l-Alkyl-2-(carbomethoxy)pyridinyl Radicals: Monomers and Dimers Defined through Chemical and Photochemical Properties and Electron Paramagnetic Resonance

Joshua Hermolin, Menachem Levin, Yusaku Ikegami, Masako Sawayanagi, Edward M. Kosower

Research output: Contribution to journalArticlepeer-review

Abstract

l-Methyl-2-(carbomethoxy)pyridinium ion (2+) is reduced to the radical dimer (2-2) which is in equilibrium with a small amount of l-methyl-2-(carbomethoxy)pyridinyl radical (2). Distillation of the dimer leads to thin films (at 77 K) of the radical 2., which dimerizes in solution. Chemical properties and EPR spectra of dimer solutions reflect the presence of the monomeric radical in equilibrium with the dimer; 2+ polarography also indicates a monomer-dimer equilibrium. Laser pulse dissociation of the dimer in solution gives the monomer reversibly, with kassoc of ca. 106 M-1 s-1. The dimerization constant, Ka(titration method), is 4 X 105 M-1; electrochemical techniques lead to a Ka value of at least 7 X 104 M-1, a kassoc value of ca. 1-4 X 104 M-1 s-1, and a kdisooc value of 1 s-1. The activation energy for dimer dissociation (from EPR spectral broadening) is 7.7 kcal/mol (CH3CN) or 9.4 kcal/mol (2-MTHF). The 1-methyl-2-acetylpyridinyl radical [(CH3CO)2.] is easily prepared and distilled and has a stability similar to that of the 1 -methyl-4-(carbomethoxy)pyridinyl radical (4.). EPR spectra of the 2-acetyl and 2-(carbomethoxy)pyridinyls are presented and analyzed. A convenient electrochemical cell for the generation and analysis of air-sensitive one-electron reduced species (such as the pyridinyl radicals or unstable pyridinyl radical dimers) is described.

Original languageEnglish
Pages (from-to)4795-4800
Number of pages6
JournalJournal of the American Chemical Society
Volume103
Issue number16
DOIs
StatePublished - Aug 1981

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