Transferability and Physicochemical Interpretation of Canonical Force Fields in Redundant Internal Coordinates:  Pyridazine and 3,6-Dichloropyridazine

Abstract

The behavior of various force fields for pyridazine and 3,6-dichloropyridazine coming from different origins (i.e., HF/6-31G*, MP2/6-311G**, BLYP/6-31G*, B3LYP/6-31G* previously scaled) and empirical force fields transferred from the benzene molecule is investigated. These potentials are subjected to a linear least-squares refinement with the available experimental information considered as observed data. Moreover, the transferability properties of the force fields for these related molecules are analyzed. From the refined force fields which gave the most satisfactory results in the fitting procedure, the physicochemical meaning of the force constants in terms of redundant simple internal coordinates in the unambiguous canonical form is studied. Although the results are not completely decisive for the pyridazine series, significant conclusions are drawn pertaining to their force fields, some of which are calculated for the first time. First, the utility of canonical force fields is shown through the force fields tranferred from benzene, which produce results for some symmetry blocks that are comparable to those obtained from quantum mechanical force fields. Second, we report the refined force fields in simple valence internal coordinates and observe that they give values for the ringstretching force constants that agree with the electronic structure of this molecule, for which the aromatic character is 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 with the low aromatic character of the pyridazine ring, especially as compared with the other azines. In addition, the calculated values for the torsion and CH and CCl bending force constants also are explained in the electronic structure of these systems.

Refined, scaled and canonical force fields for the cis- and trans-3-fluorostyrene conformers. An interplay between theoretical calculations, IR/Raman and INS data

“…Several papers have recently discussed the canonical force field. 3,4 One of the main advantages of such a force field is that the corresponding force constants are unambiguous, thus providing a very suitable method for making meaningful comparisons between force constants obtained through different systems.…”

CH-stretching Overtone Spectra of a Fast Rotating Methyl Group: 2-CH3 and 2-CHD2 Pyridines

“…This vibrational energy part e CH (θ) is calculated using nine internal curvilinear coordinates of the methyl group, namely the displacement coordinates of the three CH/CD bonds ( r ), of the three HCH/HCD angles and of the three HCC/DCC angles deformations (α). As previously stated, this set of internal coordinates offers a powerful tool to test the physicochemical meaning of the force constants. They also allow transferrability and interpolation of the force fields between related molecules.…”

A study on the properties and transferability of force fields in redundant and independent coordinates

“…This matter has been treated in the literature from different points of view. The transferability of the canonical force constants has been studied in a wide variety of molecules [19][20][21][22][23] and it has been shown that the canonical force fields are transferable between molecules with the same geometry [17]. The work described here concerns an approach to obtain transferable force fields in terms of redundant and independent symmetry coordinates for molecules with a common group of atoms.…”