THE ULTRAVIOLET PHOTODISSOCIATION DYNAMICS OF PYRROLE

Photofragment translational spectroscopy was used to study the photodi ssociation of pyrrole at 193 and 248 nm under collision-free condition s. Five primary dissociation channels were observed at 193 nm. Two cha nnels resulted from cleavage of the N-H bond to yield H + pyrrolyl rad ical with one channel following internal conversion (IC) to the ground state (almost-equal-to 21%) and the other originating from electronic ally excited pyrrole (almost-equal-to 30%). Two dissociation channels involved elimination of HCN following IC. One channel producing HCN vinylmethylene (almost-equal-to 25%) following ring opening and hydrog en migration and the other proceeding via a bridged 3H-pyrrole interme diate to form HCN + cyclopropene (almost-equal-to 24%). The last chann el at 193 nm involved IC to the ground state followed by ring opening and N-C bond cleavage to form NH + CHCCHCH2 (<1%). At 248 nm three dis sociation channels were observed, all of which involved the eliminatio n of atomic hydrogen. Analogous to the results at 193 nm, two of these channels resulted from cleavage of the N-H bond with one channel foll owing IC (almost-equal-to 42%) and the other dissociating from an exci ted electronic state (almost-equal-to 47%). The third dissociation cha nnel at 248 nm involved the cleavage of one of the two C-H bonds in el ectronically excited pyrrole (almost-equal-to 11%). Translational ener gy distributions were determined for all observed dissociation channel s. From consideration of the maximum translational energy of the photo fragments D0(N-H) = 88+/-2 kcal/mol, D0(C-H) = 112.5+/-1 kcal/mol and DELTAH(f)(pyrrolyl radical) = 62+/-2 kcal/mol were determined.