The rotational spectra of the four isotopomers [32S]-thiophene···H35Cl, [32S]-thiophene···H37Cl, [32S]-thiophene···D35Cl and [34S]-thiophene···H35Cl of a complex formed between thiophene and hydrogen chloride have been observed by using a pulsed-nozzle, Fourier-transform microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl-nuclear quadrupole coupling constants χaa, χbb-χcc and χab were determined. Interpretation of the spectroscopic constants led to the conclusion that the observed complex has Cs symmetry, with the Cl atom of HCl lying almost directly above the centre of mass of the thiophene ring but with the H atom of HCl pointing at the π-electron density near to the S atom. The S···H–Cl nuclei are almost collinear [ϑ=0.9(6)°] but the relatively large distance r(S···H)=2.7474(29) A indicates that the S···H interaction is weak. The angle φ between the C2 axis of thiophene and the S···H internuclear line was found to be 64.53(16)°. The distance r(*···Cl)=3.693 A from the centre of mass (*) of the thiophene ring to Cl and the angle (S*Cl)=98.9° are very similar in magnitude to the corresponding quantities in thiophene···Ar. Indeed, a comparison of thiophene···Ar, thiophene···HCl, benzene···Ar and benzene···HCl revealed a strong family relationship between the geometries of these four complexes. It is concluded that the non-bonding electron pair carried by S in thiophene is so weakly nucleophilic that when thiophene forms a hydrogen bond with HCl it does so via the aromatic π-electron system. In this respect, thiophene resembles benzene and is in stark contrast to its oxygen analogue, furan, with which HCl forms a hydrogen-bonded complex of C2v symmetry via the non-bonding electron pair on O.
{"title":"Rotational spectrum of thiophene···HCl Does thiophene act as an aromatic π-type electron donor or an n-type electron donor in hydrogen-bond formation?","authors":"S. Cooke, G. Corlett, A. Legon","doi":"10.1039/A800767E","DOIUrl":"https://doi.org/10.1039/A800767E","url":null,"abstract":"The rotational spectra of the four isotopomers [32S]-thiophene···H35Cl, [32S]-thiophene···H37Cl, [32S]-thiophene···D35Cl and [34S]-thiophene···H35Cl of a complex formed between thiophene and hydrogen chloride have been observed by using a pulsed-nozzle, Fourier-transform microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl-nuclear quadrupole coupling constants χaa, χbb-χcc and χab were determined. Interpretation of the spectroscopic constants led to the conclusion that the observed complex has Cs symmetry, with the Cl atom of HCl lying almost directly above the centre of mass of the thiophene ring but with the H atom of HCl pointing at the π-electron density near to the S atom. The S···H–Cl nuclei are almost collinear [ϑ=0.9(6)°] but the relatively large distance r(S···H)=2.7474(29) A indicates that the S···H interaction is weak. The angle φ between the C2 axis of thiophene and the S···H internuclear line was found to be 64.53(16)°. The distance r(*···Cl)=3.693 A from the centre of mass (*) of the thiophene ring to Cl and the angle (S*Cl)=98.9° are very similar in magnitude to the corresponding quantities in thiophene···Ar. Indeed, a comparison of thiophene···Ar, thiophene···HCl, benzene···Ar and benzene···HCl revealed a strong family relationship between the geometries of these four complexes. It is concluded that the non-bonding electron pair carried by S in thiophene is so weakly nucleophilic that when thiophene forms a hydrogen bond with HCl it does so via the aromatic π-electron system. In this respect, thiophene resembles benzene and is in stark contrast to its oxygen analogue, furan, with which HCl forms a hydrogen-bonded complex of C2v symmetry via the non-bonding electron pair on O.","PeriodicalId":17286,"journal":{"name":"Journal of the Chemical Society, Faraday Transactions","volume":"20 1","pages":"1565-1570"},"PeriodicalIF":0.0,"publicationDate":"1998-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84818337","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The different reaction routes of n-pentoxy radicals in O2/N2 mixtures were investigated in the temperature range 423–523 K under atmospheric pressure. Flow experiments were performed in several reactors, with wall efficiencies increasing from passivated quartz to Pyrex, and with a great variety of mol fractions of O2 (% of O2), which was varied from ∽0 to 100% O2; the products, a pentanal-hydroperoxide C5H10O3 of Mr 118, pentanal, a methylfuranone (γ-valerolactone), pentanol, and three methylfurans were analyzed and identified by GC-MS. Pentanal-hydroperoxide, also called 4-hydroperoxypentanal, OCH(CH2)2CH(OOH)CH3, is the major product in passivated quartz vessels. It is formed by two consecutive isomerizations: (i) a fast one, RO→R-HOH, via a six-membered