Gaseous phosphorus compounds : Part V. The dissociation energy and heat of formation of carbon monophosphide

The chemical equilibrium CP(g) + P(g) ? P₂(g) + C(g) has been studied by means of the Knudsen effusion technique combined with mass spectrometric analysis of the vapor. The enthalpy of reaction, ΔH^O₂₉₈₋ was determined as 6.3 ± 4.0 kcal/mole. Combined with the literature value for the dissociation energy of P₂, the dissociation energy of gaseous carbon monophosphide was calculated as D^O₂₉₈ = 123.2 ± 4.0 kcal/mole or D^O_O = 122.1 ± 4.0 kcal/mole. The corresponding value for the standard heat of formation is ΔH^O_f.298 = 127.5±4.5 kcal/mole. This compares with the selected JANAF value of 111.7 ± 23.1 kcal/mole.     

The energy of the N→O bond in 3-nitroisoxazoline N-oxide

The enthalpy of combustion of 3-nitroisoxazoline has been determined as H _c ^298.15 =–414±0.3 kcal/mole and that of 3-nitroisoxazoline N-oxide as H _c ^298.15 =–406.6±0.5 kcal/mole. From the values for the heats of combustion and evaporation, the standard enthalpies of formation have been calculated and the energy of the NO bond has been evaluated at 64±3 kcal/mole.     

An NMR study of proton exchange in the system p-nitrosophenol—p-quinone monooxime

The solvent used was dimethylformamide at neutral and alkaline pH. The equilibrium constants are determined by spectrophotometry. The rate of proton exchange has been measured as a function of temperature and concentration. The rate constants and activation energies have been measured; for uncatalyzed exchange k_n=(1.5±0.5) ·· 10³ M^–1 sec^–1, E=8±1 kcal/mole, while base-catalyzed exchange has k=(0.3±0.1) · 10⁶ M^–1 sec^–1 and E=6±1 kcal/mole.We are indebted to A. I. Brodskii for assistance in this work, and to V. I. Oshkaderov and L. A. Kichakova for recording the NMR spectra.     

Kinetics and mechanism of oxidation ofd-xylose by Mn(III) pyrophosphate

The oxidation ofd-xylose by Mn(III) pyrophosphate in sulphuric acid has been found to be first order with respect to [Mn(III)]. Variation of rate with [d-xylose] suggests the rapid formation of reversible cyclic complex between Mn(III) and id-xylose, which further disproportionates in a slow rate determining step. Oxidation rate has been found to increase with [H⁺]. Retardation of rate due to [pyrophosphate] and increase due to [Mn(II)] have been also observed. The value of thermodynamic parameters E, S, and G have been found to be 17.6±0.1 kcal/mole, –10.1±0.1 e.u. and 20.6±0.1 kcal/ mole respectively. A mechanism involving a free radical has been proposed for the reaction under study.     

Die Bildungswärmen von MnSi1,73, MnSi,Mn5Si3 und Mn3Si

By means of vapor pressure measurements using theTorker method (Torsion-Knudsen Effusion Recoil) as well as theKnudsen tanspiration technique in connection with a mass spectrometer, the heats of formation of the following compounds were found to be –6.5±0.7 kcal/g atom (Mn_0.366Si_0.634), –7.8±0.6 kcal/g atom (Mn_0.50Si_0.50), –7.3±0.6 kcal/g atom (Mn_0.625Si_0.375), and –6.6±0.6 kcal/g atom (Mn_0.75Si_0.25).

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Mit 7 Abbildungen     

Quasiclassical trajectory studies of H(D)+HBr(DBr) abstraction and exchange reactions

The abstraction and exchange reaction dynamics for H(D)+HBr(DBr) systems have been investigated on three LEPS potential-energy surfaces whose features are in accord with the surface topography suggested by recent molecular-beam and thermal experiments (abstraction barrier less than 1.0 kcal/mole, exchange reaction barriers of =5.0 kcal/mole, and no attractive wells with a depth greater than 0.209 kcal/mole). The surfaces differ primarily in the magnitude of the abstraction barrier which varies from 0.19 to 1.01 kcal/mole. Reaction cross sections have been computed on each surface as a function of relative collision energy from the results of 139000 quasiclassical trajectoris. Comparison of these results with measured relative abstraction cross sections suggests that the true abstraction barrier is very small, perhaps between 0.0 and 0.25 kcal/mole. However, thermal rate coefficients computed on the - best- surface at 300 K are about a factor of 2 larger than the most recently measured values. The calculated (H,D)/(D,H) isotope ratio at 300 K lies between the two reported experimental results. The computed thermal activation energy for abstraction is 835 cal/mole, which is in good agreement with a very early measurements but a factor of 2.5 less than the most recently reported experimental result. These results suggest that the molecular-beam and thermal rate measurements are inconsistent. The average fraction of the available energy which is partitioned into internal product modes <f_E> is found to be nearly independent of relative collision energy and the small topographical differences present in the potential surfaces used in these calculations. We find <f_E> = 0.40. In all reactions, the differential scattering cross sections are peaked in the backward direction for the molecular products, indicating a rebound mechanism.     

