The elusive cis tautomer of free-base porphyrins has recently been isolated and structurally characterized in the form of a supramolecular complex. The question as to whether a suitable set of peripheral substituents might lead to a stable cis tautomer in the absence of supramolecular interactions, however, remains unanswered and is one we have attempted to address here by means of density functional theory calculations. The fact that many antipodally β-tetrasubstituted tetraphenylporphyrin derivatives exhibit localized central protons attached to the β-unsubstituted pyrrole rings led us to surmise that β-tetrasubstitution of adjacent pyrrole rings might lead to a porphyrin cis tautomer, an idea that proved fruitful. Indeed, for the "adjacently" substituted tetraphenylporphyrin derivative H2[adj-(CF3)4(CH3)4TPP], the global energy minimum proved to be a highly saddled cis tautomer, with the trans tautomer about 0.07 eV higher in energy. It is important to underscore, however, that the asymmetric β-substitution pattern is far from the only factor contributing to the stability of the cis tautomer for this porphyrin. A strongly saddled conformation resulting from meso-β steric interactions also helps alleviate the repulsion between the two central NH protons, thereby stabilizing the cis tautomer relative to the trans.

中文翻译：

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带有局部NH质子的游离碱卟啉。单独的取代基能否稳定难以捉摸的顺式互变异构体？

游离碱卟啉的难以捉摸的顺式互变异构体最近已被分离，并以超分子复合物的形式在结构上进行了表征。然而，关于在没有超分子相互作用的情况下，合适的周边取代基是否可能导致稳定的顺式互变异构体的问题仍然没有得到解答，这是我们试图通过密度泛函理论计算解决的问题。许多对立的β-四取代四苯基卟啉衍生物都具有连接在β-未取代的吡咯环上的局部中心质子这一事实使我们推测相邻的吡咯环的β-四取代可能会导致卟啉顺式互变异构体，这一想法被证明是有成果的。实际上，对于“相邻”取代的四苯基卟啉衍生物H2 [adj-（CF3）4（CH3）4TPP]，事实证明，最低限度的全球能源是顺式互变异构体，反式互变异构体的能量高约0.07 eV。然而，重要的是要强调，不对称的β取代模式远不是促成该卟啉顺式互变异构体稳定性的唯一因素。由内消旋-β空间相互作用产生的强烈鞍形构象也有助于减轻两个中心NH质子之间的排斥，从而使顺式互变异构体相对于反式稳定。