The energetics of proton transfer in the (H3CH...CH3)- COMPlex, which is one stage in the gas-phase reaction CH4 + CH3- --> CH3- + CH4, have been investigated with ab initio calculations employing a 4-31G basis. The minimum-energy reaction path was determined by a steepest-descent technique in mass-weighted coordinates from the symmetric saddle point (1.35 kcal/mol above separated CH4 + CH3-) to the reactant/product association well (9.25 kcal/mol below separated CH4 + CH3-). Motion along this path is found to occur in two relatively distinct phases: motion of the proton between two fixed carbons followed by separation of the two hydrocarbon fragments. Rate constants computed by variational transition state theory, including an adiabatic approximation for the vibrational modes transverse to the reaction coordinate, demonstrate the importance of including the contributions from tunneling at energies below the top of the 7.88 kcal/mol vibrationally adiabatic barrier for low to moderate temperatures. In addition, incorporating the effects of reaction-path curvature in the tunneling calculation is found to be important, especially at low temperature. However, due to the dual nature of the reaction path, various model barriers fit-to the saddle point information yield rate constants that are much too large at low temperatures, thus underscoring the importance of computing the reaction path explicitly.

Ab initio methods are used to probe the proton-bound complex involving a water molecule and an amide, modeled by formamide or acetamide. A polarized basis set was applied in conjunction with MP2 treatment of electron correlation. This approach affords a good reproduction of experimental proton affinities of the species involved. The O atom of the amide is the preferred site of protonation or complexation with the water, with acetamide binding most strongly to the water. The proton-transfer potential of each complex contains a single minimum corresponding to H2NHCOH+...OH2, due to the more basic character of the amide oxygen. A second minimum, wherein the proton is bound to the water, occurs when the two molecules are further apart than their equilibrium separation. The energy barrier for proton transfer between the two minima grows rapidly as the two molecules are further removed from one another. The high barriers lead to very slow removal of the proton from an amide at room temperature.

Ab initio methods are used to study the interactions between H2C = CH2 and H2C = NH and their deprotonated anions. (H2CCH-H...CHCH2)- is the most weakly bound with a complexation energy of 5.6 kcal/mol at the correlated MP2 level as compared to the stronger interaction of 10.3 for (HNCH-H...CHNH)- where the peripheral C atom has been replaced by N. The strongest interaction of 15.4 kcal/mol is observed in (H2CN-H...NCH2)- where N atoms participate directly in the H-bond. (H2CCH-H...CHCH2)- contains the longest intermolecular separation while the N...N distance in the latter complex is the shortest. This separation between subunits undergoes a contraction between 0.5 and 0.9 angstrom as the proton reaches the transfer midpoint. The highest proton transfer barrier of 13 kcal/mol is observed for (H2CCH-H...CHCH2)-. In contrast, the small barrier in (H2CN-H...NCH2)- is eliminated altogether when zero-point vibrations are considered. Transfer rates are computed using modified RRKM theory. These results are placed within the broader context of other complexes in which the atoms participating in the H-bond are single- and triple-bonded within their respective subunits so as to arrive at systematic conclusions regarding the effects of such multiple bonding upon the energetics of H-bond formation and proton transfer.