共查询到20条相似文献,搜索用时 15 毫秒
1.
Yuan‐Pin Chang Ping‐Chen Lee King‐Chuen Lin Prof. C. H. Huang B. J. Sun A. H. H. Chang Prof. 《Chemphyschem》2008,9(8):1137-1145
The Br2 elimination channel is probed for 1,2‐C2H2Br2 in the B –X transition upon irradiation at 248 nm by using cavity ring‐down absorption spectroscopy (CRDS). The nascent vibrational population ratio of Br2(v=1)/Br2(v=0) is obtained to be 0.7±0.2, thus indicating that the Br2 fragment is produced in hot vibrational states. The obtained Br2 products are anticipated to result primarily from photodissociation of the ground‐state cis isomer via four‐center elimination or from cis/trans isomers via three‐center elimination, each mechanism involving a transition state that has a Br? Br distance much larger than that of ground‐state Br2. According to ab initio potential energy calculations, the pathways that lead to Br2 elimination may proceed either through the electronic ground state by internal conversion or through the triplet state by intersystem crossing. Temperature‐dependence measurements are examined, thereby supporting the pathway that involves internal conversion—which was excluded previously by using product translational spectroscopy (PTS). The quantum yield for the Br2 elimination reaction is determined to be 0.12±0.1, being substantially contributed by the ground‐state Br2 product. The discrepancy of this value from that (of 0.2) obtained by PTS may rise from the lack of measurements in probing the triplet‐state Br2 product. 相似文献
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Ping‐Chen Lee Po‐Yu Tsai Ming‐Kai Hsiao King‐Chuen Lin Prof. C. H. Huang A. H. H. Chang Prof. 《Chemphyschem》2009,10(4):672-679
A process of elimination : Optical spectra of the Br2 product eliminated from the photodissociation of 1,1‐dibromoethylene at 248 nm are obtained (see picture), and the subsequent dissociation pathways are proposed with the aid of ab initio potential energy calculations. The behavior of the compound is compared with that of 1,2‐dibromoethylene.
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Yong Du Jiadan Xue Ming‐De Li Xiangguo Guan David W. McCamant David Lee Phillips 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(23):6961-6972
A combined femtosecond transient absorption (fs‐TA) and nanosecond time‐resolved resonance Raman (ns‐TR3) spectroscopic investigation of the photoreaction of 2‐benzoylpyridine (2‐BPy) in acetonitrile and neutral, basic and acidic aqueous solvents is reported. fs‐TA results showed that the nπ* triplet 2‐BPy is the precursor of the photocyclisation reaction in neutral and basic aqueous solvents. The cis triplet biradical and the cis singlet zwitterionic species produced during the photocyclisation reaction were initially characterised by ns‐TR3 spectroscopy. In addition, a new species was uniquely observed in basic aqueous solvent after the decay of the cis singlet zwitterionic species and this new species was tentatively assigned to the photocyclised radical anion. The ground‐state conformation of 2‐BPy in acidic aqueous solvent is the pyridine nitrogen‐protonated 2‐BPy cation (2‐BPy‐NH+) rather than the neutral form of 2‐BPy. After laser photolysis, the singlet excited state (S1) of 2‐BPy‐NH+ is generated and evolves through excited‐state proton transfer (ESPT) and efficient intersystem crossing (ISC) processes to the triplet exited state (T1) of the carbonyl oxygen‐protonated 2‐BPy cation (2‐BPy‐OH+) and then photocyclises with the lone pair of the nitrogen atom in the heterocyclic ring. Cyclisation reactions take place both in neutral/basic and acidic aqueous solvents, but the photocyclisation mechanisms in these different aqueous solvents are very different. This is likely due to the different conformation of the precursor and the influence of hydrogen‐bonding of the solvent on the reactions. 相似文献
