Materials and Process Simulation Center, California Institute of Technology , Pasadena, California 91125, United States.
J Am Chem Soc. 2012 Aug 8;134(31):12970-8. doi: 10.1021/ja300545e. Epub 2012 Jul 27.
Experimental results, such as NO2 hydrolysis and the hypergolicity of hydrazine/nitrogen tetroxide pair, have been interpreted in terms of NO2 dimers. Such interpretations are complicated by the possibility of several forms for the dimer: symmetric N2O4, cis-ONO-NO2, and trans-ONO-NO2. Quantum mechanical (QM) studies of these systems are complicated by the large resonance energy in NO2 which changes differently for each dimer and changes dramatically as bonds are formed and broken. As a result, none of the standard methods for QM are uniformly reliable. We report here studies of these systems using density functional theory (B3LYP) and several ab initio methods (MP2, CCSD(T), and GVB-RCI). At RCCSD(T)/CBS level, the enthalpic barrier to form cis-ONO-NO2 is 1.9 kcal/mol, whereas the enthalpic barrier to form trans-ONO-NO2 is 13.2 kcal/mol, in agreement with the GVB-RCI result. However, to form symmetric N2O4, RCCSD(T) gives an unphysical barrier due to the wrong asymptotic behavior of its reference function at the dissociation limit, whereas GVB-RCI shows no barrier for such a recombination. The difference of barrier heights in these three recombination reactions can be rationalized in terms of the amount of B2 excitation involved in the bond formation process. We find that the enthalpic barrier for N2O4 isomerizing to trans-ONO-NO2 is 43.9 kcal/mol, ruling out the possibility of such an isomerization playing a significant role in gas-phase hydrolysis of NO2. A much more favored path is to form cis-ONO-NO2 first then convert to trans-ONO-NO2 with a 2.4 kcal/mol enthalpic barrier. We also propose that the isotopic oxygen exchange in NO2 gas is possibly via the formation of trans-ONO-NO2 followed by ON(+) migration.
实验结果，如 NO2 水解和肼/四氧化二氮对的自燃性，已经根据 NO2 二聚体进行了解释。这种解释由于二聚体可能有几种形式而变得复杂：对称的 N2O4、顺式 ONONO2 和反式 ONONO2。由于 NO2 中的共振能很大，对于每个二聚体的变化不同，并且随着键的形成和断裂而剧烈变化，因此这些系统的量子力学（QM）研究变得复杂。结果，QM 的标准方法都不是完全可靠的。我们在这里报告使用密度泛函理论（B3LYP）和几种从头算方法（MP2、CCSD（T）和 GVB-RCI）对这些系统的研究。在 RCCSD（T）/CBS 水平上，形成顺式 ONONO2 的焓垒为 1.9 kcal/mol，而形成反式 ONONO2 的焓垒为 13.2 kcal/mol，与 GVB-RCI 结果一致。然而，为了形成对称的 N2O4，RCCSD（T）由于其参考函数在离解极限处的错误渐近行为而给出了一个非物理的势垒，而 GVB-RCI 对于这种重组则没有势垒。这三个重组反应的势垒高度差异可以根据键形成过程中涉及的 B2 激发量来合理化。我们发现 N2O4 异构化为反式 ONONO2 的焓垒为 43.9 kcal/mol，排除了这种异构化在气相 NO2 水解中起重要作用的可能性。更有利的途径是首先形成顺式 ONONO2，然后以 2.4 kcal/mol 的焓垒转化为反式 ONONO2。我们还提出，NO2 气体中的同位素氧交换可能是通过形成反式 ONONO2 然后进行 ON(+)迁移。
