Individual Reaction Pathways Research Articles

Photochemical reactions in organic semiconductor thin films

We investigate the dissociation and the photochemical behavior of different organic semiconductor materials typically employed in organic light emitting diodes (OLED) and other organic electronic devices, using laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS). The study is subdivided into two parts: The first part is an investigation of several hole conductive materials from the class of triarylamines commonly used in OLEDs and organic solar cells. Upon LDI measurements, this class of materials does not show any photochemical reactions. The second part of the work deals with two typical electron conductive materials: tris(8-hydroxyquinolinato)-aluminum (Alq 3) and 4,7-diphenyl-1,10-phenanthroline (BPhen). These materials show photochemical reactions in terms of oligomerization. The individual reaction pathways will be discussed in detail. We compare these reactions by further studying two similar molecules (1,1′-biphenyl-4′-oxy)bis(8-hydroxy-2-methylquinolinato)-aluminum (BAlq) and bathocuproine (BCP). Finally, we discuss a possible relation of the results to degradation mechanisms taking place in organic devices.

Kinetic Isotope Effects in Complex Reaction Networks: Formic Acid Electro‐Oxidation

The determination of kinetic isotope effects (KIEs) for different reaction pathways and steps in a complex reaction network, where KIEs may affect the overall reaction in various different ways including dominant and minority pathways or the buildup of a reaction-inhibiting adlayer, is demonstrated for formic acid electrooxidation on a Pt film electrode by quantitative electrochemical in situ IR spectroscopic measurements under controlled mass-transport conditions. The ability to separate effects resulting from different contributions--which is not possible using purely electrochemical kinetic measurements--allows conclusions on the nature of the rate-limiting steps and their transition state in the individual reaction pathways. The potential-independent values of approximately 1.9 for the KIE of formic acid dehydration (CO(ad) formation) in the indirect pathway and approximately 3 for the CO(ad) coverage-normalized KIE of formic acid oxidation to CO2 (direct pathway) indicate that 1) C-H bond breaking is rate-limiting in both reaction steps, 2) the transition states for these reactions are different, and 3) the configurations of the transition states involve rather strong bonds to the transferred D/H species, either in the initial or in the final state, for the direct pathway and--even more pronounced--for formic acid dehydration (CO(ad) formation).

Ab initio study of the unimolecular decomposition of 2‐butenenitrile: Molecular elimination channels

AbstractAb initio molecular orbital calculations have been performed for the unimolecular decomposition of 2‐butenenitrile (CH3CHCHCN), especially for HCN and H2 molecular elimination channels. Structures and energies of the reactants, products, and relevant species in the individual reaction pathways were determined by MP2 gradient optimization and MP4 CCSD(T) single‐point energy calculations. Direct 1,1 and 1,2 molecular eliminations and H or CN migration followed by elimination channels were identified. Dissociation rates for the individual reaction pathways were calculated from vibrational frequencies at the ab initio transition state geometries by employing Rice–Ramsperger–Kassel–Marcus theory, from which channel branching ratios were determined. It was concluded that the most important reaction channel should be the direct 1,1 three‐center molecular elimination of HCN. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

An ab initio Study on the Molecular Elimination Reactions of Methacrylonitrile

Ab initio quantum chemical molecular orbital calculations have been performed for the unimolecular decomposition of methacrylonitrile (), especially for HCN and molecular elimination channels. Structures and energies of the reactants, products, and relevant species along the individual reaction pathways were determined by MP2 gradient optimization and MP4 single point energy calculations. Direct four-center elimination of HCN and three-center elimination of H2 channels were identified. In addition, H or CN migration followed by HCN or H2 elimination channels via the methylcyanoethylidene intermediate was also identified. Unlike the case of crotonitrile previously studied, in which the dominant decomposition process was the direct three-center elimination of HCN, the most important reaction pathway should be the direct threecenter elimination of in the case of methacrylonitrile.뤀댐�⨀螑㌄쀅�⨀䀠ﶖ⨀聢�⨀Ă䀠煥獣愠獵扨敡瀀ｿｿｿｿ䂂�⨀䂂�⨀?늀Ѐက섂돀遠�⨀堘�⨀肴�⨀ဈ�⨀餂돐側�⨀墵�⨀／ࠀ䀈�⨀夂덐蠀㠀駚돀遠�⨀ꁠﲖ⨀�⨀䢕�⨀돐梕�⨀��⨀炕�⨀뤏덐肕�⨀렏頏ꢕ�⨀਀ꢕ�⨀㢖�⨀਀㢖�⨀좖�⨀਀좖�⨀뢗�⨀਀뢗�⨀�⨀਀က�⨀줁?⨀؀က�⨀਀�⨀

Decomposition mechanism of 3-N-morpholinosydnonimine (SIN-1)--a density functional study on intrinsic structures and reactivities.

