3-Ethylindole electronic spectroscopy: S1 and cation torsional potential surfaces

Abstract

The tryptophan derivative 3-ethylindole was studied in the first excited electronic state and the cation ground state using resonance enhanced multiphoton ionization (REMPI) and zero electron kinetic energy (ZEKE) spectroscopy. Weakly bound clusters of 3-ethylindole with argon (n=1–3) have also been studied. The monomer spectroscopy revealed that two conformations of the 3-ethylindole exist in the jet cooled sample. Density-functional theory (DFT) calculations have been used to calculate the ground- and ionic-state geometries and energies. The calculations reveal the two conformations to be a planar and a nonplanar orientation of the ethyl substituent relative to the indole plane. The ZEKE spectrum of the nonplanar form has an extensive progression in the vibration associated with ethyl torsion and indicates a significant geometry change in the ethyl chain torsion upon ionization. The ethyl chain torsional potential is mapped out using DFT calculations, and the ion surface is adjusted such that calculated frequencies and Franck–Condon factors reproduce the experimental ZEKE spectra. The conformer interconversion barrier height is calculated in the ground and ion states. This is compared to an experimentally determined barrier height in the ion. The spectroscopy of the argon complexes reveals interesting results with regard to the cooling of the 3-ethylindole conformations.