EPR Studies on Carboxylic Esters. Part 20. EPR Spectra and Spin Densities in Radical Anions of Isocoumarin, Benzocoumarin and their Sulfur Analogues

Abstract

The radical anions of isocoumarin, benzocoumarin and the six corresponding sulfur analogues have been generated by internal electroreduction and studied by EPR spectrosopy. The spin density distributions have been evaluated from the proton hyperfine structure coupling constants and by MO calculations. They are discussed with respect to the spin density distributions in related radical anions.

Why does thionating a carbonyl molecule make it a better electron acceptor?

“…The significant weight of p à C¼S reflects the pronounced LUMO-lowering effect by CQO -CQS substitution, and is consistent with the positive shift in g values of the radical anion of these molecules, resultant from a large spin density on the heavy sulfur atom. 45,46,50 Taken together, the NBO analysis of (thio)carbonyl 1-4 indicates that the LUMO-lowering of these molecules originates from weaker (2p p ) C /(3p p ) S interactions, despite sulfur in CQS being less electronegative than oxygen in CQO (see also Fig. S2-S4 in ESI † for the effect of electronegativity on molecular electrostatic potential surface and Mulliken charges).…”

“…To investigate the electronic effect of conjugated O/S, we used isochromen-1-one ( 5a ) and isothiochromen-1-one ( 6a ), whose reduction potentials have been reported, as the model system for comparison (Table 2). 46 The “thioester” version of the isochromenone derivative ( 6a ) displays a noticeably less negative E RED = −1.27 V compared to E RED = −1.41 V for “ester” 5a . The enhancement in the electron-accepting power by –C(O)O– → –C(O)S– is clear and consistent with the implication of the Hammett substituent constants σ p of –OCH 3 and –SCH 3 and the relative LUMO energy.…”

“…To investigate the electronic effect of conjugated O/S, we used isochromen-1-one (5a) and isothiochromen-1-one (6a), whose reduction potentials have been reported, as the model system for comparison (Table 2). 46 The ''thioester'' version of the isochromenone derivative (6a) displays a noticeably less negative E RED = À1.27 V compared to E RED = À1.41 V for ''ester'' While the p à C¼O NBO contributes significantly to the LUMO of 5a and 6a (20%, along with other antibonding CQC NBOs), the lone-pair NBO of the conjugated O/S atom also has 5-9% importance. This lone-pair orbital orients perpendicular to the molecular plane, adopting the p symmetry, and can mix with the (2p p ) C of carbonyl (Fig.…”

“…The significant weight of reflects the pronounced LUMO-lowering effect by CO → CS substitution, and is consistent with the positive shift in g values of the radical anion of these molecules, resultant from a large spin density on the heavy sulfur atom. 45,46,50…”