ring transition state and (ii) a much slower one, involving an OOR-HOH radical, OOR-HOH→HOOR-2HOH, via a seven-membered ring intermediate: CCCCCO ——min CCCCCOH ——minCC(OO)CCCOH ——min CC(OOH)CCCOH ——min CC(OOH)CCCO+HO2. A chemical mechanism, taking into account all of the experimental results is proposed for reactions of the n-pentoxy radical in oxygen. An analytical steady state solution, based on the Laplace transform, has been helpful in rejecting or validating candidate models. Rate constants appearing in the proposed mechanism have been evaluated by the use of an optimization method. This analysis shows that (i) isomerization is the predomi-nant reaction route, accounting for the diversity of end products; (ii) homogeneous pentanal is formed by reaction with O2 of anisomerized pentoxy radical: CCCCCO ——min CCCCCOH ——min CCCCCOH ——min CCCCCO+HO2; (iii) methylfurans are also homogeneously produced through an analogous reaction; (iv) γ-valerolactone is formed by a heterogeneous reaction; pentanal and methylfurans are produced in significant quantities through heterogeneous reactions, especially at low concentrations of oxygen (<5% O2). The implications of heterogeneous reactions of RO radicals in atmospheric chemistry and in combustion are discussed for those reactions can correspond to a sink for radicals.
{"title":"Homogeneous and heterogeneous reactions of the n-C5H11O, n-C5H10OH and OOC5H10OH radicals in oxygen. Analytical steady state solution by use of the Laplace transform","authors":"O. Perrin, A. Heiss, F. Doumenc, K. Sahetchian","doi":"10.1039/A803340D","DOIUrl":"https://doi.org/10.1039/A803340D","url":null,"abstract":"The different reaction routes of n-pentoxy radicals in O2/N2 mixtures were investigated in the temperature range 423–523 K under atmospheric pressure. Flow experiments were performed in several reactors, with wall efficiencies increasing from passivated quartz to Pyrex, and with a great variety of mol fractions of O2 (% of O2), which was varied from ∽0 to 100% O2; the products, a pentanal-hydroperoxide C5H10O3 of Mr 118, pentanal, a methylfuranone (γ-valerolactone), pentanol, and three methylfurans were analyzed and identified by GC-MS. Pentanal-hydroperoxide, also called 4-hydroperoxypentanal, OCH(CH2)2CH(OOH)CH3, is the major product in passivated quartz vessels. It is formed by two consecutive isomerizations: (i) a fast one, RO→R-HOH, via a six-membered ring transition state and (ii) a much slower one, involving an OOR-HOH radical, OOR-HOH→HOOR-2HOH, via a seven-membered ring intermediate: CCCCCO ——min CCCCCOH ——minCC(OO)CCCOH ——min CC(OOH)CCCOH ——min CC(OOH)CCCO+HO2. A chemical mechanism, taking into account all of the experimental results is proposed for reactions of the n-pentoxy radical in oxygen. An analytical steady state solution, based on the Laplace transform, has been helpful in rejecting or validating candidate models. Rate constants appearing in the proposed mechanism have been evaluated by the use of an optimization method. This analysis shows that (i) isomerization is the predomi-nant reaction route, accounting for the diversity of end products; (ii) homogeneous pentanal is formed by reaction with O2 of anisomerized pentoxy radical: CCCCCO ——min CCCCCOH ——min CCCCCOH ——min CCCCCO+HO2; (iii) methylfurans are also homogeneously produced through an analogous reaction; (iv) γ-valerolactone is formed by a heterogeneous reaction; pentanal and methylfurans are produced in significant quantities through heterogeneous reactions, especially at low concentrations of oxygen (<5% O2). The implications of heterogeneous reactions of RO radicals in atmospheric chemistry and in combustion are discussed for those reactions can correspond to a sink for radicals.","PeriodicalId":17286,"journal":{"name":"Journal of the Chemical Society, Faraday Transactions","volume":"07 1","pages":"2323-2335"},"PeriodicalIF":0.0,"publicationDate":"1998-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77330836","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Mixing Fe2O3 with Pt/NaMor of similar grain size, followed by grinding in a mortar and calcination in O2, leads to a remarkable enhancement of the reducibility of the Fe2O3 with hydrogen. The TPR profile of such mixtures is virtually identical with that of Fe2O3 onto which Pt was deposited chemically. It is concluded that in the ground and calcined mixtures Pt migration from the zeolite to the iron oxide is crucial. Upon varying the amount of deposited Pt in Pt/Fe2O3 between 0.001% and 1%, TPR profiles are obtained showing two discrete peaks characterizing a Pt promoted and an unpromoted reduction of Fe2O3 respectively. No Pt migration occurs in mixtures of prereduced Pt/NaMor with Fe2O3; this shows that surface migration of P clusters is negligible, but