The collinearity of terminal p orbitals of a diene with that of a dienophile is required for an effective overlap to result in s bond formation during the Diels-Alder reaction. The ease of the DA reaction of a cyclic diene with a given dienophile, therefore, must also depend on the distance between the termini of the diene. A distance larger than the unsaturated bond of the dienophile is expected to raise the energy of activation. This scenario has been amply demonstrated from the study of reactions of several dienes, some designed to serve the purpose, with different dienophiles. The five-ring heterocycles pyrrole, furan, thiophene and selenophene possess varying aromatic character for the varied resonance participation of the heteroatom lone pair with ring p bonds. The aromaticity decreases in the same order due to: (a) the increasing s_C-X (X = heteroatom) bond length lifts the bond uniformity required for ring current, hence aromaticity, such as in benzene and (b) size-mismatch of the interacting lone pair orbital and the ring p orbitals, especially in thiophene and selenophene, both allowing poor overlap in the ground state structures. It is demonstrated that increase alone in the activation energies of the DA reactions of pyrrole, furan, thiophene and selenophene cannot be considered a measure of relative aromaticity as often done and even theoretically attempted in many ways to prove just that. The separation of the termini of the diene has a much larger role in the determination of activation energy, especially in thiophene and selenophene, than their aromaticity profile. There cannot be a measure better than the relative intensity of heteroatom lone pair overlap with ring p bonds, giving rise to a six-electron like system in following Hückel’s 4n+2 rule, to assess the relative aromaticity.

中文翻译：

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吡咯，呋喃，噻吩和硒基苯的相对芳香度及其Diels-Alder立体选择性的计算研究

端子p的共线性 有效重叠需要二烯的轨道与亲二烯的轨道，以在狄尔斯-阿尔德反应期间形成s键。因此，环状二烯与给定的亲二烯体的DA反应的难易程度还必须取决于二烯末端之间的距离。预期大于亲二烯体的不饱和键的距离会增加活化能。通过研究几种二烯（不同目的是为了实现该目的）对二烯的反应已充分证明了这种情况。五环杂环吡咯，呋喃，噻吩和硒烯具有不同的芳族特性，可用于带有环p键的杂原子孤对的不同共振参与。由于：（a）s _C-X的增加，芳香性以相同的顺序降低。（X =杂原子）键的长度提高了环电流所需的键均匀性，从而提高了芳香性，例如在苯中的芳香度；（b）相互作用的孤对轨道与环p的尺寸不匹配轨道，尤其是噻吩和硒烯，在基态结构中都允许较差的重叠。已经证明，吡咯，呋喃，噻吩和硒烯的DA反应的活化能单独增加不能像通常那样被认为是相对芳香度的量度，甚至在理论上也试图以多种方式尝试证明这一点。二烯末端的分离在确定活化能方面，特别是在噻吩和亚硒吩中，比其芳香性分布具有更大的作用。没有什么方法比杂原子孤对与环p键重叠的相对强度更好，它遵循Hückel的4n + 2规则产生了类似六电子的体系，以评估相对芳香性。