Hydrogen bond of radicals: Interaction of HNO with HCO,HNO, and HOO

Ab initio quantum mechanics methods are employed to investigate hydrogen bonding interactions between HNO and HCO, HOO radicals, and closed‐shell HNO. The systems were calculated at MP2/6‐311++G (2d, 2p) level and G2MP2 level. The topological and NBO analysis were investigated the origin of hydrogen bonds red‐ or blue‐shifts. In addition, the comparisons were performed between HNO‐opened‐shell radical (HCO, HOO) complexes and HNO‐corresponding closed‐shell molecule (H₂CO, HOOH) complexes. It is found that the stabilities of complexes increase from HNO‐HCO to HNO‐HOO. There are blue‐shifts of N? H, C? H stretching vibrational frequencies and a red‐shift of O? H stretching vibrational frequency in the complexes. Rehybridization and electron density redistribution contribute to the blue‐shifts of C? H and N? H stretching vibrational frequencies. Compared with the closed‐shell H₂CO, HCO is weaker proton donor and weaker proton acceptor. For the HOO, it is stronger proton donor and weaker proton acceptor than the HOOH is. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

肽链体系中的长程电子转移: 酪氨酸与色氨酸间电子转移的理论研究

用电子转移的半经典模型和量子化学半经验方法对色氨酸-酪氨酸二肽体系进行电子转移动力学参数计算.用AM1方法分别优化给体、受体和桥体几何构型,用线性反应坐标的构造了给体和受体分子间电子转移的双势阱,得到两透热势能面交叉处的反应坐标为R=(约等于)0.10,并确定了反应的内重组能及反应热.对色氨酰酪氨酸和酪氨酰色氨酸体系进行闭壳层HF自洽场计算,按Koopmans定理计算体系分子轨道分裂能值A(三角形),在R约为0处发现了A(三角形)的极小值,从而获得色氨酰酪氨酸及酪氨酰色氨酸体系分子内电子转移的电子转移矩阵元V~D~A分别为0.96kJ.mol^-^1和0.87kJ.mol^-^1.采用Marcus双球模型估算反应的溶剂重组能为64.60kJ.mol^-^1。     

Hydrogen transfer in the presence of amino acid radicals

Quantum chemical model studies of hydrogen transfer between amino acids in the presence of radicals have been performed using the density functional theory method B3LYP. These studies were made to investigate alternative mechanisms to the conventional electron transfer-proton transfer mechanisms. The model reactions studied are such that the net result of the reaction is a transfer of one neutral hydrogen atom. Simple models are used for the amino acids. Three different mechanisms for hydrogen transfer were found. In the first of these, a transition state with a protonated intermediate residue is found, in the second, the proton and electron take different paths and in the third, a neutral hydrogen atom can be identified along the reaction pathway. A key feature of these mechanisms is that charge separation is always kept small in contrast to the previous electron transfer-proton transfer mechanisms. It is therefore proposed that the processes normally considered as electron transfer in the biochemical literature could in fact be better explained as hydrogen atom transfer, at least in cases where a suitable hydrogen bonded chain pathway is present in the protein. The presence of such chains in principle allows the protein to define the path of net hydrogen transfer. Another important conclusion is that standard quantum chemical methods can be used to treat these mechanisms for hydrogen transfer, allowing for an accurate representation of the geometric changes during the reactions. Received: 10 February 1997 / Accepted: 11 February 1997     

Theoretical studies of electron and hydrogen transfer reactions between semiquinone radicals and oxygen

 To explore the interactions between ubiquinones and oxygen in living organisms, the thermodynamics of a series of electron and hydrogen transfer reactions between semiquinone radicals, as well as their corresponding protonated forms, and oxygen, singlet or triplet, were studied using the hybrid Hartree–Fock–density functional theory method Becke's three parameter hybrid method with the Lee, Yang, and Parr correlation functional. Effects of the solvent and of the isoprenyl tail on the electron and hydrogen transfer reactions were also investigated. It is found that semiquinone radicals (semiquinone anion radicals or protonated semiquinone radicals) cannot react with triplet oxygen to form the superoxide anion radical O₂ ⁻. In contrast, neutral quinones can scavenge O₂ ⁻ efficiently. In the gas phase, only protonated semiquinone radicals can react spontaneously with singlet oxygen to produce peroxyl radical (HO₂). However, both semiquinone anion radicals and protonated semiquinone radicals can react with singlet oxygen to produce harmful oxygen radicals (O₂ ^{− a l l b u l l} and HO₂, respectively) in aqueous and protein environments. The free-energy changes of the corresponding reactions obtained for different ubiquinone systems are very similar. It clearly shows that the isoprenyl tail does not influence the electron and hydrogen transfer reactions between semiquinone radicals and oxygen significantly. Results of electron affinities, vertical ionization potentials, and proton affinities also show that the isoprenyl tail has no substantial effect on the electronic properties of ubiquinones. Received: 3 July 2000 / Accepted: 6 September 2000 / Published online: 21 December 2000     