Selected intrinsic aspects of the mode of action of 3- N-morpholinosydnonimine (SIN-1) and its follow-up products are investigated by means of density functional theory. Besides the well known radical-cationic Feelisch-Schoenafinger pathway, an alternative anionic route via a neutral radical is proposed and included in the study. The individual reaction pathways are followed. Most notably, the overall activation barrier for the cationic route is calculated to be 184.04 kcal mol(-1), while the one for the anionic route is predicted to be more favorable with 14.09 kcal mol(-1).

Methanol Electrooxidation over Pt/C Fuel Cell Catalysts: Dependence of Product Yields on Catalyst Loading

We have investigated the influence of the catalyst loading (7−35 μgPt·cm-2) on methanol oxidation product yields over Vulcan XC72 supported Pt catalyst by differential electrochemical mass spectrometry (DEMS), both under potentiodynamic and potentiostatic (0.6 VRHE) conditions. Experiments were performed at room temperature and under continuous flow conditions (0.1 M CH3OH in 0.5 M H2SO4 solution). Absolute calibration of the DEMS signal allowed determination of current efficiencies for the final reaction product CO2 and for the reaction intermediates formic acid and formaldehyde. These could be converted into turnover frequencies (TOFs) on the basis of the physical (transmission electron microscopy) and electrochemical (CO stripping, HUPD) determination of the absolute particle surface area. The good agreement between these values indicates 100% utilization of the catalyst. The electrochemical efficiencies, product distribution, and the turnover frequencies of partial reactions (methanol oxidation to formaldehyde, formic acid, and CO2), during the methanol oxidation reaction (MOR) show a pronounced dependence on the catalyst loading. With increasing Pt loading the current efficiency for methanol oxidation to formaldehyde decays significantly (40% to almost 0%), while that for complete oxidation to CO2 increases from 50 to 80%. The variation in current efficiency for methanol oxidation to formic acid is small (ca. 10%). The product distribution varies accordingly; the absolute numbers, however, are different: Low (5−20%) formic acid yields are accompanied by a decaying formaldehyde yield (60% to zero) and increasing CO2 yield (30−80%) with increasing catalyst loading. The total TOF values calculated from the values for the individual reaction pathways and products are significantly higher than the Faradaic TOF numbers at low catalyst loadings, while at high loadings, where complete oxidation of methanol to CO2 is preferred, they agree satisfactorily. The pronounced variations in product distribution/TOF values are attributed to increasing readsorption and subsequent complete oxidation of desorbed reaction intermediates (formaldehyde and formic acid) at higher catalyst loadings, while at low loadings or, even more pronounced, on smooth electrodes these intermediates are more likely to survive. This has to be considered for predictions of MOR characteristics under fuel cell conditions on the basis of studies on smooth electrodes.

Natural Product-like Combinatorial Libraries Based on Privileged Structures. 2. Construction of a 10 000-Membered Benzopyran Library by Directed Split-and-Pool Chemistry Using NanoKans and Optical Encoding

Having developed a reliable and versatile solid-phase strategy for the split-and-pool synthesis of naturally occurring and designed derivatives of the benzopyran template (see preceding paper), we now report the construction of a 10 000-membered natural product-like compound library for chemical biology studies. Concomitantly, we report an early application of the IRORI NanoKan optical encoding system for the high throughput nonchemical tagging and sorting of library members during split-and-pool synthesis. The overall synthetic strategy for library construction is discussed and the individual reaction pathways are examined in the context of specific library members, illustrating reaction conditions as well as yields and purities. The issues of building block selection and quality control of library members are also addressed and, finally, potential applications of the library to chemical biology are discussed.

