Conformational Changes in 5-Methoxyindole: Effects of Thermal, Vibrational, and Electronic Excitations

Abstract

The molecule of 5-methoxyindole (5MOI) may adopt two conformational states, syn and anti, with respect to the relative orientation of the NH and OCH groups. The structure of monomeric 5MOI was characterized spectroscopically, in mid- and near-infrared domains. The conformational composition of 5MOI could be controlled in three different ways. Thermally, two conformers of 5MOI could be trapped in xenon matrixes at 16 K. Upon annealing the xenon matrix to temperatures about 30-40 K, the higher-energy syn form converted to the ground-state anti conformer. Vibrational excitations in the near-infrared domain, at the frequency of the first NH stretching overtone, 6853 cm, afforded the inverse conformational transformation, and a part of the anti conformer was upconverted to the syn form. Electronic excitations in the UV domain, at 315-310 nm, resulted in a total consumption of the syn form again, in favor of anti. Upon further irradiations at 308 nm, a partial repopulation of the syn form, at the expense of anti, was observed. We propose a mechanistic explanation of the observed transformations, which is based on computations of the vibrational spectra of the two conformers and also on computations of the ground state S and the first excited state S potential energy surfaces along the coordinate for conformational isomerization. The highlights of the present work are the first experimental observation of the minor syn conformer of 5MOI, evidence of the long-range vibrational energy transfer resulting in conformational isomerization upon excitation of the NH stretching overtone, and the possibility of partial conformational control of 5MOI by using electronic excitations.

Photoinduced transformations of indole and 3-formylindole monomers isolated in low-temperature matrices

“…The idea was similar to that employed for rotation of the methoxy group of 6-methoxyindole or 5-methoxyindole by near-IR excitations of the 2νN1H overtone. 69,70 For this purpose, the near-IR spectrum of 3FOI isolated in an Ar matrix was recorded (Fig. S5 of the supplementary material).…”

Conformational isomerizations triggered by vibrational excitation of second stretching overtones

“…separated by several bonds from the group acting as an antenna. Cases were described for the flip of a remote light H atom, 18,19 and heavy hydroxymethyl (-CH 2 OH), 20 methoxy (-OCH 3 ) 21,22 and aldehyde (-CHO) 23 fragments.…”

Matrix Isolation in Heterocyclic Chemistry

“…Methoxy-substituted indoles in position C5 or C6 have been studied by Lopes Jesus et al [37,38]. Both molecules possess two conformers differing in the orientation of the methoxyl The IR spectra of compounds isolated in noble gas matrices recorded immediately after isolation are represented in Figures 12.10 and 12.11.…”