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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Medium Effect on the rotational barrier of N,N,N'-trimethylurea and N,N,N'- trimethylthiourea: Experimental and theoretical study
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Medium Effect on the rotational barrier of N,N,N'-trimethylurea and N,N,N'- trimethylthiourea: Experimental and theoretical study

机译：对N，N，N'-三甲基脲和N，N，N'-三甲基硫脲的旋转势垒的介质效应：实验和理论研究

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摘要

The solvent effect for rotation about the conjugated C-N(CH _3)_2 bond has been studied for N,N,N'-trimethylurea (TMU) and N,N,N'-trimethylthiourea (TMT) by dynamic NMR and theoretical calculations. The experimental part comprised the measurement of activation parameters (δH~?, δS~?) by DNMR in CD _2Cl_2, CD_3OD, and D_2O/CD _3OD solutions. In methanol, TMU and TMT present similar rotational barriers, 11.3 ± 0.6 and 10.5 ± 0.3 kcal/mol, respectively. However, in D_2O/CD_3OD solution TMU has its barrier raised to 12.4 kcal/mol, whereas that of TMT remains unchanged. Molecular dynamics simulations combined to quantum chemical methods (HF, B3LYP, B3LYP-D, M06-2X, and MP2) showed that hydrogen bonding affects the rotational barriers of TMU and TMT in markedly different ways. For TMU, the rotational barrier increases due to hydrogen bonding whereas for TMT it decreases. This behavior is a consequence of the distinct ways the ground and the transition states for rotation experience hydrogen bonding. More specifically, the ground state of TMU can form strong hydrogen bonds at the carbonyl group, which stabilize the ground state relative to the transition state. On the other hand, the sulfur of TMT showed to be a poor proton acceptor, in such a way that only the nitrogen of the twisted transition state is involved in hydrogen bonding.
机译：通过动态NMR和理论计算，研究了N，N，N'-三甲基脲（TMU）和N，N，N'-三甲基硫脲（TMT）绕共轭C-N（CH _3）_2键旋转的溶剂效应。实验部分包括在CD _2Cl_2，CD_3OD和D_2O / CD _3OD溶液中通过DNMR测量活化参数（δH〜α，δS〜α）。在甲醇中，TMU和TMT表现出相似的旋转势垒，分别为11.3±0.6和10.5±0.3 kcal / mol。但是，在D_2O / CD_3OD溶液中，TMU的势垒提高到12.4 kcal / mol，而TMT的势垒保持不变。分子动力学模拟结合量子化学方法（HF，B3LYP，B3LYP-D，M06-2X和MP2）显示，氢键以明显不同的方式影响TMU和TMT的旋转势垒。对于TMU，旋转势垒由于氢键而增加，而对于TMT，旋转势垒减小。这种行为是旋转的基态和过渡态经历氢键结合的不同方式的结果。更具体地，TMU的基态可以在羰基上形成强氢键，这使基态相对于过渡态稳定。另一方面，TMT的硫显示为弱的质子受体，其方式为仅扭曲过渡态的氮参与氢键。

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