Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160, USA.
Org Biomol Chem. 2013 Dec 28;11(48):8306-23. doi: 10.1039/c3ob42063a. Epub 2013 Nov 7.
The conformations of eight neo-confused porphyrin (neo-CP) structures and seven related benzo-neo-confused porphyrins (B-neo-CPs) were minimized using DFT-B3LYP/6-311++G(d,p). In addition, in order to provide contrasts to the B-neo-CP tautomers, a series of twelve benzoporphyrin (BP) tautomers and twelve benzocarbaporphyrins (BCPs) were also analyzed, and two N-confused porphyrin (NCP) tautomers were also considered. The relative stability of the tautomers for each series was computed, and the bond lengths and bond angles were calculated. Surprisingly, all of the neo-CP and B-neo-CP tautomers were near planar. The fully conjugated members of these series showed a significant degree of bond length alternation, unlike porphyrins or the aromatic forms of NCP, BP or BCP. The most stable tautomer of neo-CP was calculated to be 27.04 kcal mol(-1) higher in energy than porphyrin and 9.95 kcal mol(-1) higher in energy than the most favored NCP tautomer. The most stable tautomer of B-neo-CP was similarly shown to be 26.40 kcal mol(-1) higher in energy than benzoporphyrin. The favored tautomers also place the internal NH opposite to the neo-confused ring to minimize steric and lone pair-lone pair interactions, and to provide improved hydrogen bonding interactions. Nucleus independent chemical shifts (NICS) demonstrated that the fully conjugated neo-CP and B-neo-CP tautomers exhibit a significant amount of diatropic character, but these values are somewhat lower than the results obtained for porphyrins, BPs or BCPs. BCPs have three internal hydrogens, but the favored tautomer places the two pyrrolic hydrogens on each side of the indene unit to maximize hydrogen bonding interactions. Tautomers with an internal CH2 unit are also feasible, but these are energetically less favorable, have reduced diatropicity, and show significant bond length alternation. The results indicate that 18π electron delocalization pathways that pass through a fused benzene ring are less favorable than alternative delocalization pathways that bypass this unit.
使用 DFT-B3LYP/6-311++G(d,p)对八个新迷叠卟啉（neo-CP）结构和七个相关的苯并新迷叠卟啉（B-neo-CP）进行了构象最小化。此外，为了与 B-neo-CP 互变异构体形成对比，还分析了一系列十二个苯并卟啉（BP）互变异构体和十二个苯并咔巴卟啉（BCP），并考虑了两个 N-迷叠卟啉（NCP）互变异构体。计算了每个系列互变异构体的相对稳定性，并计算了键长和键角。令人惊讶的是，所有的 neo-CP 和 B-neo-CP 互变异构体都是近乎平面的。这些系列的完全共轭成员表现出显著的键长交替程度，与卟啉或 NCP、BP 或 BCP 的芳香形式不同。neo-CP 的最稳定互变异构体的能量比卟啉高 27.04 kcal/mol(-1)，比最有利的 NCP 互变异构体高 9.95 kcal/mol(-1)。B-neo-CP 的最稳定互变异构体也同样比苯并卟啉高 26.40 kcal/mol(-1)。有利的互变异构体还将内部 NH 置于与新迷叠环相对的位置，以最小化空间位阻和孤对-孤对相互作用，并提供改进的氢键相互作用。核独立化学位移（NICS）表明，完全共轭的 neo-CP 和 B-neo-CP 互变异构体表现出显著的反磁各向异性特征，但这些值略低于卟啉、BP 或 BCP 获得的结果。BCP 有三个内部氢，但有利的互变异构体将两个吡咯氢放在茚单元的每一侧，以最大限度地提高氢键相互作用。具有内部 CH2 单元的互变异构体也是可行的，但这些在能量上不利，反磁各向异性降低，并且表现出显著的键长交替。结果表明，通过融合苯环传递的 18π 电子离域途径不如绕过该单元的替代离域途径有利。
