DFT study of α- and β-d-allopyranose at the B3LYP/6-311++G^** level of theory

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• 作者：Udo Schnupf ; J.L. Willett ; Wayne B. Bosma ; Frank A. Momany
• 关键词：B3LYP/6-311++G^** ; Allose ; Chair ; Boat ; Skew ; Conformation ; Hessian ; Relative free energy
• 刊名：Carbohydrate Research
• 出版年：2007
• 期刊代码：15_00086215
• 类别：ch
• 出版时间：5 February 2007
• 卷：342
• 期：2
• 页码：196-216
• 文件大小：2592 K

摘要

One hundred and two conformations of α- and β-d-allopyranose, the C-3 substituted epimer of glucopyranose, were geometry optimized using the density functional, B3LYP, and the basis set, 6-311++G^**. Full geometry optimization was performed on different ring geometries and on the hydroxymethyl rotamers (gg/gt/tg). Analytically derived Hessians were used to calculate zero point energy, enthalpy, and entropy. The lowest energy and free energy conformation found is the α-tg(g−)-⁴C₁-c conformation, which is only slightly higher in electronic (0.2 kcal/mol) and free energy than the lowest energy α-d-glucopyranose. The in vacuo calculations showed a small (0.3 kcal/mol) energetic preference for the α- over the β-anomer for allopyranose in the ⁴C₁ conformation, whereas in the ¹C₄ conformation a considerable (1.6 kcal/mol) energetic preference for the β- over the α-anomer for allopyranose was encountered. The results are compared to previous aldohexose calculations in vacuo. Boat and skew forms were found that remained stable upon gradient optimization although many starting boat conformations moved to other skew forms upon optimization. As found for glucose, mannose, and galactose the orientation and interaction of the hydroxyl groups make the most significant contributions to the conformation/energy relationship in vacuo. A comparison of different basis sets and density functionals is made in the Discussion section, confirming the appropriateness of the level of theory used here.