The kinetic isotope effect in the photo-dissociation reaction of excited-state acids in aqueous solutions

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23 Scopus citations

Abstract

The effect of isotope substitution (H/D) on the kinetics of acid - base reactions has long been a primary mechanistic tool in proton-transfer research.1 - 7 The relative ease with which such experiments may be carried out in aqueous solutions of strong acids has made this mechanistic approach very appealing to experimentalists. From the standpoint of theory, the isotope effect on proton transfer dynamics gives access to the microscopic mechanistic details of the process. Laser-induced photoacidity has been recognized for decades as the major experimental tool in the research of bimolecular proton transfer reactions in aqueous solutions. In such experiments, which were pioneered by Förster and Weller,8,9 one uses suitable organic molecules which are weak acids in the electronic ground state. Upon optical excitation the acidity of the molecules increases considerably and rivals the acidity of strong mineral acids. Molecules which undergo such a transition in their acidity are usually termed photoacids.10 The most commonly used photoacids are simple hydroxyarenes (phenol-like photoacids) such as the 1- and 2-naphthols which increase their acidity by more than a factor of a million when excited to their first electronic singlet state.11 - 16 Combining isotope substitution with photoacid research is potentially a very promising avenue toward unveiling the important mechanistic details of bimolecular proton transfer reactions. Despite extensive experimental and theoretical efforts many principal questions about bimolecular proton-transfer reactions in solutions are still open: in particular, is the reaction coordinate in the transition-state region the hydrogen-bond coordinate or, as expected for sufficiently highly exothermic and thereby almost barrierless reaction, is it the coordinate of solvent motion? Some related questions are: (a) what is the rate limiting step of proton transfer in solution; (b) how is proton motion coupled to the solvation process; (c) how important is proton tunneling; (d) are bimolecular proton-transfer reactions between acids and bases in aqueous solutions through-solvents or innersphere processes? Particularly valuable information about molecular mechanism can be obtained by determining the influence of the isotopic substitution on the rate constants and the activation energy of proton transfer. Below we consider the basic kinetic models for acid - base reactions in solutions and apply them to the proton-dissociation reaction of photoacids.

Original languageEnglish
Title of host publicationIsotope Effects in Chemistry and Biology
PublisherCRC Press
Pages451-464
Number of pages14
ISBN (Electronic)9781420028027
ISBN (Print)9780824724498
DOIs
StatePublished - 1 Jan 2005

ASJC Scopus subject areas

  • General Chemistry
  • General Medicine
  • General Biochemistry, Genetics and Molecular Biology

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