Energetics and mechanism of the decomposition of trifluoromethanol.
Energetics and mechanism of the decomposition of trifluoromethanol.
复制标题
三氟甲醇分解的能量学和机理。
DOI：
--
复制
发表时间：
2008
影响因子：
2.9
通讯作者：
D. Dixon
中科院分区：
文献类型：
--
作者：
M. Nguyen;M. Matus;Thi Ngan Vu;R. Haiges;K. Christe;D. Dixon
α-氟醇的热不稳定性通常归因于 HF 的单分子 1,2-消除，但 CF3OH 消除分子内 HF 的障碍预计为 45.1 +/- 2 kcal/mol。使用耦合簇理论 (CCSD(T)) 计算三氟甲醇的热化学参数，并外推至完整基组极限。预计 CH2FOH、CHF2OH 和 CF3OH 的单分子分解的高势垒分别为 42.9、43.1 和 45.0 kcal/mol。如果涉及络合能约为 5 kcal/mol 的 CF3OH 环状氢键二聚体，这些势垒会大大降低。六元环二聚体的能量势垒为28.7 kcal/mol，八元环二聚体的能量势垒为32.9 kcal/mol。 CF3OH 与 HF 的络合物产生强氢键合的二聚体、三聚体和四聚体，络合能分别约为 6、11 和 16 kcal/mol。二聚体 CH3OH:HF 和三聚体 CF3OH:2HF 和 (CH3OH)2:HF 的分解能垒分别为 26.7、20.3 和 22.8 kcal/mol。四聚体 (CH3OH:HF)2 会消除两个 HF 分子，势垒为 32.5 kcal/mol。 CF3OH 或 HF 均可充当通过 H 转移中继消除 HF 的催化剂。由于 HF 是分解产物之一，因此分解反应变得自催化。如果 CF3OH-HF 加合物络合产生的能量没有耗散，则消除 HF 的有效势垒将从约 20 kcal/mol 降低至约 9 kcal/mol，这使计算结果与实验观察到的稳定性相一致。
The thermal instability of alpha-fluoroalcohols is generally attributed to a unimolecular 1,2-elimination of HF, but the barrier to intramolecular HF elimination from CF3OH is predicted to be 45.1 +/- 2 kcal/mol. The thermochemical parameters of trifluoromethanol were calculated using coupled-cluster theory (CCSD(T)) extrapolated to the complete basis set limit. High barriers of 42.9, 43.1, and 45.0 kcal/mol were predicted for the unimolecular decompositions of CH2FOH, CHF2OH, and CF3OH, respectively. These barriers are lowered substantially if cyclic H-bonded dimers of CF3OH with complexation energies of approximately 5 kcal/mol are involved. A six-membered ring dimer has an energy barrier of 28.7 kcal/mol and an eight-membered dimer has an energy barrier of 32.9 kcal/mol. Complexes of CF3OH with HF lead to strong H-bonded dimers, trimers and tetramers with complexation energies of approximately 6, 11, and 16 kcal/mol, respectively. The dimer, CH3OH:HF, and the trimers, CF3OH:2HF and (CH3OH)2:HF, have decomposition energy barriers of 26.7, 20.3, and 22.8 kcal/mol, respectively. The tetramer (CH3OH:HF)2 gives rise to elimination of two HF molecules with a barrier of 32.5 kcal/mol. Either CF3OH or HF can act as catalysts for HF-elimination via an H-transfer relay. Because HF is one of the decomposition products, the decomposition reactions become autocatalytic. If the energies due to complexation for the CF3OH-HF adducts are not dissipated, the effective barriers to HF elimination are lowered from approximately 20 to approximately 9 kcal/mol, which reconciles the computational results with the experimentally observed stabilities.