Taubert Stefan, Konschin Henrik, Sundholm Dage
Department of Chemistry, University of Helsinki, P.O. Box 55 A.I. Virtanens plats 1, FIN-00014 Helsinki, Finland.
Phys Chem Chem Phys. 2005 Jul 7;7(13):2561-9. doi: 10.1039/b505546f. Epub 2005 Jun 9.
The (13)C NMR chemical shifts for alpha-D-lyxofuranose, alpha-D-lyxopyranose (1)C(4), alpha-D-lyxopyranose (4)C(1), alpha-D-glucopyranose (4)C(1), and alpha-D-glucofuranose have been studied at ab initio and density-functional theory levels using TZVP quality basis set. The methods were tested by calculating the nuclear magnetic shieldings for tetramethylsilane (TMS) at different levels of theory using large basis sets. Test calculations on the monosaccharides showed B3LYP(TZVP) and BP86(TZVP) to be cost-efficient levels of theory for calculation of NMR chemical shifts of carbohydrates. The accuracy of the molecular structures and chemical shifts calculated at the B3LYP(TZVP) level is comparable to those obtained at the MP2(TZVP) level. Solvent effects were considered by surrounding the saccharides by water molecules and also by employing a continuum solvent model. None of the applied methods to consider solvent effects was successful. The B3LYP(TZVP) and MP2(TZVP)(13)C NMR chemical shift calculations yielded without solvent and rovibrational corrections an average deviation of 5.4 ppm and 5.0 ppm between calculated and measured shifts. A closer agreement between calculated and measured chemical shifts can be obtained by using a reference compound that is structurally reminiscent of saccharides such as neat methanol. An accurate shielding reference for carbohydrates can be constructed by adding an empirical constant shift to the calculated chemical shifts, deduced from comparisons of B3LYP(TZVP) or BP86(TZVP) and measured chemical shifts of monosaccharides. The systematic deviation of about 3 ppm for O(1)H chemical shifts can be designed to hydrogen bonding, whereas solvent effects on the (1)H NMR chemical shifts of C(1)H were found to be small. At the B3LYP(TZVP) level, the barrier for the torsional motion of the hydroxyl group at C(6) in alpha-D-glucofuranose was calculated to 7.5 kcal mol(-1). The torsional displacement was found to introduce large changes of up to 10 ppm to the (13)C NMR chemical shifts yielding uncertainties of about +/-2 ppm in the chemical shifts.
使用TZVP质量基组，在从头算和密度泛函理论水平上研究了α-D-来苏呋喃糖、α-D-来苏吡喃糖(1)C(4)、α-D-来苏吡喃糖(4)C(1)、α-D-葡萄糖吡喃糖(4)C(1)和α-D-葡萄糖呋喃糖的(13)C NMR化学位移。通过使用大基组在不同理论水平上计算四甲基硅烷(TMS)的核磁屏蔽对这些方法进行了测试。对单糖的测试计算表明，B3LYP(TZVP)和BP86(TZVP)是计算碳水化合物NMR化学位移的经济高效的理论水平。在B3LYP(TZVP)水平计算得到的分子结构和化学位移的准确性与在MP2(TZVP)水平得到的相当。通过用水分子围绕糖类以及采用连续介质溶剂模型来考虑溶剂效应。所应用的考虑溶剂效应的方法均未成功。在不进行溶剂和振转校正的情况下，B3LYP(TZVP)和MP2(TZVP)的(13)C NMR化学位移计算结果与测量位移之间的平均偏差分别为5.4 ppm和5.0 ppm。通过使用结构上类似于糖类的参考化合物（如纯甲醇），可以在计算化学位移和测量化学位移之间获得更紧密的一致性。通过将从B3LYP(TZVP)或BP86(TZVP)与单糖测量化学位移的比较中推导出来的经验常数位移添加到计算的化学位移中，可以构建碳水化合物的精确屏蔽参考。O(1)H化学位移约3 ppm的系统偏差可归因于氢键作用，而溶剂对C(1)H的(1)H NMR化学位移的影响较小。在B3LYP(TZVP)水平上，计算得到α-D-葡萄糖呋喃糖中C(6)处羟基扭转运动的势垒为7.5 kcal mol(-1)。发现扭转位移会使(13)C NMR化学位移产生高达10 ppm的大变化，导致化学位移的不确定性约为±2 ppm。
