The ultraviolet photodissociation dynamics of d1-vinyl chloride

We have used partially deuterated vinyl chloride to elucidate the photodissociation mechanism of this molecule. We have found that 75% of the HCl fragments are produced by three-center α,α elimination. Surprisingly, the rotational energy distributions of HCl are the same for both elimination pathways. One possible explanation is that H atom migration in the parent molecule results in similar transition states for the two pathways. Alternatively, dissociation and fragment isomerization may be strongly coupled so that the product angular momentum distribution is determined late in the reaction coordinate. Very different rotational state distributions were observed for v″=0 and v″≳0. We speculate that this dichotomy is caused by formation of a hydrogen-bonded π complex between HCl(v″≳0) and acetylene. We have also determined that HCl+ ions are produced primarily by an α,β mechanism. This mechanism consists of electronic excitation of vinyl chloride, followed by photoelimination and photoionization of electronically excited HCl*. Finally, we determined that H atoms are preferentially produced by detachment from the β carbon, as predicted from the relative stabilities of the α- and β-chlorovinyl radicals.