Li An Yong, Yan Xiu Hua
School of Chemistry and Chemical Engineering, Southwest University, 400715, Chongqing, P. R. China.
Phys Chem Chem Phys. 2007 Dec 21;9(47):6263-71. doi: 10.1039/b712001j. Epub 2007 Oct 17.
H-bonding angle angleYHX has an important effect on the electronic properties of the H-bond Y...HX, such as intra- and intermolecular hyperconjugations and rehybridization, and topological properties of electron density. We studied the multifurcated bent H-bonds of the proton donors H3CZ (Z = F, Cl, Br), H2CO and H2CF2 with the proton acceptors Cl(-) and Br(-) at the four high levels of theory: MP2/6-311++G(d,p), MP2/6-311++G(2df,2p), MP2/6-311++G(3df,3pd) and QCISD/6-311++G(d,p), and found that they are all blue-shifted. These complexes have large interaction energies, 7-12 kcal mol(-1), and large blue shifts, delta r(HC) = -0.0025 --0.006 A and delta v(HC) = 30-90 cm(-1). The natural bond orbital analysis shows that the blue shifts of these H-bonds Y...HnCZ are mainly caused by three factors: rehybridization; indirect intermolecular hyperconjugation n(Y) -->sigma*(CZ), in that the electron density from n(Y) of the proton acceptor is transferred not to sigma*(CH), but to sigma*(CZ) of the donor; intramolecular hyperconjugation n(Z) -->sigma*(CH), in that the electron density in sigma*(CH) comes back to n(Z) of the donor such that the occupancy in sigma*(CH) decreases. The topological properties of the electron density of the bifurcated H-bonds Y...H2CZ are similar to those of the usual linear H-bonds, there is a bond critical point between Y and each hydrogen, and a ring critical point inside the tetragon YHCH. However, the topological properties of electron density of the trifurcated H-bonds Y...H3CZ are essentially different from those of linear H-bonds, in that the intermolecular bond critical point, which represents a closed-shell interaction, is not between Y and hydrogen, but between Y and carbon.
氢键角∠YHX 对氢键 Y…HX 的电子性质有重要影响，如分子内和分子间的超共轭作用以及再杂化，还有电子密度的拓扑性质。我们在 MP2/6-311++G(d,p)、MP2/6-311++G(2df,2p)、MP2/6-311++G(3df,3pd) 和 QCISD/6-311++G(d,p) 这四个高水平理论下研究了质子供体 H3CZ（Z = F、Cl、Br）、H2CO 和 H2CF2 与质子受体 Cl(-) 和 Br(-) 形成的多分叉弯曲氢键，发现它们均发生了蓝移。这些配合物具有较大的相互作用能，为 7 - 12 kcal mol(-1)，且有较大的蓝移，Δr(HC) = -0.0025 --0.006 Å 和 Δv(HC) = 30 - 90 cm(-1)。自然键轨道分析表明，这些氢键 Y…HnCZ 的蓝移主要由三个因素引起：再杂化；间接分子间超共轭作用 n(Y)→σ*(CZ)，即质子受体的 n(Y)上的电子密度不是转移到 σ*(CH)，而是转移到供体的 σ*(CZ)；分子内超共轭作用 n(Z)→σ*(CH)，即 σ*(CH) 中的电子密度回到供体的 n(Z)，使得 σ*(CH) 中的占据数减少。分叉氢键 Y…H2CZ 的电子密度拓扑性质与通常的线性氢键相似，在 Y 与每个氢之间存在一个键临界点，在四边形 YHCH 内部存在一个环临界点。然而，三叉氢键 Y…H3CZ 的电子密度拓扑性质与线性氢键本质上不同，代表闭壳层相互作用的分子间键临界点不在 Y 与氢之间，而是在 Y 与碳之间。
