An investigation of the molecular structure of cycloheptanone by gas phase electron diffraction

The electron diffraction data of cycloheptanone, collected at 371 K, can be explained using a model of partial pseudorotation, with the symmetrical twist—chair as the mean structure. Ther_g, r_α-structure is characterized by r(C-C) = 1.536 Å, r(C=O) = 1.219 Å, r(C-H) = 1.124 Å, xxxCC(sp²)C = 117.3°, xxx(CCC = 115.5° and xxx(HCH = 103.2°. Approximate values for the constants of the pseudorotation potential are included.     

The gas phase photolysis of CHFCl2

The photolysis of CHFCl₂ at 300 K has been investigated at 213.9, 163.3 and 147 nm. Methane, Br₂, HBr and HCl were added as free radical interceptors in order to unravel the primary photodecomposition processes. Analysis of the data shows that at 213.9 and 163.3 nm the photodissociative process

occurs with a quantum yield of 0.9 – 1.0, giving stable CHFCl radicals. At shorter wavelengths the quantum yield of CHFCl shows a drastic decrease with concurrent appearances of species such as CFCl, CHF and CF. The laboratory experiments indicate that CF is mainly formed via the dissociative process

The CF radicals react with CH₃ to yield C₂H₂ (CF + CH₃ → C₂H₂ + HF) while the CHF species insert readily into HCl to yield CH₂FCl. In the presence of Br₂, CF and CHF undergo reactions which result in the formation of CFBr₃ and CHFBr₂ respectively.     

Effect of water on product yields of 2-azidobenzoic acid photolysis in aprotic solvents

The effect of water admixtures on the formation of 2-oxo-(3H)-azepine-3-carboxylic acid and 2,1-benzisoxazol-3(1H)-one in the photolysis of 2-azidobenzoic acid in aprotic solvents has been investigated. It has been shown that the addition of nucleophilic agents to the reaction mixture determines the yields of not only 3H-azepines, the products of benzene ring expansion, but also the intramolecular cyclization product 2,1-benzisoxazol-3(1H)-one. Participation of the formed 2,1-benzisoxazol-3(1H)-one in the reaction has been suggested as an explanation of the effect of nucleophilic compounds (water, ethanol, and 2,1-benzisoxazol-3(1H)-one) on the product yield of the monomolecular reaction.     

Appearance of the magnetic isotope effect during the photolysis of aqueous solutions of cycloheptanone

A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 117912 Moscow. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 6, pp. 1448–1449, June, 1992.     

Temperature dependence of the fluorescence quantum yields of diphenylsiloxane‐based copolymers in dilute solutions

The monomer and excimer fluorescence quantum yields of well‐defined poly(dimethylsiloxane‐co‐diphenylsiloxane)s with different diphenylsiloxane (Ph₂SiO) contents have been determined, along with those of 1,1,3,3‐tetraphenyl‐1,3‐dimethyldisiloxane and 1,1,3,3,5,5‐hexaphenyltrisiloxane‐1,5‐diol used as model compounds, in a dilute organic solvent at different temperatures. The measured fluorescence quantum yields of the copolymers are correlated with the fraction of the ? (CH₃)₂SiO? (Ph₂SiO)_n? (CH₃)₂SiO? structures. The monomer fluorescence yield for copolymers with low Ph₂SiO contents is dominated mainly by the isolated ? (CH₃)₂SiO? (Ph₂SiO)? (CH₃)₂SiO? unit, and the apparent mean binding energy of the excimer does not increase significantly with increasing Ph₂SiO content. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 854–861, 2002     

Photodecomposition quantum yields of 3-chloro-3-methyldiazirine in the gas phase

A careful integration technique has allowed the determination of photodecomposition quantum yields Φ_D of 3-chloro-3-methyldiazirine at several wavelengths using conventional equipment. The results obtained are

A change of collision frequency from 5.0 × 10⁷ to 5.7 × 10¹⁰ does not alter the quantum yield which is also unaltered by oxygen. Our data are explained by a mechanism which includes two photodecomposition paths. The first is a direct photodissociation from the electronic state reached by the initial transition and the second involves methylchlorocarbene.     

