Crossed-beam reactive scattering of F2 plus C6H6: Heat of formation of i p s o-fluorocyclohexadienyl radical

Abstract

Angular distributions and relative total cross sections were measured for products of the collisions of F2 with C6 H6 at relative collision energies of 14 to 26 kcal mol−1 . The most conspicuous product is the ipso-fluorocyclohexadienyl radical i-C6 H6 F ̇, which displays a rather narrow peak very near the center-of-mass angle at all collision energies studied. Product C6 H5 F is masked at M/e=96 by dissociative ionization of C6 H6 F ̇ near the center-of-mass angle, but becomes observable at smaller angles to which little or no C6 H6 F ̇ is scattered. This means that at least one reaction C6 H6 +F2 →C6 H5 F+HF occurs, and does so in such a way that the products acquire high translational energies. The dependence on collision energy of the total cross section for the production of C6 H6 F ̇ was probably measured, and displays a well-marked onset at 13.9±0.3 kcal mol−1. The center-of-mass system means final state translational energy as a function of collision energy was obtained from the angular distributions and goes to zero at 13.9±0.5 kcal mol−1 . This probably means that the onset is a true threshold, from which the heat of formation of the ipso-fluorocyclohexadienyl radical is calculated to be 14.8±0.4 kcal mol−1. The mean final state translational energy asymptotically approaches 3.7 kcal mol−1 as the collision energies are increased, which is so large that only a fraction of the vibrational modes of C6 H6 F ̇ can be involved in the energy partitioning. This conclusion is confirmed by the center-of-mass angular distribution, which favors forward scattering of product F atoms, but only by about 10%–30%. This work shows that the crossed-beam method can be used to make precision thermochemical measurements of gaseous polyatomic free radicals. It also reports the investigation of a polyatomic–polyatomic reaction in crossed beams, for which type of study there are still very few examples.

Dynamics of the F2 reaction with the simplest π-bonding molecule

“…Although there have been quite a number of experimental [2][3][4][5][6][7][8][9][10][11] and theoretical [12][13][14][15][16][17][18][19] investigations on the reaction of fluorine with ethylene, the reaction mechanism remains unclear. If this simple reaction can be understood, the mechanism may be extended to larger systems.…”

“…Grover et al 11 studied the reaction of F 2 with benzene in a crossed molecular beam experiment and determined the energy threshold of generating C 6 H 6 F + F products to be 13.9Ϯ 0.5 kcal/ mol. The angular distribution of the C 6 H 6 F product was observed to be slightly backward with respect to the F 2 beam direction.…”

Dynamics of the F2+CH3SCH3 reaction: A molecule-molecule reaction without entrance barrier

“…In all previous investigations of interactions between F 2 and closed-shell molecules ͑e.g., I 2 , ICl, HI, CH 3 I, C 2 H 4 , and C 6 H 6 ͒ in crossed molecular beams, a considerable collision energy was required to promote a reaction. [1][2][3][4][5][6][7][8] Of many reactions of F 2 with closed-shell molecules, the rate constants were found to be much smaller than those of corresponding F 2 reactions with radical species. 9 However, an extraordinarily large rate constant of ͑1.6± 0.5͒ ϫ 10 −11 cm 3 molecule −1 s −1 was observed at room temperature for the F 2 +CH 3 SCH 3 reaction.…”

Barrierless reactions between two closed-shell molecules. II. Dynamics of F2+CH3SSCH3 reaction

“…4-9͒ and unsaturated hydrocarbons ͑C 2 H 4 , C 6 H 6 ͒. 10,11 In these reactions, however, only a process analogous to channel II was observed with a significant threshold for collision energy. The high reactivity between F 2 and DMS is unusual for a typical molecule-molecule interaction, yet it may be quite general for the reactions of F 2 with molecules having a loosely bound lone pair of electrons.…”