transport of PtO2 either through the gas phase or via the surface is likely. Pt migration is also detectable at room temperature in mixtures stored for weeks in a moist atmosphere; in this case the data suggest surface migration of hydrated Pt2+ ions; the TPR profiles are distinctly different from those of the mixtures calcined in O2. TPR also permits discrimination between the promotion of oxide reduction by migrating Pt and ‘true’ hydrogen spillover. The latter phenomenon requires transport of H atoms via protons and electrons and is realized with powder mixtures containing a semiconducting oxide, such as TiO2. Its TPR signature is a broad peak located between those for unpromoted and Pt promoted reduction. Physical mixtures of Fe2O3 and Pt/NaMor catalyze the reduction of acetic acid vapor to acetaldehyde via a Mars–van Krevelen mechanism. In this case Pt migration helps to regenerate oxygen vacancies in the Fe3O4 surface, whereas direct contact of CH3CO2H vapor with Pt results in the formation of methane and higher hydrocarbons. The promoting effect of Pt is not observed after prereduction of Pt/NaMor, because P does not migrate effectively under the conditions used.
{"title":"Reduction enhancement of Fe2O3 in physical mixtures with Pt/mordenite via Pt migration or ‘hydrogen spillover’","authors":"G. Fröhlich, W. Sachtler","doi":"10.1039/A800200B","DOIUrl":"https://doi.org/10.1039/A800200B","url":null,"abstract":"Mixing Fe2O3 with Pt/NaMor of similar grain size, followed by grinding in a mortar and calcination in O2, leads to a remarkable enhancement of the reducibility of the Fe2O3 with hydrogen. The TPR profile of such mixtures is virtually identical with that of Fe2O3 onto which Pt was deposited chemically. It is concluded that in the ground and calcined mixtures Pt migration from the zeolite to the iron oxide is crucial. Upon varying the amount of deposited Pt in Pt/Fe2O3 between 0.001% and 1%, TPR profiles are obtained showing two discrete peaks characterizing a Pt promoted and an unpromoted reduction of Fe2O3 respectively. No Pt migration occurs in mixtures of prereduced Pt/NaMor with Fe2O3; this shows that surface migration of P clusters is negligible, but transport of PtO2 either through the gas phase or via the surface is likely. Pt migration is also detectable at room temperature in mixtures stored for weeks in a moist atmosphere; in this case the data suggest surface migration of hydrated Pt2+ ions; the TPR profiles are distinctly different from those of the mixtures calcined in O2. TPR also permits discrimination between the promotion of oxide reduction by migrating Pt and ‘true’ hydrogen spillover. The latter phenomenon requires transport of H atoms via protons and electrons and is realized with powder mixtures containing a semiconducting oxide, such as TiO2. Its TPR signature is a broad peak located between those for unpromoted and Pt promoted reduction. Physical mixtures of Fe2O3 and Pt/NaMor catalyze the reduction of acetic acid vapor to acetaldehyde via a Mars–van Krevelen mechanism. In this case Pt migration helps to regenerate oxygen vacancies in the Fe3O4 surface, whereas direct contact of CH3CO2H vapor with Pt results in the formation of methane and higher hydrocarbons. The promoting effect of Pt is not observed after prereduction of Pt/NaMor, because P does not migrate effectively under the conditions used.","PeriodicalId":17286,"journal":{"name":"Journal of the Chemical Society, Faraday Transactions","volume":"18 1","pages":"1339-1346"},"PeriodicalIF":0.0,"publicationDate":"1998-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90759658","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Monte Carlo simulations of a restricted primitive model (RPM) electrolyte on the 2D surface of a sphere are reported. The initial quasi-random ion positions were generated using a Halton sequence. Equilibration of the system from the quasi-random starting configuration was quite rapid and typically required less than 10 successful moves per particle. The internal energy, U, and ln γ±, where γ± is the ionic activity coefficient, did not depend upon N, the total number of ions, but the heat capacity, Cv, did. The internal energy also scaled linearly with r±−1, where r± is the ionic radius. The internal energy, as a function of concentration for 1:1 and 2:2 electrolytes agreed unusually well with the energies calculated from 3D cubic geometries, especially when differences in the ionic radii were accounted for. The ln γ± values agreed reasonably with the values from 3D cubic geometries at lower concentrations, but the reported 3D values exhibit an unrealistic upward curvature at higher concentrations which our 2D results did not. The cause was found to be the use of the particle insertion method. The hybrid