Kinetic study of anhydride formation during thermal degradation of isotactic and atactic polymethacrylic acid

A kinetic study of anhydride formation in isotactic and atactic polymethacrylic acids (PMA) has been performed at about 200° using a vacuum thermobalance. The results obey first order kinetics and show that the rate of water loss is about four times higher for the isotactic PMA than for the atactic isomer. The activation energies are almost identical (42 ± 1 kcal per mole); it is concluded that the arrangement of neighbouring carboxyl groups is more favourable for the formation of a cyclic anhydride by intramolecular reaction in the case of the isotactic PMA.     

MNDO estimates of the heats of isomerization of 2,5-oxazolines

MNDO calculations were carried out to estimate the heats of isomerization of 2,5-oxazolines to give 3,5-oxazolidones. The heat of this reaction (15 kcal/mole) is comparable with the heat of isomerization of O-arylisourea to give N-arylurea (19 kcal/mole).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1666–1668, July, 1990.     

Heterolytische Anlagerung an Acetylen-Verbindungen, 1. Mitt.: Stereochemie und Kinetik der Anlagerung im System Diphenylacetylen-N-Brom-Succinimid—Essigsäure—Wasser

Zusammenfassung Die Umsetzung des Diphenylacetylens mit N-Brom-succinimid in 80–90proz. Essigsäure führt zu Brom-acetoxystilben (70% d. Th.,cis-:trans-Form=2575) und Dibrom-desoxybenzion (15–20% d. Th.). Bis zu einem Umsatz von ca. 30% befolgt die Umsetzung eine kinetische Gleichung zweiter Ordnung, dann steigt die Geschwindigkeitskonstante weiter an. Für 25° bis 50°C werden eine Aktivierungsenergie vonE _a =12,6±±1,0 kcal/Mol und einArrhenius-Faktor von lgA=7,4±±0,6 gefunden.

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The reaction of diphenylacetylene with N-bromosuccinimide and 80–90% aqueous acetic acid gives bromoacetoxystilbene (70% yield, mixture ofcis- andtrans-isomers 2575) and dibromodesoxybenzoin (15–20% yield). Up to about 30% conversion the reaction follows a second-order rate equation, but further the rate increases. For the temperature interval 25–50°C activation energyE _a =12,6±1,0 kcal/mole, andArrhenius-factor logA=7,4±0,6 are calculated.

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Mit 5 Abbildungen     

Relaxation during internal rotation ethane and hydrogen peroxyde

An ab initio SCF-LCAO-MO study of the relaxation process during internal rotation has been performed for ethane and hydrogen peroxyde. A large gaussian basis set has been used, with polarization functions. The total energy has been optimized with respect to the bond lengths and bond angles. The computed barrier for the ethane molecule is 3.07 kcal/mole with the optimized geometry (experimental 2.93 kcal/mole). For hydrogen peroxyde, this yields a cis-barrier of 10.9 kcal/mole (experimental 7.0 kcal/mole) and a trans barrier of 0.6 kcal/mole (experimental 1.1 kcal/mole), with a dihedral angle equal to 123 (experimental 111–120). The eclipsed or cis conformations are found to have more open structures than the staggered or equilibrium conformations.

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Zusammenfassung Der Relaxationsproze\ wÄhrend der inneren Rotation wurde für Äthan und Wasserstoffperoxid mit Hilfe einer ab initio SCF-LCAO-MO-Rechnung untersucht. Dabei wurde eine gro\e Basis von Gau\funktionen mit Polarisationsfunktionen benutzt. Die Gesamtenergie wurde unter Variation der BindungslÄnge und Bindungswinkel optimiert. Die berechnete Rotationsbarriere mit der optimalen Geometrie betrÄgt 3,07 kcal/Mol für Äthan (experimentell 2,93 kcal/Mol). Für Wasserstoffperoxid ergibt sich eine cis-Barriere von 10,9 kcal/Mol (experimentell 7,0 kcal/Mol) und eine trans-Barriere von 0,6 kcal/Mol (experimentell 1,1 kcal/Mol) sowie ein Verdrillungswinkel von 123 (experimentell 111–120). Die verdeckten oder cis-Konformationen besitzen mehr offene Strukturen als die gestaffelten oder die Gleichgewichtskonformationen.

