Transferability and physicochemical interpretation of canonical force fields in redundant internal coordinates: Pyridazine and 3,6-dichloropyridazine

The behavior of various force fields for pyridazine and 3,6-dichloropyridaz ine coming from different origins (i.e.. HF/6-31G*, MP2/6-311G**, BLYP/6-31 G*, B3LYP/6-31G* previously scaled) and empirical force fields transferred from the benzene molecule is investigated. These potentials are subjected t o a linear least-squares refinement with the available experimental informa tion considered as observed data. Moreover, the transferability properties of the force fields for these related molecules are analyzed. From the refi ned force fields which gave the most satisfactory results in the fitting pr ocedure, the physicochemical meaning of the force constants in terms of red undant simple internal coordinates in the unambiguous canonical form is stu died. Although the results are not completely decisive for the pyridazine s eries, significant conclusions are drawn pertaining to their force fields, some of which are calculated for the first time. First, the utility of cano nical force fields is shown through the force fields tranferred from benzen e, which produce results for some symmetry blocks that are comparable to th ose obtained from quantum mechanical force fields. Second, we report the re fined force fields in simple valence internal coordinates and observe that they give values for the ring-stretching force constants that agree with th e electronic structure of this molecule, for which the aromatic character i s shifted toward one of the Kekule structures that has double bonds between the C and N atoms and in the CC meta bond. All of this is in accordance wi th the low aromatic character of the pyridazine ring, especially as compare d with the other azines. In addition, the calculated values for the torsion and CH and CCl bending force constants also are explained in the electroni c structure of these systems.