are accelerated, indicating negative activation volumes for all these reactions. Considering the bond-formation nature of these reactions, the observed acceleration under pressure is not surprising. Apparently, different mechanisms are in operation in this dimerization, for both the symmetry-allowed [4+2] and [6+4] and forbidden [2+2] dimers, according to the Woodward-Hoffmann rules, were observed. The symmetry-allowed [4+2] cycloadducts and the [6+4]-ene adduct could, in principle, be formed through either a concerted or stepwise mechanism, whereas the symmetry-forbidden [2+2] cycloadducts are usually expected to be formed via a stepwise mechanism under thermal conditions (Figure ). The main difference between the concerted and stepwise mechanisms, in terms of activation volume, is that two bonds are formed at once in a concerted mechanism and only one bond is initially formed in a stepwise mechanism, so that the concerted transition state (TS) structure is usually more compact than the stepwise TS structure. Therefore, the concerted reaction usually has a more negative activation volume than a stepwise reaction.