Sadhukhan Saikat, Mondal Soumen, 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 Phys Chem A. 2025 Aug 7;129(31):7146-7159. doi: 10.1021/acs.jpca.5c03732. Epub 2025 Jul 28.
Computational chemistry is used to examine the unimolecular isomerization kinetics of isolated methylhydroxycarbene (MHC) and compare with that of its catalyzed bimolecular isomerization in the presence of ammonia (NH) and formic acid (FA) over the 180 K-380 K temperature range. The two catalysts, one being a base and the other being an acid, both exhibit hydrogen bonding. For all the three processes studied, the potential energies along the reaction path are calculated at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level. For the unimolecular isomerization of isolated MHC, canonical variational transition state theory (CVTST) with small curvature tunneling (SCT) corrections reveals that while acetaldehyde (Ald) is the main product at low temperatures, vinyl alcohol (VA) becomes the dominant product above 310 K. The computed barrier heights for VA and Ald formation are ∼23.1 and ∼28.8 kcal/mol, respectively, and are consistent with earlier findings. For the NH-catalyzed isomerization of MHC, the energetically most favorable path produces solely the Ald product via a double hydrogen atom transfer (DHAT) mechanism. This reaction path is effectively barrierless, with the transition state (TS) being ∼0.47 kcal/mol below the total energy of the isolated MHC + NH reactants. By contrast, the singlet MHC insertion mechanism is negligible for the NH-catalyzed reaction. The FA-catalyzed reaction is found to proceed through three distinct paths (channel-I, channel-II, and channel-III) instead of the two (channel-I and channel-II), reported previously. The newly identified channel-III is effectively barrierless and involves a DHAT mechanism. All three channels in the FA catalyzed reaction predominately produce the Ald product. The temperature-dependent kinetics using the master equation solver for multienergy well reactions (MESMER) reveals that the overall rate constant for Ald production via both the NH- and FA-catalyzed reactions is ∼10 cm molecule s across the examined temperature range. A comparison of the relative rates for the unimolecular isomerization of isolated MHC versus that for the NH- and FA-catalyzed isomerization suggests that even for relatively low catalyst concentrations, 10 ppbv, the bimolecular isomerization dominates over the unimolecular isomerization process.
计算化学用于研究孤立的甲基羟基卡宾（MHC）的单分子异构化动力学，并将其与在180 K - 380 K温度范围内氨（NH）和甲酸（FA）存在下催化的双分子异构化动力学进行比较。两种催化剂，一种是碱，另一种是酸，都表现出氢键作用。对于所研究的所有三个过程，沿着反应路径的势能在CCSD(T)/6 - 311++G(3df,3pd)//M06 - 2X/6 - 311++G(3df,3pd)水平上进行计算。对于孤立的MHC的单分子异构化，具有小曲率隧道效应（SCT）校正的正则变分过渡态理论（CVTST）表明，虽然乙醛（Ald）在低温下是主要产物，但乙烯醇（VA）在310 K以上成为主要产物。计算得到的VA和Ald形成的势垒高度分别约为23.1和28.8 kcal/mol，与早期研究结果一致。对于NH催化的MHC异构化，能量上最有利的路径通过双氢原子转移（DHAT）机制仅产生Ald产物。该反应路径实际上没有势垒，过渡态（TS）比孤立的MHC + NH反应物的总能量低约0.47 kcal/mol。相比之下，单重态MHC插入机制对于NH催化的反应可以忽略不计。发现FA催化的反应通过三个不同的路径（通道I、通道II和通道III）进行，而不是先前报道的两个路径（通道I和通道II）。新发现的通道III实际上没有势垒，并且涉及DHAT机制。FA催化反应中的所有三个通道主要产生Ald产物。使用多能阱反应主方程求解器（MESMER）的温度相关动力学表明，在所研究的温度范围内，通过NH和FA催化反应生成Ald的总速率常数约为10 cm³分子⁻¹ s⁻¹。孤立的MHC单分子异构化与NH和FA催化异构化的相对速率比较表明，即使对于相对较低的催化剂浓度（10 ppbv），双分子异构化也比单分子异构化过程占主导地位。
