Hayashi Nobuyuki, Ujihara Tomomi, Ikeda Hirotaka
Food Research Institute, National Agriculture and Food Research Organization (NARO) , 2-1-12 Kannondai, Tsukuba, Ibaraki 305-8642, Japan.
Materials Science Department, MOLSIS Inc. , 1-28-38 Shinkawa, Chuo-ku, Tokyo 104-0033, Japan.
J Phys Chem A. 2017 Nov 9;121(44):8484-8494. doi: 10.1021/acs.jpca.7b08085. Epub 2017 Oct 27.
The geometries of the two hydroxy groups in 1,2-ethanediol or 2,3-butanediols are more stable in a gauche orientation than those in an anti orientation. This has been generally explained in terms of the gauche effect, which is stabilization due to antiperiplanar electron delocalization between an antibonding orbital of the C-O bond (σ*) and a bonding orbital of the C-H or C-C bond (σ or σ). However, a C-C single bond rotation simultaneously determines the geometries of the six vicinal bonds. Therefore, it is important to understand the effects on conformational stability of other interactions of the bond orbitals adjacent to the rotating C1-C2 bond. Bond model analysis revealed that antiperiplanar bond orbital interactions as a whole contribute to the higher stabilities of hydroxy/hydroxy gauche conformers, where the C-O/C-H or C-O/C-C combination including the σ*/σ or σ*/σ delocalization is not the dominant interaction stabilizing hydroxy/hydroxy gauche conformers. Rather, our results show that a large destabilization due to the antiperiplanar C-O/C-O combination in hydroxy/hydroxy anti conformers relatively increases the stabilities of hydroxy/hydroxy gauche conformers. This destabilization results mainly from the repulsion between the antiperiplanar bonding orbitals (σ/σ), which have a larger overlap compared to the synclinal σ/σ combination. The sum of the interbond energies between the vicinal bond orbitals of these 1,2-alkanediols is more advantageous for stability in gauche conformers. In addition, interactions between the gauche-oriented hydroxy groups provide large stabilization energies and the corresponding interactions in anti conformers are negligible. The relative conformational stabilities of 1,2-ethanediol and erythro-2,3-butanediol can be explained by the interactions between the antiperiplanar bond orbitals, between the vicinal bond orbitals, or between the hydroxy groups in addition to the combination of interactions between the vicinal bond orbitals and between the hydroxy groups. In contrast, in threo-2,3-butanediol, differences in the relative stabilities of the three conformers can be understood by the combination of the interactions between the vicinal bond orbitals and between the hydroxy groups.
1,2 - 乙二醇或2,3 - 丁二醇中两个羟基的几何构型在邻位交叉取向时比反式取向更稳定。这通常用邻位交叉效应来解释，即由于碳 - 氧键的反键轨道（σ*）与碳 - 氢或碳 - 碳键的成键轨道（σ或σ）之间的反平行电子离域而产生的稳定作用。然而，碳 - 碳单键的旋转同时决定了六个相邻键的几何构型。因此，了解与旋转的C1 - C2键相邻的键轨道的其他相互作用对构象稳定性的影响很重要。键模型分析表明，整体上反平行键轨道相互作用有助于羟基/羟基邻位交叉构象体具有更高的稳定性，其中包括σ*/σ或σ*/σ离域的碳 - 氧/碳 - 氢或碳 - 氧/碳 - 碳组合并非稳定羟基/羟基邻位交叉构象体的主要相互作用。相反，我们的结果表明，羟基/羟基反式构象体中由于反平行碳 - 氧/碳 - 氧组合导致的较大去稳定作用相对增加了羟基/羟基邻位交叉构象体的稳定性。这种去稳定作用主要源于反平行成键轨道（σ/σ）之间的排斥，与顺错的σ/σ组合相比，它们具有更大的重叠。这些1,2 - 链烷二醇的相邻键轨道之间的键能总和在邻位交叉构象体中对稳定性更有利。此外，邻位交叉取向的羟基之间的相互作用提供了较大的稳定能，而反式构象体中的相应相互作用可忽略不计。1,2 - 乙二醇和赤藓糖型2,3 - 丁二醇的相对构象稳定性除了可以用相邻键轨道之间以及羟基之间的相互作用组合来解释外，还可以用反平行键轨道之间、相邻键轨道之间或羟基之间的相互作用来解释。相比之下，在苏阿糖型2,3 - 丁二醇中，三种构象体相对稳定性的差异可以通过相邻键轨道之间以及羟基之间的相互作用组合来理解。
