Faculty of Chemistry, Maria Curie-Skłodowska University, pl. Marii Curie-Skłodowskiej 3, 20-031 Lublin, Poland.
J Magn Reson. 2012 Jan;214(1):1-9. doi: 10.1016/j.jmr.2011.08.042. Epub 2011 Sep 8.
Quantum chemistry SCF/GIAO calculations were carried out on a set of compounds containing diastereotopic protons. Five molecules, including recently synthesized 1,3-di(2,3-epoxypropoxy)benzene, containing the chiral or pro-chiral center and the neighboring methylene group, were chosen. The rotational averages (i.e. normalized averages with respect to the rotation about the torsional angle τ with the exponential energy weight at temperature T) calculated individually for each of the methylene protons in 1,3-di(2,3-epoxypropoxy)benzene differ by ca. 0.6 ppm, which is significantly less than the value calculated for the lowest energy conformer. This value turned out to be low enough to guarantee the proper ordering of theoretical chemical shifts, supporting the interpretation of the (1)H NMR spectrum of this important compound. The rotational averages of chemical shifts for methylene protons for a given type of conformer are shown to be essentially equal to the Boltzmann averages (here, the population-weighted averages for the individual conformers representing minima on the E(τ) cross-section). The calculated Boltzmann averages in the representative conformational space may exhibit completely different ordering as compared to the chemical shifts calculated for the lowest-energy conformer. This is especially true in the case of molecules, for which no significant steric effects are present. In this case, only Boltzmann averages account for the experimental pattern of proton signals. In addition, better overall agreement with experiment (lower value of the root-mean-square deviation between calculated and measured chemical shifts) is typically obtained when Boltzmann averages are used.
量子化学 SCF/GIAO 计算在一组包含非对映质子的化合物上进行。选择了五个分子，包括最近合成的含有手性或前手性中心和相邻亚甲基的 1,3-二（2,3-环氧丙氧基）苯。对于 1,3-二（2,3-环氧丙氧基）苯中每个亚甲基质子单独计算的旋转平均值（即相对于扭转角 τ 旋转的归一化平均值，在温度 T 下具有指数能量权重）相差约 0.6 ppm，这明显小于计算的最低能量构象的数值。这个值低到足以保证理论化学位移的正确排序，支持对这种重要化合物的 (1)H NMR 谱的解释。对于给定构象类型的亚甲基质子的旋转化学位移平均值被证明与玻尔兹曼平均值基本相等（此处，代表 E(τ) 截面上最小值的各个构象的加权平均值）。在代表性构象空间中计算的玻尔兹曼平均值与为最低能量构象计算的化学位移可能表现出完全不同的排序。对于没有明显空间位阻效应的分子尤其如此。在这种情况下，只有玻尔兹曼平均值才能解释质子信号的实验模式。此外，当使用玻尔兹曼平均值时，通常可以获得与实验更好的整体一致性（计算和测量化学位移之间的均方根偏差值较低）。