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Arnab Bagchi Yu‐Hsuan Huang Z. F. Xu P. Raghunath Yuan T. Lee Chi‐Kung Ni M. C. Lin Yuan‐Pern Lee 《化学:亚洲杂志》2011,6(11):2961-2976
The photodissociation of gaseous benzaldehyde (C6H5CHO) at 193, 248, and 266 nm using multimass ion imaging and step‐scan time‐resolved Fourier‐transform infrared emission techniques is investigated. We also characterize the potential energies with the CCSD(T)/6‐311+G(3df,2p) method and predict the branching ratios for various channels of dissociation. Upon photolysis at 248 and 266 nm, two major channels for formation of HCO and CO, with relative branching of 0.37:0.63 and 0.20:0.80, respectively, are observed. The C6H5+HCO channel has two components with large and small recoil velocities; the rapid component with average translational energy of approximately 25 kJ mol?1 dominates. The C6H6+CO channel has a similar distribution of translational energy for these two components. IR emission from internally excited C6H5CHO, ν3 (v=1) of HCO, and levels v≤2, J≤43 of CO are observed; the latter has an average rotational energy of approximately 13 kJ mol?1 and vibrational energy of approximately 6 kJ mol?1. Upon photolysis at 193 nm, similar distributions of energy are observed, except that the C6H5+HCO channel becomes the only major channel with a branching ratio of 0.82±0.10 and an increased proportion of the slow component; IR emission from levels ν1 (v=1) and ν3 (v=1 and 2) of HCO and v≤2, J≤43 of CO are observed; the latter has an average energy similar to that observed in photolysis at 248 nm. The observed product yields at different dissociation energies are compared to statistical‐theory predicted results based on the computed singlet and triplet potential‐energy surfaces. 相似文献
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Coumarin Photocaging Groups Modified with an Electron‐Rich Styryl Moiety at the 3‐Position: Long‐Wavelength Excitation,Rapid Photolysis,and Photobleaching 下载免费PDF全文
Dr. Qiuning Lin Lipeng Yang Zhiqiang Wang Yujie Hua Dasheng Zhang Bingkun Bao Prof. Chunyan Bao Prof. Xueqing Gong Prof. Linyong Zhu 《Angewandte Chemie (International ed. in English)》2018,57(14):3722-3726
A new class of coumarin photocaging groups modified with an electron‐rich styryl moiety at the 3‐position was constructed. The large π‐conjugated structure and stabilization of the carbocation intermediates by electron donors endowed the new photocaging groups with excellent long‐wavelength absorption, large two‐photon absorption cross‐sections, and high uncaging quantum yields. Moreover, the new photocaging groups displayed unique photobleaching properties after photocleavage as a result of the intramolecular cyclization rearrangement of a carbocation intermediate to form five‐membered ring byproducts and block the styryl conjugation at the 3‐position. These superior properties of the new photocaging groups are extremely beneficial for high‐concentration samples and thick specimens, thus extending the application of photocaging groups in many fields. 相似文献
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Yuzhu Liu Qiusha Zheng Yan Zhang Rongrong Zhang Yanmei Wang Dr. Bing Zhang Prof. 《Chemphyschem》2009,10(5):830-834
Velocity ion imaging (see figure) is used to analyze the photodissociation mechanism of 1‐C4H9I (left image) and 2‐C4H9I (right image) at 266 nm. For 2‐C4H9I, a new channel for formation of I and I* atoms is observed. It is attributed to the repulsive mode along the C? I stretch, coupled with some bending motions.
