No abstract

No abstract

No abstract

-The addition of polyenes to excited anthracenes may yield several novel adducts derivable from both concerted and stepwise pathways according to the Woodward-Hoffmann Rules. The role of exciplex intermediates in these photocycloadditions is discussed. The concerted pathway in these excited state reactions is found to be favored relative to stepwise pathway(s) by polar solvente. It is suggested that the key step responsible for the observed polar solvent effect is a potential surface 1 crossing from the exciplex derived from the B 2 u (s 1 ) state of anthracene to a low lying doubly excited state.

Rate constants and mechanisms have been determined for proton-transfer reactions of the type AH+ + M <=; MH + + A, where A is propylene, isobutene, írani-2-butene, cyclopentene, and cyclohexene. In order to avoid competing side effects the AH+ reactant ions are generated in alkanes and alkyl halides. It is observed that the rate constants for exothermic direct proton transfer reactions, from AH+ to M or from MH+ to A, are equal to the collision rate only when the total rotational, vibrational, and electronic entropy change associated with the reaction is positive, and w hen the exothermicity of the reaction exceeds 2-4 kcal/mol. On the basis of rate constants in the forward and reverse direction for multiple reaction pairs, internally consistent values are obtained for the proton affinities of propylene, isobutene, and cyclopentene. Protonation of trans-2-C4Hx by H30+ (A/Vr" = -10 kcal/mol, 41.84 kJ/mol) yields sec-C4Hg+ exclusively. However, carbon skeletal rearrangement to the Z-C4H9+ configuration becomes important when the AH of the proton transfer is <10 kcal/mol. The experimental evidence indicates that the isomerization occurs before departure of C4Hg+ from the reaction complex. The isomerization process which also occurs upon collision of iec-C4Hg+ with polar molecules is dependent on the dipole moment of M. For 5coC4H9+/ trans-2-C4H8 and c-C6H n+/c-CaH ¡o, where these isomerization processes are especially important, thermochemical information is derived from qualitative considerations of trends in reaction rates. Scales of relative proton affinities of the olefins and relative heats of formation of the alkyl ions are derived and compared with data on absolute heats of formation. Taking a value of A//f(r-C4H9+) = 162.1 ± 0.8 kcal/mol (678.2 kJ/mol), the values derived for the heats of formation in kcal/mol