Time-Resolved IR Spectroscopy of 1,3-Dicyanophenylcyclopentane-1,3-diyl Diradicals: CN Stretching Wavenumber as a Vibrational Signature of Radical Character.
Time-Resolved IR Spectroscopy of 1,3-Dicyanophenylcyclopentane-1,3-diyl Diradicals: CN Stretching Wavenumber as a Vibrational Signature of Radical Character.
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1,3-二氰基苯基环戊烷-1,3-二基双自由基的时间分辨红外光谱：CN 伸缩波数作为自由基特征的振动特征。
DOI：
10.1021/jp500636j
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发表时间：
2014
期刊：
影响因子：
--
通讯作者：
Taka-aki
中科院分区：
文献类型：
--
作者：
Maeda;Akihiro; Oshita;Takehide; Abe;Manabu; Ishibashi;Taka-aki
用时间分辨红外光谱观察了环戊烷-1，3-二基自由基单重态和三重态(1，3-di(4-cyanophenyl)-2，2-dimethoxyoctahydropentalene-1，3-diyl和1，3-di(4-cyanophenyl)-2，2-dimethyloctahydropentalene-1，3-diyl)的CN伸缩谱带。单重态和三重态双自由基的CN伸缩波数分别比相应的闭合环状化合物的伸缩波数下移了10和19 cm-1。观察到的下移归因于CN键的键级降低，这是由于在自由基碳的对位上具有累积双键(CCN·)的共振结构的贡献。只有当分子有未配对的电子时，这种共振结构才有可能；因此，波数下移可以被视为双自由基自由基性质的实验表现。观察到的CN伸缩波数表明，单重态双自由基的自由基性质不如三重态自由基特征明显。较小的自由基特征归因于单重态双自由基中的两性离子和π-单键共振结构的贡献。用6-31G(D)基组在B3LYP理论水平上进行的无限制密度泛函计算再现了单重态和三重态双自由基的波数下移之间的小/大关系，但高估了单重态双自由基的位移。
CN stretch bands of singlet and triplet cyclopentane-1,3-diyl diradicals (1,3-di(4-cyanophenyl)-2,2-dimethoxyoctahydropentalene-1,3-diyl and 1,3-di(4-cyanophenyl)-2,2-dimethyloctahydropentalene-1,3-diyl) were observed by time-resolved IR spectroscopy. CN stretching wavenumbers of the singlet and triplet diradicals were downshifted by 10 and 19 cm–1from those of their corresponding ring-closed compounds, which are in closed-shell electronic states, respectively. The observed downshifts are attributed to the bond-order decrease in the CN bonds due to the contribution from a resonance structure that has cumulative double bonds (CCN•) at the para-positions of the radical carbons. This resonance structure is only possible when the molecules have unpaired electrons; thus, the wavenumber downshift can be regarded as an experimental manifestation of the radical character of the diradicals. The observed CN stretching wavenumbers indicate that the radical character of the singlet diradical is less significant than that of the triplet. The smaller radical character is ascribed to the contribution from the zwitterionic and π-single bonding resonance structures in the singlet diradical. Unrestricted DFT calculations at the B3LYP level of theory with the 6-31G(d) basis set reproduced the small/large relationship between the wavenumber downshifts of the singlet and triplet diradicals; however, the shift of the singlet diradical was overestimated.