The Role of Singlet and Triplet Dioxygen Molecules in the EPR and 1H NMR Spectrum Line Broadening

In the Mo^(VI)/H₂O₂/H₂O system, the relaxation time (T ₁) of protons in a water molecule and in a CH₃ group decreases 10 to 30 times under conditions of dismutation of H₂O₂ with the formation of ¹O₂(¹_g). It is experimentally found that the overequilibrium concentration of triplet dioxygen cannot be the reason behind a decrease in T ₁ in the ¹H NMR spectra. Neither can it explain the anomalous line broadening in ESR spectra under conditions of ¹O₂(¹_g) formation in the systems V^(V)/H₂O₂/AcOH and Mo^(VI)/H₂O₂/H₂O. Ab initio calculations showed that it is principle possible that the ³O₄(³·^- _g-¹_g) molecule exists in a snake-like form and is formed by the reaction between ³O₂(³·^- _g) and ¹O₂(¹_g), which is the product of H₂O₂ decomposition in the systems V^(V)/H₂O₂/AcOH and Mo^(IV)/H₂O₂/H₂O. The interaction of ¹O₂ with the ·OOH radical is exothermic (Q = 2.30 kcal/mol) and leads to the formation of ·OOOOH. It is assumed that the paramagnetic species of type ·OOOOH or ³O₄(³ A ₁) that is formed in the reaction might be responsible for the spectral effects observed.     

Cerium-mediated Free Radicals in Contractile Proteins. EPR and DSC study

The effect of free radicals obtained in hydroxyl and cerium(IV)-nitrilotriacetic acid free radical generating systems on contractile proteins (actin, myosin and their complexes in glycerinated muscle fibres) was studied using differential scanning calorimetry and spin trapping electron paramagnetic resonance technique. The analysis of spectra showed that selective attack of thiol groups – Cys-257 and Cys-374 residues of actin, and among others Cys-707 residue of myosin – and random attack of sidechains of the main proteins of muscle tissue produced structural and functional changes, which affected the ATP hydrolysis cycle and very likely the dynamics of actin. The melting curves obtained on protein systems support the view that global conformational changes accompany the local damage of free radicals. This revised version was published online in July 2006 with corrections to the Cover Date.     

EPR Detection of Free Radicals in UV-lrradiated Skin: Mouse Versus Human

Abstract— Ultraviolet radiation produces free radicals in Skh-1 mouse skin, contributing to photoaging and carcinogenesis. If a mouse model is a general indicator of free radical processes in human skin photobiology, then radical production observed in mouse and human skin should be directly comparative. In this work we show that UV radiation (Λ > 300 nm, 14 μW/cm² UVB; 3.5 mW/cm² UVA) increases the ascorbate free radical (Asc^.-) electron paramagnetic resonance (EPR) signal in both Skh-1 mouse skin (45%) and human facial skin biopsies (340%). Visible light (Λ > 400 nm; 0.23 mW/cm² UVA) also increased the Asc^.- signal in human skin samples (45%) but did not increase baseline mouse Asc^.-, indicating that human skin is more susceptible to free radical formation and that a chromophore for visible light may be present. Using EPR spin-trapping techniques, UV radiation produced spin adducts consistent with trapping lipid alkyl radicals in mouse skin (α-[4-pyridyl 1-oxide]-N-tert-butyl nitrone/alkyl radical adduct; a^N= 15.56 G and a^H= 2.70 G) and lipid alkoxyl radicals in human skin (5,5-dimethylpyrroline-l-oxide/alkoxyl radical adduct; a^N= 14.54 G and a^H= 16.0 G). Topical application of the iron chelator Desferal to human skin significantly decreases these radicals (±50%), indicating a role for iron in lipid peroxidation; Desferal has previously been shown to decrease radical production in mouse skin. This work supports the use of the Skh-1 mouse as a predictive tool for free radical formation in human skin. These results provide the first direct evidence for UV radiation-induced free radical formation at near physiological temperatures in human skin and suggest that iron chelators may be useful as photoprotective agents.     