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Dr. Joel Torres‐Alacan Oliver Krahe Prof. Dr. Alexander C. Filippou Prof. Dr. Frank Neese Prof. Dr. Dirk Schwarzer Prof. Dr. Peter Vöhringer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(10):3043-3055
The photochemistry of iron azido complexes is quite challenging and poorly understood. For example, the photochemical decomposition of [FeIIIN3(cyclam‐ac)]PF6 ([ 1 ]PF6), where cyclam‐ac represents the 1,4,8,11‐tetraazacyclotetradecane‐1‐acetate ligand, has been shown to be wavelength‐dependent, leading either to the rare high‐valent iron(V) nitrido complex [FeVN(cyclam‐ac)]PF6 ([ 3 ]PF6) after cleavage of the azide Nα? Nβ bond, or to a photoreduced FeII species after Fe? Nazide bond homolysis. The mechanistic details of this intriguing reactivity have never been studied in detail. Here, the photochemistry of 1 in acetonitrile solution at room temperature has been investigated using step‐scan and rapid‐scan time‐resolved Fourier transform infrared (FTIR) spectroscopy following a 266 nm, 10 ns pulsed laser excitation. Using carbon monoxide as a quencher for the primary iron‐containing photochemical product, it is shown that 266 nm excitation of 1 results exclusively in the cleavage of the Fe? Nazide bond, as was suspected from earlier steady‐state irradiation studies. In argon‐purged solutions of [ 1 ]PF6, the solvent‐stabilized complex cation [FeII(CH3CN)(cyclam‐ac)]+ ( 2 red ) together with the azide radical (N3.) is formed with a relative yield of 80 %, as evidenced by the appearance of their characteristic vibrational resonances. Strikingly, step‐scan experiments with a higher time resolution reveal the formation of azide anions (N3?) during the first 500 ns after photolysis, with a yield of 20 %. These azide ions can subsequently react thermally with 2 red to form [FeIIN3(cyclam‐ac)] ( 1 red ) as a secondary product of the photochemical decomposition of 1 . Molecular oxygen was further used to quench 1 red and 2 red to form what seems to be the elusive complex [Fe(O2)(cyclam‐ac)]+ ( 6 ). 相似文献
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Zhen‐Zhou Cao Zheng‐Rong Wei Dr. Lin‐Qiang Hua Chang‐Jin Hu Dr. Song Zhang Dr. Bing Zhang Prof. 《Chemphyschem》2009,10(8):1299-1304
Photodissociation dynamics and rotational wave packet coherences of o‐bromofluorobenzene are studied by femtosecond time‐resolved photoelectron imaging (see figure). The decay of different photoelectron rings shows the population decay of states from which the lifetimes of different states are determined. The variation of photoelectron angular distributions reflects the evolution of rotational coherences.
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Details on the reactions of: (1) Pd+ + CH3CHO → PdCO+ + CH4 and (2) Pd+ + CH3CHO → PdH + CH3CO+ in the gas phase were investigated using density functional theory (B3LYP), in conjunction with the LANL2DZ+6‐311+G(d) basis set. Three encounter complexes were located on the potential energy surfaces and the calculations indicated that both the C? C and aldehyde C? H bond activation of acetaldehyde could lead to the dominant demethanation reaction. The charge transfer process for PdH abstraction was caused by an intramolecular PdH rearrangement of the newly found η1‐aldehyde attached complex. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 相似文献
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Kristoffer Haldrup Dr. Morten Christensen Marco Cammarata Dr. Qingyu Kong Dr. Michael Wulff Prof. Simon O. Mariager Klaus Bechgaard Prof. Robert Feidenhans'l Prof. Niels Harrit Prof. Martin M. Nielsen Prof. 《Angewandte Chemie (International ed. in English)》2009,48(23):4180-4184
Molecular movies : Time‐resolved X‐ray scattering provides direct structural information on an electronically excited complex while it is formed in the bimolecular reaction between excited octahydrogen[tetrakis‐μ‐diphosphito‐1κP:2κP′‐diplatinate](4‐) (PtPOP*) and thallium ions. In the exciplex one thallium(I) and two platinum(II) ions are found to be collinear.
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Liwei Zheng Xiaojuan Dai Hongmei Su Marc M. Greenberg 《Angewandte Chemie (International ed. in English)》2020,59(32):13406-13413
Guanine radicals are important reactive intermediates in DNA damage. Hydroxyl radical (HO.) has long been believed to react with 2′‐deoxyguanosine (dG) generating 2′‐deoxyguanosin‐N1‐yl radical (dG(N1‐H).) via addition to the nucleobase π‐system and subsequent dehydration. This basic tenet was challenged by an alternative mechanism, in which the major reaction of HO. with dG was proposed to involve hydrogen atom abstraction from the N2‐amine. The 2′‐deoxyguanosin‐N2‐yl radical (dG(N2‐H).) formed was proposed to rapidly tautomerize to dG(N1‐H).. We report the first independent generation of dG(N2‐H). in high yield via photolysis of 1 . dG(N2‐H). is directly observed upon nanosecond laser flash photolysis (LFP) of 1 . The absorption spectrum of dG(N2‐H). is corroborated by DFT studies, and anti‐ and syn‐dG(N2‐H). are resolved for the first time. The LFP experiments showed no evidence for tautomerization of dG(N2‐H). to dG(N1‐H). within hundreds of microseconds. This observation suggests that the generation of dG(N1‐H). via dG(N2‐H). following hydrogen atom abstraction from dG is unlikely to be a major pathway when HO. reacts with dG. 相似文献