Simultaneous deactivation by coke and sulfur of bimetallic Pt–Re(Ge, Sn)/Al 2O 3 catalysts for n-hexane reforming

The simultaneous deactivation by coke and sulfur of monometallic Pt/Al 2O 3 and bimetallic Pt–Re(Ge, Sn)/Al 2O 3 catalysts was studied using n-hexane reforming as bifunctional test reaction and thiophene as poisoning molecule. The residual activities in the activity decay curves were used for measuring the catalyst sensitivity to coke formation and sulfur poisoning. Sulfur and carbonaceous deposits accumulated essentially on the metallic fraction and affected the catalyst activity for both monofunctional metallic and bifunctional metal–acid catalyzed reactions. The overall deactivation rate for n-hexane conversion increased in the order Pt–Ge<Pt⪡Pt–Sn≤Pt–Re. This deactivation trend resulted from the combination of the catalyst resistance to each individual deactivation process. Pt–Ge/Al 2O 3 was the most stable catalyst essentially because of its high thiotolerance for n-hexane transformation reactions and also because it showed low activity for dehydrogenation reactions leading to the formation of coke precursors. Sulfur poisoning on Pt/Al 2O 3 decreased monofunctional metal-catalyzed reactions but concomitantly increased the activity for acid-controlled skeletal rearrangement reactions; as a result, n-hexane conversion was only slightly diminished by the addition of sulfur. Pt–Sn/Al 2O 3 showed high resistance to coke deactivation but was severely poisoned by the addition of sulfur. The Pt–Re/Al 2O 3 activity was significantly decreased by both deactivation processes. Changes in catalyst selectivity are interpreted in terms of selective deactivation by coke and sulfur of individual reaction pathways involved in the n-hexane reforming mechanism.

Reactions of α-olefins of different chain length added during Fischer–Tropsch synthesis on a cobalt catalyst in a slurry reactor

Based on the picture of Fischer–Tropsch synthesis as an ideal polymerization reaction it is easily noticeable that primarily formed product olefins can undergo secondary reactions, which generally lead to carbon number dependencies of certain olefin reaction pathways and thereby to modifications of product distributions. To obtain more insight into the role of secondary olefin reactions during Fischer–Tropsch synthesis and the reason for their chain length dependence, extensive studies on their extent and selectivity and particularly their chain length dependence at different reaction conditions (variation of CO partial pressure and reaction temperature) were performed by co-feeding α-olefins (C 2–C 11) during FT synthesis with cobalt catalysts in a gradientless slurry reactor. Conversion of the added olefins revealed a strong carbon number dependence exhibiting a sharp minimum at C 3. The observed chain length dependencies of individual reaction pathways (hydrogenation and incorporation) are in agreement with the selectivities observed in the base case experiments without olefin addition and mainly due to carbon number dependent solubility of the olefins in the liquid reaction product.

Sequential clustering reactions of Si+ with SiD4: Identification of a bottleneck preventing rapid growth of hydrogenated silicon particles

Sequential clustering of atomic silicon-29 cations with SiD4 is observed at room temperature in the ion cell of a Fourier transform mass spectrometer. The clustering reactions proceed in a highly specific fashion. Si+ grows initially by sequential addition of three –SiD2 units. The measured reaction rates for these three steps are, respectively, 8.1±0.4, 0.36±0.04, and 2.0±0.3×10−10 cm3 molecule−1 s−1. A back reaction which results in loss of the silicon-29 isotopic label also occurs for these three reactions and represents ∼12%–15% of the reaction products, depending on the reaction step. This cluster growth mechanism then encounters a critical bottleneck and ceases. Further aggregation occurs only by slow bimolecular attachment of SiD4 at a rate of 1.0±0.3×10−13 cm3 molecule−1 s−1 at p(SiD4)=2.0×10−7 Torr. The fundamental mechanisms and energetics for the individual reaction pathways have been calculated by Raghavachari using ab initio electronic structure theory and are presented in a companion paper. The clustering mechanism involves insertion of a cluster ion into a SiD bond of SiD4 followed by elimination of D2. Addition of –SiD2 serves to increasingly saturate the bonds of all of the silicon centers which leads to the observed growth limitations at Si4D+6. The accuracy of these calculated potential surfaces is tested using statistical phase space theory. Since both the forward and reverse reaction rates are measured using isotopic labeling, the phase space theory calculations are used to determine both the insertion and the elimination transition state energies for each of the first three clustering steps as well as a lower limit for the well depth of the [Si5D+10] intermediate complex. Overall, good agreement is found between the transition state energies obtained from phase space theory and those determined by Raghavachari. These results indicate that Si+ clustering with SiD4 encounters an early bottleneck which prevents rapid formation of the critical nucleus size required for spontaneous growth of large hydrogenated silicon particulates.