Pulsed field ionization-ZEKE spectroscopy of 4-aminobenzonitrile–H2O. Hydrogen-bonding interaction in the amino site

Abstract

Pulsed field ionization (PFI)-zero-kinetic-energy (ZEKE) photoelectron spectra are measured for 4-aminobenzonitrile (4ABN) and “NH₂-donor isomer” of 4ABN–H₂O 1 : 1 complex in which the oxygen atom of water is bonded to the amino hydrogen (K. Sakota, N. Yamamoto, K. Ohashi, H. Sekiya, M. Saeki, S. Ishiuchi, M. Sakai and M. Fujii, Chem. Phys. Lett., 2001, 341, 70.). We have investigated changes in intermolecular interaction and structure of the NH₂-donor isomer upon photoionization. The adiabatic ionization potentials of 4ABN and NH₂-donor isomer are determined to be 66 493 and 62 843 cm⁻¹, respectively. The oxygen atom of water is hydrogen bonded to an amino hydrogen atom in the D₀ state of NH₂ donor isomer as well as in the S₁ and S₀ states. However, the hydrogen bond is suggested to be much stronger in the D₀ state than in the S₁ state from a substantial decrease (3650 cm⁻¹) in the adiabatic potential due to the formation of a hydrogen bond. Normal mode analysis has been made for the intermolecular vibrations in the D₀ state with the aid of density functional theory calculations at the B3LYP/aug-cc-pVDZ level. The observation of a prominent progression of the intermolecular stretch mode in the PFI-ZEKE spectrum via the electronic origin of the S₁–S₀ transition is consistent with the increased hydrogen bond strength in the D₀ state.