A study of reactions of C nH +m ions ( n = 4, 5, 6; m = 0–6) with H 2 and CO at 300 K and 80 K

Abstract

The reactions of ions in the series C n H + m ( n = 4, 5, 6, m = 0–6), derived by electron impact on benzene vapour, have been studied with H 2 and CO in a selected ion flow tube apparatus at 80 K and 300 K (and in some cases also at 210 K and 475 K) and over limited pressure ranges of helium carrier gas; both the rate coefficients and the product ions of the reactions have been determined. In general only the less-hydrogenated ions ( m ⩽ 1) showed significant reactivity with H 2 and CO, a notable exception being C 6H + 5, which additionally was generated in two differently reacting forms. Further, the ions C 4H + 2, C 4H + 3 and C 5H + 2 were observed to react only with CO. The reactions with H 2 proceed mainly by H-atom abstraction producing ions possessing an additional H atom. Thus hydrogenation of the bare carbon chain ions can only proceed to m = 2 by these reactions. The reaction with CO were exclusively association reactions producing ions of the form C n H m CO +. In addition, the bare carbon chain ions were the only ones observed to add two CO molecules. Some of these association reactions are very fast, especially at 80 K, the effective two-body rate coefficient being close to the collisional limiting value and independent of the helium pressure over the available range. Therefore only lower limits to the three-body rate coefficients can be given. The intriguing possibility arises that some of these fast association reactions may be occurring in two-body collisions, the binding energy being released by the emission of photons. This would have major implications for the synthesis of carbon—oxygen chain molecules in interstellar clouds.