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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Theoretical Study of Alternative Ring Forms of α-L-Fucopyranose
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Theoretical Study of Alternative Ring Forms of α-L-Fucopyranose

机译：α-L-富二糖替代环形式的理论研究

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The ring puckering of α-L-fucopyranose was studied by the MM2~*-LMOD (low mode) conformational search technique built into the Macromodel program. The molecular geometries of lowest energy (within a 10 kcal/mol energy window) were analyzed at the HF/3-21G, HF/6-31G(d), and generalized gradient approximation (GGA) levels of density functional theory (DFT). Our results show that the MM2~* method tends to overestimate the number of stable conformers in the high-energy region. The MM2~* method detected 33 stable conformers within the aforementioned energy window, and this number was reduced to 25 after re-minimization by the HF and GGA DFT methods. The low-energy conformers yielded by the MM2~* method are in qualitative agreement with the ab initio results. In these conformers, intramolecular chains of hydroxyl groups are formed in counterclockwise or clockwise directions. The best calculations predict the most stable ~1C_4 chair conformer to be lower in energy by ～6 kcal/mol than the most stable non-~1C_4 conformer. The zero-point energy differences are very small. We provide a detailed study of the basis set (up to 6-311 + G-(2d, p) basis set) and correlation effects (MP2 and hybrid functional study) on the relative energies and molecular geometries. The converged results are presented wherever it is possible. The largest difference between the various methods is ～2.7 kcal/mol; however, agreement well within 1 kcal/mol is experienced for most of the conformers. We discuss the geometric consequences of the exoanomeric effect, and show how the calculated and X-ray experimental results can be brought into accordance with each other. The geometric aspects of the O…H interactions (hydrogen bonding) are described with the aid of a polar coordinate system centered at the acceptor oxygen atom. The equatorial-equatorial and the equatorial-axial type of OH interactions can be readily distinguished using the proposed geometric parameters. We present the shape of the contour surface of the Laplacian of the electron density around the oxygen atom and shown how this shape influences the various types of O…H interactions. The strength and weakness of the Van den Enden's π-, sp~3-, and σ-type interactions were readily explained using the aforementioned surface.
机译：通过Macromodel程序中内置的MM2〜* -LMOD（低模）构象搜索技术研究了α-L-呋喃果糖的环褶皱。在HF / 3-21G，HF / 6-31G（d）和密度泛函理论（DFT）的广义梯度逼近（GGA）级别分析了最低能量的分子几何结构（在10 kcal / mol能量窗口内）。 。我们的结果表明，MM2〜*方法倾向于高估高能区域中稳定构象体的数量。 MM2 *方法在上述能量窗口内检测到33个稳定构象异构体，通过HF和GGA DFT方法将其最小化后，该数目减少至25。 MM2〜*方法产生的低能构象异构体与从头算的结果在质量上是一致的。在这些构象异构体中，沿逆时针或顺时针方向形成羟基的分子内链。最佳计算结果表明，最稳定的〜1C_4椅子构型异构体的能量比最稳定的〜1C_4椅子构型异构体的能量低〜6 kcal / mol。零点能量差很小。我们提供了基础集（最多6-311 + G-（2d，p）基础集）和相关能量（MP2和混合功能研究）对相对能量和分子几何结构的详细研究。会尽可能显示收敛的结果。各种方法之间的最大差异为〜2.7 kcal / mol;但是，对于大多数构象异构体来说，都必须在1 kcal / mol之内达成一致。我们讨论了exoanomeric效应的几何后果，并展示了如何将计算得出的结果和X射线实验结果相互结合。 O…H相互作用（氢键）的几何方面是借助以受体氧原子为中心的极坐标系进行描述的。使用提议的几何参数可以轻松地区分OH相互作用的赤道-赤道和赤道-轴向类型。我们介绍了氧原子周围电子密度的拉普拉斯算子的轮廓表面的形状，并显示了这种形状如何影响各种O…H相互作用。 Van den Enden的π，sp〜3-和σ型相互作用的优缺点很容易用上述表面解释。

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