Department of Chemistry and Chemical Engineering, New Jersey Institute of Technology , Newark, New Jersey 07102, United States.
J Phys Chem A. 2014 May 1;118(17):3147-67. doi: 10.1021/jp412590g. Epub 2014 Apr 17.
The formation of cyclic ethers is a major product in the oxidation of hydrocarbons, and the oxidation of biomass derived alcohols. Cyclic ethers are formed in the initial reactions of alkyl radicals with dioxygen in combustion and precombustion processes that occur at moderate temperatures. They represent a significant part of the oxygenated pollutants found in the exhaust gases of engines. Cyclic ethers can also be formed from atmospheric reactions of olefins. Additionally, cyclic ethers have been linked to the formation of the secondary organic aerosol (SOA) in the atmosphere. In combustion and thermal oxidation processes these cyclic ethers will form radicals that react with (3)O2 to form peroxy radicals. Density functional theory and higher level ab initio calculations are used to calculate thermochemical properties and bond dissociation enthalpies of 3 to 5 member ring cyclic ethers (oxirane, yC2O, oxetane, yC3O, and oxolane, yC4O), corresponding hydroperoxides, alcohols, hydroperoxy alkyl, and alkyl radicals which are formed in these oxidation reaction systems. Trends in carbon-hydrogen bond dissociation energies for the ring and hydroperoxide group relative to ring size and to distance from the ether group are determined. Bond dissociation energies are calculated for use in understanding effects of the ether oxygen in the cyclic ethers, their stability, and kinetic properties. Geometries, vibration frequencies, and enthalpies of formation, ΔH°f,298, are calculated at the B3LYP/6-31G(d,p), B3LYP/6-31G(2d,2p), the composite CBS-QB3, and G3MP2B3 methods. Entropy and heat capacities, S°(T) and Cp°(T) (5 K ≤ T ≤ 5000), are determined using geometric parameters and frequencies from the B3LYP/6-31G(d,p) calculations. The strong effects of ring strain on the bond dissociation energies in these peroxy systems are also of fundamental interest. Oxetane and oxolane exhibit a significant stabilization, 10 kcal mol(-1), lower ΔfH°298 when an oxygen group is on the ether carbon relative to the isomer with the oxygen group on a secondary carbon. Relative to alkane systems the ether oxygen decreases bond dissociation energies (BDEs) on carbon sites adjacent to the ether by ∼5 kcal mol(-1), and increases BDEs on nonether carbons ∼1 kcal mol(-1). The cyclic structures have significant effects on the C-H, CO-OH, COO-H, and CO-H bond dissociation enthalpies. These values can be used to help calibrate calculations of larger more complex bicyclic and tricyclic hydrocarbon and ether species.
环醚的形成是烃类氧化和生物质衍生醇氧化的主要产物。环醚在燃烧和预燃烧过程中烷基自由基与氧气的初始反应中形成，这些过程发生在中等温度下。它们是发动机废气中发现的含氧污染物的重要组成部分。环醚也可以通过烯烃的大气反应形成。此外，环醚与大气中二次有机气溶胶（SOA）的形成有关。在燃烧和热氧化过程中，这些环醚将形成与（3）O2 反应形成过氧自由基的自由基。密度泛函理论和更高水平的从头算计算用于计算 3 至 5 元环环醚（环氧乙烷、yC2O、氧杂环丁烷、yC3O 和四氢呋喃、yC4O）、相应的过氧化物、醇、过氧烷基和烷基自由基的热化学性质和键离解焓，这些键是在这些氧化反应系统中形成的。确定了相对于环大小和与醚基团的距离的碳-氢键离解能的环和过氧化物基团的趋势。计算了键离解能，以用于理解环醚中醚氧的作用、它们的稳定性和动力学性质。在 B3LYP/6-31G(d,p)、B3LYP/6-31G(2d,2p)、复合 CBS-QB3 和 G3MP2B3 方法上计算了几何形状、振动频率和生成焓，ΔH°f,298。使用 B3LYP/6-31G(d,p)计算中的几何参数和频率确定熵和热容，S°(T)和 Cp°(T)（5 K ≤ T ≤ 5000）。过氧系统中环应变对键离解能的强烈影响也具有重要的基础性。与氧位于仲碳的异构体相比，当氧位于醚碳上时，氧杂环丁烷和四氢呋喃的生成焓显著降低了 10 kcal mol(-1)，ΔfH°298。相对于烷烃体系，醚氧降低了与醚相邻碳原子上的键离解能（BDE）约 5 kcal mol(-1)，增加了非醚碳原子上的 BDE 约 1 kcal mol(-1)。环结构对 C-H、CO-OH、COO-H 和 CO-H 键离解焓有显著影响。这些值可用于帮助校准更大更复杂的双环和三环烃和醚类物质的计算。
