N-H and α(C-H) bond dissociation enthalpies of aliphatic amines
N-H and α(C-H) bond dissociation enthalpies of aliphatic amines
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DOI：
10.1021/ja0204168
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发表时间：
2002-08-14
影响因子：
15
通讯作者：
Fouassier, JP
中科院分区：
文献类型：
--
作者：
Lalevée, J;Allonas, X;Fouassier, JP
用光声量热法测定了一系列脂肪胺（21）的键离解能（BIDES）。尽管在伯胺、仲胺和叔胺系列中研究了不同的结构，但发现a（C-H）BIDES对于不受约束的胺非常相似，其值非常接近91 kcal/mol。α C-和N-烷基化或引入羟基对溴化二苯醚的影响很小，这一事实与在不同CBS水平上进行的计算完全一致。这证明了涉及N和alphaC（.）轨道另一方面，当从伯胺到仲胺时，N-H BIDE降低。这一结果被解释在sigmaC-C键，这导致了稳定的胺基自由基的超共轭。对于环化胺，BIDES取决于单个占据的α C（.）轨道相对于N原子，不利于两轨道三电子相互作用。然而，这样的结构可以表现出贯穿键相互作用。对于拥挤的结构，如三异丙胺，其中α C（.）由于BIDE与氮原子轨道不共面，应变能的弛豫使得BIDE可以与柔性结构相媲美。
Bond dissociation enthalpies (BIDES) of a large series of aliphatic amines (21) were measured by means of photoacoustic calorimetry. Despite the different structures studied in the primary, secondary, and tertiary amine series, the a(C-H) BIDES were found to be very similar for unconstrained amines with values very close to 91 kcal/mol. alphaC- and N-alkylation or introduction of an hydroxy group only slightly affect the BDEs, a fact in perfect agreement with calculations performed at different CBS levels. This demonstrates the predominance of the two-orbital-three-electron interaction involving the N and alphaC(.) orbitals. On the other hand, the N-H BIDE decreases when going from primary to secondary amines. This result is interpreted in term of a hyperconjugation in sigmaC-C bonds, which leads to a stabilization of the aminyl radical. For cyclized amines, the BIDES depend on the relative geometry of the singly occupied alphaC(.) orbital with respect to that of the N atom, disfavoring the two-orbital-three-electron interaction. However, such structures can exhibit through-bond interaction. For a crowded structure such as triisopropylamine, for which the alphaC(.) orbital is not coplanar with the nitrogen one, the relaxation of a strain energy allows the BIDE to be comparable to flexible structures.