Garavelli Marco, Bernardi Fernando, Cembran Alessandro, Castaño Obis, Frutos Luis Manuel, Merchán Manuela, Olivucci Massimo
Dipartimento di Chimica G. Ciamician, Università di Bologna, Via Selmi 2, 40126 Bologna, Italy.
J Am Chem Soc. 2002 Nov 20;124(46):13770-89. doi: 10.1021/ja020741v.
We use ab initio CASSCF and CASPT2 computations to construct the composite multistate relaxation path relevant to cycloocta-1,3,5,7-tetraene singlet photochemistry. The results show that an efficient population of the dark excited state (S(1)) takes place after ultrafast decay from the spectroscopic excited state (S(2)). A planar D(8)(h)-symmetric minimum represents the collecting point on S(1). Nonadiabatic transitions to S(0) appear to be controlled by two different tetraradical-type conical intersections, which are directly accessible from the S(1) minimum following specific excited-state reaction paths. The higher-energy conical intersection belongs to the same type of intersections previously documented in linear and cyclic conjugated hydrocarbons and features a triangular -(CH)(3)- kink. This point mediates both cis --> trans photoisomerization and cyclopropanation reactions. The lowest energy conical intersection has a boat-shaped structure. This intersection accounts for production of semibullvalene or for double-bond shifting. The mapping of both photochemical and thermal reaction paths (including also Cope rearrangements, valence isomerizations, ring inversions, and double-bond shifting) has allowed us to draw a comprehensive reactivity scheme for cyclooctatetraene, which rationalizes the experimental observations and documents the complex network of photochemical and thermal reaction path interconnections. The factors controlling the selection and accessibility of a number of conjugated hydrocarbon prototype conical intersections and ground-state relaxation channels are discussed.
我们使用从头算CASSCF和CASPT2计算来构建与环辛-1,3,5,7-四烯单重态光化学相关的复合多态弛豫路径。结果表明，在从光谱激发态（S(2)）超快衰减后，暗激发态（S(1)）发生了有效布居。一个平面D(8)(h)对称极小值代表S(1)上的汇聚点。向S(0)的非绝热跃迁似乎由两个不同的四自由基型锥形交叉点控制，它们可通过特定的激发态反应路径从S(1)极小值直接到达。能量较高的锥形交叉点属于先前在直链和环状共轭烃中记录的同一类型交叉点，其特征是具有一个三角形-(CH)(3)-扭结。这一点介导了顺式→反式光异构化和环丙烷化反应。能量最低的锥形交叉点具有船形结构。这个交叉点导致了半环戊二烯或双键迁移的产生。对光化学和热反应路径（包括科普重排、价键异构化、环反转和双键迁移）的映射使我们能够绘制出环辛四烯的综合反应性方案，该方案使实验观察结果合理化，并记录了光化学和热反应路径相互连接的复杂网络。讨论了控制一些共轭烃原型锥形交叉点和基态弛豫通道的选择和可达性的因素。