The temperature dependence of the cage effect in the gas phase

The photodissociation of iodine has been studied in the gas phase by laser flash photolysis. The decrease of the quantum yield with increasing ethane and propane pressure has been interpreted in terms of the cage effect. As the temperature is increased a less pronounced cage effect is observed. The trends in the measured quantum yields with changing temperature and pressure agree with model calculations for dissociation in a viscous continuum. However the simple model applied is not useful for quantitative predictions. A small decrease of the second order rate constant for iodine atom recombination has been observed with increasing temperature.     

Magnetic field dependence of delayed fluorescence of pyrene in solution

The magnetic field effect on the intensities of monomer (I_M) and excimer (I_D) delayed fluorescence of pyrene has been found to be identical. This result suggests a common spin-selective step in the processes giving rise to I_M and I_D, and is discussed in the light of previously proposed mechanisms. An unusual solvent effect is found on the I_M (I_D) versus field strength curves. This is attributed to photochemical doublet-radical formation. Doublet-radical formation affects the field dependence of I_M and I_D through spin selectivity in the triplet quenching reaction.     

Calculated and measured transient product yields in pulse radiolysis of aqueous solutions: Concentration dependence

Pulse radiolysis of aqueous solutions was modeled by using 54 equations for the reaction of water radiolysis intermediates with carefully selected rate coefficients. Yields of products formed in the hydrated electron+solute and hydroxyl radical+solute reactions were calculated and compared with the measured yields in wide concentration range. These reactions are in competition with the reactions of the water radiolysis intermediates with each other and with H₂O, H⁺ and H₂O₂. An empirical equation was developed for the calculation of scavenged product yields that can be used in cases when due to low rate coefficient, low solubility or very high absorbance, low solute concentrations are applied and a considerable fraction of the water radiolysis intermediates does not react with the solute.     

The photolysis of ortho-nitrophenols: a new gas phase source of HONO

Formation of nitrous acid (HONO) in the gas phase has been observed for the first time in a flow tube photoreactor upon irradiation (lambda = 300-500 nm) of 2-nitrophenol and methyl substituted derivatives using a selective and sensitive instrument (LOPAP) for the detection of HONO. Formation of HONO by heterogeneous NO2 photochemistry has been excluded, since production of NO2 under the experimental conditions is negligible. Variation of the surface to volume ratio and the nitrophenol concentration showed that the photolysis occurred in the gas phase indicating that HONO formation is initiated by intramolecular hydrogen transfer from the phenolic OH group to the nitro group. From the measured linear dependence of the HONO formation rate on the reactant's concentration and photolysis light intensity, a non-negligible new HONO source is proposed for the urban atmosphere during the day. Unexpectedly high HONO mixing ratios have been observed recently in several field campaigns during the day. It is proposed that the photolysis of aromatic compounds containing the ortho-nitrophenol entity could help to explain, at least in part, this high contribution of HONO to the oxidation capacity of the urban atmosphere.     

Direct and sensitized photolysis of bicyclo[4.2.0]octa-2,4-diene in the gas phase

The direct photolysis of bicyclo[4.2.0]octa-2,4-diene in the gas phase at 280–300 nm produces mainly 1,3,5-cyclooctatriene and benzene plus ethylene. The yield of the former product is enhanced by added gases, and it is proposed that it is formed in a vibrationally excited state which can revert to bicyclooctadiene unless the excess energy is removed in collisions. Computer modelling of the direct photolysis yielded quantitative agreement with the experimental results, but only when large, arbitrary adjustments were made to the calculated rate constants for the interconversion of cyclooctratriene and bicyclooctadiene. The Hg(6³P₁) sensitized reaction of bicyclooctadiene produces mainly benzene plus ethylene, a process which is also enhanced by added gases.     

