Out-of-Plane Vibrations of [1H5]- and [2H5]Pyridines

Abstract

Assignments of the out-of-plane normal vibrations of [1H5]- and [2H5]pyridines were reexamined based on the polarization behavior of the Raman bands and also through the normal coordinate calculation.

A priori results for molecular geometry, scaled quantum mechanical (SQM) force field, and vibrational spectra of pyridazine

“…The a priori spectrum predicts two fundamentals of about 1300 cm"1: One of them is of large IR intensity (1264 cm"1, B2 species), and the other one (1297 cm"1, A| species) is expected to be weak. With respect to this we have interchanged the IR bands of Stidham and Tucci21 at 1272 and 1319 cm"1 between the symmetry species A| and B2, in accordance with the modifications of Ozono et al 24 The experimental C-D stretching frequencies agree better with the a priori ones than the C-H frequencies, as expected from the lower anharmonicity of the former.…”

“…Presently, the a priori SQM force field of pyridazine is considered as the best harmonic force field for pyridazine. With the help of this a priori SQM force field the experimental assignments of Ozono et al 24 have been confirmed basically; however, assignments have been changed in six and four cases for pyridazine-h4 and pyridazine-rf4, respectively. Computed IR intensities are generally in agreement with experiments at a qualitative level.…”

“…See ref 25. 4 See ref 21.' See ref 24. spectrum prefers the assignment of Ozono et al 24 to that of Stidham and Tucci.21 As we have previously noticed,108•11 the C-H stretching vibrations present a special problem partly due to the strong anharmonicity and resonance effects and partly due to a minor uncertainty of the C-H correction for the reference geometry.…”

“…Note also that the Bt fundamental of highest frequency is expected around 960 cm"1 according to our a priori vibrational spectrum; thus, the Raman band at 842 cm"121 has been excluded, too. At this place the assignments of Ozono et al 24 have also been altered: we interchanged the Raman bands at 970 and 986 cm"1 between the symmetry species A2 and Bi.…”

“…Their calculations23 do not support the A2 assignments of Stidham and Tucci21 for pyridazine. Ozono et al 24 reexamined the previous assignments for pyridazineâ nd pyridazme-i/4 on the basis of polarization behavior of the Raman and IR bands and also through normal-coordinate calculation that used an improved modification of the Urey-Bradley and VFF force fields. Assignments reported in refs 21 and 24 are inconsistent.…”