FT EPR of Short-Lived Radicals in Aqueous Solution

The primary 1-methylthymine cation radical (44) and three successor radicals, the N3-deprotonated cation radical (45), the C6-OH 5-yl radical (46) and a C6-0P032 5-yl addition radical (47), were characterized using Fourier Transform EPR spectroscopy (FT EPR) in aqueous solution at room temperature.36 

Radical 44 is generated by electron transfer from the parent pyrimidine to the optically generated (308 nm laser pulse) triplet states of anthroquinone-2,6-disulfonic acid. The experimental method detects transient species in the time range of 40 ns to 10 ps after the laser pulse. At pH = 1.0, the primary cation radical is the only radical found 40 ns after the pulse. The spectrum of this radical disappears at a time shorter than 1.0 ps after the pulse. A second spectrum observed 5 ps after the pulse, is attributed to radical 46, which was thought to form by net OH- addition to C6 of the cation radical. At pH = 7.0, the N3 

In a comment on this work,39 Close expresses doubt regarding the DFT calculations that indicate a lack of planarity in pyrimidine radical anions, contrary to what is experimentally found in low temperature single crystal studies. He notes that using an extended basis set results in reasonable agreement between the experimental and calculated values of hyperfine coupling constant for the a-proton without the need to move the proton significantly out of the 

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