Vibrational Spectroscopy of the Pyridazine Cation in the Ground State

Abstract

Vibrational structure of the pyridazine cation in the ground state has been revealed by a vacuum-ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. The adiabatic ionization energy is precisely measured to be 70241 ( 6 cm -1 (8.7088 ( 0.0007 eV). The origin is very weakly observed, while a long progression of the ν 9 + (a 1 ) band of which the fundamental vibrational frequency is 647 cm -1 is predominantly observed. The ν 9 + (a 1 ) mode progression combined with one quantum of the ν 3 + (a 1 ) band at 1698 cm -1 is found to be even stronger. Many other weakly observed vibrational features of the pyridazine cation are identified in the vibrational energy of 0-3500 cm -1 . The structural change of pyridazine upon ionization, reflected in the vibrational spectrum obtained by the one-photon direct ionization process, is theoretically predicted by ab initio calculations. Ring distortion including contraction of the NdN bond should be responsible for strong excitations of ν 3 + and ν 9 + modes. Franck-Condon analysis is given for the comparison of the experiment and theory.

Electron impact ionization and cationic fragmentation of the pyridazine molecules

“…For example, the obtained bond lengths and angles, rN(1)N(2) = 1.3240Å, rN(2)C(3) = 1.3252Å, rC(3)C(4) = 1.3921Å, rC(4)H(8) = 1.0711Å, ∠C(6)N(1)N(2) = 120.112 • and ∠N(1)N(2)C(3) = 122.564 • (see Fig. 1 for labelling of atoms) are in excellent agreement with the RHF calculations [35] and also in quite satisfactory agreement with the DFT calculations, which used the aug-CC-pVDZ basis set [36]. The obtained structural parameters are also consistent with the results derived from electron diffraction [37] and electron diffraction, microwave and liquid-crystal NMR spectra [38].…”

Structure of Pyridazine in the S1 State: Experiment and Theory

“…In Figure 1 b-h, MATI spectra taken from various intermediate S 1 vibronic states are presented to give highly resolved vibronic features of pyridazine in the D 0 state. The ab initio [UB3LYP/6-311 [17] calculation for the D 0 pyridazine is carried out to properly assign the vibrational bands observed in the MATI spectra. Then, the S 1 vibronic states are reversely assigned using the fact that the strongest band in each MATI spectrum, following the Franck-Condon principle, represents a vibrational mode of the S 1 state used as an intermediate state for the (1 + 1') MATI spectrum ( Figure 1).…”

“…The large intensity enhancement of the combination 6b + 6a + band in the MATI spectrum, taken via the 6a mode of S 1 as an intermediate state, also supports the in-plane structural distortion of the S 1 pyridazine, as it is well known that both neutral and cationic ground states belong to the C 2v symmetry. [17] The 6b + band excitation in the MATI spectrum taken via the S 1 -S 0 origin (Figure 1 b) should reflect the structural change of the distorted planar geometry of S 1 to the C 2v planar structure of D 0 . [17] Assignments for other strongly observed bands in R2PI and MATI spectra have been straightforwardly done with the help of ab initio calculations, and described in our former publication.…”

“…[17] The 6b + band excitation in the MATI spectrum taken via the S 1 -S 0 origin (Figure 1 b) should reflect the structural change of the distorted planar geometry of S 1 to the C 2v planar structure of D 0 . [17] Assignments for other strongly observed bands in R2PI and MATI spectra have been straightforwardly done with the help of ab initio calculations, and described in our former publication. [16] For example, the 758 cm À1 band of S 1 correlates to the 1078 cm À1 band of D 0 , which indicates that these bands are due to the 6b 2 (S 1 ) and 6b 2 + (D 0 ) modes, respectively.…”

Ultrafast resonance-enhanced multiphoton ionization in the azabenzenes: Pyridine, pyridazine, pyrimidine, and pyrazine

“…Due to their fundamental similarity to benzene, their prevalence in herbicides 2 and pharmaceutical drugs, 3 and pyrimidine's function as the base structure for the DNA/RNA nucleotides, the electronic structure of the azabenzenes has been studied thoroughly. Transitions, energies, and symmetry assignments have been investigated through electron-energy loss experiments and UV absorption, [4][5][6][7] zero-kinetic-energy (ZEKE) electron spectroscopy, [8][9][10][11] and more recently, absorption of vacuum ultraviolet (VUV) synchrotron radiation 12 (Refs. 4-7, and 12 include comprehensive tables of the available electronic transitions in the azabenzenes).…”