David H. Volman

Rate constants for CN reactions with hydrocarbons and the product HCN vibrational populations : examples of heavy-light-heavy abstraction reactions

The rate constants for the reactions of CN radicals with methane, ethane, propane, cyclo‐propane, isobutane, and neopentane have been measured over a temperature range from 275 to 455 K. Laser photolysis was used to produce the radicals and time delayed laser induced fluorescence was used to follow the radical concentration as a function of time. The temperature dependence of the observed rate constants could be fitted with a three‐parameter Arrhenius plot. The activation energies that were observed were all small and in some cases they were negative. Time resolved ir emission was used to follow the formation of the HCN(0n2) and HCN(0n’1) product emission. The time dependence of the relative emission intensities, as well as computer modeling of the decay curves, suggest that vibrational population inversion occurs for all of the hydrocarbons studied except methane and cyclopropane. These observations are discussed in terms of the current theories for these type of reactions.

Photochemistry of the gaseous hydrogen peroxide—carbon monoxide system. II: Rate constants for hydroxyl radical reactions with hydrocarbons and for hydrogen atom reactions with hydrogen peroxide

Abstract Previous studies on the photochemistry of the system hydrogen peroxide—carbon monoxide—oxygen—isobutane, from which we obtained the rate constant for the reaction of hydroxyl with isobutane, have been continued with other aliphatic hydrocarbons and additional hydroxyl radical rate constants have been obtained. The constants obtained were: C3H6, 13.4 × 10−12; C3H8, 2.2 × 10−12; n-C4H10, 2.9 × 10−12; c-C4H8, 1.2 × 10−12; and c-C6H12, 6.7 × 10−12 cm3 molecule−1 s−1. For the reactions: by a treatment of earlier data on the photochemistry of the system hydrogen peroxide—carbon monoxide, we have obtained k5 = 5.7 × 10−15 and k6 = 3.1 × 10−15 cm3 molecule−1 s−1.

Decomposition of Di‐t‐butyl Peroxide and Kinetics of the Gas Phase Reaction of t‐butoxy Radicals in the Presence of Ethylenimine

The reaction of di‐t‐butyl peroxide with ethylenimine has been studied between 129° and 154°C. The t‐butoxy radicals formed by the rupture of the O–O bond of the peroxide, (CH3)3COOC(CH3)3→ lim k12(CH3)3CO, are postulated to enter into the following reactions: (CH3)3CO→ lim k2CH3COCH3+CH32CH3→ lim k3C2H6CH3+(CH2)2NH→ lim k4(CH2)2N+CH4(CH3)3CO+(CH2)2NH→ lim k5(CH2)2N+(CH3)3COH. The rate constant for the unimolecular decomposition of the peroxide into t‐butoxy radicals is found to be in the range k1=6×1014e−36,000/RT to k1=6×1016e−40,000/RT sec−1. The activation energies E4—E3/2=4.8 kcal/mole and E2—E5=12 kcal/mole are calculated from the kinetic data. The complete rate expression for the capture of imine H atom by CH3 radical, k4=4.5×1010e−4800/RT (moles/cc)−1 sec−1, is obtained by using Gomer and Kistiakowskys values for k3=4.5×1013 (moles/cc)−1 sec−1 and E3=0. The activation energy for the thermal decomposition of t‐butoxy radical is about 17 kcal/mole if E4 and E5 of similar type H capture mechanisms a...

Photochemistry of the gaseous hydrogen peroxide-carbon monoxide system: Rate constants for hydroxyl radical reactions with hydrogen peroxide and isobutane by competitive kinetics

Abstract The reaction between hydrogen peroxide and carbon monoxide initiated by absorption of 254 nm light at 298 K in a static system has been studied. Hydroxyl radicals were formed in the primary process and reacted with carbon monoxide to yield carbon dioxide and hydrogen atoms. Hydrogen atoms reacted with hydrogen peroxide either to yield H2O and OH or to yield H2 and HO2. For these processes the rate constant ratio was found to be kH2O/kH2 = 3.0 ± 1.0. With sufficient added oxygen hydrogen atoms reacted only with oxygen to yield HO2 and it was possibel to obtain a rate constant for the reaction of OH with H2O2. With both oxygen and isobutane added, it was possible to obtain a rate constant for the reaction of OH with (CH3)3CH. The constants obtained were kOH+H2O2 = 1.2 ± 0.3 × 10−12 and kOH + (CH3)3CH = 3.5 ± 0.8 × 10−12 cm3 molecule−1 sec−1.

