Magnitude and Mechanism of Charge Enhancement of CH••O Hydrogen Bonds
Magnitude and Mechanism of Charge Enhancement of CH••O Hydrogen Bonds
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DOI：
10.1021/jp4081788
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发表时间：
2013-10-10
影响因子：
2.9
通讯作者：
Scheiner, Steve
中科院分区：
文献类型：
--
作者：
Adhikari, Upendra;Scheiner, Steve
量子化学计算表明，中性的甲胺和硫醚与N-甲基乙酰胺（NMA）形成了结合能为2-5 kcal/mol的复合物。这种相互作用被放大了4-9倍，使结合能高达20千卡/摩尔，当一个CH 3+基团被添加到质子供体。络合物优选三叉排列，其中三个独立的甲基基团向O受体提供一个质子。当系统浸入极性增加的溶剂中时，结合能降低，但离子络合物即使在水中也保持其有利的状态。当甲基被较长的烷基链取代时，结合能降低。质子受体倾向于与那些尽可能靠近形式正电荷的SIN中心的CH基团缔合。单一线性CH中心点中心点O氢键（H-键）不如具有三个独立甲基的三分叉有利。具有相同甲基的三个H原子的三叉排列甚至更不有利。各种分析手段，包括NBO、SAPT、NMR和电子密度位移，都将+CH中心点中心点O相互作用确定为真正的氢键。
Quantum calculations find that neutral methylamines and thioethers form complexes, with N-methylacetamide (NMA) as proton acceptor, with binding energies of 2-5 kcal/mol. This interaction is magnified by a factor of 4-9, bringing the binding energy up to as much as 20 kcal/mol, when a CH3+ group is added to the proton donor. Complexes prefer trifurcated arrangements, wherein three separate methyl groups donate a proton to the O acceptor. Binding energies lessen when the systems are immersed in solvents of increasing polarity, but the ionic complexes retain their favored status even in water. The binding energy is reduced when the methyl groups are replaced by longer alkyl chains. The proton acceptor prefers to associate with those CH groups that are as close as possible to the SIN center of the formal positive charge. A single linear CH center dot center dot O hydrogen bond (H-bond) is less favorable than is trifurcation with three separate methyl groups. A trifurcated arrangement with three H atoms of the same methyl group is even less favorable. Various means of analysis, including NBO, SAPT, NMR, and electron density shifts, all identify the +CH center dot center dot O interaction as a true H-bond.