Experimental and Theoretical Analysis of the Vibrational Spectra and Theoretical Study of the Structures of 3,6-Dichloropyridazine and 3,4,5-Trichloropyridazine

摘要

The infrared and Raman spectra of 3,6-dichloropyridazine and 3,4,5-trichloropyridazine were recorded. These spectra were taken in the solid phase and with various solvents (HCCl_3, CS_2, CCl_4), giving wavenumbers and relative intensities of the bands and the qualitative Raman depolarization ratios. Conventional 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 functional theory (DFT) using the 6-31G~* basis functions and the BLYP and B3LYP functionals, were used to predict the geometries and to calculate the harmonic frequencies and force fields of these molecules. In addition, coupled cluster calculations, i.e., CCSD and CCSD(T), using the 6-311G~(**) basis set were included to evaluate the geometries of pyridazine and 3,6-dichloropyridazine, analyzing in detail the structure of these systems. To fit the calculated 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 (set A), and (2) a set directly calibrated from the pyridazine molecule (set B). The calculations indicate that pyridazine and its chlorinated derivatives show less aromatic character than expected. The B3LYP exchange-correlation functional with the 6-31G~* basis functions gave the most reliable structural parameters for both molecules. The a priori scaled spectra were sufficiently decisive to get a complete assignment of the vibrational spectra. Both series of scale factors led to similar results. However, although the standard set (set A) proved to be an useful tool to carry out the initial assignments, further detailed analysis of the spectra required specific empirical parameters derived from the pyridazine molecules. This was especially true for the most congested spectral areas.