Non-bonded Interactions of Acetylene

The acetylenic proton is well known to be more acidic than its alkyl or alkenyl analogues. It should, therefore, be more susceptible to H-bond formation with suitable acceptor atoms, e.g. C—H—O. Indeed, the result of theoretical studies on the acetylene-water system d37 indicated that acetylene does form H-bonds to water oxygens. This 

Molecular packing in the low-temperature orthorhombic phase of perdeutero-acetylene shows a layer structure with a T-shaped interaction between pairs of molecules within a layer. The D—C distances form an isosceles triangle with edges of 2.738, 2.737 A and a D to C=C midpoint distance of 2.672 A. The D—C contacts are some 0.2 A shorter than those expected from van der Waals radii considerations. The geometrical arrangement clearly implies a donor-acceptor interaction between D and the triple-bond TT-electrons, a situation described as long ago as 1972 to account for close C=C—I interactions in crystals of diiodo-acetylene. More recently five O—H—C C and five NH—C=C TT-interactions with mean H—C distances of 2.69 (for O—H) and 2.61 (for N—H) have been located in the CSD. 

This T-shaped geometry is also identified in a microwave study of the C2H2.HF complexwhere the H—C=C midpoint distance is short at 2.19 A consistent with the higher acidity of HF by comparison with acetylene. Microwave and infrared studies of acetylene dimer also reveal a T-shape deviating by 27° from the fully perpendicular arrangementThe distance between the centres of mass of the two monomer units is given as 4.38 A. 

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