Dimers with two interaction sites

In the preceding section, we have seen that the dimer (HC=CH, HCN), for example, has two structural isomers, each of which presumably corresponds to a minimum on the interaction potential energy surface. The lower and higher energy minima can be referred to as arising from the primary (p) and secondary (s) interactions, respectively, between the component molecules. Thus, for component molecules labelled 1 and 2, fej = N, 2  

When k kl, only the primary isomer is observed in low temperature jet experiments. On the other hand, when k xkl, two possibilities exist, namely either two separate isomers are observed (see above) or, if the geometries of the two components permit, advantage is taken of both interactions to give a dimer held together by a two-site interaction. An example will clarify the genejal argument. 

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