Bernard Jürgen, Köck Eva-Maria, Huber Roland G, Liedl Klaus R, Call Ludwig, Schlögl Robert, Grothe Hinrich, Loerting Thomas
Institute of Physical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria. Email:
Max-Planck-Institut für chemische Energiekonversion, D-45470 Mülheim an der Ruhr, Germany.
RSC Adv. 2017 Apr 21;7(36):22222-22233. doi: 10.1039/c7ra02792c.
The monoesters of carbonic acid are deemed to be unstable and decompose to alcohol and carbon dioxide. In spite of this, we here report the isolation of the elusive carbonic acid monoethyl ester (CAEE) as a pure solid from ethanolic solutions of potassium bicarbonate. The hemiester is surprisingly stable in acidic solution and does not experience hydrolysis to carbonic acid. Furthermore, it is also stable in the gas phase, which we demonstrate by subliming the hemiester without decomposition. This could not be achieved in the past for any hemiester of carbonic acid. In the gas phase the hemiester experiences conformational isomerism at 210 K. Interestingly, the thermodynamically favored conformation is only reached for the torsional movement of the terminal ethyl group, but not the terminal hydrogen atom on the millisecond time scale. Accordingly, IR spectra of the gas phase trapped in an argon matrix are best explained on the basis of a 5 : 1 mixture of monomeric conformers. Our findings necessitate reevaluation of claims of the formation of a carbonic acid polymorph in methanolic solution, which is the subject of a forthcoming publication.
碳酸单酯被认为是不稳定的，会分解为醇和二氧化碳。尽管如此，我们在此报告，已从碳酸氢钾的乙醇溶液中分离出难以捉摸的碳酸单乙酯（CAEE），并得到了纯净的固体。该半酯在酸性溶液中出奇地稳定，不会水解为碳酸。此外，它在气相中也很稳定，我们通过升华该半酯而不发生分解来证明这一点。过去，对于任何碳酸半酯都无法做到这一点。在气相中，该半酯在210 K时会发生构象异构。有趣的是，仅在毫秒时间尺度上，末端乙基的扭转运动才能达到热力学上有利的构象，而末端氢原子则不能。因此，基于5∶1的单体构象混合物，才能最好地解释捕获在氩气基质中的气相红外光谱。我们的发现有必要重新评估关于在甲醇溶液中形成碳酸多晶型物的说法，这将是即将发表的一篇论文的主题。
