H-atom abstraction dynamics of reactions between Cl atoms and heterocyclic organic molecules

The HCl(v' = 0, J') rotational state distributions produced in the photoinitiated reactions of ground spin-orbit state Cl(P-2(3/ 2)) atoms with the organic heterocycles oxirane (c-C2H4O), thiirane (c-C2H4S) and oxetane (c-C3H6O) have been measured using the technique of 2+1 resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. In contrast to previous studies of H-atom abstraction dynamics by Cl atoms with linear functionalized molecules, the nascent HCl( v' = 0, J') products of the reactions of the three-membered heterocycles oxirane and thiirane are formed rotationally cold, with mean rotational energies < E-rot> of ( 0.28 +/- 0.02) kcal mol(-1) and (0.31 +/- 0.02) kcal mol(-1), respectively. The reaction of Cl atoms with the four-membered heterocycle oxetane, however, results in more rotationally excited HCl(v' = 0, J') products for which < E-rot > ( 0.78 similar to 0.03) kcal mol(-1). Ab initio calculations were performed at the G2//MP2/6-311G(d,p) level to characterize molecular complexes and transition states on the reactive pathways. In all cases, the reactions were calculated to be barrierless, and weakly bound pre- and post- reaction complexes were located on the potential energy surfaces. The observed distributions and calculated reaction profiles are compared with previous studies of the reactions of Cl atoms with linear functionalized organic molecules and the consequences of the constrained molecular geometry on the HCl(v' = 0, J') rotational distribution discussed in terms of the dipole-dipole interaction model proposed previously.