distinguished by the geometry of the dihalogen IY molecular axis located at either a planar or a perpendicular arrangement with respect to the imidazolethione, NCS, plane. The planar conformations have been found to be more stable than the perpendicular ones, since they allow a more favourable geometry for an additional interaction between the iodine atom and the amidic H. The results reveal interesting correlations of various structural and frequency features like the C-S and S-I bonding distances and the N-H stretching frequency shifts with the enthalpy of complexation, the electron donor-acceptor capacity of the dihalogen and the electronegativity of the Y atom and the strength of the interaction between the amidic hydrogen and the inner I.