The Clar Structure in Inorganic BN Analogues of Polybenzenoid Hydrocarbons: Does it Exist or Not?

The Clar structure of polybenzenoid hydrocarbons (PBHs) have attracted considerable interest of both theoretical and experimental chemists since it was proposed in the 1950s. However, it remains unclear whether the Clar structure could exist in inorganic PBHs, the boron nitride (BN) analogues where the alternate boron and nitrogen atoms are used to replace the carbon atoms of PBHs. Here, we carry out thorough density functional theory (DFT) calculations to probe the possibility of Clar structures in BN analogues of PBHs. A strong correlation (r²=0.975) between the ring number (n=3–10) of BN analogues of [n]acenes and energy differences between the most and least stable isomers is identified, suggesting the existence of Clar structures in inorganic PBHs. In addition, the slightly weaker correlations in comparison to that (r²=0.989) of the organic PBHs is rationalized by the reduced aromaticity, which is revealed by two aromatic indices: ELF_π and SCI.     

Unexpected 1,2‐Migration in Metallasilabenzenes: Theoretical Evidence for Reluctance of Silicon to Participate in π Bonding

Density functional theory (DFT) calculations were carried out to investigate the 1,2‐migration in metallasilabenzenes. The results suggested that the chloride migration of metallabenzenes is unfavorable due to the loss of aromaticity in the nonaromatic analogues. In sharp contrast, such a migration in metallasilabenzenes is favorable due to the reluctance of silicon to participate in π bonding. The migration of hydride and methyl group from the metal center to the silicon atom in metallasilabenzenes is computed to be also feasible. In addition, the π donor ligand and the third row transition metal can stabilize metallasilabenzenes. Thus, such a migration becomes less favorable thermodynamically and kinetically. These findings could be very helpful for synthetic chemists to realize the first metallasilabenzene.     

Theoretical Investigation on Triplet Excitation Energy Transfer in Fluorene Dimer

Triplet-triplet energy transfer in fluorene dimer is investigated by combining rate theories with electronic structure calculations. The two key parameters for the control of energy transfer, electronic coupling and reorganization energy, are calculated based on the diabatic states constructed by the constrained density functional theory. The fluctuation of the electronic coupling is further revealed by molecular dynamics simulation. Succeedingly, the diagonal and off-diagonal fluctuations of the Hamiltonian are mapped from the correlation functions of those parameters, and the rate is then estimated both from the perturbationtheory and wavepacket diffusion method. The results manifest that both the static and dynamic fluctuations enhance the rate significantly, but the rate from the dynamic fluctuation is smaller than that from the static fluctuation.     

Non-Condon电子转移速率理论与含时波包方法

随着电子转移理论在化学、材料科学、生物医学等领域的广泛应用,人们针对不同体系提出了多种电子转移理论模型。本文主要总结了近年来我们在non-Condon电子转移理论以及含时波包方法等方面的相关工作。首先阐述包含non-Condon效应的电子转移速率理论并用于二噻吩四硫富瓦烯有机半导体迁移率的计算。而后介绍了包含量子相干效应的含时波包方法,并初步用于研究二聚芴分子三三态能量转移过程。另外,本文还阐述了如何采用量子化学计算获得电子转移速率的结构参数。     

Gaussian软件在红外光谱教学中的应用——同位素效应

借助Gaussian软件探讨了同位素效应对分子特征官能团红外光谱的影响,帮助学生在课堂上认识同位素取代可能造成的红外特征吸收峰的光谱频移,并进一步促进学生在课后独立思考,发挥主观能动性,自行设计有效的同位素取代方案,利用软件工具将课堂知识应用于化合物组分的鉴别中。     

Super-exchange and Exchange-Enhanced Reactivity in Fe4S4-Mediated Activation of SAM by Radical SAM Enzymes?

[4Fe-4S]-dependent radical S-adenosylmethionine (SAM) proteins are a superfamily of oxidoreductases that can catalyze a series of challenging transformations using the common 5-dAdo radical intermediate. Although the structures and functions of radical SAM enzymes have been extensively studied, the electronic state-dependent reactions of the [4Fe-4S] clusters in these enzymes are still elusive. Herein we performed QM/MM calculations to elucidate the electronic state-dependent reactivity of the [4Fe-4S] cluster in pyruvate-formate lyase activating enzyme. Our calculations show that the electronic state-dependent SAM activation by the [4Fe-4S] clusters in radical SAM enzyme is determined by both the super-exchange and exchange-enhanced reactivities. The super-exchange coupling in the [4Fe-4S] cluster favors the antiferromagnetic coupling between two neighbouring pairs, which results in the \begin{document}$\alpha$\end{document}-electron rather than the \begin{document}$\beta$\end{document}-electron donation from the [4Fe-4S]\begin{document}$^{1+}$\end{document} cluster toward the SAM activation. Meanwhile, in the most favorable electronic state for the reductive cleavage of S\begin{document}$-$\end{document}C5\begin{document}$'$\end{document}, Fe4 would donate its \begin{document}$\alpha$\end{document}-electron to gain the maximum exchange interactions in the Fe4-block. Such super-exchange and exchange-enhanced reactivity could be the general principles for reactivities of [4Fe-4S] cluster in RS enzymes.     

Theoretical Study of Electronic Structure,Formation Mechanism and Intramolecular Sulfoxide Imidation Reactivity of Iron Phthalocyanine Nitrene Complex

Density functional theory(DFT) calculations are performed to investigate recent experimentally studied ring-closing sulfoxide imidation catalyzed by Fe(Ⅱ)-phthalocyanine(FeⅡPc). Our results reveal that the ground state of iron phthalocyanine nitrene intermediate(PcFeNR, R =(CH_2)_3(SO)Ph), which is believed to mediate the intramolecular imitation, is triplet state featuring a diradical structure. The formation of Pc Fe NR is the result of a denitrification process with a calculated high-barrier of 23.4 kcal/mol which is in good agreement with the experimentally observed high reaction temperature of 100 ℃. The generated Pc Fe NR undergoes a low-barrier intramolecular nucleophilic attack by proximal nitrogen atom on the sulfur accomplishing the cyclization of sulfoxide. This study provides theoretical insights into the mechanism-based design of useful catalysts for nitrene transfer reactions.