Real-Time Observation of Bimodal Proton Transfer in Acid-Base Pairs in Water

• Published in: Science (American Association for the Advancement of Science) Vol. 301; no. 5631; pp. 349 - 352
• Main Authors: Rini, Matteo, Magnes, Ben-Zion, Pines, Ehud, Erik T. J. Nibbering
• Format: Journal Article
• Language: English
• Published: Washington, DC American Association for the Advancement of Science 18.07.2003; The American Association for the Advancement of Science
• Subjects: Acetates; Acid-base chemistry; Acids; Analysis; Chemical bases; Chemical reactions; Chemistry; Deuterons; Exact sciences and technology; General and physical chemistry; Ground state; Hydrogen bonds; Infrared radiation; Inhalants; Molecules; Organic Chemistry; Protons; Reaction mechanisms; Real time; Solution properties; Solutions; Solvents; Spectroscopy; Transfer Rates (College); Water; Germany; Water; Reaction mechanism; Acids; Base; Proton transfer; Hydrogen bond
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.1085762

The neutralization reaction between an acid and a base in water, triggered after optical excitation, was studied by femtosecond vibrational spectroscopy. Bimodal dynamics were observed. In hydrogen-bonded acid-base complexes, the proton transfer proceeds extremely fast (within 150 femtoseconds). In encounter pairs formed by diffusion of uncomplexed photoacid and base molecules, the reaction upon contact was an order of magnitude slower, in agreement with earlier reported values. These results call for a refinement of the traditional Eigen-Weller picture of acid-base reactions: A three-stage model is introduced to account for all observed dynamics.