| Abstract
| - The hydride affinities of 80 various p- and o-quinones in DMSO solution were predicted by using B3LYP/6-311++G (2df,p)//B3LYP/6-31+G* and MP2/6-311++G**//B3LYP/6-31+G* methods, combined withthe PCM cluster continuum model for the first time. The results show that the hydride affinity scale ofthe 80 quinones in DMSO ranges from −47.4 kcal/mol for 9,10-anthraquinone to −124.5 kcal/mol for3,4,5,6-tetracyano-1,2-quinone. Such a long scale of the hydride affinities (−47.4 to −124.5 kcal/mol)indicates that the 80 quinones can form a large and useful library of organic oxidants, which can providevarious organic hydride acceptors that the hydride affinities are known for chemists to choose in organicsyntheses. By examining the effect of substituent on the hydride affinities of quinones, it is found thatthe hydride affinities of quinones in DMSO are linearly dependent on the sum of the Hammett substituentparameters σ: ΔGH-(Q) ≈ −16.0Σσi − 70.5 (kcal/mol) for p-quinones and ΔGH-(Q) ≈ −16.2Σσi −81.5 (kcal/mol) for o-quinones only if the substituents have no large electrostatic inductive effect andlarge ortho-effect. Study of the effect of the aromatic properties of quinone on the hydride affinitiesshowed that the larger the aromatic system of quinone is, the smaller the hydride affinity of the quinoneis, and the decrease of the hydride affinities is linearly to take place with the increase of the number ofbenzene rings in the molecule of quinones, from which the hydride affinities of aromatic quinones withmultiple benzene rings can be predicted. By comparing the hydride affinities of p-quinones and thecorresponding o-quinones, it is found that the hydride affinities of o-quinones are generally larger thanthose of the corresponding p-quinones by ca. 11 kcal/mol. Analyzing the effect of solvent on the hydrideaffinities of quinones showed that the effects of solvent (DMSO) on the hydride affinities of quinonesare mainly dependent on the electrostatic interaction of the charged hydroquinone anions (QH-) withsolvent (DMSO). All the information disclosed in this work should provide some valuable clues to chemiststo choose suitable quinones or hydroquinones as efficient hydride acceptors or donors in organic synthesesand to predict the thermodynamics of hydride exchange between quinones and hydroquinones in DMSOsolution.
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