Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822.
W. M. Keck Laboratory in Astrochemistry, University of Hawaii at Manoa, Honolulu, HI 96822.
Proc Natl Acad Sci U S A. 2022 Jan 4;119(1). doi: 10.1073/pnas.2111938119.
Geminal diols-organic molecules carrying two hydroxyl groups at the same carbon atom-have been recognized as key reactive intermediates by the physical (organic) chemistry and atmospheric science communities as fundamental transients in the aerosol cycle and in the atmospheric ozonolysis reaction sequence. Anticipating short lifetimes and their tendency to fragment to water plus the aldehyde or ketone, free geminal diols represent one of the most elusive classes of organic reactive intermediates. Here, we afford an exceptional glance into the preparation of the previously elusive methanediol [CH(OH)] transient-the simplest geminal diol-via energetic processing of low-temperature methanol-oxygen ices. Methanediol was identified in the gas phase upon sublimation via isomer-selective photoionization reflectron time-of-flight mass spectrometry combined with isotopic substitution studies. Electronic structure calculations reveal that methanediol is formed via excited state dynamics through insertion of electronically excited atomic oxygen into a carbon-hydrogen bond of the methyl group of methanol followed by stabilization in the icy matrix. The first preparation and detection of methanediol demonstrates its gas-phase stability as supported by a significant barrier hindering unimolecular decomposition to formaldehyde and water. These findings advance our perception of the fundamental chemistry and chemical bonding of geminal diols and signify their role as an efficient sink of aldehydes and ketones in atmospheric environments eventually coupling the atmospheric chemistry of geminal diols and Criegee intermediates.
偕二醇——碳原子上带有两个羟基的有机分子——已被物理（有机）化学和大气科学界公认为气溶胶循环和大气臭氧分解反应序列中的基本瞬态的关键反应中间体。由于预期寿命短且易于断裂为水加醛或酮，游离偕二醇代表最难以捉摸的有机反应中间体之一。在这里，我们通过低温甲醇-氧气冰的能量处理，为以前难以捉摸的甲醇[CH(OH)]瞬态——最简单的偕二醇——的制备提供了一个难得的视角。通过异构图选择性光致电离反射飞行时间质谱结合同位素取代研究，在气相升华过程中鉴定出了甲醇。电子结构计算表明，甲醇通过激发态动力学形成，通过电子激发的原子氧插入甲醇甲基的碳-氢键，然后在冰基质中稳定化。甲醇的首次制备和检测证明了其气相稳定性，这是由于阻止其向甲醛和水的单分子分解的显著势垒。这些发现提高了我们对偕二醇基本化学和化学键的认识，并表明它们作为大气环境中醛和酮的有效汇，最终将偕二醇和 Criegee 中间体的大气化学联系起来。
