Experimentally measured permanent dipoles induced by hydrogen bonding. The Stark spectrum of indole–NH3

Abstract

Hydrogen bond pairs involving the chromophore indole have been extensively studied in the gas phase. Here, we report high resolution electronic spectroscopy experiments on the indole–NH₃ hydrogen bond pair in the absence and presence of an electric field. The S₁–S₀ origin band of this complex recorded in zero field at high resolution reveals two overlapping spectra; a consequence of NH₃ hindered internal rotation. The barrier to internal rotation is predicted by theory to be less than 20 cm⁻¹ in the ground state, therefore requiring a non-rigid rotor Hamiltonian to interpret the spectra. Conducting the experiment in the presence of an applied electric field further perturbs the already congested spectrum of the complex, but makes possible the measurement of the permanent electric dipole moments in its S₀ and S₁ states. These values reveal significant changes in electron distribution that arise from hydrogen bonding effects.