Probing the Low-Barrier Hydrogen Bond in Hydrogen Maleate in the Gas Phase: A Photoelectron Spectroscopy and ab Initio Study

Abstract
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-HO2CCHCHCO2-) were obtained at low temperatures and at 193 nm photon energy. Vibrational structure was observed for trans-HO2CCHCHCO2- due to the OCO bending modes; however, cis-HO2CCHCHCO2- yielded a broad and featureless spectrum. The electron binding energy of cis-HO2CCHCHCO2- is about 1 eV blue-shifted relative to trans-HO2CCHCHCO2- 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-HO2CCHCHCO2-. Combining experimental and theoretical calculations yields an estimate of 21.5 ± 2.0 kcal/mol for the intramolecular hydrogen bond strength in hydrogen maleate.