Investigation of the complex vibronic structure in the first excited and ionic ground states of 3-chloropyridine by means of REMPI and MATI spectroscopy and Franck–Condon analysis

Abstract

3-Chloropyridine (3-CP) has been investigated by means of resonance-enhanced multi-photon ionization (REMPI) and mass-analyzed threshold ionization (MATI) spectroscopy to elucidate the effect of m-chlorine substitution on the vibronic structure of the first electronically excited and ionic ground states. The S₁ excitation energy has been determined to be 34 840 ± 2 cm⁻¹ (4.3196 ± 0.0002 eV) with a difference of less than 0.2 cm⁻¹ between both isotopomers, which is the first reported value for this transition in the gas phase so far. The S₁ state has been assigned to the ¹π* ← n transition. It is subject to strong vibronic coupling via ν_16b to one or both of the lowest ¹ππ* states. In addition, strong coupling via at least one more non-totally symmetric vibration is very likely to exist but the vibration could not be identified yet. Overall, the coupling results in a minimum S₁ structure with C₁ symmetry. The adiabatic ionization energy of the n_N-LP orbital (14a′) has been determined to be 75 879 ± 6 cm⁻¹ (9.4078 ± 0.0007 eV) with a difference of less than 2 cm⁻¹ between the two isotopomers, which is the first value reported for this state so far. The ionic ground state exhibits a distinct vibronic coupling via ν_16a and ν_10a to either the D₁ state (4a′′) and/or D₂ state (3a′′), which results in a twisted D₀ geometry with C₁ symmetry. As a consequence of the warped geometry in both S₁ and D₀ states, very complicated MATI spectra were obtained when exciting S₁ states at higher wavenumbers.