New boryl radicals derived from N-heteroaryl boranes: generation and reactivity

Recently, boryl radicals have been the subject of revived interest. These structures were generated by hydrogen-abstraction reactions from the corresponding boranes (i.e., from amine or phosphine boranes). However, the classical issue remains their high B--H bond-dissociation energy (BDE), thereby preventing a very efficient hydrogen-abstraction process. In the present paper, new N-heteroaryl boranes that exhibiting low B--H BDE are presented; excellent hydrogen-transfer properties have been found. Both the generation and the reactivity of the associated boryl radicals have been investigated through their direct observation in laser flash photolysis. The boryl radical interactions with double bonds, oxygen, oxidizing agent, and alkyl halides have been studied. Some selected applications of N-heteroaryl boryl radicals as new polymerization-initiating structures are proposed to evidence their high intrinsic reactivity.     

四氰基乙烯-四甲基乙烯体系光诱导电子转移双势阱的从头算研 究

以两态模型为基础,用从头算方法,在DZP[所有原子带极化函数的Dunning(9s,5p)/(3s,2p)]基组水平上对四氰基乙烯与四甲基乙烯间的电子转移进行理论计算。通过孤立给体和受体的几何构型优化,计算了给体的电离能和受体的电子亲和能。计算表明,在光诱导电荷分离之后的返回电子转移处于高放热的Marcus反转区。通过碰撞配合物的结构优化和电荷分离处理,在线性反应坐标近似下得到四甲基乙烯-四氰基乙烯配合物电荷分离反应的双势阱,进而获得反应热,键重组能,以及跃迁能。     

Contra-thermodynamic behavior in intermolecular hydrogen transfer of alkylperoxy radicals.

Quantum chemical investigation of bimolecular hydrogen transfer involving alkylperoxy radicals, a key reaction family in the free-radical oxidation of hydrocarbons, was performed to establish structure-reactivity relationships. Eight different reactions were investigated featuring four different alkane substrates (methane, ethane, propane and isobutane) and two different alkylperoxy radicals (methylperoxy and iso-propylperoxy). Including forward and reverse pairs, sixteen different activation energies and enthalpies of reaction were used to formulate structure-reactivity relationships to describe this chemistry. We observed that the enthalpy of formation of loosely bound intermediate states has a strong inverse correlation with the overall heat of reaction and that this results in unique contra-thermodynamic behavior such that more exothermic reactions have higher activation barriers. A new structure-reactivity relationship was proposed that fits the calculated data extremely well: E(A)=E(o)+alphaDeltaH(rxn) where alpha=-0.10 for DeltaH(rxn)<0, and alpha=1.10 for DeltaH(rxn)>0 and E(o)=3.05 kcal mol(-1).     

Dendronized perylenetetracarboxdiimides with peripheral triphenylamines for intramolecular energy and electron transfer

Novel perylene-3,4,9,10-tetracarboxdiimides (PDI) dyes functionalized with polyphenylene dendrimers attached at the bay region are reported. Derivatives of PDI bearing polyphenylene dendrimers up to the second generation, substituted with an increasing number of triphenylamine (TPA) moieties at the periphery, as well as a related nondendronized model compound were prepared. Intramolecular energy transfer was demonstrated by the observation of PDI emission on excitation of the triphenylamines, and electron transfer was detected by comparing photoluminescence quenching in solvents of different polarity.     

Computational studies on electron and proton transfer in phenol‐imidazole‐base triads

The electron and proton transfer in phenol‐imidazole‐base systems (base = NH₂^? or OH^?) were investigated by density‐functional theory calculations. In particular, the role of bridge imidazole on the electron and proton transfer was discussed in comparison with the phenol‐base systems (base = imidazole, H₂O, NH₃, OH^?, and NH₂^?). In the gas phase phenol‐imidazole‐base system, the hydrogen bonding between the phenol and the imidazole is classified as short strong hydrogen bonding, whereas that between the imidazole and the base is a conventional hydrogen bonding. The n value in spⁿ hybridization of the oxygen and carbon atoms of the phenolic CO sigma bond was found to be closely related to the CO bond length. From the potential energy surfaces without and with zero point energy correction, it can be concluded that the separated electron and proton transfer mechanism is suitable for the gas‐phase phenol‐imidazole‐base triads, in which the low‐barrier hydrogen bond is found and the delocalized phenolic proton can move freely in the single‐well potential. For the gas‐phase oxidized systems and all of the triads in water solvent, the homogeneous proton‐coupled electron transfer mechanism prevails. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Kinetics of the interaction of ketyl and semiquinone radicals with dioxygen. Concerted electron and proton transfer involving ketyl and semiquinone radicals

