C-H and N-H bond dissociation energies of small aromatic hydrocarbons
C-H and N-H bond dissociation energies of small aromatic hydrocarbons
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DOI：
10.1021/ja982454q
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发表时间：
1999-01-27
影响因子：
15
通讯作者：
Hadad, CM
中科院分区：
文献类型：
--
作者：
Barckholtz, C;Barckholtz, TA;Hadad, CM
对计算煤中代表官能团的单环芳烃分子中C-H键和N-H键的均裂键离解能（BDE）的方法进行了综述。这些包括六元环（苯、吡啶、哒嗪、嘧啶、吡嗪）和五元环（呋喃、噻吩、吡咯、恶唑）。通过计算结果与实验值的比较，选择B3 LYP/6- 31 G（d）水平计算了多环芳烃（PAHs）的溴化二苯醚，包括含碳多环芳烃（萘、蒽、芘、晕苯）和杂原子多环芳烃（苯并呋喃、苯并噻吩、吲哚、苯并恶唑、喹啉、异喹啉、二苯并呋喃、咔唑）. C-H或N-H键的断裂产生σ自由基，其通常位于氢原子被去除的位点。然而，当C-H键与氮杂苯中的氮原子相邻时，未成对电子的离域导致自由基相对于苯基的形成类似于7 kcal.mol（-1）的稳定。由于自旋密度和几何因素的局部化，五元环的自由基比六元环的自由基稳定性低6 kcal.mol（-1）。杂原子在芳环中的位置比芳族网络的大小更显著地影响C-H键强度。因此，一般情况下，单环芳香族分子可以用来预测1 kcal.mol（-1）以内的大分子多环芳烃的C-H BDE。
A survey of computational methods was undertaken to calculate the homolytic bond dissociation energies (BDEs) of the C-H and N-H bonds in monocyclic aromatic molecules that are representative of the functionalities present in coal. These include six-membered rings (benzene, pyridine, pyridazine, pyrimidine, pyrazine) and five-membered rings (furan, thiophene, pyrrole, oxazole). By comparison of the calculated C-H BDEs with the available experimental values for these aromatic molecules, the B3LYP/6-31G(d) level of theory was selected to calculate the BDEs of polycyclic aromatic hydrocarbons (PAHs), including carbonaceous PAHs (naphthalene, anthracene, pyrene, coronene) and heteroatomic PAHs (benzofuran, benzothiophene, indole, benzoxazole, quinoline, isoquinoline, dibenzofuran, carbazole). The cleavage of a C-H or a N-H bond generates a sigma radical that is, in general, localized at the site from which the hydrogen atom was removed. However, delocalization of the unpaired electron results in similar to 7 kcal.mol(-1) stabilization of the radical with respect to the formation of phenyl when the C-H bond is adjacent to a nitrogen atom in the azabenzenes. Radicals from five-membered rings are similar to 6 kcal.mol(-1) less stable than those formed from six-membered rings due to both localization of the spin density and geometric factors. The location of the heteroatoms in the aromatic ring affects the C-H bond strengths more significantly than does the size of the aromatic network. Therefore, in general, the monocyclic aromatic molecules can be used to predict the C-H BDE of the large PAHs within 1 kcal.mol(-1).