N···C═O n → π* Interaction: Gas-Phase Electronic and Vibrational Spectroscopy Combined with Quantum Chemistry Calculations

Abstract

Herein, we have used gas-phase electronic and vibrational spectroscopic techniques for the first time to study the N···C═O n → π* interaction in ethyl 2-(2-(dimethylamino) phenyl) acetate (NMe2-Ph-EA). We have measured the electronic spectra of NMe2-Ph-EA in the mass channels of its two distinct fragments of m/z = 15 and 192 using a resonant two-photon ionization technique as there was extensive photofragmentation of NMe2-Ph-EA. Identical electronic spectra obtained in the mass channels of both fragments confirm the dissociation of NMe2-Ph-EA in the ionic state, and hence, the electronic spectrum of the fragment represents that of NMe2-Ph-EA only. UV–UV hole-burning spectroscopy proved the presence of a single conformer of NMe2-Ph-EA in the experiment. Detailed quantum chemistry calculations reveal the existence of a N···C═O n → π* interaction in all six low-energy conformers of NMe2-Ph-EA. A comparison of the IR spectrum of NMe2-Ph-EA acquired from the gas-phase experiment with those obtained from theoretical calculations indicates that the experimentally observed conformer has a N···C═O n → π* interaction. The present finding might be further valuable in drug design and their recognition based on the N···C═O n → π* interaction.