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Effects of local Coulomb potentials on acid and base protonation–deprotonation rates and equilibria
Author(s) -
Smuczyńska Sylwia,
Simons Jack
Publication year - 2009
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.22230
Subject(s) - deprotonation , protonation , chemistry , proton , amine gas treating , computational chemistry , potential energy surface , yield (engineering) , proton affinity , ab initio , chemical physics , photochemistry , organic chemistry , thermodynamics , quantum mechanics , ion , physics
Abstract Ab initio electronic structure methods, even when performed at a very modest level, applied to the energy profiles for deprotonation of an organic acid or of a protonated amine in the presence of proximal charged groups yield data that suggest the following: 1. The proton affinity of an amine tethered to a surface can be altered substantially by the presence of proximal positively charged groups. 2. Barriers (above the thermodynamic energy requirement) along the amine protonation–deprotonation pathway arise when positive charges are tethered nearby. 3. A competition between the attractive intrinsic NH bonding potential and the repulsive Coulomb potential between the departing proton and the proximal positive charges is the primary source of the above two results. 4. There is much less change in the equilibrium constant K or the deprotonation and protonation rate constants k 1 and k −1 for an organic acid tethered to a surface when in the presence of proximal negative charges. 5. The attractive intrinsic OH bonding potential and the attractive Coulomb potential between the proton and the proximal and nascent negative charges act in concert in the organic acid case. So, unlike the amine example, there is no competition between the dominant potentials and, as a result, no pronounced barriers occur along the reaction path. The relevance of these findings to acid–base properties of surfaces and nanoparticles are discussed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009