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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Quadricyclane Radical Cation Q~+: Formation and Isomerization in Liquid Methylcyclohexane
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Quadricyclane Radical Cation Q~+: Formation and Isomerization in Liquid Methylcyclohexane

机译：四环烷自由基阳离子Q〜+：在液态甲基环己烷中的形成和异构化

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摘要

The radical cation of quadricyclane (Q) was studied by pulse radiolysis at 133 K with methylcyclohexane (MCH) as solvent, saturated with N_2O. The known solvent radical cations, MCH~+ and its precursor M~+, are expected to produce Q~(+*) by charge transfer. Although Q~+ was known not to absorb in the visible λ-range, there was a very early absorption band at λ_(max) = 720 nm (the transient is called Q~(+*)), which eventually transformed into the cation of norbornadiene (NBD~+) with λ_(max) = 650 nm. The analysis of the geminate ion kinetics with the semiempirical t~(-0.6) kinetic law revealed that Q~(+*) decays faster than the isomer NBD~+ is built up. Q~(+*) must be a precursor to the nonabsorbing Q~+, which eventually isomerizes to NBD~+, followed by a back reaction with Q to re-form Q~+. The quantitative analysis revealed that a substantial amount of the cations is lost before NBD~+ is formed. This loss to a fragment or isomer (called F~+) occurs from Q~(+*). As this loss dropped drastically for very low [Q], Q~(+*) must increasingly be bypassed by lowering [Q]. It turns out that Q~(+*) is produced from M~(+*) only (the higher energy precursor of the solvent radical cation MCH~+) in competition with the transformation of M~(+*) to MCH~+, the latter becoming dominant at low [Q], increasingly producing Q~+ directly from MCH~+ without going through Q~(+*). The loss yield (F~+) correspondingly loses. The complete mechanism is given (Scheme 3). All the rate constants and the free ion contributions of all cations were determined and, together with the known G_(fi) value, the absorption coefficients were derived. Comparing these results with a mechanism proposed recently by Adam et al. (J. Am. Chem. Soc. 1995, 117, 9693) suggests that Q~(+*) corresponds to their cation Q~+(l), where the lateral bonds are oxidized, Q~+ to their cation Q~+(i), where the internal bonds are oxidized, and F~+ to the Q~+ isomer BHD~+ (the bicyclo[3.2.0]hepta-2,6-diene cation). The precursor ion M~(=*) of the solvent, which is responsible for the Q~(+*) production must be of higher energy than MCH~+; however, its structure remains unknown. The two precursor cations, M~(+*) and Q~(+*), are critically compared and discussed.
机译：以甲基环己烷（MCH）为溶剂，在N_2O饱和条件下，于133 K下通过脉冲辐射分解研究了四环烷（Q）的自由基阳离子。预期已知的溶剂自由基阳离子，MCH-+及其前体M-+会通过电荷转移产生Q〜（+ *）。尽管已知Q〜+在可见的λ范围内不吸收，但在λ_（max）= 720 nm处有一个很早的吸收带（瞬态称为Q〜（+ *）），最终转变为阳离子。降冰片二烯（NBD〜+）的λ_（max）= 650 nm。用半经验t〜（-0.6）动力学定律分析离子动力学，发现Q〜（+ *）的衰变快于异构体NBD〜+的建立。 Q〜（+ *）必须是未吸收Q〜+的前体，后者最终异构化为NBD〜+，然后与Q发生逆反应以重新形成Q〜+。定量分析表明，在NBD〜+形成之前，大量的阳离子损失了。片段或异构体（称为F〜+）的这种损失是从Q〜（+ *）发生的。由于此损耗在非常低的[Q]下急剧下降，因此必须通过降低[Q]越来越多地绕过Q〜（+ *）。事实证明，Q〜（+ *）仅由M〜（+ *）（溶剂自由基阳离子MCH〜+的高能前体）产生，与M〜（+ *）向MCH〜+的转化竞争，后者在[Q]低时成为优势，越来越多地直接从MCH〜+产生Q〜+，而无需经过Q〜（+ *）。损失产量（F〜+）相应地损失。给出了完整的机制（方案3）。测定所有阳离子的所有速率常数和自由离子贡献，并与已知的G_（fi）值一起得出吸收系数。将这些结果与Adam等人最近提出的机制进行比较。 （J. Am。Chem。Soc。1995，117，9693）表明，Q〜（+ *）对应于它们的阳离子Q〜+（l），其中侧键被氧化，Q〜+对应于它们的阳离子Q〜+ （i），其中内部键被氧化，并且F + +到Q +异构体BHD +（双环[3.2.0]庚-2,6-二烯阳离子）。负责Q〜（+ *）产生的溶剂前体离子M〜（= *）必须比MCH〜+具有更高的能量;但是，其结构仍然未知。严格地比较和讨论了两个前体阳离子M〜（+ *）和Q〜（+ *）。

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