Description of the S1(ππ*) state and the reinterpretation of the (1+1) REMPI spectra of N-methylpyrrole as based on the ab intio calculation: a computational spectroscopy

The ab intio calculation was performed to establish the assignment of the title spectra by such as searching for stationary points belonging to lower excited states. The lowest excited state was confirmed to be of ππ* type with an A″ symmetry of a molecular point group C_s (against the previous assumption of πΣ* type) trapped in deep potential minima at the nonplanar staggered conformation (also against the current belief on the involvement of internal rotation). Thus, lower ‘vibrational’ levels in the S₁ state were shown to be tunnel-split levels with various symmetry species for a molecular symmetry group G₁₂. Based on this finding, the spectral data as reported by Philis [Chem. Phys. Lett. 353 (2002) 84] were reassigned while applying the formalism as will be presented in Appendix A.