particle method was found to give more consistent and realistic values. The internal energy also depended upon the charge product (qiqj) and inversely upon the solvent permittivity, although neither relationship was purely linear. Distributions of ion numbers lying within the Bjerrum distance and pair correlation functions clearly indicated ion association and the formation of strings or chains of ions with alternating charge. These structures were confirmed by plotting the ion positions on the surface of the sphere. The fact that dimensionality and curvature had no apparent effect upon the results may be due partly to the fact that ion association gives rise to predominantly 1D structures (chains) which will not be seriously affected by 2D or 3D spaces.
{"title":"Monte Carlo simulations of restricted primitive electrolytes in a 2D non-Euclidean geometry","authors":"T. Vandernoot, A. Panayi","doi":"10.1039/A801268G","DOIUrl":"https://doi.org/10.1039/A801268G","url":null,"abstract":"Monte Carlo simulations of a restricted primitive model (RPM) electrolyte on the 2D surface of a sphere are reported. The initial quasi-random ion positions were generated using a Halton sequence. Equilibration of the system from the quasi-random starting configuration was quite rapid and typically required less than 10 successful moves per particle. The internal energy, U, and ln γ±, where γ± is the ionic activity coefficient, did not depend upon N, the total number of ions, but the heat capacity, Cv, did. The internal energy also scaled linearly with r±−1, where r± is the ionic radius. The internal energy, as a function of concentration for 1:1 and 2:2 electrolytes agreed unusually well with the energies calculated from 3D cubic geometries, especially when differences in the ionic radii were accounted for. The ln γ± values agreed reasonably with the values from 3D cubic geometries at lower concentrations, but the reported 3D values exhibit an unrealistic upward curvature at higher concentrations which our 2D results did not. The cause was found to be the use of the particle insertion method. The hybrid particle method was found to give more consistent and realistic values. The internal energy also depended upon the charge product (qiqj) and inversely upon the solvent permittivity, although neither relationship was purely linear. Distributions of ion numbers lying within the Bjerrum distance and pair correlation functions clearly indicated ion association and the formation of strings or chains of ions with alternating charge. These structures were confirmed by plotting the ion positions on the surface of the sphere. The fact that dimensionality and curvature had no apparent effect upon the results may be due partly to the fact that ion association gives rise to predominantly 1D structures (chains) which will not be seriously affected by 2D or 3D spaces.","PeriodicalId":17286,"journal":{"name":"Journal of the Chemical Society, Faraday Transactions","volume":"65 1","pages":"1939-1945"},"PeriodicalIF":0.0,"publicationDate":"1998-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89833605","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
FT-Raman spectroscopy has been used to probe single-component and binary-component adsorption of benzene, p-xylene and cyclohexane in silicalite-1. It was shown that FT-Raman is not only able to probe the sorbate–framework interaction related to the phase transitions commonly observed when organic species are sorbed within silicalite-1, but also provides a sensitive and direct probe of sorbate–sorbate interactions within the zeolite framework. Single-component adsorption of benzene, p-xylene and cyclohexane in silicalite-1 has been considered at various loadings. In the case of benzene adsorption, FT-Raman spectroscopy was shown to detect the transformation in sorbate interactions associated with the transformation in crystal symmetry from monoclinic P21/n.1.1 to orthorhombic Pnma at a sorbate loading of 4 molecules per unit cell. We also confirmed the results of Huang (J. Am. Chem. Soc., 1996, 118, 7233) in detecting the sorbate-induced crystal-phase transition from orthorhombic Pnma to orthorhombic P212121 for the case of p-xylene adsorption at loadings in excess of 4 molecules per unit cell. No evidence of a crystal-phase transition as a function of sorbate loading was observed for the case of cyclohexane adsorption, consistent with earlier studies. It was shown that, in the case of benzene–p-xylene co-adsorption, benzene and p-xylene access the sites most favoured during the respective single-component adsorption processes. In contrast, cyclohexane when co-adsorbed with either benzene or p-xylene was seen to compete for the same sites, forcing benzene and p-xylene into less favoured adsorption sites.