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Roberto E. Di Paolo Dr. João Seixas de Melo Prof. João Pina Hugh D. Burrows Prof. Jorge Morgado Prof. António L. Maçanita Prof. 《Chemphyschem》2007,8(18):2657-2664
Two p‐phenylenevinylene (PV) trimers, containing 3′‐methylbutyloxyl (in MBOPV3) and 2′‐ethylhexyloxyl (in EHOPV3) side chains, are used as model compounds of PV‐based conjugated polymers (PPV) with the purpose of clarifying the origin of fast (picosecond time) components observed in the fluorescence decays of poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylenevinylene] (MEH‐PPV). The fluorescence decays of MBOPV3 and EHOPV3 reveal the presence of similar fast components, which are assigned to excited‐state conformational relaxation of the initial population of non‐planar trimer conformers to lower‐energy, more planar conformers. The rate constant of conformational relaxation kCR is dependent on solvent viscosity and temperature, according to the empirical relationship kCR=aηo?α?exp(?αEη/RT), where aηo?α is the frequency factor, ηo is the pre‐exponential coefficient of viscosity, Eη is the activation energy of viscous flow. The empirical parameter α, relating the solvent microscopic friction involved in the conformational change to the macroscopic solvent friction (α=1), depends on the side chain. The fast component in the fluorescence decays of MEH‐PPV polymers (PPVs), is assigned to resonance energy transfer from short to longer polymer segments. The present results call for revising this assignment/interpretation to account for the occurrence of conformational relaxation, concurrently with energy transfer, in PPVs. 相似文献
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《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(11):3030-3034
Localized singlet diradicals are key intermediates in bond homolyses. The singlet diradicals are energetically much less stable than the σ‐bonded species. In general, only one‐way reactions from diradicals to σ‐bonded species are observed. In this study, a thermal equilibrium between a singlet 1,2‐diazacyclopentane‐3,5‐diyl diradical and the corresponding σ‐bonded species was directly observed. The singlet diradical was more stable than the σ‐bonded species. The solvent effect clarified key features, such as the zwitterionic character of the singlet diradical. The effect of the nitrogen atoms is discussed in detail. 相似文献
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Dr. Christian van der Linde Wai‐Kit Tang Dr. Chi‐Kit Siu Prof.Dr. Martin K. Beyer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(36):12684-12687
Gas‐phase reactions of CO3.? with formic acid are studied using Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry. Signal loss indicates the release of a free electron, with the formation of neutral reaction products. This is corroborated by adding traces of SF6 to the reaction gas, which scavenges 38 % of the electrons. Quantum chemical calculations of the reaction potential energy surface provide a reaction path for the formation of neutral carbon dioxide and water as the thermochemically favored products. From the literature, it is known that free electrons in the troposphere attach to O2, which in turn transfer the electron to O3. O3.? reacts with CO2 to form CO3.?. The reaction reported here formally closes the catalytic cycle for the oxidation of formic acid with ozone, catalyzed by free electrons. 相似文献
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Gas‐Phase Peptide Structures Unraveled by Far‐IR Spectroscopy: Combining IR‐UV Ion‐Dip Experiments with Born–Oppenheimer Molecular Dynamics Simulations 下载免费PDF全文
Sander Jaeqx Prof. Dr. Jos Oomens Dr. Alvaro Cimas Prof. Dr. Marie‐Pierre Gaigeot Dr. Anouk M. Rijs 《Angewandte Chemie (International ed. in English)》2014,53(14):3663-3666
Vibrational spectroscopy provides an important probe of the three‐dimensional structures of peptides. With increasing size, these IR spectra become very complex and to extract structural information, comparison with theoretical spectra is essential. Harmonic DFT calculations have become a common workhorse for predicting vibrational frequencies of small neutral and ionized gaseous peptides. 1 Although the far‐IR region (<500 cm?1) may contain a wealth of structural information, as recognized in condensed phase studies, 2 DFT often performs poorly in predicting the far‐IR spectra of peptides. Here, Born–Oppenheimer molecular dynamics (BOMD) is applied to predict the far‐IR signatures of two γ‐turn peptides. Combining experiments and simulations, far‐IR spectra can provide structural information on gas‐phase peptides superior to that extracted from mid‐IR and amide A features. 相似文献