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