Spectra Of Pyridazine Research Articles

Millimeter-wave and high-resolution infrared spectroscopy of the low-lying vibrational states of pyridazine isotopologues.

The gas-phase rotational spectrum from 8 to 750GHz and the high-resolution infrared (IR) spectrum of pyridazine (o-C4H4N2) have been analyzed for the ground and four lowest-energy vibrationally excited states. A combined global fit of the rotational and IR data has been obtained using a sextic, centrifugally distorted-rotor Hamiltonian with Coriolis coupling between appropriate states. Coriolis coupling has been addressed in the two lowest-energy coupled dyads (ν16, ν13 and ν24, ν9). Utilizing the Coriolis coupling between the vibrational states of each dyad and the analysis of the IR spectrum for ν16 and ν9, we have determined precise band origins for each of these fundamental states: ν16 (B1) = 361.213 292 7 (17) cm-1, ν13 (A2) = 361.284 082 4 (17) cm-1, ν24 (B2) = 618.969 096 (26) cm-1, and ν9 (A1) = 664.723 378 4 (27) cm-1. Notably, the energy separation in the ν16-ν13 Coriolis-coupled dyad is one of the smallest spectroscopically measured energy separations between vibrational states: 2122.222 (72) MHz or 0.070 789 7 (24) cm-1. Despite ν13 being IR inactive and ν24 having an impractically low-intensity IR intensity, the band origins of all four vibrational states were measured, showcasing the power of combining the data provided by millimeter-wave and high-resolution IR spectra. Additionally, the spectra of pyridazine-dx isotopologues generated for a previous semi-experimental equilibrium structure (reSE) determination allowed us to analyze the two lowest-energy vibrational states of pyridazine for all nine pyridazine-dx isotopologues. Coriolis-coupling terms have been measured for analogous vibrational states across seven isotopologues, both enabling their comparison and providing a new benchmark for computational chemistry.

A study by ultraviolet spectroscopy on the self-association of diazines in aqueous solution

The self-association of pyridazine and pyrazine was studied in aqueous solution at acidic, neutral and basic pH values, by ultraviolet spectroscopy. The spectra of pyridazine in the mid-ultraviolet region did not show any variation in molar absorptivity upon concentration of this compound, indicating that self-association is not important. By contrast, deviations from Beer–Lambert law with increasing concentration were found in pyrazine at the pH values studied. A single hypochromic effect was detected at pH<0, 0.6 and 11.0, while a double hypochromic effect was observed at pH 6.9. These results were interpreted in terms of self-association of pyrazine leading to the formation of dimers at acidic and basic pH, and of dimers and polymers at neutral pH. From the fitting of the experimental curves of hypochromic effects, self-association constants were calculated. The self-association of 3-methylpyridazine, 2-methylpyrazine, and 2,3-dimethylpyrazine was also analyzed, concluding that the introduction of a methyl group has no influence upon the self-association of the pyridazine ring, but the self-association of pyrazine is enhanced by the introduction of two methyl groups. A comparative discussion of the self-association behavior of pyridazine, pyrazine and pyrimidine in aqueous solution has been carried out in terms of the differences in the molecular structure of the three diazines.

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 ν 7 and ν 10 modes. The INS spectrum has been analyzed using the C limax program, and a set of independent symmetry coordinates (C 2v) 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.

Vibrational spectrum of pyridazine

The infrared spectrum of pyridazine (1,2-diazine) was obtained in the pure liquid and in solution (4000–200 cm-1), in the gas phase and as a polycrystalline film (4000–400 cm-1). Additionally, the Raman spectrum of the liquid together with qualitative depolarization ratios were obtained. Using the new experimental evidence, an ab initio scaled computation of the vibrational spectrum at the HF/3–21G* and HF/6–31G** levels of theory clarified the spectral regions most often discussed in the literature. The IR spectra in CS2 and CCl4 solutions showed a splitting in the CH stretching region. The IR spectrum in polycrystalline film together with the scaled calculations were decisive in the assignment of the A2 fundamentals. The IR spectrum in the gas phase showed, first, an A-type contour at 665 cm-1 which determined the ν9(A1) and ν24(B2) assignments and, second, the lowest wavenumber ring vibration ν23 at 1027 cm-1 which was related to the band at 1043 cm-1 in polycrystalline film. © 1998 John Wiley & Sons, Ltd.

Additivity of 13C-1H and 1H-1H spin-spin coupling constants in six-membered aromatic nitrogen-containing heterocycles

We have analyzed proton-coupled 13C NMR and PMR spectra of pyridazine, pyrimidine, pyrazine, 1,3,5-triazine, and 1,2,4-triazine for one-molar solutions of the compounds in DMSO-D6. Comparison of the values obtained for the 13C-1H and 1H-1H spin-spin coupling constants with the values calculated on the basis of the spin-spin coupling constants in pyridine and benzene has shown that it is possible to predict the constants using linear additive equations. Substantial nonlinear deviations (5–6 Hz) are observed for 1JCH when the carbon atom is located between two adjacent nitrogen atoms.

The proton magnetic resonance spectrum of pyridazine

Proton–proton couplings for pyridazine have been extracted by AA′XX′ analysis and checked against the 13C–H satellite multiplets. Partial disagreement with previous results is reported. The low-field shift of the signals with increasing concentration is discussed.

H-H and13C-H coupling constants in pyridazine

Analysis of the proton and 13C-H satellite N.M.R. spectra of pyridazine has given the following H-H coupling constants: J 34 = 5·05 c.p.s., J 35 = 1·85 c.p.s., J 45 = 8·00 c.p.s., J 36 = ± 1·40 c.p.s.; the 13C-H couplings are 183 c.p.s. and 168 c.p.s. respectively for the 3 (or 6) and 4 (or 5) positions.

Pyridazines. IX. Proton magnetic resonance studies of pyridazine, pyrazine, and substituted pyridazines.

Pyridazine, pyrazine and substituted pyridazines have been studied by proton magnetic resonance spectroscopy. The spectra of pyridazine and pyrazine were analyzed completely with the aid of information obtained from the C13-satellites (in natural isotopic abundance), whereas the spectra of substituted pyridazines were analyzed easily by the usual treatments. The proton magnetic resonance parameters of diazines were discussed in connection with the electronegativity of the nitrogen atoms. Effects of some substituents upon the ring proton shifts of pyridazine were discussed. The total methyl effect on the ring protons of pyridazine (0.54 p.p.m.) falls in the normal aromatic range. Electronic charge distributions in diazine molecules were discussed by comparing the π-electron densities calculated by the Huckel MO method with those obtained from the ring proton chemical shifts. Discrepancies in these electronic densities were ascribed to the possible paramagnetic anisotropy effect arising from the n-π* transition of the lone-pair electrons of the nitrogen atoms.