Probing the Low-Barrier Hydrogen Bond in Hydrogen Maleate in the Gas Phase: A Photoelectron Spectroscopy and ab Initio Study Hin-Koon Woo, Xue-Bin Wang, and Lai-Sheng Wang* Department of Physics, Washington State UniVersity, 2710 UniVersity DriVe, Richland, Washington 99354, and W. R. Wiley EnVironmental Molecular Sciences Laboratory and Chemical Sciences DiVision, Pacific Northwest National Laboratory, MS 8-88, P.O. Box 999, Richland, Washington 99352 Kai-Chung Lau The James Franck Institute and Department of Chemistry, UniVersity of Chicago, Chicago, Illinois 60637 ReceiVed: September 19, 2005; In Final Form: October 19, 2005 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 CCHdCHCO 2 - ) were obtained at low temperatures and at 193 nm photon energy. Vibrational structure was observed for trans-HO 2 CCHdCHCO 2 - due to the OCO bending modes; however, cis-HO 2 CCHdCHCO 2 - yielded a broad and featureless spectrum. The electron binding energy of cis-HO 2 CCHdCHCO 2 - is about 1 eV blue-shifted relative to trans-HO 2 CCHdCHCO 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 CCHdCHCO 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. The study of the hydrogen bond has long been a topic of intensive scientific research due to its relevance in materials science, biochemistry, organic chemistry, inorganic chemistry, solid-state physics, and molecular medicine. 1-5 In recent years, the short-strong hydrogen bond, usually referred as the low- barrier hydrogen bond (LBHB), has attracted considerable attention because of its possible role in enzyme catalysis. 6-9 It has been suggested that LBHB can supply up to 20 kcal/mol to stabilize the transition states in enzymatic reactions. 7 However, the importance of LBHBs in enzyme catalysis has been a controversial issue, 10-12 although the LBHBs have been shown to be involved in several enzyme systems. 13-15 One system that has been widely studied for LBHB is the maleic/fumaric acid monoanion (cis-/trans-HO 2 CCHd CHCO 2 - ), 8,16-23 due to the existence of a LBHB in the maleic acid monoanion (hydrogen maleate). Several experimental and theoretical studies have been designed to measure the hydrogen bond strength in hydrogen maleate. 20-23 The reported values cover a rather wide energy range. Schwartz and Drueckham- mer 20 measured the equilibrium constant between hydrogen maleate and the corresponding trans-monoanion isomer, as well as the cis-/trans-neutral diacids. The difference in the relative free energies of the cis- and trans-isomers was used as an estimate of the relative strength of the intramolecular hydrogen bond in hydrogen maleate. Their equilibrium measurement showed that the energy difference between the cis- and trans- forms of hydrogen maleate ranged from 0.5 to 5.5 kcal/mol, depending on the solvent used, with lower polarity solvents favoring stronger hydrogen bonding. However, this estimate of the hydrogen bond strength is significantly lower than the 20 kcal/mol value suggested for a LBHB. Theoretically, Lluch et al. 21 have calculated that the hydrogen maleate is 14.14 kcal/ mol more stable than the fumarate. The gas-phase hydrogen bond strength in the monoanion was estimated to be 18.35 kcal/ mol higher than that of the normal hydrogen bond in the neutral diacid. Using 1a in Chart 1 as the reference structure (non- hydrogen bond case in hydrogen maleate), McAllister 22 calcu- lated the gas-phase hydrogen bond strength to be approximately 27 kcal/mol. Bach et al. 23 have computed the hydrogen bond strength on the basis of different reference structures (1a, 1b, and 1c in Chart 1). They concluded that the gas-phase intramolecular hydrogen bond strength in hydrogen maleate is in the range 14-28 kcal/mol, depending on the choice of reference structures. * To whom correspondence should be addressed. E-mail: ls.wang@ pnl.gov. CHART 1 10633 2005, 109, 10633-10637 Published on Web 11/04/2005 10.1021/jp0553277 CCC: $30.25 © 2005 American Chemical Society