Computational study of the initial stage of diborane pyrolysis.
Computational study of the initial stage of diborane pyrolysis.
复制标题
乙硼烷热解初始阶段的计算研究。
DOI：
10.1021/ic4001957
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发表时间：
2013
影响因子：
4.6
通讯作者：
M. Mckee
中科院分区：
文献类型：
--
作者：
Baili Sun;M. Mckee
用几种方法研究了两种硼烷缔合生成乙硼烷的速率常数。最复杂的方法是可变反应坐标-变分过渡态理论（VRC-VTST），它已被开发用于处理没有势垒的反应。在545 K时，计算的速率常数为8.2 × 10（-11）cm（3）·molecule（-1）·s（-1），与实验符合较好。速率常数也使用常规VTST与G4复合方法计算。二硼烷热解的多步机制的两种变化。一个是由步骤B2 H6 → 2 BH 3开始，而另一个是以2 B2 H6 → B3 H9 + BH 3作为初始基本步骤开始。这两种变化在乙硼烷中为3/2级，并且具有相同的活化能（G4，420 K时为28.65 kcal/mol）。相比之下，涉及具有C3 v对称性的B3 H9中间体的传统机制具有更高的活化能（33.37 kcal/mol）。这两种变化涉及一个C2对称的五配位B3 H9结构，虽然它是一个电子最小值，但它不是B2 H6 + BH 3 → B3 H7 + H2之间自由能路径上的一个稳定点。虽然计算的活化势垒高于最近确定的实验势垒，但报告值的变化很大（22.0-29.0千卡/摩尔）。我们讨论了实验和理论之间的分歧可能的来源。
The rate constants for the association of two boranes to form diborane are investigated using several methods. The most sophisticated method is the variable reaction coordinate-variational transition state theory (VRC-VTST) which has been developed to handle reactions with no enthalpic barriers. The calculated rate constant of 8.2 × 10(-11) cm(3)·molecule(-1)·s(-1) at 545 K is in good agreement with experiment. The rate constant was also computed using conventional VTST with the G4 composite method. Two variations of the multistep mechanisms for diborane pyrolysis are presented. One is initiated by the step B2H6 ⇄ 2 BH3 while the other begins with 2 B2H6 ⇄ B3H9 + BH3 as the initial elementary step. Both variations are 3/2 order in diborane and have the same activation energy (G4, 28.65 kcal/mol at 420 K). In contrast, the traditional mechanism involving a B3H9 intermediate with C3v symmetry has a higher activation energy (33.37 kcal/mol). The two variations involve a C2-symmetry penta-coordinate B3H9 structure that, while an electronic minimum, is not a stationary point on the free energy path between B2H6 + BH3 → B3H7 + H2. While the calculated activation barrier is higher than the recently determined experimental barrier, the variation in reported values is large (22.0-29.0 kcal/mol). We discuss possible sources of disagreement between experiment and theory.