Mass analyzed threshold ionization spectroscopy of 5-methylindole and 3-methylindole cations and the methyl substitution effect.

The vibrationally resolved mass analyzed threshold ionization spectra of jetcooled 5-methylindole (5MI) and 3-methylindole (3MI) have been recorded by ionizing via various vibronic levels of each species. The adiabatic ionization energies (IEs) of 5MI and 3MI are determined to be 61,696+/-5 and 60,679+/-5 cm(-1), which are less than that of indole by 895 and 1912 cm(-1), respectively. Comparing these data with those of 1-methylindole and indole suggests that the methyl substitution on the pyrrole part leads to a greater redshift in the IE than on the benzene part. These experimental findings are well supported by the theoretical calculations. Analysis on these new data shows that many active vibrations of the 5MI cation are related to the CH(3) torsion and in-plane ring bending vibrations. In contrast, the observed vibrational bands of the 3MI cation are very weak due to unfavorable Franck-Condon transition.