Charge radical cations

The low-temperature EPR experiments used to determine the DNA ion radical distribution make it very clear that electron and hole transfer occurs after the initial random ionization. What then determines the final trapping sites of the initial ionization events To determine the final trapping sites, one must determine the protonation states of the radicals. This cannot be done in an ordinary EPR experiment since the small hyperfine couplings of the radicals only contribute to the EPR linewidth. However, detailed low-temperature EPR/ENDOR (electron nuclear double resonance) experiments can be used to determine the protonation states of the low-temperature products [17]. These proto-nation/deprotonation reactions are readily observed in irradiated single crystals of the DNA base constituents. The results of these experiments are that the positively charged radical cations tend to deprotonate and the negatively charged radical anions tend to protonate. 

Charged radical cations and anions are often indicated by the symbol, formula, or structure with a superscript dot followed by a plus or minus sign. However, in mass spectrometry, the reverse is used. Therefore, use the order of dots and signs for charges that is appropriate for the context. 

Historically, the first Ps formation mechanism was suggested by Ore for the purpose of interpretation of experiments on e+ annihilation in gases [9]. It implies that the hot positron, e+, having some excess kinetic energy, pulls out an electron from molecule M, thereby forming a Ps atom and leaving behind a positively charged radical-cation M+ ... 

A radical ion is a free radical species that carries a negative charge (radical anion) or a positive charge (radical cation). When a neutral, spin-paired species gains a single electron it becomes a radical anion. Likewise, when a neutral, spin-paired species loses an electron it becomes a radical cation. 

Each of the carbocations discussed to this point has been a species in which all of the electrons were spin-paired. Another type of positively charged reactive intermediate is the radical cation—a species that has both an impaired electron and a positive charge. Radical cations play important roles in many radiochemical and photochemical reactions, and they may also be important in biological processes, including photosynthesis and the biosynthesis of natural products. ... 

The main difference between the reactions of radical cations and of neutral radicals thus arises from the acidity of the former, due to their positive charge. Radical cations have a very strong tendency to lose protons or to add anions, forming neutral radicals which in turn are further oxidized to cations. One does not therefore as a rule see products arising from bimolecular reactions of radical cations, e.g., combination or disproportionation, since their lives are too short under the conditions where they are formed. 

Thus two electrons exit the reaction zone, leaving a positively charged species (M ) called an ion (in this case, a molecular ion). Strictly, M" is a radical-cation. This electron/molecule interaction (or collision) was once called electron impact (also El), although no impact actually occurs. 

Electrostatic potential map for 2-methyl-2-butanol radical cation shows most positively-charged regions (in blue) and less positively-charged regions (in red). 

Use geometries, electrostatic potential maps and spin densities to help you draw Lewis structures for butanal radical cation, the transition state and product. Where is the positive charge and the unpaired electron in each Is the positive charge (the unpaired electron) more or less delocalized in the transition state than in the reactant In the product ... 

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