{"title":"FT-Raman studies of single-component and binary adsorption in silicalite-1","authors":"Sunil Ashtekar, J. J. Hastings, L. Gladden","doi":"10.1039/A708285A","DOIUrl":"https://doi.org/10.1039/A708285A","url":null,"abstract":"FT-Raman spectroscopy has been used to probe single-component and binary-component adsorption of benzene, p-xylene and cyclohexane in silicalite-1. It was shown that FT-Raman is not only able to probe the sorbate–framework interaction related to the phase transitions commonly observed when organic species are sorbed within silicalite-1, but also provides a sensitive and direct probe of sorbate–sorbate interactions within the zeolite framework. Single-component adsorption of benzene, p-xylene and cyclohexane in silicalite-1 has been considered at various loadings. In the case of benzene adsorption, FT-Raman spectroscopy was shown to detect the transformation in sorbate interactions associated with the transformation in crystal symmetry from monoclinic P21/n.1.1 to orthorhombic Pnma at a sorbate loading of 4 molecules per unit cell. We also confirmed the results of Huang (J. Am. Chem. Soc., 1996, 118, 7233) in detecting the sorbate-induced crystal-phase transition from orthorhombic Pnma to orthorhombic P212121 for the case of p-xylene adsorption at loadings in excess of 4 molecules per unit cell. No evidence of a crystal-phase transition as a function of sorbate loading was observed for the case of cyclohexane adsorption, consistent with earlier studies. It was shown that, in the case of benzene–p-xylene co-adsorption, benzene and p-xylene access the sites most favoured during the respective single-component adsorption processes. In contrast, cyclohexane when co-adsorbed with either benzene or p-xylene was seen to compete for the same sites, forcing benzene and p-xylene into less favoured adsorption sites.","PeriodicalId":17286,"journal":{"name":"Journal of the Chemical Society, Faraday Transactions","volume":"104 1","pages":"1157-1161"},"PeriodicalIF":0.0,"publicationDate":"1998-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89936275","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The structure, bonding and relative stabilities of the ground and low-lying excited states of Fe2O2 have been studied by the hybrid B3LYP density-functional and coupled-cluster molecular orbital methods. Calculations indicate that the (µ-O)2 rhombic 7B2u state is the ground state for Fe2O2. Stable molecular diiron oxo Fe2-O2 complexes in distorted tetrahedral and planar side-on modes have been also located on the potential-energy hypersurfaces of Fe2O2. The calculated IR-active frequencies corresponding to two in-plane deformations of the rhombic ring agree well with the observed values. The bonding features of the (µ-O)2 rhombic Fe2O2 have been discussed based on natural bond orbital and Bader topological analyses. These analyses show that an effective Fe–Fe bonding across the ring exists in the 7B2u ground state.
{"title":"Low-lying electronic states and molecular structure of Fe2O2","authors":"Z. Cao, M. Duran, M. Solà","doi":"10.1039/A803725F","DOIUrl":"https://doi.org/10.1039/A803725F","url":null,"abstract":"The structure, bonding and relative stabilities of the ground and low-lying excited states of Fe2O2 have been studied by the hybrid B3LYP density-functional and coupled-cluster molecular orbital methods. Calculations indicate that the (µ-O)2 rhombic 7B2u state is the ground state for Fe2O2. Stable molecular diiron oxo Fe2-O2 complexes in distorted tetrahedral and planar side-on modes have been also located on the potential-energy hypersurfaces of Fe2O2. The calculated IR-active frequencies corresponding to two in-plane deformations of the rhombic ring agree well with the observed values. The bonding features of the (µ-O)2 rhombic Fe2O2 have been discussed based on natural bond orbital and Bader topological analyses. These analyses show that an effective Fe–Fe bonding across the ring exists in the 7B2u ground state.","PeriodicalId":17286,"journal":{"name":"Journal of the Chemical Society, Faraday Transactions","volume":"7 1","pages":"2877-2881"},"PeriodicalIF":0.0,"publicationDate":"1998-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90847824","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Catalytic performance measurements showed that CH4 conversion and C2 selectivity were higher over SrF2/Nd2O3 catalyst than over pure Nd2O3 for the oxidative coupling of methane (OCM). XRD indicated that there was some exchange of F− and O2−, leading to the formation of tetragonal and rhombohedral NdOF phases during the catalyst preparation. After a certain amount of SrF2 was added to Nd2O3, the increase in surface basicity and conductivity, as well as the role of F− on the dispersion of surface active sites, would be favourable to a decrease in the secondary deep oxidation of methyl radicals and C2 hydrocarbons, giving improved C2 selectivity. Since there is a certain correlation between methane conversion and C2 selectivity for an oxygen-limited OCM reaction, methane conversion is also expected to increase with the increase in C2 selectivity.
