Müller Beat, Bally Thomas, Gerson Fabian, de Meijere Armin, von Seebach Malte
Department of Chemistry, University of Fribourg, Switzerland.
J Am Chem Soc. 2003 Nov 12;125(45):13776-83. doi: 10.1021/ja037252v.
Bicyclopropylidene 1a and its octamethyl derivative 1b are subjected to ionization by X-irradiation in solid argon. In accord with previous experiments, this treatment leads to the spontaneous opening of both cyclopropylidene rings, as does ionization of 1b by gamma-irradiation in CFCl(3) at 77 K. The resulting tetramethyleneethane (or bisallyl) radical cations 2a+* and 2b+* are distinguished by a broad band in the NIR. In the case of 2a+, wavelength-selective photolyses reveal the presence of two interconvertible species with very similar yet distinct spectra. Based on DFT and CASSCF/CASPT2 calculations, these spectra are assigned to two "electromeric" forms of 2a+ which differ in the nature of the singly occupied MO. The NIR bands correspond to charge-resonance transitions between states with fully delocalized spin and charge. Calculations predict that similar electromers should also exist in 2b+* which shows a much weaker NIR band, but no corresponding experimental evidence could be found. On the other hand, the ESR spectrum of 2b+* indicates that, in contrast to 2a+, the spin is largely localized in one of the two allylic moieties in 2b+. Although no theoretical method is presently available that would permit an accurate modeling of the opposing factors favoring localized or delocalized structures in molecules such as 2a+* or 2b+*, the observed trends can be satisfactorily rationalized on the basis of semiquantitative considerations. In particular, the important role of vibronic coupling in shaping the potential surfaces for such systems is emphasized.
双环丙叉基1a及其八甲基衍生物1b在固态氩中通过X射线辐照进行电离。与之前的实验一致，这种处理会导致两个环丙叉基环自发开环，在77 K的CFCl₃中用γ射线辐照1b时也会发生电离。生成的四亚甲基乙烷（或双烯丙基）自由基阳离子2a⁺*和2b⁺在近红外区有一个宽带特征。对于2a⁺，波长选择性光解揭示存在两种可相互转化的物种，它们具有非常相似但又明显不同的光谱。基于密度泛函理论（DFT）和完全活性空间自洽场/完全活性空间微扰理论二级近似（CASSCF/CASPT2）计算，这些光谱被归因于2a⁺*的两种“电子互变异构”形式，它们在单占据分子轨道的性质上有所不同。近红外波段对应于自旋和电荷完全离域的状态之间的电荷共振跃迁。计算预测2b⁺*中也应该存在类似的电子互变异构体，其近红外波段要弱得多，但未找到相应的实验证据。另一方面，2b⁺*的电子顺磁共振（ESR）光谱表明，与2a⁺*不同，自旋在很大程度上定域在2b⁺*的两个烯丙基部分之一中。尽管目前没有理论方法能够准确模拟诸如2a⁺*或2b⁺*等分子中有利于局域或离域结构的相反因素，但基于半定量考虑，可以令人满意地解释所观察到的趋势。特别是，强调了振动-电子耦合在塑造此类系统的势能面中的重要作用。
