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The O2 vibration of dioxygen adducts of Fe and Co model complexes of a,a,ac,a-tetrapivalamidophenylporphyrin ("picket fence" porphyrin, TpivPP) with 1-methylimidazole and 1-tritylimidazole as axial bases are reported, obtained with difference techniques between 1'602,O180, 160-.80 A continuing controversy exists over the nature of ligand bonding to hemoproteins and metalloporphyrins. While it has been well established that both iron and cobalt bind dioxygen in a bent end-on fashion (1, 2), physical measurements (3-6) and theoretical calculations (7-10) have been used to support both the spin-paired dioxygen (M"1-02) (11) and the electron transfer superoxide (MIII-02-) (12) models. Here we present infrared data on dioxygen "picket-fence" porphyrin complexes of iron and cobalt. These results strongly support the MIn-02-formalism. The present data correct the preliminary report of an oxygen vibration at 1385 cm-' (3,13) which has since proved to be an artifact* (14). We are unaware of any previous reports of Po2 in model Fe and Co porphyrins. In addition, we discuss the nature of CO binding to ferrous porphyrins and the implication this has for the role of the apoprotein in biological oxygen carriers. It is our thesis that the distal binding site in hemoproteins functions to lower the carbon monoxide affinity.MATERIALS AND METHODS Benzene (spectroquality, Baker "Photrex") solutions of the "picket fence" porphyrin complexes (3, 14), FelImesotetra(a,a,a,a-o-pivalamidophenyl)porphyrinato (TpivPP) and Coll (aa,a,a-TpivPP), 17 mM and 19 mM containing 5 and 2 equivalents of 1-tritylimidazole (NTrIm), respectively, were Abbreviations: TpivPP, meso-tetra (a,a,a,a-o-pivalamidophenyl

Nucleophilic displacement reactions (the SN2 reaction) of ions in the gas phase are a prototypical reaction system that allows a study of dynamics, mechanisms, and structure-energy relations. This article reviews aspects of the kinetics (especially the applicability of statistical reaction rate theory), the relation of structure and reactivity, and the effects of small numbers of solvent molecules on the reaction and compares the behavior of the ionic reaction in the gas phase with that in solution.

The gas-phase S(N)2 reactions of chloride with ethyl and neopentyl chlorides and their alpha-cyano derivatives have been explored with B3LYP, CBS-QB3, and PDDG/PM3 calculations. Calculations predict that the steric effect of the tert-butyl group raises the activation energy by about 6 kcal/mol relative to methyl in both cases. Solvent effects have been computed with QM/MM Monte Carlo simulations for DMSO, methanol, and water, as well as with a polarizable continuum model, CPCM. Solvents cause a large increase in the activation energies of these reactions but have a very small differential effect on the ethyl and neopentyl substrates and their cyano derivatives. The theoretical results contrast with previous conclusions that were based upon gas-phase rate measurements.