Abstract
Background: The vibronic structures of the first electronically excited state (S1) and ionic ground state (D0) of 2-, 3- and 4-picoline were investigated by means of resonance enhanced multi photon ionization (REMPI) and mass analyzed threshold ionization (MATI) spectroscopy.
Objective: The experimental results were supported by geometry optimizations and frequency analyses at the (time-dependent) density functional theory ((TD)DFT) level of theory. Furthermore, the (TD)DFT results were used to calculate the vibrational intensities in the REMPI and MATI spectra by a multidimensional Franck-Condon approach.
Method: The experimental data and theoretical predictions matched well for 2- and 4-picoline whereas deviations were observed in the case of 3-picoline. The latter was attributed to a vibronic coupling of the S1 state (1π*←n) to a higher lying 1π*←π state, which is likely to occur for all picolines with 3-picoline exhibiting the strongest coupling. The S1 excitation energies of 2-, 3- and 4-picoline were determined to be 34766 ± 3 cm-1 (4.3104 ± 0.0004 eV), 34660 ± 3 cm-1 (4.2973 ± 0.0004 eV) and 35177 ± 4 cm-1 (4.3614 ± 0.0005 eV) respectively.
Results: They are in excellent agreement with the literature. The MATI spectra yielded an adiabatic ionization energy of 72371 ± 5 cm-1 (8.9729 ± 0.0006 eV), 72613 ± 5 cm-1 (9.0029 ± 0.0006 eV) and 72932 ± 4 cm-1 (9.0424 ± 0.0005 eV) for 2-, 3- and 4-picoline respectively.
Conclusions: These are the most accurate values reported to this day. In addition, the geometry and vibronic structure of the first electronically excited state and ionic ground state have been determined.
Keywords: REMPI, MATI, pyridine, picoline, excitation energy, ionization energy, vibronic structure.
Graphical Abstract
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