Bonding rearrangements of hydrogen-bonded complexes involving alkynes

Molecules containing a C-C triple bond, such as HCtriple bond]CH, FCtriple bond]CF, and the Ctriple bond]CH radical, are allowed to interact with a partner molecule of H2O, NH3, or HF. Quantum chemical calculations show that these Ctriple bond]CH...X H-bonded complexes are bound by up to 4 kcal x mol(-1). More importantly, they can rearrange in such a way that the partner molecule adds to the triple bond so as to form a double C=C bond. Whereas this process is strongly exoergic, there is a high-energy barrier to this rearrangement process. On the other hand, when a second water molecule is added to the complex, it can shuttle protons from the donor part of the complex to the acceptor, and thereby greatly reduce the rearrangement energy barrier. In the case of CCH + 2H2O, this barrier is computed to be less than 4 kcal x mol(-1).