A Computational Investigation of Cooperativity in Weakly Hydrogen Bonded Assemblies

The non-covalent forces present in ethyne oligomers and ethyne-water aggregates containing C-H ... pi(C drop C) interactions, are investigated using ab initio calculations. The C-H ... pi(C drop C) interaction is found to be a very weak hydrogen bonding interaction, in accordance with previously reported work, with an enthalpy of interaction of around -4 kJ mol(-1). The potential surface of this interaction in the ethyne T-shaped dimer demonstrates that the interaction energy is relatively insensitive to the position of the donating proton along the bond vector of the accepting triple bond as well as to the tilt angle of the major axis of the acetylene molecule.

The strength of the O-H ... pi(C drop C) contact is found to be consistent with a very weak hydrogen bonding interaction with an enthalpy of interaction of around -6 kJ mol(-1) which is very similar to that of the ethyne T-shaped dimer although ethyne is a much poorer hydrogen bond donor. The interaction energy per C-H ... pi(C drop C) interaction in ethyne trimers, both cyclic and linear, as compared to that in the ethyne dimer does not appear to suggest that there are large gains in stabilisation through cooperativity and the use of a more polarised surface-that of a water molecule-to create larger polarisation effects again resulted in small cooperative gains (0-10% range) suggesting that hydrogen-bonded arrays containing terminal alkynes are incapable of exhibiting significant cooperative enhancements. It is demonstrated that the shifts in the stretching frequency of the C-H bond of propyne in different intermolecular C-H ... O hydrogen bonding environments are insignificant relative to the effects of the accepting strength of the oxygen and therefore infra-red spectroscopic data may not provide sufficient evidence to prove that large cooperative effects operate in these hydrogen bonding arrays.