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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >The Thermodynamic Basis for Enantiodiscrimination: Gas-Phase Measurement of the Enthalpy and Entropy of Chiral Amine Recognition by Dimethyldiketopyridino-18-crown-6
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The Thermodynamic Basis for Enantiodiscrimination: Gas-Phase Measurement of the Enthalpy and Entropy of Chiral Amine Recognition by Dimethyldiketopyridino-18-crown-6

机译：对映歧化的热力学基础：气相测量的二甲基二酮基吡啶基-18冠-6手性胺识别的焓和熵。

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摘要

Discrimination between the enantiomers of 1-phenylethylamine (PhEt) and α(1-naphthyl)ethylamine (NapEt) by the chiral ligand protonated dimethyldiketopyridino-18-crown-6 was studied using Fourier transform ion cyclotron resonance mass spectrometry to perform variable-temperature equilibrium (van't Hoff) experiments in the gas phase. The heterochiral complexes [(S,S)-ligand with (R)-amine, for example] have more-favorable enthalpy in both studied cases than the homochiral complexes; the differences are -6.7 ± 0.7 kJ mol~(-1) for the PhEt enantiomers and -10.0 ± 1.2 kJ mol~(-1) for the NapEt enantiomers. Entropy disfavors the heterochiral complexes by -14.8 ± 2.2 J mol~(-1) K~(-1) for PhEt and by -20.0 ± 3.9 J mol~(-1) K~(-1) for NapEt; entropy-enthalpy compensation is evident. These results suggest that enantiodiscrimination in these complexes is enthalpic and that locking of methyl rotors in the thermodynamically disfavored complexes is probably not important. Computational methods were also used to determine complex geometries at the HF/6-31+G~* level (diffuse functions on O and N atoms only), and energies at these geometries were determined using the same basis set with MP2 and B3LYP methods. The computed geometries have shorter hydrogen-bonding distances in the heterochiral complexes than those in the homochiral ones. The computational results also correctly predict that the heterochiral complexes are energetically favored. The calculations at most levels fail to reproduce the experimental finding that enantiodiscrimination of NapEt is greater than that of PhEt.
机译：使用傅里叶变换离子回旋共振共振质谱法研究了手性配体质子化的二甲基二酮基吡啶基-18-crown-6对1-苯基乙胺（PhEt）和α（1-萘基）乙胺（NapEt）对映体的区分。 （无法霍夫）在气相中进行实验。在两个研究案例中，杂手性配合物[（（S，S）-配体与（R）-胺]的焓均高于同手性配合物。 PhEt对映体的差异为-6.7±0.7 kJ mol〜（-1），NapEt对映体的差异为-10.0±1.2 kJ mol〜（-1）。熵对PhEt不利于-14.8±2.2 J mol〜（-1）K〜（-1），对NapEt不利于-20.0±3.9 J mol〜（-1）K〜（-1）。熵-焓补偿是明显的。这些结果表明，这些复合物中的对映异构是焓的，并且热力学不利的复合物中甲基转子的锁定可能并不重要。还使用计算方法确定HF / 6-31 + G〜*能级的复杂几何形状（仅在O和N原子上具有扩散函数），并使用与MP2和B3LYP方法相同的基础确定这些几何形状处的能量。所计算的几何形状在异手性配合物中比在同手性配合物中具有更短的氢键距离。计算结果还正确地预测了异手性配合物在能量上是有利的。在大多数水平上的计算都无法重现NapEt的对映歧义大于PhEt的对映歧义的实验发现。

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