Showing papers in "Journal of the American Chemical Society in 1970"

pi-Cation radicals and dications of metalloporphyrins.

Abstract: Zinc tetraphenylporphyrin (ZnTPP) and magnesium octaethylporphyrin (MgOEP) undergo reversible oneand two-electron oxidations. Removal of the first electron by controlled potential electrolysis or by oxidation with xenon difluoride or bromine results in cation radicals whose esr spectra clearly indicate electron abstraction from the porphyrin ring. The observed hyperfine structure is assigned, with the help of deuterium labeling, to interaction of the unpaired electron with the meso protons of MgOEP+ and with the protons on the phenyl groups of ZnTPP+. Electronic absorption spectra of the radicals, dications, and of a dimer of MgOEP+ which exists at low temperatures are presented. The results of an SCF-MO study are compared with the experimental esr and optical data. These calculations suggest that a radical may occupy either of two close-lying ground states. We conclude that the ground-state symmetries of ZnTPP+ and MgOEP+ are, respectively, 2AZu and 2A1u of the D4h point group. C steps in photosynthetic and metabolic processes involve metalloporphyrin redox properties. Prior studies of heme function have stressed the redox characteristics of the central metal; s, 4 in contradistinction, primary photosynthetic events involving (I) Aspects of this work were previously presented: (a) J. Fajer. D. C. Borg. D. Dolphin. and R. H. Felton. Abstracts. 157th National Meeting of the American Chemical Society. Minneapolis, Minn .• April 1969. p 130; (b) R. H. Felton. D. Dolphin. D. C. Borg. and J. Fajer. J. A mer. Chern. Soc .• 91. 196 (1969). (2) (a) Department of Applied Science, Brookhaven National Laboratory; (b) Medical Department. Brookhaven ::-.lational Laboratory; (cl Harvard University; (d) Georgia Institute of Technology. (3) J. E. Falk, \"Porphyrins and Metalloporphyrins,\" Elsevier Publishing Co., New York, N. Y., 1964. (4) B. Chance, R. W. Estabrook. and T. Yonetani, Ed .• \"Hemes and Hemoproteins.\" Academic Press, New York, N. Y .• 1966. chlorophyll appear to depend upon ring redox properties.o· 6 A striking feature of the porphyrin ring is its ability to undergo facile reduction and oxidation. Reductions of chlorophylF and metalloporphyrins,8 dissolved in apr otic solvents. have been accomplished byelectrochemical and chemical techniques, and the results (5) B. Kok, Biochirn. Biophys. Acta, 48. 527 (1961). (6) J. Weikard, A. Muller. and H. T. Witt. Z. Natur/orsch .• 18b. 139 (1963). (7) R. H. Felton, G. W. Sherman, and H. Linschitz. Nature, 203, 631 (1964). (8) G. L. Closs and L. E. Closs. J. Arner. Chem. Soc., 85, 818 (1963); N. Hush and J. Dodd. J. Chern. Soc .• 4601 (1964); D. Clack and N. Hush. J. Amer. Chern. Soc., 87, 4238 (1965); R. H. Felton and H. Linschitz, ibid., 88, 1113 (1966). Fajer, Borg, Forman, Dolphin, Felton / Dications of Metalloporphyrins

Nuclear Magnetic Resonance Spectroscopy. Carbon-13 Chemical Shifts in Acyclic and Alicyclic Alcohols

Abstract: The chemical shifts of ^(13)C in a variety of acyclic and alicyclic alcohols have been determined by high resolution nmr spectroscopy with the aid of proton decoupling It has been found that there are rather good linear relationships between carbon chemical shifts in alcohols and analogously constituted hydrocarbons, wherein a methyl group replaces the hydroxyl group The linear correlation coefficients for relationships of the general type δ_CROH = Aδ_CRCHa + B are better than 098 for shifts of corresponding ɑ (directly attached hydroxyl), β, y, and δ carbons for a variety of primary, secondary, and tertiary acyclic alcohols and cyclohexanols carrying both axial and equatorial substituents The shifts of the carbons of a number of cycloalkanols have been investigated in hope of providing information about conformations in medium-sized ring compounds

Nuclear magnetic resonance spectroscopy. Carbon-13 chemical shifts in norbornyl derivatives

Abstract: The ^(13)C chemical shifts for a number of norbornyl derivatives have been obtained in natural abundance. The compounds studied were the series of methylnorbornanes, methylnorbornenes, methyl-2-norbornanones, methyl-2,2-difluoronorbornanes, and a variety of exo- and endo-2-substituted norbornanes. The observed shifts have been interpreted in terms of inductive, bond length, and steric effects. The ɑ- and β-substituent effects are shown to be dependent on the extent of substitution at both the α and β carbon atoms and changes in bond lengths have been used to rationalize the shift changes. Steric interactions between the endo-2 and endo-6 positions, and the exo-2 and syn-7 positions have been shown to generate important chemical- shift changes and these changes can be used as a semiquantitative measure of steric interactions in norbornyl derivatives. Studies with electron-withdrawing groups have revealed a new long-range cmr y effect produced by exo-2 substituents on C-6. These shifts appear to provide a sensitive and quantitative measure of the electronic demands of a neighboring group as a function of stereochemistry. The substituent shifts obtained from the monosubstituted derivatives have been used to predict the shifts of each of the 1- and 3-methyl-2-norbornanols. The agreement between observed and calculated values is generally good and supports the general approach. Several ^(13)C-^(13)C coupling constants for norbornane, nortricyclene, and quadricyclene are reported and interpreted in terms of the s character of the C-C bonds.