GAS-PHASE AND SOLUTION-PHASE POTENTIAL-ENERGY SURFACES FOR CO2+NH2O(N=1,2)

Ab initio and free energy perturbation simulations are used to characterize both the gas-phase and solution-phase potential energy surfaces for the reaction of CO2 with H2O. The 3-21G, 6-31G, and 6-31G⋆⋆ basis sets were used to locate and fully optimize all critical points for the reactions of one and two H2O molecules with CO2. Where possible, correlation corrections were determined at the MP4/6-31G⋆⋆//6-31G⋆⋆ level, otherwise, MP2/6-31G⋆⋆//6-31G⋆⋆ was used. From these calculations our best estimate (MP4/6-31G⋆⋆//6-31G⋆⋆) for the activation energy for the reaction of CO2 with one water molecule is 54.5 kcal/mol and for the dehydration reaction it is 46 kcal/mol. Our best estimate (MP2/6-31G⋆⋆/6-31G⋆⋆) for two water molecules reacting with CO2 is 32.3 kcal/mol and for the reverse reaction it is 25.5 kcal/mol. Zero-point and entropic corrections determined with use of 3-21G vibrational frequencies give free energies of activation (ΔGact) for the one and two-water reactions of 56.7 ± 1.0 kcal/mol (reverse 41.1 ±1.0 kcal/mol) and 40.6 ± 1.0 kcal/mol (reverse 25.2 ± 1.0 kcal/mol), respectively. Thus, we predict that the reaction of two water molecules with CO2 is more facile than with one in the gas phase. This can be rationalized by considering that the reaction of a single water molecule results in a four-electron process (Woodward-Hoffman forbidden), while the reaction of two water molecules involves six electrons (Woodward-Hoffman allowed). The unfavorable entropic contribution for the two-water reaction is not great enough to disfavor this reaction over the one-water case. In order to assess how the gas-phase ab initio results reflect what might be occurring in solution we have developed molecular mechanical parameters that enable us to determine approximate solution-phase ΔGact for these reactions. Using the 6-31G⋆⋆//3-21G level of theory we have determined interaction energies between the critical points along the reaction profile and used these to determine a set of force field parameters that reproduce these energies. We then used these parameters to evaluate relative free energies of solvation for the critical points using free energy perturbation theory. These combined with the ab initio free energies allowed us to construct an approximate free energy profile for these reactions. We find that the forward ΔGact for the one-water reaction is 49.1 ± 1.4 kcal/mol, while that for two is 21.2 ± 1.3 kcal/mol. The ΔGact for the reverse reactions were found to be 44.1 ± 1.0 and 15.2 ± 1.0 kcal/mol, respectively. The experimental value for the forward and reverse solution-phase ΔGact is 18.4 and 15.3 kcal/mol, respectively. The two-water reaction is in better agreement with experiment and suggests that this reaction is the dominant one in solution. © 1990, American Chemical Society. All rights reserved.