Parandaman Arathala, Perez Josue E, Sinha Amitabha
Department of Chemistry and Biochemistry , University of California-San Diego , La Jolla , California 92093 , United States.
J Phys Chem A. 2018 Dec 13;122(49):9553-9562. doi: 10.1021/acs.jpca.8b09316. Epub 2018 Nov 29.
Quantum chemistry calculations are used to investigate the energetics and kinetics of CFOH decomposition catalyzed by a single formic acid (FA) molecule acting alone and in conjunction with a single water (HO) molecule to form the products carbonyl fluoride (CFO) and hydrofluoric acid (HF). While the uncatalyzed reaction has a barrier of ∼44.7 kcal/mol, the presence of a FA molecule reduces the barrier to 6.4 kcal/mol, while the presence of both a FA and HO molecule acting in unison decreases the barrier to -1.6 kcal/mol measured relative to the separated reactants. For comparison, we have also examined the decomposition of CFOH catalyzed by HO and HO + HO, which have been suggested in the literature to be an important atmospheric catalyst for CFOH decomposition. In addition, we have also examined the loss of CFOH via its bimolecular reaction with OH radicals. The rate constants for these various reactions were calculated using canonical variational transition state theory coupled with small curvature tunneling corrections over the temperature range between 200 and 300 K. Our results show that the rates for the CFOH + FA and CFOH + FA + HO reactions are ∼10 times faster compared, respectively, to the corresponding reactions involving CFOH + HO and CFOH + HO + HO at 300 K. Further, we find that, although the CFOH + FA reaction has a higher barrier compared to CFOH + FA + HO, measured relative to the separated reagents, its effective first order rate for CFOH decomposition is significantly faster for temperatures above 240 K compared to that of CFOH + FA + HO. This trend arises from the higher unimolecular reaction barrier for the reactant complex associated with the CFOH + HO + FA reaction compared to that for CFOH + FA, as well as the lower concentration of reactant dimer complexes for CFOH + HO + FA compared to the concentration of the monomer FA reactant in the CFOH + FA reaction. Finally, our calculations show that the rate for CFOH decomposition catalyzed by FA is ∼10 times faster relative to the loss of CFOH via its bimolecular reaction with OH radicals over the 200-300 K temperature range. Thus, the present study suggests that, among the various known loss mechanisms, a unimolecular reaction catalyzed by FA is likely the dominant gas phase decomposition pathway for CFOH in the troposphere.
量子化学计算用于研究由单个甲酸（FA）分子单独作用以及与单个水分子（HO）共同作用催化CFOH分解形成产物羰基氟（CFO）和氢氟酸（HF）的能量学和动力学。虽然未催化反应的势垒约为44.7千卡/摩尔，但FA分子的存在将势垒降低至6.4千卡/摩尔，而FA和HO分子共同作用时，相对于分离的反应物，势垒降低至 -1.6千卡/摩尔。为作比较，我们还研究了由HO以及HO + HO催化的CFOH分解，文献中认为它们是CFOH分解的重要大气催化剂。此外，我们还研究了CFOH通过与OH自由基的双分子反应而损失的情况。使用正则变分过渡态理论并结合小曲率隧道效应校正，计算了200至300 K温度范围内这些不同反应的速率常数。我们的结果表明，在300 K时，CFOH + FA和CFOH + FA + HO反应的速率分别比涉及CFOH + HO和CFOH + HO + HO的相应反应快约10倍。此外，我们发现，尽管相对于分离的试剂，CFOH + FA反应的势垒比CFOH + FA + HO高，但在240 K以上的温度下，其CFOH分解的有效一级速率比CFOH + FA + HO快得多。这种趋势源于与CFOH + FA相比，与CFOH + HO + FA反应相关的反应物络合物的单分子反应势垒更高，以及与CFOH + FA反应中单体FA反应物的浓度相比，CFOH + HO + FA的反应物二聚体络合物浓度更低。最后，我们的计算表明，在200 - 300 K温度范围内，由FA催化的CFOH分解速率相对于CFOH通过与OH自由基的双分子反应而损失的速率快约10倍。因此，本研究表明，在各种已知的损失机制中，由FA催化的单分子反应可能是对流层中CFOH主要的气相分解途径。
