Solvent effects on the thioamide rotational barrier: An experimental and theoretical study
Solvent effects on the thioamide rotational barrier: An experimental and theoretical study
复制标题
DOI：
10.1021/ja003586y
复制
发表时间：
2001-03-07
影响因子：
15
通讯作者：
Rush, DJ
中科院分区：
文献类型：
--
作者：
Wiberg, KB;Rush, DJ
用从头算理论和核磁共振波谱研究了N，N-二甲基硫代甲酰胺(DMTF)和N，N-二甲基硫代乙酰胺(DMTA)的C-N转动势垒的溶剂效应。利用选择性反转恢复核磁共振实验测量了一系列溶剂中的转动势垒。这些数据与G2(MP2)理论水平上的从头计算结果进行了比较。后者被修正为大幅度的振动运动，以给出自由能的差异。计算的气相势垒与实验值吻合较好。用反应场理论计算了溶剂化效应。这种方法已经发现，对于许多不与溶质分子发生特定相互作用的非质子化、非芳香族溶剂，这种方法提供的势垒与实验非常一致。用上述溶剂计算的硫代酰胺的溶液相势垒也与观察到的势垒符合得很好。硫代酰胺的基态偶极矩较大，且偶极矩随溶剂极性的增加而变化较大，因此溶剂对硫代酰胺转动势垒的影响大于酰胺。酰胺类和硫代酰胺类化合物的过渡态偶极矩比较相似。讨论了C-N转动势垒的起源及其与酰胺“共振”概念的关系。
The solvent effect on the C-N rotational barriers of N,N-dimethylthioformamide (DMTF) and N,N-dimethylthioacetamide (DMTA) has been investigated using ab initio theory and NMR spectroscopy. Selective inversion recovery NMR experiments were used to measure rotational barriers in a series of solvents. These data are compared to ab initio results at the G2(MP2) theoretical level. The latter are corrected for large amplitude vibrational motions to give differences in free energy. The calculated gas phase barriers are in very good agreement with the experimental values. Solvation effects were calculated using reaction field theory. This approach has been found to give barriers that are in good agreement with experiment for many aprotic, nonaromatic solvents that do not engage in specific interactions with the solute molecules. The calculated solution-phase barriers for the thioamides using the above solvents are also in good agreement with the observed barriers. The solvent effect on the thioamide rotational barrier is larger than that for the amides because the thioamides have a larger ground-state dipole moment, and there is a larger change in dipole moment with increasing solvent polarity. The transition-state dipole moments for the amides and thioamides are relatively similar. The origin of the C-N rotational barrier and its relation to the concept of amide "resonance" is examined.