Study of ultraviolet photolysis of bis(trichloromethyl)Mercury in the gas phase and in solution

Conclusions The UV photolysis of Hg(CCl₃)₂ in the gas phase at 80°, and in dimethoxyethane solutions at 18°, leads to the respective formation of C₂Cl₆ and HCCl₃ as the main products, which testifies to the predominant free-radical decomposition of the starting molecules involving CCl₃.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2146–2148, September, 1974.     

Probing the pyrolysis of methyl formate in the dilute gas phase by synchrotron radiation and theory

The thermal dissociation of the atmospheric constituent methyl formate was probed by coupling pyrolysis with imaging photoelectron photoion coincidence spectroscopy (iPEPICO) using synchrotron VUV radiation at the Swiss Light Source (SLS). iPEPICO allows threshold photoelectron spectra to be obtained for pyrolysis products, distinguishing isomers and separating ionic and neutral dissociation pathways. In this work, the pyrolysis products of dilute methyl formate, CH₃OC(O)H, were elucidated to be CH₃OH + CO, 2 CH₂O and CH₄ + CO₂ as in part distinct from the dissociation of the radical cation (CH₃OH^+• + CO and CH₂OH⁺ + HCO). Density functional theory, CCSD(T), and CBS-QB3 calculations were used to describe the experimentally observed reaction mechanisms, and the thermal decomposition kinetics and the competition between the reaction channels are addressed in a statistical model. One result of the theoretical model is that CH₂O formation was predicted to come directly from methyl formate at temperatures below 1200 K, while above 1800 K, it is formed primarily from the thermal decomposition of methanol.     

Probing the pyrolysis of ethyl formate in the dilute gas phase by synchrotron radiation and theory

The thermal decomposition of the atmospheric constituent ethyl formate was studied by coupling flash pyrolysis with imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet (VUV) radiation at the Swiss Light Source (SLS). iPEPICO allows photoion mass-selected threshold photoelectron spectra (ms-TPES) to be obtained for pyrolysis products. By threshold photoionization and ion imaging, parent ions of neutral pyrolysis products and dissociative photoionization products could be distinguished, and multiple spectral carriers could be identified in several ms-TPES. The TPES and mass-selected TPES for ethyl formate are reported for the first time and appear to correspond to ionization of the lowest energy conformer having a cis (eclipsed) configuration of the O = C (H)– O – C (H₂)–CH₃ and trans (staggered) configuration of the O= C (H)– O – C (H₂)– C H₃ dihedral angles. We observed the following ethyl formate pyrolysis products: CH₃CH₂OH, CH₃CHO, C₂H₆, C₂H₄, HC(O)OH, CH₂O, CO₂, and CO, with HC(O)OH and C₂H₄ pyrolyzing further, forming CO + H₂O and C₂H₂ + H₂. The reaction paths and energetics leading to these products, together with the products of two homolytic bond cleavage reactions, CH₃CH₂O + CHO and CH₃CH₂ + HC(O)O, were studied computationally at the M06-2X-GD3/aug-cc-pVTZ and SVECV-f12 levels of theory, complemented by further theoretical methods for comparison. The calculated reaction pathways were used to derive Arrhenius parameters for each reaction. The reaction rate constants and branching ratios are discussed in terms of the residence time and newly suggest carbon monoxide as a competitive primary fragmentation product at high temperatures.     

Wavelength dependence of HCO formation in the photolysis of acetaldehyde

The wavelength dependence of HCO(0,0,0) formation in the photodissociation of acetaldehyde was measured using narrow (0.1 nm) bandwidth laser excitation and time-resolved intracavity laser detection (TRILD). A sharp energetic onset at 320 ± 1 nm (89.3 ± 0.3 kcal) for HCO(0,0,0) formation was found. The maximum concentration of HCO(0,0,0) occurs between 100 and 250 μs after excitation depending on the wavelength of excitation