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Résumé Une étude des phénomènes de relaxation liés à la rotation interne a été effectuée pour l'ethane et l'eau oxygénée par la méthode ab initio SCF-LCAO-MO. Avec une base étendue de fonctions gaussiennes comprenant des fonctions de polarisation, on minimise l'énergie par rapport aux longueurs et aux angles des liaisons. La barrière calculée pour l'ethane est de 3,07 kcal/mole (valeur expérimentale 2,93 kcal/mole). Pour l'eau oxygénée, on trouve pour les barrières cis 10,9 kcal/mole, trans 0,6 kcal/mole et pour l'angle dièdre 123 (values expérimentales 7,0 et 1,1 kcal/mole et 111–120). Les conformations éclipsée ou cis possèdent des structures plus «ouvertes» que les conformations en étoile ou trans.

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Quantum Mechanical Calculations on Barriers to Internal Rotation. Part VI. Preceding paper, Ref. [35].     

Energy of rehydration of natural clinoptilolite and its lithium form as a function of the dehydration temperature

Natural clinoptilolite (CL) is practically reversibly rehydrated below (350±50)°C, and is only 10% sintered at 500°C. LiCL is reversibly dehydrated up to (250±20)°C, and is sintered much more rapidly than CL at higher temperatures (by 50% at 400°C, for example). The heats of rehydration pass through maxima as a function of the calcination temperature: for CL (250±10) J/g at 300–400°C, and for LiCL (300±15) J/g at 250°C. The corresponding half-widths of the maxima are >500 and (330±20)°C. The maximum values of the average molecular integral heats of adsorption of water vapors are (73±3) kJ/mole on CL and (76±3) kJ/mole in LiCL.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2407–2409, November, 1989.We would like to thank T. M. Guliev for assisting in the measurements.     

Hydrogenation of difurfurylideneacetone on a raney nickel catalyst

1.5-Difurylpent-1-en-3-one, 1,5-difurylpentan-3-one, 1,5-ditetrahydrofurylpentan3-one, and l,5-ditetrahydrofuryl-pentan-3-ol were obtained by hydrogenation of difurfuryldieneacetone on a Raney nickel catalyst at 20–65°C and atmospheric pressure. It was established that the hydrogenation of difurfuryldieneacetone is a zero-order reaction with respect to the concentration of the starting compound. The rate constants of the reaction were found; the activation energy found for 20–65°C is 6.2±0.5 kcal/mole.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 462–465, April, 1979.     

Thermal investigation of the basic copper(II) nitrate Cu(OH)1.5(NO3)0.5

A thermal investigation of basic copper(II) nitrate was made under dynamic heating conditions as well as in an isothermal environment. Data analysis was applied to calculate the heat of thermal decomposition of gerhardtite (21.6±0.1 kcal/mole), which does not depend on the heating rate from 4/min to 20/min.     

Investigation of the reaction of 4-methyl-7-azaindoline with acetic and trifluoroacetic acids in methylene chloride by PMR and IR spectroscopy

The relative position of the equilibria of proton transfer via a hydrogen bond and dissociation of the H complexes to give solvated ions was determined on the basis of a study of the dependence of the chemical shifts and the spin-spin coupling constants of aromatic protons in the PMR spectrum of 4-methyl-7-azaindoline on the acetic and trifluoracetic acid concentrations in methylene chloride. The protolytic equilibrium in the amine-CF₃COOH system is shifted completely to favor H complexes of the ion pair type when the reagent ratio is nearly equimolar. A molecular complex ion pair equilibrium is observed in the reaction with CH₃COOH under the same conditions. The results were confirmed by measurement of the IR spectra of the investigated systems. The enthalpy (-H = 2.9 ± 0.7 kcal/mole) and entropy (-S = 8.0 ± 2 kcal/mole·deg) of proton transfer via a hydrogen bond were determined from the PMR spectra measured at +25° to –50°.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 76–82, January, 1976.     

Mechanism of hydroformylation,part II Study of the formation of hydrocobalttetracarbonyl by the reaction of Co2(CO)8 and H2

The kinetics and the position of the equilibrium of the reaction Co₂(CO)₈+H₂2 HCo(CO)₄ were studied in the range of 80–160 °C and 50–100 atm. by means of in situ IR spectroscopy.The reaction is reversible first order with respect to CO₂(CO)₈ and HCo(CO)₄ and the energies of activation of the forward and the reverse reaction are found to be 17,3 cal/mole, and 11.0 kcal/mole resp.The reaction is slightly endothermic with H=6.6 kcal/mole and S=14.6 e.u. The heat of formation of HCo(CO)₄ and the bond strength between hydrogen and cobalt in HCo(CO)₄ were found to be—146.1 kcal/mole and 54.7 kcal/mole resp.With 8 FiguresPart of the Ph.D. Dissertation 1974.Part I see Ref.¹³     

