Gas to liquid to solid transition in halogen hot atom chemistry. 3. Evidence for an excited reaction intermediate in the (n,.gamma.)-activated reactions of iodine with acetylene

Abstract

Publication costs assisted by the U.S. Energy Research and Development AdminlstratlonThe reactions of lZsI activated by radiative neutron capture with acetylene occur primarily through an addition channel forming an electronically excited reaction intermediate. Acetylene is unlike olefins in that no photochemical addition occurs under conditions simulating sample handling. In an excess of gaseous acetylene, at l atm 14.9 f 0.6% of lZsI is stabilized as organic activity to the following extent: CH3I 0.49%, CH& 0.70%; C2H31 0.40%, CzH5I 9.30%; i-C3H7I 2.71%; CH~CH~CHICHS 1.10%, CH3(CH2)3I 0.20%. The effects of rare gas additives in moderating lZ8I with gaseous CzH2 were determined in an effort to ascertain the nature of the activation process. Only the C2H51281 yield is decreased by the presence of rare gases, suggesting that both hot lZsI ions and thermal I+(lD2) and other excited ions are involved in the primary addition to acetylene. The gas to condensed phase studies showed a depletion of all C1, vinyl iodide, Cs, and C4 128I-labeled products; where the only product observed in high pressure gas and condensed phase systems was C2H51281. A reaction scheme is proposed which postulates the existence of an excited complex reaction intermediate. The properties of the proposed complex are compared qualitatively to those predicted by RRKM, "caged" complex, and caging radical theories of enhancement yields.

Hot Atom Chemistry 1976

“…However in any application of the technique, possible chemical effects of the decay preceeding the y-y cascade must be considered. This has been recognized in PAC studied on 181 Hf- [14] and 111 In-labelled [20] complexes in aqueous solutions. Time-differential PAC measurements have been performed on 181 Hf-labelled and neutron irradiated dl complexes of Hf with mandelic acid.…”

“…Such states are either unexistent and/or feebly populated in the two other isotopes [52], The relative importance of cage and molecular reactions 1 "5R of hot I with C 2 H 2 has been investigated with the density variation technique, i. e. by measuring the product yields from gas to liquid to solid phase. The data suggest the formation of an intermediate excited complex of iodine and acetylene [111], Cryptâtes possess cage-like structures in which electropositive elements can be trapped if they have the right size. It is foreseen that the recoil imparted by a (η, γ) activation should be high enough to pull the radioactive atom out of the complex.…”

Computer Simulation Evaluating Kinetic Theory Parameters in Hot Atom Chemistry

“…In the high pressure and condensed phase enhancement reactions become important. There have been many studies of these multi-molecular reactions throughout the gas to condensed phase transition [26][27][28][29][30][31][32][33], Enhancement reactions appear to be of two types: cage-radical, cage-ion reactions involving semi-random recombinations [26,30] and caged-complex reactions [29,31 ] involving the stabilization of an electronically excited cage complex. Evidence exists that these reactions may occur simultaneously [32,33], Although enhancement reactions are complex and description difficult as a result of the inseparability of enhancement events from molecular reactions, several attempts have been made to develop a semi-emperical description of enhancement events [34][35][36], These have considered the variation of bulk [34] and molecular [35] parameters in photolytically-induced cage radical recombinations and the implantation of a hot atom in a lattice matrix [33], No attempt had been made to simulate the effects of collapsing molecular environment on a chemical system which exhibits enhancement reactions.…”

“…While many two component systems provide well-defined physical parameters necessary for the first requirement, the presence of enhancement reactions contributing to diverse product formation routes makes chemical simplicity a difficult criterion to fulfill. The (n, -^-activated I28 I-acetylene system [31] is unusual in that only one product is found in the high pressure and condensed phases. All products found in the low pressure system with the exception of ethyl iodide decrease to zero in the gas to condensed phase transition.…”

“…All products formed throughout the range studied proceed via one reaction intermediate formed by the addition of iodine to an acetylene molecule. For simulation purposes, the system could be reduced to the modelling of the reaction intermediate [31].…”