Kinetics of the interaction of ketyl and neutral semiquinone radicals with dioxygen was studied by the flash photolysis technique. The reactivity of neutral semiquinone radicals in the transfer of a hydrogen atom to O₂ is lower than that of ketyl radicals and increases as the reduction ability of the radicals increases, which give evidence for the charge transfer from the radicals to O₂ in the transition state of the reaction. The deuterium kinetic isotope effect of the reaction (up to 2.6) suggests considerable weakening of the O−H bond of the seminquinone radical in the transition state. A cyclic structure of the transition state similar to that in the reactions of ketyl radicals with hydrogen atom acceptors is proposed. In aprotic volvents, solvation has essentially no effect on the reactivity of neutral anthrasemiquinone radicals up to solvent nucleophilicityB≈240. In solvents with higher nucleophilicity and in protic solvents, their reactivity drops sharply. Hydrogen atom transfer reactions involving ketyl and neutral semiquinone radicals are shown to involve concerted electron and proton transfers, and to have transition states in which the partial transfer of an electron and a proton from the ketyl or semiquinone radical to an acceptor occurs. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1131–1137, June, 1997.     

Photoinduced intramolecular electron transfer and triplet energy transfer in a steroid-linked norbornadiene-carbazole dyad

Bichromophoric compound 3 beta-((2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17 beta-yl-[2-(N-carbazolyl)acetate] (NBD-S-CZ) was synthesized and its photochemistry was examined by fluorescence quenching, flash photolysis, and chemically induced dynamic nuclear polarization (CIDNP) methods. Fluorescence quenching measurements show that intramolecular electron transfer from the singlet excited state of the carbazole to the norbornadiene group in NBD-S-CZ occurs with an efficiency (Phi SET) of about 14 % and rate constant (kSET) of about 1.6 x 10(7) s-1. Phosphorescence and flash photolysis studies reveal that intramolecular triplet energy transfer and electron transfer from the triplet carbazole to the norbornadiene group proceed with an efficiency (TET + TT) of about 52 % and rate constant (kTET + kTT) of about 3.3 x 10(5) s-1. Upon selective excitation of the carbazole chromophore, nuclear polarization is detected for protons of the norbornadiene group (emission) and its quadricyclane isomer (enhanced absorption); this suggests that the isomerization of the norbornadiene group to the quadricyclane proceeds by a radical-ion pair recombination mechanism in addition to intramolecular triplet sensitization. The long-distance intramolecular triplet energy transfer and electron transfers starting both from the singlet and triplet excited states are proposed to proceed by a through-bond mechanism.     

生物体系中电子转移机理的研究Ⅱ: 分子间电子转移及 电子供、受体间不同基 团的影响

利用自编程序MOPAC-ET中AM1方法,及KT(Koopman'sTheorem)法,研究了二苯负离子体系的分子间电子转移现象,计算了其电子供、受体在不同距离下的V~A~B及它们之间的相关性,另外,还对两苯环间不同介入基团对电子转移的影响做了初步研究,发现不同的介入基团存在着较大的差异。     

Persistent hydrogen-bonded and non-hydrogen-bonded phenoxyl radicals

The production of stable phenoxyl radicals is undoubtedly a synthetic chemical challenge. Yet it is a useful way to gain information on the properties of the biological tyrosyl radicals. Recently, several persistent phenoxyl radicals have been reported, but only limited synthetic variations could be achieved. Herein, we show that the amide-o-substituted phenoxyl radical (i.e. with a salicylamide backbone) can be synthesised in a stable manner, thereby permitting easy synthetic modifications to be made through the amide bond. To study the effect of H-bonding on the properties of the phenolate/phenoxyl radical redox couple, simple H-bonded and non-H-bonded o,p-tBu-protected salicylamidate compounds have been prepared. Their redox properties were examined by cyclic voltammetry and showed a fully reversible one-electron oxidation process to the corresponding phenoxyl radical species. Remarkably, the redox potential appears to be correlated, at least partially, with H-bond strength, as relatively large differences (ca. 300 mV) in the redox potential between H-bonded and non-H-bonded phenolate salts are observed. The corresponding phenoxyl radicals produced electrochemically are persistent at room temperature for at least an hour; their UV/Vis and EPR characterisation is consistent with that of phenoxyl radicals, which makes them excellent models of biological tyrosyl radicals. The analyses of the experimental data coupled with theoretical calculations indicate that both the deviation from planarity of the amide function and intramolecular H-bonding influence the oxidation potential of the phenolate. The latter H-bonding effect appears to be predominantly exerted on the phenolate and not (or only a little) on the phenoxyl radical. Thus, in these systems the H-bonding energy involved in the phenoxyl radical appears to be relatively small.