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(2)CCH=CHCO(2)(-)) were obtained at low temperatures and at 193 nm photon energy. Vibrational structure was observed for trans-HO(2)CCH=CHCO(2)(-) due to the OCO bending modes; however, cis-HO(2)CCH=CHCO(2)(-) yielded a broad and featureless spectrum. The electron binding energy of cis-HO(2)CCH=CHCO(2)(-) is about 1 eV blue-shifted relative to trans-HO(2)CCH=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(2)CCH=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.