Edward P. Rack

Reactions of High‐Energy, Excited I128 Ions with Gaseous Molecules

The manner in which molecular additives inhibit the reaction of (n, γ) activated I128 with CH4 was determined in an effort to observe indirectly reactions of I128 with the additives. The data suggest that (1) O2, N2, and CF4 serve only to remove excess I128 kinetic energy; (2) the ionization potential of O2 is greater than 12.16 ev, the potential energy of I+(1D2); (3) the ionization potential of C2F6 is less than 12.16 ev; (4) CH3I, CF3I, n‐C3H7I, and C6H6 inhibit the reaction principally as a result of I++additive ion‐molecule reactions and/or physical quenching.

Stereochemical Consequences of Recoil Halogen Atom Substitution

The substitution reactions of Br-for-Br generated by the 8 2 m Br———> Br nuclear transformation, were studied in (2S,3R)and (2S,3S)-bromolluorobutane and (2S,3R)and (2S,3S)-dibromobutane. Experiments were carried out in the gas phase for all compounds and in the condensed phase and in solutions of n-hexane and acetonitrile for bromofluorobutane. The results show for gas phase systems for Br-for-Br substitution a net retention of configuration in the (2S,3S)-halobutanes and net inversion of configuration in (2S,3R)-halobutane in the gas phase. In the condensed phase Br-for-Br substitution at the chiral centers resulted in nearly equal retention and inversion of configuration.

Liquid Phase Hot Atom Chemistry: At Crossroads

The state of current research in liquid phase hot atom chemistry is discussed. Four classes of experimental approaches are highlighted. These include 1) primary physical data for (η, γ)-activated 1 2 Ί , (I. T.)-activated 1 3 0 I and effects on chemical reactivity; 2) the density-variation technique involving iodine reactions with saturated and unsaturated hydrocarbons; 3) stereochemistry experiments on chlorocarbon molecules with single and multiple chiral centers; and 4) experiments employing dilute aqueous solutions of halogenerated biomolecules in the ice state, exposed to neutron irradiation.

Stereochemical Consequences of Recoil Halogen Atom Substitution IV : Effect of Kinetic Energy Moderator Concentration on the 18F-for-X (X=Br, F) Reactions in Gaseous Diastereomeric 2,3-Bromofluorobutanes

The stereochemical consequences of recoil F-for-X (X = Br, F) atom substitutions in (2S,3R)-dland (2S,3S)-rf/-bromofluorobutanes (BFBs) were investigated in the gas phase as a function of kinetic energy moderator concentration. Results from exothermic F-for-Br substitutions in the BFB stereoisomers revealed that the reaction stereochemistry changed from high retention of configuration to predominant inversion of configuration with increasing kinetic energy moderator concentration. An inspection of individual yields revealed that the inversion product channel was insensitive to kinetic energy moderator concentration, while the retention product channel decreased in importance as the moderator concentration was increased. In contrast, results from thermoneutral F-for-F substitutions in BFBs revealed that reaction stereochemistry was insensitive to kinetic energy moderator. An inspection of product yields revealed that channels for retention and inversion product formation exhibited the same moderator dependence.

Molecular mechanics calculations for dihalogenated butanes, pentanes, and hexanes

Abstract The conformational preferences of a series of vicinal dihaloalkanes are calculated by means of the MM2 (85) force field program. These data are of interest with regard to the stereochemistry of ldhot atomrd halogen for halogen exchange reactions, which may be related to ground state geometry of the molecule. The free space or ldaccessibilityrd of each chiral center for substitution by attack of a hot atom with retention and with inversion is calculated from the molecular mechanics data. With exceptions, the calculations predict a more accessible approach to C* with retention of configuration. These results are qualitatively in accord with experimental results for“hot atom” substitution reactions.

Stereochemical Consequences of Halogen-for-Halogen Substitutions with Gaseous Enatiomers

by recoil atoms at sp -hybridized carbon in the gas phase is important to investigate since it can provide insight into the dynamics of hot-atom substitution reactions. Previous studies of recoil tritium-for-hydrogen [1—7] and recoil halogen-for-halogen [8—18] substitution centered mainly with diastereomers in gas, liquid and solid systems. All recoil tritium substitution reactions, regardless of substrate molecules studied or phase, occur with predominant retention of configuration. Gas phase substitutions by recoil fluorine [8], chlorine [4,9,10], bromine [2,11], and iodine [11] atoms on substrate molecules containing two chiral centers were also observed to yield high levels of retention. Subsequent studies by this group on gas-phase substitution of 3 4 Cl on substrate molecules possessing a single chiral center yielded significantly higher levels of inversion [13,19].