Hydrogen bonding and proton transfers involving triply bonded atoms. Acetylene and hydrocyanic acid

The formation of the H-bonded complexes (HCCH˙˙˙CCH)-, (NCH˙˙˙CN)-, (CNH˙˙˙NC)-, (NCH˙˙˙NC)-, and (CNH˙˙˙CN)- and the subsequent proton transfers taking place within them are studied by ab initio methods. When all H bonds are constrained to the same length, the proton-transfer barrier diminishes as the acceptor group becomes more basic. Hence, the "intrinsic" barrier for proton transfer between C atoms is lower than that for internitrogen transfer. However, greater acidity of the proton donor group leads to a stronger and hence shorter H bond in which the proton needs to traverse a smaller distance from donor to acceptor. Consequently, when the length of the H bond is freed of external restraint, the dominant factor controlling the height of the transfer barrier switches from the acceptor to the donor group. While the full potential energy surface characterizing proton transfer between C atoms contains a pair of equivalent minima separated by an energy barrier, internitrogen transfer occurs in the absence of a barrier. Only one minimum, corresponding to (NCH˙˙˙NC)-, is present in the potential of the asymmetric system. These principles explain the previously observed difference in proton transfer behavior between C acids and normal acids containing N and 0 atoms.

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