Five-membered ring compounds from the ortho-benzyne plus methyl radical reaction under interstellar conditions

Reactive open-shell species, such as radicals and biradicals, are key intermediates in the formation of (poly)cyclic hydrocarbon species in a variety of interstellar environments, ranging from cold molecular clouds to the outflows of carbon-rich stars. In this work, we identify the products of the o-benzyne + methyl radical reaction isomer-selectively by photoion mass-selected threshold photoelectron spectroscopy. We assign the benzyl (C7H7.) radical as the sole intermediate of the association reaction. Subsequent hydrogen-atom loss from benzyl yields the five-membered ring species fulvenallene (FA), 1-ethynylcyclopentadiene (1ECP) and 2-ethynylcyclopentadiene (2ECP), which have recently been detected in the cold molecular cloud TMC-1. We report a comprehensive C7H7 potential energy surface of the title reaction and show that the products form via direct barrierless addition followed by ring contraction and hydrogen elimination. A statistical model predicts 89% 1ECP, 8% FA and 3% 2ECP branching ratios at 0 K. Astrochemical simulations of TMC-1 incorporating this reaction result in the excellent reproduction of the abundance of a five-membered ring species, 1ECP, and provide strong evidence for the in situ "bottom-up' formation of small cyclic species in cold cores. Last, we put the results in context of the recent detection of fulvenallene in TMC-1.