Hydration of a cationic amide group: a ZEKE spectroscopic study of trans-formanilide–H2O

Abstract

The gas-phase structures of the two isomers (N–H⋯OH₂ bonded and CO⋯HOH bonded) of the trans-formanilide–water complex have been investigated by two-colour (1 + 1′) resonance enhanced multiphoton ionisation (REMPI) and zero electron kinetic energy (ZEKE) spectroscopy. Excitation energies were determined for the S₁ 0⁰ transitions of trans-formanilide–water to be 35 787 ± 0.2 cm⁻¹ (NH-bound) and 36 118 ± 0.2 cm⁻¹ (CO-bound) which correspond to a red shift of 218 cm⁻¹ and a blue shift of 114 cm⁻¹, respectively. This is in excellent agreement with recent one-colour measurements of Dickinson et al., J. Chem. Phys. A, 1999, 103, 6938; and Fedorov et al., J. Chem. Phys. A, 2000, 104, 4943. The D₀ 0⁰ origins of NH-bound and CO-bound trans-formanilide–water were observed by ZEKE spectroscopy at 64 225 ± 5 cm⁻¹ and 64 230 ± 20 cm⁻¹ corresponding to red shifts of 3182 cm⁻¹ and 3177 cm⁻¹, respectively. Both the REMPI and the ZEKE spectra show numerous intermolecular transitions which were assigned on the basis of comparison to similar complexes and to the trans-formanilide monomer. To support these assignments, ZEKE spectra of trans-formanilide–water (NH-bound) were recorded via several vibrationally excited S₁ intermediate states, and ab initio CASSCF calculations were also performed.