The Ultraviolet Absorption Spectra of Gaseous Diazomethane and Diazoethane. Evidence for the Existence of Ethylidine Radicals in Diazoethane Photolysis

The absorption of gaseous diazoethane begins at about 5400A, increases to a broad maximum of e=3.5 at 4500A and falls to a minimum of e=0.15 at 3200A. At wavelengths shorter than 3000A a second region of rapidly increasing absorption is found. The absorption curve of diazomethane is similar in general shape to this curve but shifted somewhat toward shorter wavelengths. The absorption of diazoethane is continuous over its entire region while in the visible region diazomethane shows a number of broad and very diffuse bands from about 4300A to 3200A which probably overlie a continuum. The presence of 2‐butene in the photolysis products of diazoethane is explained by the combination of ethylidine radicals formed by the primary photochemical process CH3CHN2+hν→CH3CH+N2.

The Vapor‐Phase Photo Decomposition of Hydrogen Peroxide

The decomposition of hydrogen peroxide vapor initiated by light of wave‐length 2537A was studied. The quantum yield of the reaction was found to be 1.7±0.4, essentially independent of pressure of peroxide and intensity of absorbed light. The reaction products were water and oxygen only. The rate of the reaction was found to be independent of temperature and was not affected by the addition of oxygen, nitrogen or water to the reaction mixture. The reaction scheme proposed is H2O2+hν→2 OHOH+H2O2→H2O+HO2HO2+HO2→H2O2+O2 this leads to a maximum quantum yield of 2.

Reactions of Free Radicals with Aldehydes. The Reactions of Methyl and t‐Butoxy Radicals with Acetaldehyde and Acrolein

The reaction in the gas phase of radicals derived from the thermal decomposition of di‐t‐butyl peroxide with acetaldehyde and acrolein has been studied. The activation energy for H atom abstraction from acetaldehyde by methyl was found to be 7.5±0.3 kcal/mole. The steric factor was about 3.7×10−3, in agreement with many similar reactions. Evidence for chain ending steps other than the recombination of methyl radicals is given. Acrolein polymerizes as well as decomposes in the presence of radicals. The polymerization follows the kinetics to be expected from chain propagation by radical‐monomer interaction and chain termination by radical combination. Evidence that the decomposition may be the splitting off of carbon monoxide from the polymerizing chain is presented.

The Mercury Photo‐Sensitized Reaction Between Hydrogen and Oxygen

The mercury sensitized reaction between hydrogen and oxygen in a flowing system was studied. The quantum yield of the reaction was found to be less than unity, and no evidence of a chain reaction at 40° was found. Hydrogen peroxide was the principal product, and yields of peroxide above 90 percent were found for many of the experiments. It was shown that in the gas phase reaction between ozone and hydrogen peroxide, water was rapidly formed. The yield of product was found to increase with increasing oxygen flow rate, with increasing total flow rate for a fixed hydrogen to oxygen ratio, and to increase and then decrease with increasing hydrogen to oxygen ratio. The reaction scheme proposed is Hg+hν→Hg*,Hg*+H2→H+H+Hg,Hg*+O2→O2+Hg,H+O2→HO2,H+M→HM,HM+HM→H2+M,HO2+HO2→H2O2+O2. This leads to the rate equation, d(H2O2)dt=k3/k4′′(O2)k3/k4′′(O2)+1Iak1/k2(H2)k1/k2(H2)+(O2), using the flow rates of hydrogen and oxygen. k1/k2 is the ratio of quenching efficiencies of hydrogen and oxygen for activated mercury or 1.62. ...

Photochemical Evidence Relative to the Excited States of Oxygen

Recent results on the formation of ozone from oxygen photosensitized by mercury vapor at 2537 A show that the effects of foreign gases on deactivating the excited state of oxygen formed by energy transfer are He, A, N2, and CO2 in order of increasing efficiency. However the efficiencies of A and N2 are about the same. Consideration of the process of deactivation by collision indicate that the 3Σu+ state of O2 is not the excited state involved, and that the excited state is vibrationally excited O2 in the ground state, 3Σg−. Analogous arguments applied to the unsensitized reactions at 1849 A, where the effects of the added gases are in opposite order, show that after the absorption of light predissociation occurs from the 3Σu− state of O2.

Photochemical Oxygen‐Hydrogen Reaction at 1849 A

The photochemical reaction between oxygen and hydrogen was studied at atmospheric pressure in a flowing system using the 1849.6 A mercury resonance line as a light source. The yields of hydrogen peroxide, water and ozone were determined. The results obtained in this work above the convergence limit of the oxygen molecule absorption spectrum are in agreement with those obtained in other studies below the convergence limit. The predissociation of oxygen molecules to form oxygen atoms as the primary process following light absorption is shown to be the only reaction of optically excited oxygen. The mechanisms used to explain the formation of products, ozone, hydrogen peroxide and water, are compatible with well‐known reactions in various oxygen‐hydrogen systems.