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CO Bond Cleavage (co + bond_cleavage)

Distribution by Scientific Domains

Chemistry	100%

Selected Abstracts

Reaction Mechanisms for Renewable Hydrogen from Liquid Phase Reforming of Sugar Compounds

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1-2 2006
A. Tanksale
Hydrogen is anticipated to become a major source of energy in the future. Hydrogen is a clean burning fuel and has been described as a long-term replacement for natural gas. It has been demonstrated here that hydrogen can be produced from biomass in the temperature range of 185-220°C using a single batch reactor pressurised at 25-30 bar. The current work is based on sugars which are considered here as the biomass resource. Glucose, fructose and sucrose solutions were used for the liquid phase reforming using supported platinum catalyst. The sugar molecules might go through reversible dehydrogenation steps to give adsorbed species on metal sites. This adsorption might be either on CC or CO bond cleavage. Platinum is one of the best catalysts for the reforming of hydrocarbons due to its high selectivity for CC bond cleavage. The CC bond cleavage is the limiting factor for the reforming and leads to a high rate of formation of hydrogen. On the other hand CO bond cleavage results in formation of alcohols, acids and other organic groups. [source]

Modulation of Spectrokinetic Properties of o -Quinonoid Reactive Intermediates by Electronic Factors: Time-Resolved Laser Flash and Steady-State Photolysis Investigations of Photochromic 6- and 7-Arylchromenes

CHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2009
Jarugu, Narasimha Moorthy Prof.
Abstract A variety of differently substituted 6- and 7-arylchromenes such as that depicted undergo photoinduced CO bond cleavage to yield colored o -quinonoid intermediates. A combined analysis of ,s,ms (laser flash) and real-time kinetic data show that the o -quinonoid intermediates decay faster when the C2-aryl and C6-/C7-aryl rings contain electron-donating and electron-accepting groups, respectively. Similarly, the decay occurs slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron donating. A ready synthetic accessibility of a series of 6- and 7-arylchromenes via Pd0 -catalyzed Suzuki coupling protocol has permitted a comprehensive investigation of the thermal decay behavior of a broad set of photogenerated o -quinonoid reactive intermediates. It is shown that substantial mesomeric effect between the benzopyran nucleus and the aryl ring at C6 or C7 position of the former renders significant absorption beyond 350,nm such that they are readily photoactivated to yield colored o -quinonoid intermediates. The absorption spectra of the latter are found to be strongly influenced by the substituents on C2-, C6- and C7-aryl rings; indeed the colored absorptions can be conveniently tuned by appropriate choice of substituents. The thermal decay (bleaching phenomenon), which is important from the point of view of their application in ophthalmic lenses, was investigated in each case by ,s,ms as well as real-time absorption spectroscopy. By careful experimentation, we have extracted the decay rate constants for Z,E and E,E o -quinonoid isomers of all 6- and 7-arylchromenes in an attempt to establish a correlation between the electronic attributes with their thermokinetic behavior. From a combined analysis of ,s,ms (laser flash) and real-time kinetic data, it is shown that the colored o -quinonoid intermediates decay faster when the C2-aryl and C6-/C7-aryl rings contain electron-donating and electron-accepting groups, respectively. In the same vein, the decay was found to occur slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron-donating. Thus, any substituent on the C2-aryl ring that contributes mesomerically to the development of charge on the quinonoid oxygen, and any substituent on the C6-/C7-aryl ring that exerts ,I effect appear to expedite thermal decay. Furthermore, evidence is obtained for the first time from ,s time-resolved laser-flash spectroscopy for the formation and characterization of the trans,cis (E,Z) o -quinonoid isomer, which has heretofore eluded spectral characterization in the photochromic phenomena of pyrans. [source]

,-Carbonyl Substituent Effect on the Lifetimes of Triplet 1,4-Biradicals from Norrish-Type-II Reactions

CHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2006
Xichen Cai Dr.
Abstract Triplet 1,4-biradicals were generated by Norrish-Type-II hydrogen transfer from ,-heteroatom-substituted ,-branched butyrophenones 1,6 and detected by laser flash absorption measurements. For three oxy-substituted compounds 2,4 (R,=OH, OCOMe, OCOOEt) comparable lifetimes were determined in acetonitrile (roughly 1.5 ,s). In benzene, divergent trends were observed: for the hydroxy compound 2 a lower lifetime of 790 ns was determined, whereas for 3 and 4 the lifetimes increased to 4.9 ,s. Photolyses of the ,-amino-substituted compounds 1 and 6 resulted in transient species with significant lower lifetimes (for 1 160 ns in benzene and 450 ns in acetonitrile; for 6 <100 ns in both solvents). The mesyloxy substrate 5 undergoes rapid CO bond cleavage upon photolysis and no transient triplet species were detected. Computational (UB3,LYP/6,31G* and natural don orbital (NBO) analyses) results supported the assumption of a negative hyperconjugative interaction strongly stabilizing ,-oxy-substituted over ,-amino-substituted radicals. [source]

Comprehensive Analysis of DNA Strand Breaks at the Guanosine Site Induced by Low-Energy Electron Attachment

CHEMPHYSCHEM, Issue 1 2010
Jiande Gu Prof. Dr.
Abstract To elucidate the role of guanosine in DNA strand breaks caused by low-energy electrons (LEEs), theoretical investigations of the LEE attachment-induced CO ,-bonds and N-glycosidic bond breaking of 2,-deoxyguanosine-3,,5,-diphosphate (3,,5,-dGMP) were performed using the B3LYP/DZP++ approach. The results reveal possible reaction pathways in the gas phase and in aqueous solutions. In the gas phase LEEs could attach to the phosphate group adjacent to the guanosine to form a radical anion. However, the small vertical detachment energy (VDE) of the radical anion of guanosine 3,,5,-diphosphate in the gas phase excludes either CO bond cleavage or N-glycosidic bond breaking. In the presence of the polarizable surroundings, the solvent effects dramatically increase the electron affinities of the 3,,5,-dGDP and the VDE of 3,,5,-dGDP,. Furthermore, the solvent,solute interactions greatly reduce the activation barriers of the CO bond cleavage to 1.06,3.56 kcal,mol,1. These low-energy barriers ensure that either C5,O5, or C3,O3, bond rupture takes place at the guanosine site in DNA single strands. On the other hand, the comparatively high energy barrier of the N-glycosidic bond rupture implies that this reaction pathway is inferior to CO bond cleavage. Qualitative agreement was found between the theoretical sequence of the bond breaking reaction pathways in the PCM model and the ratio for the corresponding bond breaks observed in the experiment of LEE-induced damage in oligonucleotide tetramer CGTA. This concord suggests that the influence of the surroundings in the thin solid film on the LEE-induced DNA damage resembles that of the solvent. [source]