{"title":"Promotion by strontium fluoride of neodymium oxide catalysis of the oxidative coupling of methane","authors":"Ruiqiang Long, H. Wan","doi":"10.1039/A800367J","DOIUrl":"https://doi.org/10.1039/A800367J","url":null,"abstract":"Catalytic performance measurements showed that CH4 conversion and C2 selectivity were higher over SrF2/Nd2O3 catalyst than over pure Nd2O3 for the oxidative coupling of methane (OCM). XRD indicated that there was some exchange of F− and O2−, leading to the formation of tetragonal and rhombohedral NdOF phases during the catalyst preparation. After a certain amount of SrF2 was added to Nd2O3, the increase in surface basicity and conductivity, as well as the role of F− on the dispersion of surface active sites, would be favourable to a decrease in the secondary deep oxidation of methyl radicals and C2 hydrocarbons, giving improved C2 selectivity. Since there is a certain correlation between methane conversion and C2 selectivity for an oxygen-limited OCM reaction, methane conversion is also expected to increase with the increase in C2 selectivity.","PeriodicalId":17286,"journal":{"name":"Journal of the Chemical Society, Faraday Transactions","volume":"14 1","pages":"1129-1135"},"PeriodicalIF":0.0,"publicationDate":"1998-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90871801","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. A. Doyle, D. Hutchings, N. Lancaster, C. Wormald
A flow-mixing calorimeter has been used to measure the excess molar enthalpy HmE(p°) of (nitrogen+chloroform)(g) at standard atmospheric pressure (p°) over the temperature range 333.2–423.2 K. The measurements were analysed using pair potential parameters for nitrogen, assuming suitable combining rules for the calculation of cross-terms, and finding parameters of the Stockmayer potential for chloroform that fitted the excess enthalpy measurements. These parameters are e/k=1040 K, σ=0.306 nm and t*=0.0877. Second virial coefficients calculated from these parameters are in agreement with those obtained from (pVT) experiments.
{"title":"Second virial coefficient of chloroform from measurements of the excess molar enthalpy of (nitrogen+chloroform)(g)","authors":"J. A. Doyle, D. Hutchings, N. Lancaster, C. Wormald","doi":"10.1039/A708603B","DOIUrl":"https://doi.org/10.1039/A708603B","url":null,"abstract":"A flow-mixing calorimeter has been used to measure the excess molar enthalpy HmE(p°) of (nitrogen+chloroform)(g) at standard atmospheric pressure (p°) over the temperature range 333.2–423.2 K. The measurements were analysed using pair potential parameters for nitrogen, assuming suitable combining rules for the calculation of cross-terms, and finding parameters of the Stockmayer potential for chloroform that fitted the excess enthalpy measurements. These parameters are e/k=1040 K, σ=0.306 nm and t*=0.0877. Second virial coefficients calculated from these parameters are in agreement with those obtained from (pVT) experiments.","PeriodicalId":17286,"journal":{"name":"Journal of the Chemical Society, Faraday Transactions","volume":"9 1","pages":"1263-1265"},"PeriodicalIF":0.0,"publicationDate":"1998-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89543237","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
It has recently been suggested that benzyl radicals may play an important role in stimulating spontaneous ignition, both in diesel and in petrol engines. We examine here one of the proposed mechanisms. The energies and structures of the intermediate benzylhydroperoxide, and of the initial reactants and final products, were determined at the MP2/6-311G**//B3LYP/6-311G** level of theory. An estimate was made of the k(E) function for the unimolecular dissociation of into and thence, the relative fractions of collisions that lead directly to the formation of OH, as a function of temperature and pressure, as opposed to being stabilized to the hydroperoxide. The computed rate constants were then incorporated into a kinetic model in order to assess the importance of benzyl radicals in stimulating spontaneous ignition in hydrocarbon - air mixtures.
