Namai Hayato, Ikeda Hiroshi, Kato Nobuyuki, Mizuno Kazuhiko
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.
J Phys Chem A. 2007 May 24;111(20):4436-42. doi: 10.1021/jp068308l. Epub 2007 May 2.
Substituent effects on the energies (Eob) of electronic transitions of geminally diphenyl-substituted trimethylenemethane (TMM) radical cations 5a-k*+ and those of structurally related 1,1-diarylethyl cations 7a-k+ were determined experimentally by using electronic transition spectroscopy. In addition, transition energies of these radical cations were determined by using density functional theory (DFT) and time-dependent (TD)-DFT calculations. The electronic transition bands of 5a-k*+ and 7a-k+ have maxima (lambdaob) that appear at 500-432 and 472-422 nm, respectively. A Hammett treatment made by plotting the Eob values relative to that of the diphenyl-TMM radical cation 5d*+ (DeltaEob) vs the cationic substituent parameter sigma+ give a favorable correlation with a boundary point at sigma+ = 0.00 and a positive rho for sigma+ < 0 and a negative rho for sigma+ > 0. A comparison of the lambdaob and rho values for 5a-k*+ and 7a-k+ suggests that the chromophore of 5*+ is substantially the same as that of 7+. The results of TD-DFT calculations, which reproduce the experimental electronic transition spectra and relationships between DeltaEob and sigma+, and suggest that the absorption band of 5*+ is associated with the SOMO-X --> SOMO transition, while that of 7+ is due to the HOMO --> LUMO transition. Another interesting observation is that Cl and Br substituents in the diphenyl-substituted TMM radical cations and 1,1-diarylethyl cations 7a-k+ act as electron-donating groups in terms of their effect on the corresponding electronic transitions. The results show that the molecular structure of 5*+ is a considerably twisted and that 5*+ has a substantially localized electronic state in which the positive charge and odd electron are localized in the respective diarylmethyl and the allyl moieties.
通过电子跃迁光谱实验测定了偕二苯基取代三亚甲基甲烷（TMM）自由基阳离子5a - k*+以及结构相关的1,1 - 二芳基乙基阳离子7a - k+的电子跃迁能量（Eob）的取代基效应。此外，利用密度泛函理论（DFT）和含时（TD）- DFT计算确定了这些自由基阳离子的跃迁能量。5a - k*+和7a - k+的电子跃迁带的最大值（lambdaob）分别出现在500 - 432 nm和472 - 422 nm处。通过绘制相对于二苯基 - TMM自由基阳离子5d*+的Eob值（DeltaEob）与阳离子取代基参数sigma +的哈米特处理方法，得到了良好的相关性，在sigma + = 0.00处有一个边界点，sigma + < 0时rho为正，sigma + > 0时rho为负。5a - k*+和7a - k+的lambdaob和rho值的比较表明，5*+的发色团与7+的基本相同。TD - DFT计算结果重现了实验电子跃迁光谱以及DeltaEob与sigma +之间的关系，并表明5*+的吸收带与SOMO - X --> SOMO跃迁相关，而7+的吸收带则归因于HOMO --> LUMO跃迁。另一个有趣的观察结果是，二苯基取代的TMM自由基阳离子和1,1 - 二芳基乙基阳离子7a - k+中的Cl和Br取代基在对相应电子跃迁的影响方面表现为供电子基团。结果表明，5*+的分子结构相当扭曲，并且5*+具有基本上局域化的电子态，其中正电荷和单电子分别局域在各自的二芳基甲基和烯丙基部分。
