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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Theoretical Analysis of the Unimolecular Gas-Phase Decompositions of the Propane Molecular Ion
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Theoretical Analysis of the Unimolecular Gas-Phase Decompositions of the Propane Molecular Ion

机译：丙烷分子离子单分子气相分解的理论分析

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Although the propane ion has been a long-standing model for RRKM/QET calculations, the validity of the transition states utilized in such calculations was unclear. To remedy this, we use potential energy barriers and harmonic vibrational frequencies calculated at the ab initio QCISD(T)/6-311+G(2d,2p) and UMP2/6-31G(d) levels of theory, respectively, as parameters to compute rate constants versus internal energy curves for the losses of the H atom, CH_3~·, and CH_4 from the propane ion by a RRKM procedure. The results agree reasonably with experimental ones. The ab initio calculations confirm that H atom loss occurs from the middle carbon of the propane ion to form the sec-propyl cation. The rate constant of H atom loss increases slowly with increasing internal energy, which is surprising for a simple bond cleavage. This is shown to be due to the changes in vibrational frequencies between the propane ion and the transition state being small for this reaction. Methane elimination occurs in a stepwise fashion through a methyl radical-ethyl ion complex. Low frequencies arising from CX bond elongation in the rate determining step for this reaction give a faster rise in rate constant with increasing internal energy for this reaction than for H atom loss, despite the former reaction being an elimination. A number of frequencies are very low in the transition state for CH_3~· loss, taken to be a loosely bound methyl radical-ethyl ion complex. This gives a very rapid rise in the rate constant of this simple CC bond cleavage with increasing internal energy. Losses of single atoms by simple cleavages are predicted to be slower than most types of competing reactions due to changes in vibrational frequencies between the reactant and transition states being relatively small.
机译：尽管丙烷离子一直是RRKM / QET计算的长期模型，但尚不清楚在这种计算中使用的过渡态的有效性。为了解决这个问题，我们分别使用从头算QCISD（T）/ 6-311 + G（2d，2p）和UMP2 / 6-31G（d）的理论水平分别计算出的势能垒和谐波振动频率作为参数通过RRKM程序计算丙烷离子中H原子，CH_3〜·和CH_4的损失的速率常数与内部能量曲线的关系。结果与实验结果合理吻合。从头算计算证实，H原子的损失从丙烷离子的中间碳发生，从而形成仲丙基阳离子。 H原子损失的速率常数随着内部能量的增加而缓慢增加，这对于简单的键裂解而言是令人惊讶的。示出这是由于该反应中丙烷离子和过渡态之间的振动频率变化较小。通过甲基-乙基离子络合物逐步消除甲烷。尽管该反应被消除，但在该反应的速率确定步骤中由CX键伸长产生的低频比H原子损失更快，随着该内部能量的增加，速率常数的增加更快。 CH_3〜·损失的过渡态中的许多频率非常低，被认为是松散结合的甲基自由基-乙基离子络合物。随着内部能量的增加，这种简单的CC键断裂的速率常数迅速增加。由于反应物和过渡态之间振动频率的变化相对较小，通过简单裂解的单原子损失预计比大多数竞争反应要慢。

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