Hammerum Steen
Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
J Am Chem Soc. 2009 Jun 24;131(24):8627-35. doi: 10.1021/ja901854t.
Spectroscopic, energetic and structural information obtained by DFT and G3-type computational studies demonstrates that charged proton donors can form moderately strong hydrogen bonds to simple alkyl radicals. The presence of these bonds stabilizes the adducts and modifies their structure, and gives rise to pronounced shifts of IR stretching frequencies and to increased absorption intensities. The hydrogen bond acceptor properties of alkyl radicals equal those of many conventional acceptors, e.g., the bond length changes and IR red-shifts suggest that tert-butyl radicals are slightly better acceptors than formaldehyde molecules, while propyl radicals are as good as H(2)O. The hydrogen bond strength appears to depend on the proton affinity of the proton donor and on the ionization energy of the acceptor alkyl radical, not on the donor-acceptor proton affinity difference, reflecting that the charge-transfer aspects of hydrogen bonding are particularly conspicuous when the acceptor polarity and basicity is low.
通过密度泛函理论（DFT）和G3型计算研究获得的光谱、能量和结构信息表明，带电荷的质子供体可以与简单烷基自由基形成中等强度的氢键。这些键的存在使加合物稳定并改变其结构，导致红外伸缩频率发生明显位移并增加吸收强度。烷基自由基的氢键受体性质与许多传统受体相当，例如，键长变化和红外红移表明叔丁基自由基作为受体比甲醛分子略好，而丙基自由基与水相当。氢键强度似乎取决于质子供体的质子亲和力和受体烷基自由基的电离能，而不是供体-受体质子亲和力差，这反映出当受体极性和碱性较低时，氢键的电荷转移方面尤为显著。
