Determination of the accurate ionization energy and cationic structure of cyanopyrazine by one-photon mass-analyzed threshold ionization spectroscopy

• The substitution in pyrazine may alter the molecular orbital and ionization energy.
• The ionization energy and the cationic structure of cyanopyrazine are reported.
• The accurate molecular structure provides the intuition on molecular orbitals.

The ionization energy and cationic structure of cyanopyrazine have been determined for the first time by pulsed-field threshold ionization mass spectrometry. The ionization energy was accurately measured to be 80,305 ± 6 cm−1 (9.9566 ± 0.0007 eV) from the 0-0 band position in the mass-analyzed threshold ionization (MATI) spectrum obtained for the direct S0 → D0 transition using a vacuum ultraviolet laser pulse generated by four-wave mixing in Kr. The highest occupied molecular orbital (HOMO) and singly occupied molecular orbital (SOMO) calculations indicated the extent of conjugation of nonbonding orbitals on nitrogen atoms with the π orbital in cyanopyrazine, and clarified the effect of substitution of H with CN in pyrazine on the ionization energy of cyanopyrazine. The observed MATI spectrum was interpreted using the Franck–Condon factors and vibrational frequencies calculated through adjustment of the geometrical parameters of the cyanopyrazine cation in the ground electronic state at the B3LYP/cc-pVTZ level. From rigorous analyses, it has been concluded that upon ionization, the cyanopyrazine cation in the D0 state should retain a planar structure with Cs symmetry, similar to the ground state molecule.

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