The Reaction of Singlet and Triplet Oxygen with 2-Phenylnorbornene

The reaction of singlet oxygen with 2-phenylnorbornene ( 1 ) in aprotic solvents gives 3-formylcyclopentyl phenyl ketone ( 2 ) (10%) and uncharacterized polymer (90%). When methanol is used as solvent, endo-2-phenyl-exo-2-methoxy-exo-3-hydroperoxynorbornane ( 4 ) and endo-2-(anti-1′, 4′-epidioxy-5′,6′-epoxycyclohex-2′-enyl)-exo-2,3-epoxynorbornane ( 6 and 7 ) are obtained in addition to 2 . Triplet oxygen with 1 gave 2 , endo-2-phenyl-exo-2,3-epoxynorbornane ( 8 ), and the trimer 9 or 10 of exo-2,3-epidioxy-endo-2-phenylnorbornane. With protic solvents the amount of epoxide increased at the expense of trimer. The singlet and triplet oxygen reactions are discussed in the light of possible intermediates.     

有机发光二极管中三重态激子的单重态转换

在OLED的研究中如何充分利用三重态激子以提高器件的电-光转换效率一直是人们关注的问题,近年来出现的经热激活逆向上转换过程获得延迟荧光的办法,使OLED的研究出现了一派崭新的前景。本文综合前人的工作对有关这一领域的研究基础,如：电子跃迁、激发态的分裂、单重态/三重态的交换能量、载流子的重合和激子的形成,以及在纯有机化合物与有机-过渡金属配合物中的电荷转移（CT）问题等,进行了较详细的讨论。     

Effect of Oxygen Free Radicals on Myosin in Muscle Fibres. A DSC and EPR study

Differential scanning calorimetry (DSC) and electron paramagnetic resonance spectroscopy (EPR, both conventional and saturation transfer EPR) were used to study the motional dynamics and segmental flexibility of myosin in muscle fibres in the presence of free radical generating system. Muscle fibre bundles isolated from psoas muscle of rabbit were spin-labelled with maleimide- and isothiocyanate-based probe molecules at the reactive sulfhydryl sites (Cys-707) of the motor domain. In the presence of hydroxyl free radicals the spectral intensity of the maleimide probe molecules decreased with time following a single exponential curve. MgADP and MgATP plus orthovanadate that produce flexibility changes in the multisubunit structure of myosin enhanced the reduction of the attached nitroxide molecules in free radical generating system. The analysis of the EPR spectra of spin-labelled and oriented fibres showed that the narrow distribution of spin labels changed in the presence of hydroxyl free radicals. Spectrum analysis by computer subtraction showed that short irradiation by UV light resulted in the enhancement of the ordered population at the expense of the disordered population. This suggests a transition of myosin heads from weak- binding state into strong-binding state. DSC measurements performed on calf cardiac myosin resulted in two main transitions at 49.4 and 54.1°C, respectively. Addition of MgADP produced a decrease of the 49.4°C transition, whereas a shift towards higher temperature was detected at the 54.1°C transition. It shows that there is an inter-site communication between the domains of the myosin. Hydroxyl free radicals induced further shifts of the transition temperatures and affected the width of the heat absorption curves. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Important Role of Heterogeneous Reactions of Free Atoms and Radicals in Processes of Flame Propagation

Russian Journal of Physical Chemistry A - It is shown that patterns of flame propagation and the spectral composition of emission depend strongly on the chemical composition of the contacting...     

The Dynamics of Populating The Lowest Triplet State Of Benzophenone Following Singlet Excitation

Measurements of the build-up of triplet-triplet absorption at 5300 A have been made during and subsequent to the excitation of benzophenone by 5 psec pulses of 3545 A radiation (frequency tripled Nd³⁺: glass fundamental). The data were fitted to a simple model that included the finite gaussian pulse width and results for the assumed process S_i

T₁ are: K^?1=16.5±3 psec for benzophenone in ethanol; k^?1 = 30 ±5 psec for benzophenone in benzene. This new type of solvent effect on triplet build-up is discussed.     

The EPR Spectrum of Triplet Mesitylphosphinidene: Reassignment and New Assignment

Low‐temperature UV‐photolysis of mesitylphosphiranes under highly anaerobic conditions leads to the formation of the triplet mesitylphosphinidene (MesP). The recorded X‐band EPR spectrum of triplet MesP and the derived zero‐field splitting parameter D =4.116 cm⁻¹ differ significantly from those reported previously for this intermediate. New magnetic parameters of mesitylphosphinidene are discussed along with the results of DFT calculations.     