Estimation of activation energies from differential thermal analysis curves

An average activation energy ΔE3 of 31.7 ± 10.0 kcal/mole was calculated from exothermic peaks of urea nitrate differential thermal analysis (DTA) curves using the Murray and White equation and various other reaction rate equations developed by the authors. An average enthalpy of activation, ΔH3 of 30.8 ±9.7 kcal/mole was calculated from the same results. The values of ΔE3 and ΔH3 differed by a fraction of a kcal/mole indicating that ΔE3 <ΔH3 cannot be differentiated experimentally in our study. Application of the Kissinger method of calculating ΔE3 and ΔH3 produced respectively 21.6 ±7.9 and 20.7 ±8.0 kcal/mole, which are quite low. The values of ΔE3 and ΔH3 calculated thermogravimetrically were 28.1, ± 1.1 kcal/mole and 27.6 ± 1.2 kcal/mole which are close to those obtained from the Murray and White approach and the authors' approach to treatment of the DTA data. These results illustrate the pronounced effect of self heating on calculation of activation energies. The Kissinger method of calculating the reaction order developed for endothermic DTA peaks produced good results when applied to the present DTA study.     

Catalytic action of metallic salts in autoxidation and polymerization. IV. Polymerization of methyl methacrylate by cobalt(II) or (III) acetylacetonate–tert-butyl hydroperoxide or dioxane hydroperoxide

Polymerization of methyl methacrylate was carried out by four initiating systems, namely, cobalt(II) or (III) acetylacetonate–tert-butyl hydroperoxide (t-Bu HPO) or dioxane hydroperoxide (DOX HPO). Dioxane hydroperoxide systems were much more effective for the polymerization of methyl methacrylate than tert-butyl hydroperoxide systems, and cobaltous acetylacetonate was more effective than cobaltic acetylacetonate in both hydroperoxides. The initiating activity order and activation energy for the polymerization were as follows: Co(acac)₂–DOX HPO (E_a-9.3 kcal/mole) > Co (acac)₃–DOX HPO (E_a = 12.4 kcal/mole) > Co(acac)₂–t-Bu HPO (E_a = 15.1 kcal/mole) > Co(acac)₃–t-Bu HPO (E_a-18.5 kcal/mole). The effects of conversion and hydroperoxide concentration on the degree of polymerization were also examined. The kinetic data on the decomposition of hydroperoxides catalyzed by cobalt salts gave a little information for the interpretation of polymerization process.     

Effects of temperature on the weathering lifetime of coated polycarbonate

The lifetime of polycarbonate (PC) coated with silicone hardcoats containing UV absorber is shorter at elevated temperatures. The activation energy (E_a) for delamination was found to be 18 ± 2 kJ/mol (4.3 ± 0.5 kcal/mol) at the 95% confidence level in this study. This E_a is the consequence of the sensitivity of the substrate and the UV absorber to temperature. The E_a for PC photodegradation was previously found to be 17-21 kJ/mol (4-5 kcal/mol). The E_a for loss of absorbance in the second-generation silicone hardcoat was found to be 28.5 ± 5.4 kJ/mol (6.8 ± 1.3 kcal/mol) at the 95% confidence level. Results are consistent with experimental findings when these activation energies are used in published predictive models. Since the E_a for coating delamination depends on the E_a of UV absorber loss, coating systems different from the one in this study will need to be investigated separately.     

A theoretical study of the internal rotation and inversion in hydroxymethyl radical (CH2OH)

Ab initio SCF calculation shows that the hydroxymethyl radical (CH₂OH) is non-planar, supporting the ESR experimental evidence. The out-of-plane angle of the CH₂ group is calculated to be 25° with a barrier of 0.4 kcal/mole. The rotational barriers of the hydroxy-group of the non-planar radical are calculated to be 1.8 kcal/mole (trans) and 3.5 kcal/mole (cis), respectively.     

Self-consistent-field potential energy surface in three dimensions for the Cl + H2 → ClH + H chemical reaction

Non-empirical self-consistent-field calculations have been carried out for 38 points on the potential surface for the Cl + H₂ → ClH + H chemical reaction. A basis set of seven s, five p, and one d functions on chlorine and three s and one p on each hydrogen atom was used. The least energy path occurs for the linear Cl---H---H arrangement. A much higher barrier is found for the approach of Cl along the H---H perpendicular bisector. The linear barrier height is predicted to be 26.2 kcal/mole and the saddle point occurs for R(Cl---H) ≈ 1.46 Å, R(H---H) ≈ 0.94 Å. The experimental activation energy is 5.5 kcal/mole. It seems likely that a general feature of the Hartree-Fock approximation is an overestimation of barrier heights. The exothermicity is calculated to be −6.7 kcal/mole, compared to the near Hartree-Fock result −2.3 kcal/mole and experiment −3.0 kcal/mole.