Theory of multichannel thermal unimolecular reactions. 2. Application to the thermal dissociation of formaldehyde.
Theory of multichannel thermal unimolecular reactions. 2. Application to the thermal dissociation of formaldehyde.
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多通道热单分子反应理论。
DOI：
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发表时间：
2005
影响因子：
2.9
通讯作者：
J. Troe
中科院分区：
文献类型：
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作者：
J. Troe
甲醛的热解离在三个通道上进行，分子消除通道H2 CO--> H2 + CO（1），自由基形成键裂变通道H2 CO--> H + HCO（2），以及键裂变引发的分子内氢夺取通道H2 CO--> H.. HCO --> H2 + CO（3），也形成分子产物。该系统在单分子反应的低压范围内的动力学被证明是由碰撞通道耦合的微妙叠加来控制的，通过求解主方程来处理，通过从头计算势以及光谱和光物理确定阈值能量和高于自由基形成阈值能量的通道分支，可以实现旋转通道切换通过甲醛光解量子产率以及经典轨道计算表征。在现有资料的基础上，分析和外推在宽范围的条件下的分子和自由基片段的形成的速率系数。在反应的低压范围内，在1400-3200 K范围内，在浴气Ar中以这种方式的隧道效应（忽略隧道效应）由k 0表示，Mol/[Ar]约为9.4 × 10（-9）exp（-33，140 K/T）cm 3 molecule（-1）s（-1）和k0，Rad/[Ar]约为6.2 x 10（-9）exp（-36，980 K/T）cm 3 molecule（-1）s（-1）。分析特别依赖的浴气Kr的相应值为k 0，Mol/[Kr]约7.7 x 10（-9）exp（-33，110 K/T）和k 0，Rad/[Kr]约4.1 x 10（-9）exp（-36，910 K/T）cm 3 molecule（-1）s（-1）。虽然通道2和3的阈值能量E0，2取自光谱测量，但通道1的阈值能量E0，1基于实验比率k 0、Rad/k 0、Mol结合光解量子产率拟合。导出的E0，1（1）= 81.2（+/-0.9）kcal mol（-1）与最近的从头计算结果81.9（+/-0.3）kcal mol（-1）很好地一致，但高于早期的物理实验结果79.2（+/-0.8）kcal mol（-1）。反应的高压极限的速率系数也建模。本工作的结果显着依赖于通道2和3之间的分支比。从经典的轨迹计算和光解量子产率测量的分支比的表达式进行了测试。同时，进行了光解量子产率的建模。甲醛体系是迄今为止表征最好的多通道解离反应。它可以作为其他多通道解离反应的原型。
The thermal dissociation of formaldehyde proceeds on three channels, the molecular-elimination channel H2CO --> H2 + CO (1), the radical-forming bond-fission channel H2CO --> H + HCO (2), and the bond-fission-initiated, intramolecular-hydrogen-abstraction channel H2CO --> H...HCO --> H2 + CO (3) which also forms molecular products. The kinetics of this system in the low-pressure range of the unimolecular reaction is shown to be governed by a subtle superposition of collisional channel coupling to be treated by solving a master equation, of rotational channel switching accessible through ab initio calculations of the potential as well as spectroscopic and photophysical determinations of the threshold energies and channel branching above the threshold energy for radical formation which can be characterized through formaldehyde photolysis quantum yields as well as classical trajectory calculations. On the basis of the available information, the rate coefficients for the formation of molecular and radical fragments are analyzed and extrapolated over wide ranges of conditions. The modeled rate coefficients in the low-pressure range of the reaction (neglecting tunneling) over the range 1400-3200 K in the bath-gas Ar in this way are represented by k0,Mol/[Ar] approximately 9.4 x 10(-9) exp(-33,140 K/T) cm3 molecule(-1) s(-1) and k0,Rad/[Ar] approximately 6.2 x 10(-9) exp(-36,980 K/T) cm3 molecule(-1) s(-1). The corresponding values for the bath-gas Kr, on which the analysis relies in particular, are k0,Mol/[Kr] approximately 7.7 x 10(-9) exp(-33,110 K/T) and k0,Rad/[Kr] approximately 4.1 x 10(-9) exp(-36 910 K/T) cm3 molecule(-1) s(-1). While the threshold energy E0,2 for channels 2 and 3 is taken from spectroscopic measurements, the threshold energy E0,1 for channel 1 is fitted on the basis of experimental ratios k0,Rad/k0,Mol in combination with photolysis quantum yields. The derived value of E0,1(1) = 81.2 (+/-0.9) kcal mol(-1) is in good agreement with results from recent ab initio calculations, 81.9 (+/-0.3) kcal mol(-1), but is higher than earlier results derived from photophysical experiments, 79.2 (+/-0.8) kcal mol(-1). Rate coefficients for the high-pressure limit of the reaction are also modeled. The results of the present work markedly depend on the branching ratio between channels 2 and 3. Expressions of this branching ratio from classical trajectory calculations and from photolysis quantum yield measurements were tested. At the same time, a modeling of the photolysis quantum yields was performed. The formaldehyde system so far presents the best characterized multichannel dissociation reaction. It may serve as a prototype for other multichannel dissociation reactions.