{"title":"Theoretical study of benzyl radical reactivity in combustion systems","authors":"W. Davis, S. Heck, H. O. Pritchard","doi":"10.1039/A804131H","DOIUrl":"https://doi.org/10.1039/A804131H","url":null,"abstract":"It has recently been suggested that benzyl radicals may play an important role in stimulating spontaneous ignition, both in diesel and in petrol engines. We examine here one of the proposed mechanisms. The energies and structures of the intermediate benzylhydroperoxide, and of the initial reactants and final products, were determined at the MP2/6-311G**//B3LYP/6-311G** level of theory. An estimate was made of the k(E) function for the unimolecular dissociation of into and thence, the relative fractions of collisions that lead directly to the formation of OH, as a function of temperature and pressure, as opposed to being stabilized to the hydroperoxide. The computed rate constants were then incorporated into a kinetic model in order to assess the importance of benzyl radicals in stimulating spontaneous ignition in hydrocarbon - air mixtures.","PeriodicalId":17286,"journal":{"name":"Journal of the Chemical Society, Faraday Transactions","volume":"59 1","pages":"2725-2728"},"PeriodicalIF":0.0,"publicationDate":"1998-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89734520","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Aggregation equilibrium and the fluorescence properties of hydroxoaluminium tricarboxymonoamidephthalocyanine adsorbed on microgranular cellulose have been studied at different dye loadings. Up to a concentration of nearly 3 × 10−6 mol phthalocyanine (g cellulose)−1 diffuse reflectance spectra may be interpreted on the basis of a simple monomer–dimer equilibrium. Monomer and dimer spectra are similar to the spectra of the monomeric dye in solution. The solid-state dimer spectrum is red-shifted with respect to that of the monomer and this is attributed to the coplanarity of dimers. Fluorescence spectra and quantum yields show typical effects of re-absorption and re-emission of light. In particular, the observed fluorescence quantum yields depend on concentration and span the range 0.29–0.07. To account for these effects as well as the effect of aggregation on fluorescence quantum yields and to obtain corrected fluorescence spectra a model based on the Kubelka–Munk theory of diffuse reflectance is developed. The application of this model to the case under study yields a true fluorescence quantum yield ϕ = 0.46 ± 0.02 in the whole range of concentrations, which is slightly higher than the value found for the same dye in dimethyl sulfoxide solution.
{"title":"Modeling of fluorescence quantum yields of supported dyes Aluminium carboxyphthalocyanine on cellulose","authors":"M. Lagorio, L. Dicelio, M. Litter, E. S. Román","doi":"10.1039/A706113G","DOIUrl":"https://doi.org/10.1039/A706113G","url":null,"abstract":"Aggregation equilibrium and the fluorescence properties of hydroxoaluminium tricarboxymonoamidephthalocyanine adsorbed on microgranular cellulose have been studied at different dye loadings. Up to a concentration of nearly 3 × 10−6 mol phthalocyanine (g cellulose)−1 diffuse reflectance spectra may be interpreted on the basis of a simple monomer–dimer equilibrium. Monomer and dimer spectra are similar to the spectra of the monomeric dye in solution. The solid-state dimer spectrum is red-shifted with respect to that of the monomer and this is attributed to the coplanarity of dimers. Fluorescence spectra and quantum yields show typical effects of re-absorption and re-emission of light. In particular, the observed fluorescence quantum yields depend on concentration and span the range 0.29–0.07. To account for these effects as well as the effect of aggregation on fluorescence quantum yields and to obtain corrected fluorescence spectra a model based on the Kubelka–Munk theory of diffuse reflectance is developed. The application of this model to the case under study yields a true fluorescence quantum yield ϕ = 0.46 ± 0.02 in the whole range of concentrations, which is slightly higher than the value found for the same dye in dimethyl sulfoxide solution.","PeriodicalId":17286,"journal":{"name":"Journal of the Chemical Society, Faraday Transactions","volume":"325 1","pages":"419-425"},"PeriodicalIF":0.0,"publicationDate":"1998-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76561670","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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