Molecular elimination and three-body dissociation

In addition to single bond fission it is also possible for two or more bonds to break simultaneously and for molecular fragments to be ejected. Both three- and four-centre transition structures are possible in molecular elimination processes, examples of the former being the elimination of H2 from CH2O and (a,a) elimination of HCl from CHCICF2. A four-centre (a,jS) transition structure must be involved for the elimination of HF from CHCICF2, and this competes with the three-centre elimination channel forming HCl (the ratio of three- to four-centre elimination is 0.87 0.13). 

With higher excitation energies, several bonds may be broken in either a concerted or stepwise manner. A good example of a stepwise process is the photodissociation of acetone, i.e. 

In contrast, photodissociation of the highly symmetric molecule -tetrazine is a concerted process with all three fragments being formed simultaneously, i.e. 

The elimination of HCl from vinyl chloride (CH2CHCI), following photo-excitation at 193 nm (ArF laser) can occur via either a three-centre (a,a) or a four-centre (a,jS) transition structure. The HCl is observed to be both vibrationally (v 7) and rotation-ally hot, with a bimodal rotational distribution (i.e. Ti 500 K and Ta = 10 000 K, for v 4). The low rotation energy distribution results from four-centre elimination, whereas the hot distribution is from three-centre elimination. Low rotational excitation is expected with four-centre elimination, as the H atom has a small impact parameter for its movement towards the halogen atom. 

In addition to the molecular elimination channel, four other channels are observed, including the 

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