Effects of Formaldehyde Substituents on Potential Energy Profiles for Proton Transfer in [ABCO-H-OCXH]+

Abstract

Ab initio methods are used to discover the effects of formaldehyde substituents on potential hypersurfaces for proton transfer in the equilibrium complex (ABCO-H-OCXH)+ in which A, B, and X are electronreleasing or -withdrawing groups. The potential profiles span the full range from symmetric double well, asymmetric double well, to single well, depending on the substituents. A symmetric double well corresponds to a complex with two equivalent subunits such as (FCHO-H-OCHF)', whereas in (HFCO-H-OCH?)' only one minimum structure is obtained in the entire potential surface. When the protonation energies of the two subunits are not greatly different an asymmetric double well might form. A ratio o to represent the extent of the difference of protonation energies between the two subunits in the complex was introduced to illustrate the formation of an asymmetric double well for calculated several complexes. To determine which conformation of the two wells has lower energy, the magnitude of the addition of binding energy of the conformation and the protonation energy of the subunit nearer the central proton is a crucial factor. The bigger is the magnitude of the conformation, the deeper is the well. A deeper right well in either the trans or cis conformer of (CH,FCHO-H-OCH*)'can be clarified easily with this magnitude as a parameter. It would be puzzling if only one term of energy (either binding energy of the conformation of two wells or protonation energy of the two subunits) were used. The difference of the magnitudes in two wells represents the potential gap between the two wells. The geometries of complexes varied from the parent complex (H2CO-H-OCH*)+ are discussed briefly based on the direction of the dipole moment in the substituted subunits. The thermodynamic properties AH', AS", and AGO of the association reaction ABCOH' + HXCO -(ABCO-H-OCXH)+ at several temperatures are evaluated according to standard thermodynamic formulae that incorporate the vibrational frequencies of the various species. IntroductionProton transfer involved in chemical and biological processes has been extensively investigated.Experimental measurements yielded a linear correlation between the bond dissociation energy and the difference between the proton affinities of the proton donor and of the proton acceptor.2 Blatz et al.' measured the concentrations of the H-bonded form and of the protontransferred form of a Schiff base quantitatively as a function of temperature through the absorbance of the species. Deuterium isotope fractionation6 in the gas phase of proton-transfer reactions between DO-and substituted 2-propenes have also been measured. Quantum chemical calculations were employed to supplement the experimental work; this approach has the advantage that it yields both energies and structures of the intermediates and transition states. Such a calculation supplies directly the activation energies and is independent of a dynamic model to correlate rates withThe transition structures and energy barriers of intramolecular proton transfe...

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