The ultraviolet photodissociation dynamics of pyrrole

Photofragment translational spectroscopy was used to study the photodissociation of pyrrole at 193 and 248 nm under collision-free conditions. Five primary dissociation channels were observed at 193 nm. Two channels resulted from cleavage of the NH bond to yield H + pyrrolyl radical with one channel following internal conversion (IC) to the ground state (≈21%) and the other originating from electronically excited pyrrole (≈30%). Two dissociation channels involved elimination of HCN following IC. One channel producing HCN + vinylmethylene (≈25%) following ring opening and hydrogen migration and the other proceeding via a bridged 3H-pyrrole intermediate to form HCN+cyclopropene (≈24%). The last channel at 193 nm involved IC to the ground state followed by ring opening and NC bond cleavage to form NH+CHCCHCH₂ (<1%). At 248 nm three dissociation channels were observed, all of which involved the elimination of atomic hydrogen. Analogous to the results at 193 nm, two of these channels resulted from cleavage of the NH bond with one channel following IC (≈42%) and the other dissociating from an excited electronic state (≈47%). The third dissociation channel at 248 nm involved the cleavage of one of the two CH bonds in electronically excited pyrrole (≈11%). Translational energy distributions were determined for all observed dissociation channels. From consideration of the maximum translational energy of the photofragments D₀(NH) =88±2 kcal/mol, D₀(CH) = 112.5±1 kcal/mol and ΔH_r(pyrrolyl radical) = 62±2 kcal/mol were determined.