Effect of Bond Multiplicity upon Hydrogen Bonding and Proton Transfers. Double Bonded Atoms

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MP2 level as compared to the stronger interaction of 10.3 for (HNCH-H...CHNH)- where the peripheral C atom has been replaced by N. The strongest interaction of 15.4 kcal/mol is observed in (H2CN-H...NCH2)- where N atoms participate directly in the H-bond.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 Å 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