Muonium atom addition to pyridine and to the pyridinium cation

Muonium atoms are found to add mainly to the N-atom and to the ortho and me ta positions in pyridine in almost equal amounts, giving pyridinyl and 2-az acyclohexadienyl radicals and 3-azacyclohexadienyl radicals, respectively ( no addition was observed at the para site); on protonation, as in a sample of PyH+ BF4-, the reactivity follows the positive charge density, being enh anced for addition to the N(H+) position, reduced for the ortho/meta positi ons, and now conferring a significant yield of the para isomer. These resul ts for genuine reactions of hydrogen atoms are discussed in view of studies of radical formation in these materials, made using EPR spectroscopy, when they are exposed to)brays in cryogenic matrices, for which mechanisms invo lving charge-neutralization of initial radical ions are proposed. Copyright (C) 2001 John Wiley & Sons, Ltd.