Origin and Scope of Long-Range Stabilizing Interactions and Associated SOMO-HOMO Conversion in Distonic Radical Anions
Origin and Scope of Long-Range Stabilizing Interactions and Associated SOMO-HOMO Conversion in Distonic Radical Anions
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DOI：
10.1021/ja404279f
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发表时间：
2013-10-16
影响因子：
15
通讯作者：
Coote, Michelle L.
中科院分区：
文献类型：
--
作者：
Gryn'ova, Ganna;Coote, Michelle L.
用高水平量子化学方法研究了SOMO-HOMO转换的远子自由基阴离子中非相互共轭阴离子与自由基部分之间显著的长程稳定相互作用的范围和物理来源。在这种物种中，酸片段的去质子化可以使远程自由基稳定数万焦耳，或者类似地，形成稳定的自由基(通过抽提或均解裂解反应)使远程酸的酸度增加几个pK(A)单位。这种稳定可以广泛地归类为一种新型的极性效应，它起源于色相互作用，但与标准的极性效应不同，它持续存在于没有电荷分离(即偶极)共振贡献者的自由基中，是无方向性的，因此范围非常广泛。当高度离域的自由基与初始不太稳定的阴离子(即较弱酸的共轭碱)结合时，去质子化的稳定性最大，而对于高度局域的自由基和/或稳定的阴离子，稳定性可以忽略不计。这种影响在气相和低极性溶剂中最大，但在水中猝灭，那里的阴离子足够稳定。这些简单的规则可以用来设计各种可切换的化合物，这些化合物能够响应于pH而可逆地释放自由基，用于例如有机合成或氮氧化物介导的聚合。此外，由于其广泛的化学范围，这种效应可能会影响许多生物底物在自由基攻击下的质子化状态，并可能有助于酶催化。
High-level quantum-chemical methods have been used to study the scope and physical origin of the significant long-range stabilizing interactions between non-mutually conjugated anion and radical moieties in SOMO-HOMO converted distonic radical anions. In such species, deprotonation of the acid fragment can stabilize the remote radical by tens of kilojoules, or, analogously, formation of a stable radical (by abstraction or homolytic cleavage reactions) increases the acidity of a remote acid by several pK(a) units. This stabilization can be broadly classified as a new type of polar effect that originates in Coloumbic interactions but, in contrast to standard polar effects, persists in radicals with no charge-separated (i.e., dipole) resonance contributors, is nondirectional, and hence of extremely broad scope. The stabilization upon deprotonation is largest when a highly delocalized radical is combined with an initially less stable anion (i.e., the conjugate base of a weaker acid), and is negligible for highly localized radicals and/or stable anions. The effect is largest in the gas phase and low-polarity solvents but is quenched in water, where the anion is sufficiently stabilized. These simple rules can be employed to design various switchable compounds able to reversibly release radicals in response to pH for use in, for example, organic synthesis or nitroxide-mediated polymerization. Moreover, given its wide chemical scope, this effect is likely to influence the protonation state of many biological substrates under radical attack and may contribute to enzyme catalysis.