Linden Michael M, Wagner J Philipp, Bernhardt Bastian, Bartlett Marcus A, Allen Wesley D, Schreiner Peter R
Institute of Organic Chemistry , Justus Liebig University , Heinrich-Buff-Ring 17 , 35392 Giessen , Germany.
Center for Computational Quantum Chemistry and Department of Chemistry , University of Georgia , Athens , Georgia 30602 , United States.
J Phys Chem Lett. 2018 Apr 5;9(7):1663-1667. doi: 10.1021/acs.jpclett.8b00295. Epub 2018 Mar 19.
Disentangling internal and external effects is a key requirement for understanding conformational tunneling processes. Here we report the s- trans/ s- cis tunneling rotamerization of carbonic acid monomethyl ester (1) under matrix isolation conditions and make comparisons to its parent carbonic acid (3). The observed tunneling rate of 1 is temperature-independent in the 3-20 K range and accelerates when using argon instead of neon as the matrix material. The methyl group increases the effective half life (τ) of the energetically disfavored s- trans-conformer from 3-5 h for 3 to 11-13 h for 1. Methyl group deuteration slows the rotamerization further (τ ≈ 35 h). CCSD(T)/cc-pVQZ//MP2/aug-cc-pVTZ computations of the tunneling probability suggest that the rate should be almost unaffected by methyl substitution or its deuteration. Thus the observed relative rates are puzzling, and they disagree with previous explanations involving fast vibrational relaxation after the tunneling event facilitated by the alkyl rotor.
区分内部和外部效应是理解构象隧穿过程的关键要求。在此，我们报道了碳酸单甲酯（1）在基质隔离条件下的s-反式/s-顺式隧穿旋转异构化，并将其与母体碳酸（3）进行比较。观察到的1的隧穿速率在3 - 20 K范围内与温度无关，并且当使用氩气而非氖气作为基质材料时会加快。甲基将能量上不利的s-反式构象异构体的有效半衰期（τ）从3的3 - 5小时增加到1的11 - 13小时。甲基氘代进一步减慢了旋转异构化（τ≈35小时）。隧穿概率的CCSD(T)/cc-pVQZ//MP2/aug-cc-pVTZ计算表明，速率几乎不应受甲基取代或其氘代的影响。因此，观察到的相对速率令人困惑，并且与先前涉及烷基转子促进隧穿事件后快速振动弛豫的解释不一致。
