Calculated Effects of Formaldehyde Substituents on Proton Transfer in (H₂CO–H–OCX₂)⁺

In quantum-chemical calculations with full geometry optimization of the energetics of proton-bridged complexes (H₂CO–H–OCX₂)⁺, in which X = H, F, Cl, and CH₃, we used a polarized split-valence basis set 4-31G* with fourth-order Moller–Plesset perturbation theory (MP4) treatment for electron correlation. The presence of a fluorine substituent decreases the proton affinity of oxygen; formyl fluoride is more acidic than formaldehyde by 13–15 kcal/mol. In contrast, the methyl group in acetaldehyde increases the proton affinity of oxygen; acetaldehyde is more basic than formaldehyde by about 12 kcal/mol. The proton-transfer potentials for halogen-substituted complexes contain a single minimum corresponding to (H₂COH⁺ · · ·OCHX), whereas an asymmetric double-well potential was found in methyl-substituted complexes; the global minimum energy corresponds to the conformation (H₂CO· · ·⁺HOCHCH₃). Proton transfer proceeds with greater difficulty in fluoro-substituted complexes than in the nonsubstituted complex, whereas with much greater ease in methyl-substituted counterparts. Substituted complexes are less stable than nonsubstituted ones; the binding energies are smaller by about 3–5 kcal/mol, regardless of the nature of the substituents. The structures of the complexes vary greatly with the substituents and their positions. They are further analyzed in regard to the direction of the dipole moment of the subunit in the complexes. The transition structures in the proton-transfer potentials all have the central proton on the O–O axis, but the location depends on the type of substituent.