Chiral Transformation in Protonated and Deprotonated Adipic Acids through Multistep Internal Proton Transfer

Abstract

Protonated and deprotonated adipic acids (PAA: HOOC-(CH2)(4)-COOH2+ and DAA: HOOC-(CH2)(4)-COO-) have a charged hydrogen bond under the influence of steric constraint due to the molecular skeleton of a circular ring. Despite the similarity between PAA and DAA, it is surprising that the lowest energy structure of PAA is predicted to have (H2O center dot center dot center dot H center dot center dot center dot OH2)(+) Zundel-like symmetric hydrogen bonding, whereas that of DAA has H3O+ Eigen-like asymmetric hydrogen bonding. The energy profiles show that direct proton transfer between mirror image structures is unfavorable. Instead, the chiral transformation is possible by subsequent backbone twistings through stepwise proton transfer along multistep intermediate structures, which are Zundel-like ions for PAA and Eigen-like ions for DAA. This type of chiral transformation by multistep intramolecular proton transfers is unprecedented. Several prominent OH center dot center dot center dot O short hydrogen-bond stretching peaks are predicted in the range of 1000-1700 cm(-1) in the Car-Parrinello molecular dynamics (CPMD) simulations, which show distinctive signatures different from ordinary hydrogen-bond peaks. The O-H-O stretching peaks in the range of 1800-2700 cm(-1) become insignificant above around 150 K and are almost washed out at about 300 K.