Theoretical Study of the Influence of Electric Fields on Hydrogen-Bonded Acid-Base Complexes Mar Ramos, ²,‡ Ibon Alkorta,* ,² Jose Elguero, ² Nicolai S. Golubev, § Gleb S. Denisov, § Hans Benedict, | and Hans-Heinrich Limbach* ,| Contribution from the Instituto de Quı ´mica Me ´ dica, C.S.I.C., Juan de la CierVa 3, E-28006 Madrid, Spain, Institute of Physics, St. Petersburg State UniVersity, 198904 St. Petersburg, Russian Federation, and Institut fu ¨ r Organische Chemie, Takustrasse 3, Freie UniVersita ¨ t Berlin, D-14195 Berlin, Germany ReceiVed: August 7, 1997; In Final Form: October 2, 1997 X Matrix effects on the optimized geometries and the electronic properties of acid-base complexes XHB, with HX ) HF, HCl, HBr, HCN and B ) NH 3 , have been modeled using ab initio methods (6-31G** and 6-311++G** basis sets) in two different ways. Model A corresponds to the Onsager SCRF model, and model B corresponds to a homogeneous electric field F ) 2qe 0 /r e 2 ) 2.88 × 10 5 q V/cm of varying strength generated by two distant charges +qe 0 and -qe 0 of opposite sign placed at distances of r e ) 100 Å. In both models, the minima and reaction coordinate of proton transfer has been calculated. As the electric interactions are increased, both models predict an increase of the dipole moments associated with a proton shift from X to B, i.e., a conversion of the molecular to the zwitterionic complexes. Both models predicts double minima for some electric fields; in model B electric fields are found where the neutral complex is not stable, evolving to the ion pair complex. These fields can be used to characterize the acidity of the donor toward the base without the necessity of assuming a proton-transfer equilibrium. In both models a similar field-induced correlation between the two hydrogen bond distances r 1 ≡ X‚‚‚H and r 2 ≡ H‚‚‚B is observed for all configurations. This correlation indicates in the molecular complexes a hydrogen bond compression when the proton is shifted toward the base and in the zwitterionic complexes a widening. The minimum of the X‚‚‚B distance r 1 + r 2 occurs when the proton-transfer coordinate r 1 - r 2 ) r 01 - r 02 , where r 01 and r 02 represent the distances X‚‚‚H and H‚‚‚B + in the free donors. Introduction Hydrogen bonds are one of the most important forces for the structural organization of molecules in condensed phase. They define the 3D arrangement of macromolecules and their biological activity. 1 In hydrogen-bonded complexes XHB between an acid HX and a base B, proton transfers can take place that are the key steps of a large number of chemical and biochemical reactions. 2 Thus, numerous experimental and theoretical studies have been carried out to understand the properties of strong hydrogen bonds and the nature of the proton transfer in these bonds as a function of the environment. 3,4 Most of the ab initio calculations of hydrogen-bonded complexes XHB refer to isolated systems in the gas phase where the hydrogen bond geometries are often very different from those in condensed matter. The effect of external electric fields that can model the solvent effect on the hydrogen bonds have been studied by Scheiner et al. 5 and Eckert and Zundel. 6 In the first case, the potential energy surface of the proton transfer was studied using nonhomogeneous electric fields in systems where the heavy atom positions were fixed. The second study considered a homogeneous electrid field over the BrH‚‚‚CH 3 - NH 2 system. The energy and dipole moment surfaces were calculated as a funtion of the electric field. Kurnig and Scheiner examined the three X-H‚‚‚NH 3 cases (X ) F, Cl, Br), taking into account solvent effects. 7 They used the reaction field susceptibility g [g ) (2/a 3 )Y, where a is the radius of the sperical cavity and Y is the Onsager function] to describe the solvent and a “proton transfer parameter” F [F) Δr(XH) - Δr(NH), Δr’s being defined as differences between the complex and the isolated XH and NH 4 + ]. They show that an increase in g produces an increase in F (the proton is transferred when F> 0), the phenomenon being dependent on the nature of X (Br > Cl . F). The distance X‚‚‚N decreases (contraction) when g increases (greater ionic character). Note, finally, that Scheiner’s F is related to our r 1 -r 2 distance (Figure 1) [for the three complexes (r 1 - r 2 ) ) 0.996 + 2.712 F, r 2 ) 1.000]. Recently, Scheiner and Kar 8 and Clementi et al. 9 have modeled, using ab initio methods, solvent effects on XHB. Clementi contribution concerns very high level calculations of one of Scheiner’s complexes, Cl-H‚‚‚NH 3 . Since the experimental geometry was known (d Cl-H ) 1.30 Å, d N‚‚‚H ) 1.82 Å, d Cl‚‚‚N ) 3.13 Å), Clementi shows that MP2 or CCSD calculations with a very large basis set are necessary to reproduce exactly the experi- mental geometry (compare with Scheiner’s MINI-1 results, d Cl-H ) 1.44 Å, d N‚‚‚H ) 1.58 Å, and with our 6-311++G** results, d Cl-H ) 1.29 Å, d N‚‚‚H ) 2.01 Å). Solvent effects were studied, using the SCRF method, for different values of e, maintaining the distance d Cl‚‚‚N ) 3.13 Å. 9 In the case of Scheiner and Kar, 8 the complex was placed in the center of a spherical cavity surrounded by a continuous medium with a given dielectric constant ǫ. The reaction electric field generated by this model is proportional to the dipole moment of the solute in a medium of increasing dielectric constant, where the reactive electric field is generated using the Onsager SCRF formalism. 10 As ǫ was increased, a proton-transfer equilibrium was predicted to arise between the molecular complexes XH‚‚‚B and zwitterionic complexes X - ‚‚‚HB + , separated by an energy barrier that arises ² Instituto de Quı ´mica Me ´dica. ‡ On leave from the Departamento de Quı ´mica Orga ´nica I, Facultad de Quı ´mica, Universidad Complutense de Madrid, Ciudad Universitaria s/n, E-28040 Madrid, Spain. § Institute of Physics. | Institut fu ¨r Organische Chemie. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, November 15, 1997. 9791 J. Phys. Chem. A 1997, 101, 9791-9800 S1089-5639(97)02586-3 CCC: $14.00 © 1997 American Chemical Society