Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100, United States.
J Phys Chem A. 2010 Nov 25;114(46):12283-90. doi: 10.1021/jp105913x. Epub 2010 Oct 13.
Replacement of H by D perturbs the (13)C NMR chemical shifts of an alkane molecule. This effect is largest for the carbon to which the D is attached, diminishing rapidly with intervening bonds. The effect is sensitive to stereochemistry and is large enough to be measured reliably. A simple model based on the ground (zero point) vibrational level and treating only the C-H(D) degrees of freedom (local mode approach) is presented. The change in CH bond length with H/D substitution as well as the reduction in the range of the zero-point level probability distribution for the stretch and both bend degrees of freedom are computed. The (13)C NMR chemical shifts are computed with variation in these three degrees of freedom, and the results are averaged with respect to the H and D distribution functions. The resulting differences in the zero-point averaged chemical shifts are compared with experimental values of the H/D shifts for a series of cycloalkanes, norbornane, adamantane, and protoadamantane. Agreement is generally very good. The remaining differences are discussed. The proton spectrum of cyclohexane- is revisited and updated with improved agreement with experiment.
H 被 D 取代会扰乱烷烃分子的 (13)C NMR 化学位移。这种影响在与 D 相连的碳原子上最大，随着间隔键的增加迅速减小。这种效应对立体化学敏感，且足够大，可以可靠地测量。提出了一种基于基态（零点）振动能级且仅考虑 C-H(D)自由度的简单模型（局部模式方法）。计算了 CH 键长随 H/D 取代的变化以及伸展和两个弯曲自由度的零点能级概率分布范围的减小。用这三个自由度的变化来计算 (13)C NMR 化学位移，并对 H 和 D 分布函数进行平均。将零级平均化学位移的差异与一系列环烷烃、降冰片烷、金刚烷和原金刚烷的 H/D 位移的实验值进行比较。一般来说，一致性非常好。讨论了剩余的差异。重新考察了环己烷-的质子谱，并与实验结果进行了改进，使其一致性更好。
