Effects of external ions on the energetics of proton transfers across hydrogen bonds

Abstract

Ions of both positive and negative charge are placed in various locations around H bonds and energetics of proton transfer within the bonds calculated by ab initio molecular orbital methods. Ions examined include Li+, Na+, Cl', (NH4)+, HCOO", and point charges of both signs. These ions are found capable of producing profound effects upon the transfers, with the most marked changes occurring when the ions are positioned along the H-bond axis. Approach of each ion toward the symmetric (H20-H-0H2)+ system introduces progressively greater degrees of asymmetry into the transfer potential. As a cation approaches the nitrogen atom of (H3N-H-OH2)+, a second well appears in this potential which, in the absence of external charges, contains only one minimum. The new well becomes progressively stabilized under closer approach of the ion until the (H3N--HOH2)+ configuration is equal in energy to (H3NH--OH2)+ at a distance of 3 Á. The effects of all ions, including the point charges, are quite similar to one another, particularly for distances greater than 3 Á. The perturbations in the transfer potentials may be modeled quite closely by including the Coulombic interaction between a point charge and the proton undergoing the motion; however, the remaining atoms of the H-bonded system act to magnify to nearly double this effective field.

Effects of external ions upon proton transfer reactions: H-bonded systems containing HCOOH

“…So as to maintain consistency with prior calculations along these lines [ 13,141, ab initio calculations were carried out with at the SCF level using the 4-31G basis set [16]. While a quantitative reproduction of all the features of the systems concerned would clearly require a larger basis set and inclusion of electron correlation, the sc~/4-31G procedure is nonetheless expected to provide a satisfactory framework for studying the general trends.…”

“…Work in this laboratory has been devoted to a systematic study of this reaction in an effort to identify the fundamental principles involved [7-121. As part of this project, some of our prior calculations [13,14] have treated the effect upon the proton transfer potential of a set of ions and dipoles which are distributed in the region surrounding the H-bonded system.…”

“…The following observations [ 13,141 have suggested that the influence of the external species is electrostatic in nature: Various monocations such as Na', Li+, and NH; all have nearly identical effects. Perturbations caused by the monoanions C1-and HCOO-are similar to one another and opposite in sign to those of the cations.…”

“…The purpose of the present work is an investigation of how our prior conclusions may be altered when these small molecules are replaced by larger species. With this goal in mind, we take as a starting point the proton-bound water dimer (H,O -H OH,)' which has served as the model H-bonded system in our earlier work [13,14] and re-place one (or both) of the OH, molecules by the larger HCOOH. The more extended structure of HCOOH allows the external ion to approach a part of the molecule which is not directly involved in the H-bond.…”

The role of coupling in intramolecular proton transfer reactions. The hydrogen oxalate anion as an example

“…Given that these processes primarily involve the motion of the transfemng hydrogen, some calculations have frozen the other geometrical parameters (3,4). In other studies this important restriction has been partially lifted so that the distance between H-donor and H-acceptor atoms is fixed, all other geometrical parameters being fully optimized at each stage of the proton transfer (5)(6)(7).…”