The Anomalous Triplet Exciplex Emission of 4-phenylbenzophenones with Amines

Structured phosophorescence with 20μs life time[480nm, 510nm (max),550nm] from 4-phenyl-benzophenone (4-PBP) was quenched by trieyhylamine (TEA) in benzene. An Anomalous exciplex emission with strutureless peaking at 647nm and 30μs life time. A quenching mechanism has been shown to involve a mixed (π,π^*) and (n,π^*) triplet of 4-PBP to form an exciplex with the grond state of TEA.     

非线形CS2 分子单重态和三重态的密度泛函理论

The geometrical optimizations on nonlinear CS2 in the singlet and triplet states have been made using two Density Function Theory（DFT）methods（B3LYP and B3PW91）with 6-311+G* and aug-cc-pVTZ basis sets. Three Singlet states 1A1 and five triplet states（3A2、3B1 and 3B2）have been found,and the frequency analysis confirms that there are no image frequencies for these states,indicating that they are geniune minima at the potential energy surface. At their respective equilibrium geometries,the lowest-lying electronic states are predicted to have an energy ordering by 4t 3B2相似文献   

Substituent Effects on the Low-Lying Singlet and Triplet States of Methylene

The relative energies of the three lower-lying singlet states (here called S_a, S_b, and S_c for the sake of generality) and the lowest triplet state of CHX and CX₂ carbenes (in which X = Li, BeH, BH₂, NH₂, OH, or F) are evaluated by means of the semiempirical MNDO method as well as, for some species, by means of ab initio calculations at the 6-31G, MP3/6-31G, and MP3/6-31G* levels. Calculations for CH(CN) and C(CN)₂ are also reported. In spite of the known MNDO overestimation of the stability of the σ¹π¹ configurations of methylene, this method turns out to be satisfactory for most carbenes reported here. Emphasis is put on the appearance of the plots of the ΔH values vs. the carbene bond angles for the different states and on the seldom considered S_b states (¹B₁ for C_2v carbenes). A carbene classification is proposed on the basis of the form of these plots. For carbenes with π-acceptor substituents such as those of “type IA”, open-shell, diradical configurations are predicted for the lowest singlet states, so that no significant structural differences should be expected between their lowest singlet and triplet states. On the other hand, for carbenes with strong π-donor substituents, either “type ID” or “IID”, the closed-shell singlets appear to be the ground states, and the singlet and triplet behaviors should be much more clearly distinguishable.     

Photoexcited Singlet and Triplet States of a UV Absorber Ethylhexyl Methoxycrylene

The excited states of UV absorber, ethylhexyl methoxycrylene (EHMCR) have been studied through measurements of UV absorption, fluorescence, phosphorescence and electron paramagnetic resonance (EPR) spectra in ethanol. The energy levels of the lowest excited singlet (S₁) and triplet (T₁) states of EHMCR were determined. The energy levels of the S₁ and T₁ states of EHMCR are much lower than those of photolabile 4‐tert‐butyl‐4′‐methoxydibenzoylmethane. The energy levels of the S₁ and T₁ states of EHMCR are lower than those of octyl methoxycinnamate. The weak phosphorescence and EPR B_min signals were observed and the lifetime was estimated to be 93 ms. These facts suggest that the significant proportion of the S₁ molecules undergoes intersystem crossing to the T₁ state, and the deactivation process from the T₁ state is predominantly radiationless. The photostability of EHMCR arises from the ³ππ* character in the T₁ state. The zero‐field splitting (ZFS) parameter in the T₁ state is D** = 0.113 cm^?1.     

自由基检测技术进展

重点对主要的几种自由基检测技术的发展情况进行了讨论,并评价了各种方法的优缺点。涉及的自由基检测技术主要有分光光度吸收法与荧光方法、高效液相色谱法、化学发光法、电化学方法、自旋捕集技术和毛细管电泳方法,按间接检测方法和直接检测方法分类进行了评价。     

Metal Complexes of Free Radicals. Part III: Triplet state ESR. of magnesium(glyoxal-bis-N-t-butylimine)2

The preparation and the ESR. spectra of a 1 : 2 complex of Mg²⁺ with the radical anion of glyoxal-bis-N-t-butylimine (GLI) are reported. The ESR. spectrum of Mg(GLIR)₂ in 2-methyl-tetrahydrofuran at ?165° is interpreted as due to a randomly oriented molecular triplet. The magnetic dipole-dipole interaction of the two unpaired electrons in Mg(GLIR)₂ can be interpreted by an axially symmetric fine structure term with D = 0.021 cm^?1.