Mondal Soumen, Sadhukhan Saikat, Sinha Amitabha, Hazra Montu K
Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India.
Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400094, India.
J Am Chem Soc. 2025 Jan 8;147(1):211-222. doi: 10.1021/jacs.4c08903. Epub 2024 Dec 17.
The ultraviolet (UV) photodissociation of pyruvic acid through the absorption of solar actinic flux generates methylhydroxycarbene (MHC) in the atmosphere. It is recognized that isolated MHC can undergo unimolecular isomerization to form acetaldehyde and vinyl alcohol. However, the rates and mechanism for its possible bimolecular reactions with atmospheric constituents, which can occur in parallel with its unimolecular reaction, is not well understood. Here we investigate the energetics, kinetics, and mechanism of the reaction of MHC with three ubiquitous atmospheric molecules N, O, and HO over the 160 K-380 K temperature range. Our study, at the CCSD(T)/6-311++G(3df,3pd)//M06-2/6-311++G(3df,3pd) level, reveals that the MHC + N encounter is nonreactive, while the MHC + O reaction, which leads to CHCO + HO formation, has a rate that is significantly different from previous estimates. For the MHC + HO reaction, we find that a single HO molecule is very effective in catalyzing the isomerization of MHC to form predominantly acetaldehyde. An analysis of the computed rate for this reaction indicates that it will be an important source of tropospheric acetaldehyde ̵ a major pollutant and precursor for atmospheric reactive intermediates. Our findings are in sharp contrast to current assessments in the literature that the MHC + HO reaction is minor. Furthermore, in the MHC + HO reaction system, we find that due to the presence of the OH group on MHC, the concerted insertion mechanism, which is typically dominant in reactions involving singlet carbenes, is suppressed relative to a hydrogen bond mediated double hydrogen atom transfer mechanism.
丙酮酸通过吸收太阳光化通量发生紫外光解，在大气中产生甲基羟基卡宾（MHC）。人们认识到，孤立的MHC可发生单分子异构化形成乙醛和乙烯醇。然而，其与大气成分可能发生的双分子反应的速率和机制，这可能与其单分子反应同时发生，目前还不太清楚。在这里，我们研究了在160 K - 380 K温度范围内，MHC与三种普遍存在的大气分子N、O和HO反应的能量学、动力学和机制。我们在CCSD(T)/6 - 311++G(3df,3pd)//M06 - 2/6 - 311++G(3df,3pd)水平上的研究表明，MHC + N碰撞不发生反应，而MHC + O反应生成CHCO + HO，其速率与先前的估计有显著差异。对于MHC + HO反应，我们发现单个HO分子在催化MHC异构化形成主要为乙醛方面非常有效。对该反应计算速率的分析表明，它将是对流层乙醛的一个重要来源——乙醛是一种主要污染物和大气反应性中间体的前体。我们的发现与文献中目前认为MHC + HO反应次要的评估形成鲜明对比。此外，在MHC + HO反应体系中，我们发现由于MHC上存在OH基团，在涉及单线态卡宾的反应中通常占主导的协同插入机制相对于氢键介导的双氢原子转移机制受到抑制。
