Experimental and theoretical analysis of the vibrational spectra and theoretical study of the structures of 3,6-dichloropyridazine and 3,4,5-trichloropyridazine

The infrared and Raman spectra of 3,6-dichloropyridazine and 3,4,5-trichlor opyridazine were recorded. These spectra were taken in the solid phase and with various solvents (HCCl3, CS2, CCl4), giving wavenumbers and relative i ntensities of the bands and the qualitative Raman depolarization ratios. Co nventional ab initio methods at the Hartree-Fock and MP2 levels using the 3 -21G*, 6-31G*, 6-31G**, and 6-311G** basis sets, as well as density functio nal theory (DFT) using the 6-31G* basis functions and the BLYP and B3LYP fu nctionals, were used to predict the geometries and to calculate the harmoni c frequencies and force fields of these molecules. In addition, coupled clu ster calculations, i.e., CCSD and CCSD(T), using the 6-311G** basis set wer e included to evaluate the geometries of pyridazine and 3,6-dichloropyridaz ine, analyzing in detail the structure of these systems. To fit the calcula ted wavenumbers to the experimental ones, the B3LYP/6-31G* force field was selected to be scaled. The following two sets of scale factors were tested: (1) a standard set derived from a training set of 30 organic molecules (se t A), and (2) a set directly calibrated from the pyridazine molecule (set B ). The calculations indicate that pyridazine and its chlorinated derivative s show less aromatic character than expected. The B3LYP exchange-correlatio n functional with the 6-31G* basis functions gave the most reliable structu ral parameters for both molecules. The a priori scaled spectra were suffici ently decisive to get a complete assignment of the vibrational spectra. Bot h series of scale factors led to similar results. However, although the sta ndard set (set A) proved to be an useful tool to carry out the initial assi gnments, further detailed analysis of the spectra required specific empiric al parameters derived from the pyridazine molecule. This was especially tru e for the most congested spectral areas.