A reinvestigation of the ν7 and ν10 modes of pyridazine on the basis of the inelastic neutron scattering spectrum analysis

The inelastic neutron scattering (INS) spectrum of pyridazine has been analyzed by fitting different harmonic force fields: MP2/6-311G**, BLYP/6-31G* and B3LYP/6-31G* levels, and B3LYP/6-31G* previously scaled. The four intramolecular potentials have been refined to minimise the difference between observed and calculated spectra. In all the cases, only the diagonal force constants expressed in symmetry coordinates were fitted. The combined use of wavenumbers and INS intensities in the refinement procedure has been decisive in order to clarify the assignment of the whole of the vibrational spectrum, particularly the assignment of the ν7 and ν10 modes. The INS spectrum has been analyzed using the Climax program, and a set of independent symmetry coordinates (C2v) for describing the normal modes of the pyridazine molecule. The a priori scaled B3LYP/6-31G* force field gave the best agreement with the measured INS spectral profile.