Probing the low-barrier hydrogen bond in hydrogen maleate in the gas phase: A photoelectron spectroscopy and ab initio study

The strength of the low-barrier hydrogen bond in hydrogen maleate in the gas phase was investigated by low-temperature photoelectron spectroscopy and ab initio calculations. Photoelectron spectra of maleic and fumaric acid monoanions (cis-/trans-HO ₂CCH=CHCO ₂ ^-) were obtained at low temperatures and at 193 nm photon energy. Vibrational structure was observed for trans-HO ₂CCH=CHCO ₂ ^- due to the OCO bending modes; however, cis-HO ₂CCH=CHCO ₂ ^- yielded a broad and featureless spectrum. The electron binding energy of cis-HO ₂CCH=CHCO ₂ ^- is about 1 eV blue-shifted relative to trans-HO ₂CCH=CHCO ₂ ^- due to the formation of intramolecular hydrogen bond in the cis-isomer. Theoretical calculations (CCSD(T)/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ) were carried out to estimate the strength of the intramolecular hydrogen bond in cis-HO ₂CCH=CHCO ₂ ^-. Combining experimental and theoretical calculations yields an estimate of 21.5 ± 2.0 kcal/mol for the intramolecular hydrogen bond strength in hydrogen maleate. © 2005 American Chemical Society.