A-PRIORI RESULTS FOR MOLECULAR-GEOMETRY, SCALED QUANTUM-MECHANICAL (SQM) FORCE-FIELD, AND VIBRATIONAL-SPECTRA OF PYRIDAZINE

For the ground electronic state of pyridazine, two kinds of molecular data, the molecular geometry and the harmonic force field, have been determined theoretically at the ab initio Hartree-Fock level using the 4-21G basis set. In order to compensate for the systematic errors of both sets of computed molecular data, empirical corrections were used with the help of a few parameters whose values have been calibrated previously on model molecule(s). Thus, the resulting corrected data are a priori for pyridazine. These a priori molecular data are regarded as good estimates of the ''true'' values of the corresponding data. The a priori molecular geometry of pyridazine [r(N1N2) = 133.8 pm, r(N2C3) = 132.6 pm, r(C3C4) = 140.4 pm, r(C4C5) = 137.9 pm, r(C3H3) = 107.4 pm, r(C4H4) = 107.5 pm, alpha(NNC) = 119.8-degrees, alpha(NCC) = 123.2-degrees, alpha(CCC) = 117.1-degrees, alpha(NCH) = 115.7-degrees, and alpha(C3C4H4) = 120.6-degrees] is presently the best equilibrium structure for the gaseous molecule and shows less aromatic character for this molecule than expected: this is manifested by the significant alternation of bond lengths around the ring. Our results are in excellent agreement with the ring structure of a recent X-ray analysis of pyridazine at 100 K. The a priori scaled quantum mechanical (SQM) force field of pyridazine (transferring all the required scale factors from the benzene molecule) is regarded as physically the most correct and the most accurate harmonic force field of this molecule. On the basis of this force field, the a priori vibrational spectra of pyridazine-h4 and pyridazine-d4 have been determined as if the experimental vibrational spectra of both molecules were completely unknown. Comparison with experimental spectra, after a few reassignments, shows 15.6 and 30 cm-1 for the mean and the maximal individual deviations, respectively. (Only the non-CH/CD stretching frequencies were considered.) Computed IR intensities are generally in agreement with experiments at a qualitative level.