Influence of hydrogen bonding on hydrogen-atom abstraction reactions of dehydropyridinium cations in the gas phase.
(2010)
Journal - The journal of physical chemistry. A (United States )
Abstract :
The reactions of several substituted, positively charged dehydropyridinium cations with cyclohexane, methanol, and tetrahydrofuran have been examined in a Fourier-transform ion cyclotron resonance mass spectrometer. All of the charged monoradicals react with the neutral reagents exclusively via hydrogen atom abstraction. For cyclohexane, there is a good correlation between the reaction efficiencies and the calculated electron affinities at the radical sites; that is, the greater the electron affinity of the charged monoradical at the radical site, the faster the reaction. The reaction efficiencies with methanol and tetrahydrofuran, however, do not correlate with the calculated electron affinities. Density functional theory (DFT) calculations indicate that for these reagents a stabilizing hydrogen bonding interaction exists in the hydrogen atom abstraction transition states for some of the charged monoradicals but not for others. At both the MPW1K and G3MP2B3 levels of theory, there is a good correlation between the calculated activation enthalpies and the observed reaction efficiencies, although the G3MP2B3 method provides a slightly better correlation than the MPW1K method. The extent of enhancement in the reaction efficiencies caused by the hydrogen bonding interactions parallels the calculated hydrogen bond lengths in the transition states.
Gas-phase reactivity of protonated 2-, 3-, and 4-dehydropyridine radicals toward organic reagents.
(2009)
Journal - The journal of physical chemistry. A (United States )
Abstract :
To explore the effects of the electronic nature of charged phenyl radicals on their reactivity, reactions of the three distonic isomers of n-dehydropyridinium cation (n = 2, 3, or 4) have been investigated in the gas phase by using Fourier-transform ion cyclotron resonance mass spectrometry. All three isomers react with cyclohexane, methanol, ethanol, and 1-pentanol exclusively via hydrogen atom abstraction and with allyl iodide mainly via iodine atom abstraction, with a reaction efficiency ordering of 2 > 3 > 4. The observed reactivity ordering correlates well with the calculated vertical electron affinities of the charged radicals (i.e., the higher the vertical electron affinity, the faster the reaction). Charged radicals 2 and 3 also react with tetrahydrofuran exclusively via hydrogen atom abstraction, but the reaction of 4 with tetrahydrofuran yields products arising from nonradical reactivity. The unusual reactivity of 4 is likely to result from the contribution of an ionized carbene-type resonance structure that facilitates nucleophilic addition to the most electrophilic carbon atom (C-4) in this charged radical. The influence of such a resonance structure on the reactivity of 2 is not obvious, and this may be due to stabilizing hydrogen-bonding interactions in the transition states for this molecule. Charged radicals 2 and 3 abstract a hydrogen atom from the substituent in both phenol and toluene, but 4 abstracts a hydrogen atom from the phenyl ring, a reaction that is unprecedented for phenyl radicals. Charged radical 4 reacts with tert-butyl isocyanide mainly by hydrogen cyanide (HCN) abstraction, whereas CN abstraction is the principal reaction for 2 and 3. The different reactivity observed for 4 (as compared to 2 and 3) is likely to result from different charge and spin distributions of the reaction intermediates for these charged radicals.
Reactivity of aromatic sigma, sigma-biradicals toward riboses.
(2006)
Journal - Journal of the American Society for Mass Spectrometry (United States )
Abstract :
The gas-phase reactions of sugars with aromatic, carbon-centered sigma,sigma-biradicals with varying polarities [as reflected by their calculated electron affinities (EA)] and extent of spin-spin coupling [as reflected by their calculated singlet-triplet (S-T) gaps] have been studied. The biradicals are positively charged, which allows them to be manipulated and their reactions to be studied in a Fourier-transform ion cyclotron resonance mass spectrometer. Hydrogen atom abstraction from sugars was found to be the dominant reaction for the biradicals with large EA values, while the biradicals with large S-T gaps tend to form addition/elimination products instead. Hence, not all sigma, sigma-biradicals may be able to damage DNA by hydrogen atom abstraction. The overall reaction efficiencies of the biradicals towards a given substrate were found to be directly related to the magnitude of their EA values, and inversely related to their S-T gaps. The EA of a biradical appears to be a very important rate-controlling factor, and it may even counterbalance the reduced radical reactivity characteristic of singlet biradicals that have large S-T gaps.
| ISSN : | 1044-0305 |
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| Mesh Heading : | Cyclotrons Data Interpretation, Statistical Fourier Analysis Free Radicals Hydrocarbons, Aromatic Indicators and Reagents Mass Spectrometry Ribose |
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| Mesh Heading Relevant : | chemistry analogs & derivatives chemistry |
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