Surface Acidity of Water Probed by Free Energy Calculation for Trimethylamine Protonation

Surface acidity/basicity of water has been elusive, due to contradicting observations from various aspects. One effective definition has been proposed by investigating the protonation equilibrium (pKa) of the indicator species at the water surface. We calculated the pKa of trimethylamine at the surface by using quantum mechanics/molecular mechanics (QM/MM) and thermodynamic integration calculations and elucidated the observed pKa shift at the surface. It was revealed that the observed pKa shift reflects specific properties of the indicator species, particularly relative solvation free energy at the surface. The pKa shift appears within ∼5 A thickness. One has to properly take account of the specific indicators toward deriving a general picture of surface acidity/basicity from protonation reactions.

[1]  P. Kollman,et al.  How well does a restrained electrostatic potential (RESP) model perform in calculating conformational energies of organic and biological molecules? , 2000 .

[2]  H. Allen,et al.  Unified molecular picture of the surfaces of aqueous acid, base, and salt solutions. , 2005, The journal of physical chemistry. B.

[3]  T. Darden,et al.  Particle mesh Ewald: An N⋅log(N) method for Ewald sums in large systems , 1993 .

[4]  N. Matubayasi,et al.  Simple and exact approach to the electronic polarization effect on the solvation free energy: formulation for quantum-mechanical/molecular-mechanical system and its applications to aqueous solutions. , 2012, The Journal of chemical physics.

[5]  G. Ciccotti,et al.  Numerical Integration of the Cartesian Equations of Motion of a System with Constraints: Molecular Dynamics of n-Alkanes , 1977 .

[6]  B. C. Garrett,et al.  Molecular simulations of the transport of molecules across the liquid/vapor interface of water. , 2006, Chemical reviews.

[7]  A. Becke,et al.  Density-functional exchange-energy approximation with correct asymptotic behavior. , 1988, Physical review. A, General physics.

[8]  Parr,et al.  Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. , 1988, Physical review. B, Condensed matter.

[9]  G. Richmond,et al.  Adsorption and reaction of CO2 and SO2 at a water surface. , 2006, Journal of the American Chemical Society.

[10]  Nobuyuki Matubayasi,et al.  A quantum chemical approach to the free energy calculations in condensed systems: the QM/MM method combined with the theory of energy representation. , 2004, The Journal of chemical physics.

[11]  Michael T. Bowers,et al.  A thermodynamic analysis of solvation effects on the basicities of alkylamines. An electrostatic analysis of substituent effects , 1976 .

[12]  Leonard Kleinman,et al.  Efficacious Form for Model Pseudopotentials , 1982 .

[13]  L. Dang,et al.  Recent advances in molecular simulations of ion solvation at liquid interfaces. , 2006, Chemical reviews.

[14]  Masayoshi Takahashi,et al.  Zeta potential of microbubbles in aqueous solutions: electrical properties of the gas-water interface. , 2005, The journal of physical chemistry. B.

[15]  W. Kohn,et al.  Self-Consistent Equations Including Exchange and Correlation Effects , 1965 .

[16]  S. Trasatti.  The absolute electrode potential: an explanatory note (Recommendations 1986) , 1986 .

[17]  A. Becke.  Density-functional thermochemistry. III. The role of exact exchange , 1993 .

[18]  T. Straatsma,et al.  THE MISSING TERM IN EFFECTIVE PAIR POTENTIALS , 1987 .

[19]  P. Kollman,et al.  Atomic charges derived from semiempirical methods , 1990 .

[20]  Y. Saad,et al.  Finite-difference-pseudopotential method: Electronic structure calculations without a basis. , 1994, Physical review letters.

[21]  N. Matubayasi,et al.  Erratum: “Theory of solutions in the energy representation. II. Functional for the chemical potential” [J. Chem. Phys. 117, 3605 (2002)] , 2003 .

[22]  Hideaki Takahashi,et al.  Real Space Ab Initio Molecular Dynamics Simulations for the Reactions of OH Radical/OH Anion with Formaldehyde , 2001 .

[23]  A. Morita,et al.  Molecular dynamics analysis of interfacial structures and sum frequency generation spectra of aqueous hydrogen halide solutions. , 2007, The journal of physical chemistry. A.

[24]  Nobuyuki Matubayasi,et al.  Theory of solutions in the energy representation. III. Treatment of the molecular flexibility , 2003 .

[25]  M. Hoffmann,et al.  Proton Availability at the Air/Water Interface , 2010 .

[26]  N. Matubayasi,et al.  Chapter 9 - Development of a Quantum Chemical Method Combined with a Theory of Solutions—Free-Energy Calculation for Chemical Reactions by Condensed Phase Simulations , 2010 .

[27]  J. Tomasi,et al.  Quantum mechanical continuum solvation models. , 2005, Chemical reviews.

[28]  Gregory A. Voth,et al.  The hydrated proton at the water liquid/vapor interface , 2004 .

[29]  S. Yamaguchi,et al.  Communication: Quantitative estimate of the water surface pH using heterodyne-detected electronic sum frequency generation. , 2012, The Journal of chemical physics.

[30]  H. Berendsen,et al.  Molecular dynamics with coupling to an external bath , 1984 .

[31]  Robert Vácha,et al.  Water surface is acidic , 2007, Proceedings of the National Academy of Sciences.

[32]  Nobuyuki Matubayasi,et al.  Theory of solutions in the energy representation. II. Functional for the chemical potential , 2002 .

[33]  Tomoya Ono,et al.  Timesaving Double-Grid Method for Real-Space Electronic-Structure Calculations , 1999 .

[34]  Hideaki Takahashi,et al.  A hybrid QM/MM method employing real space grids for QM water in the TIP4P water solvents , 2001, J. Comput. Chem..

[35]  R. Saykally,et al.  Is the liquid water surface basic or acidic? Macroscopic vs. molecular-scale investigations , 2008 .

[36]  P. Kollman,et al.  An approach to computing electrostatic charges for molecules , 1984 .

[37]  J. Beattie,et al.  The surface of neat water is basic. , 2009, Faraday discussions.

[38]  M. Faubel,et al.  Behavior of hydroxide at the water/vapor interface , 2009 .

[39]  P. P. Ewald.  Die Berechnung optischer und elektrostatischer Gitterpotentiale , 1921 .

[40]  A. Volkov,et al.  Quantitative Theory of Surface Tension and Surface Potential of Aqueous Solutions of Electrolytes , 2002 .

[41]  Hideaki Takahashi,et al.  A Density Functional Study for Hydrogen Bond Energy by Employing Real Space Grids , 2000 .

[42]  P. Kollman,et al.  Structure and Properties of Neat Liquids Using Nonadditive Molecular Dynamics: Water, Methanol, and N-Methylacetamide , 1995 .

[43]  Nobuyuki Matubayasi,et al.  Theory of solutions in the energetic representation. I. Formulation , 2000 .

[44]  M. Bowers,et al.  Quantitative proton affinities, ionization potentials, and hydrogen affinities of alkylamines , 1976 .

[45]  Wu,et al.  Higher-order finite-difference pseudopotential method: An application to diatomic molecules. , 1994, Physical review. B, Condensed matter.

[46]  E. P. Hunter,et al.  Evaluated Gas Phase Basicities and Proton Affinities of Molecules: An Update , 1998 .