The fluorescence lifetimes of the methyl-substituted benzyl,triphenylmethyl and diphenylmethyl radicals

Time resolved fluorescence of some methyl-substituted benzyl, triphenylmethyl, and diphenylmethyl radicals trapped in rigid solvents at low temperature has been observed. These radicals were excited by pulsed N₂ laser at 337 nm. It is found that these radicals exhibit very long fluorescence lifetime. The long lifetimes of these radicals seem to indicate that the first doublet-doublet electronic transitions of the radicals have a forbidden character.     

Study of the rate of methyl loss and the structure of the product ion as a function of the lifetime of the molecular ion of pent-3-en-2-ol

The normalized unimolecular rate constant for loss of a methyl radical from pent-3-en-2-ol molecular ions with lifetimes ranging from 10^?11 to 10^?9 s was studied by field ionization kinetics (FIK). The normalized rate curve shows maxima at molecular ion lifetimes of 10^?10.5 and 10^?10.1 s. Based on results for deuterium and ¹³C-labelled pent-3-en-2-ol, the maximum at 10^?10.5 s is ascribed to loss of the methyl group at the 1-position by a direct cleavage reaction. The maximum at 10^?10.1 s is attributed to a 1,2-H shift-initiated rearrangement of the molecular ion, which leads to loss of the methyl group at the 5- and 1-positions. The time dependence of the processes in the form of the maxima on the normalized rate curve is discussed qualitatively in terms of a lower critical energy and a tighter transition state of the 1,2-H shift than those of the direct cleavage reaction. Collision-induced dissociation of the [C₄H₇O]⁺ product ions in combination with FIK provides evidence that at molecular ion lifetimes corresponding to the first maximum on the rate curve protonated crotonaldehyde is formed, whereas protonated methyl vinyl ketone and the butyryl cation are formed at times corresponding to the second maximum.     

Rate constants for the reactions of alkyl radicals with 1,4-cyclohexadiene

An electron paramagnetic resonance (EPR ) technique was used to show that simple alkyl radicals readily abstract hydrogen from 1,4-cyclohexadiene. Rate constants for the reaction were ca. 10⁴–10⁵ M^?1 s^?1 at 300 K and activation energies 5–7 kcal mol^?1. For the stabilized radicals, allyl and benzyl, the rate constants were <10² M^?1 s^?1 at 300 K. The data suggest that 1,4-cyclohexadiene could be used as an effective trap to probe rearrangement reactions of carbon centered radicals and biradicals.     

Rate constants for the reactions of benzyl and methyl-substituted benzyl radicals with O2 and NO

Rate constants for reactions of benzyl, o-niethylbenzyl and p-meihylbenzyl radicals with O₂ and NO have been measured at room temperature. The radicals were generated by UV flash photolysis and the time decay measured by absorption at ≈ 300 nm. The rate constants are: benzyl (0.99 ± 0.07 and 9.5 ± 1.2), o-methylbenzyl (1.2 ± 0.07 and 8.6 ± 0.8) and p-mithyl-benzyl (1.1= 0.10 and 8.9 = 0.9) for O₂ and NO respectively in units of 10^?12 cm³ molecule^?1 s^?1.     

Rate constants for the gas-phase reactions of the OH radical with a series of monoterpenes at 294 ± 1 K

Using a relative rate technique, rate constants for the gas-phase reactions of the OH radical with a series of monoterpenes have been determined in one atmosphere of air at 294 ± 1 K. Relative to a rate constant for the reaction of OH radicals with 2,3-dimethyl-2-butene of 1.12 × 10^?10 cm³ molecule^?1 sec^?1, the rate constants obtained were (in units of 10^?11 cm³ molecule^?1 sec^?1): α-Pinene, 5.45 ± 0.32; β-pinene, 7.95 ± 0.52; Δ³-carene, 8.70 ± 0.43; d-limonene, 16.9 ± 0.5; α-terpinene, 36.0 ± 4.0; γ-terpinene, 17.6 ± 1.8; α-phellandrene, 31.0 ± 7.1; myrcene, 21.3 ± 1.6; and ocimene (acis-, trans-mixture), 25.0 ± 1.9. These are the first quantitative kinetic data reported for many of these monoterpenes. The rate constants obtained are compared with the available literature data and with a priori estimates based on the number and configuration of substituents around the double bond(s). The tropospheric lifetimes of these monoterpenes with OH radicals, NO₃ radicals and O₃ are estimated and compared. Atmospheric lifetimes with respect to reaction with the OH radical are calculated to range from ～0.75 hr for α-terpinene to ～5 hr for α-pinene.     

Stable All‐Organic Radicals with Ambipolar Charge Transport

A series of neutral long‐lived purely organic radicals based on the stable [4‐(N‐carbazolyl)‐2,6‐dichlorophenyl]bis(2,4,6‐trichlorophenyl)methyl radical adduct (Cbz‐TTM) is reported herein. All compounds exhibit ambipolar charge‐transport properties under ambient conditions owing to their radical character. High electron and hole mobilities up to 10^?2 and 10^?3 cm² V^?1 s^?1, respectively, were achieved. Xerographic single‐layered photoreceptors were fabricated from the radicals studied herein, exhibiting good xerographic photosensitivity across the visible spectrum.     

Kinetics and rate constants for the reaction of tri-n-butylgermanium hydride with methyl iodide and carbon tetrachloride. The combination of methyl and trichloromethyl radicals in solution.

The kinetics of the photoinitiated reductions of methyl iodide and carbon tetrachloride by tri-n-butylgermanium hydride in cyclohexane at 25°C have been studied and absolute rate constants have been measured. Rate constants for the combination of CH₃? and CCl₃? radicals are equal within experimental error and are also equal to the values found for the self-reactions of most non-polymeric radicals in low viscosity solvents, i.e. ～1–3 × 10⁹ M^?1 sec^?1. Rate constants for hydrogen atom abstraction by CH₃? and CCl₃? radicals are both ～1?2 × 10⁵ M^?1 sec^?1. Tri-n-butyltin hydride is about 10–20 times as good a hydrogen donor to alkyl radicals as is tri-n-butylgermanium hydride. The strength of the germanium–hydrogen bond, D(n-Bu₃Ge–H) is estimated to be approximately 84 kcal/mole.     

The acetyl radical studied by flash photolysis and kinetic spectroscopy

The acetyl radical absorption spectrum is a broad band with maximum decadic extinction coefficient of (1.0 ± 0.1) × 10⁴ ? mole^?1 cm^?1 at 215 nm and an oscillator strength of 0.23 ± 0.03. Absolute rate constants were estimated as 4.5 × 10¹⁰ ? mole^?1 s^?1 for the mutual interaction of acetyl radicals, and as 7.5 × 10¹⁰ ? mole^?1 s^?1 for the cross interaction of acetyl and methyl radicals.     

Electron transfer from α-aminoalkyl radicals to methyl viologen

The reactions of tert-butoxyl radicals with amines, leading to the formation of α-aminoalkyl radicals, and the reactions of these with the electron acceptor methyl viologen have been examined using laser flash photolysis techniques. For example, the radicals CH₃?HNEt₂ and HOCH₂?H N(CH₂CH₂OH)₂ react with methyl viologen with rate constants equal to (1.3 ± 0.1) × 10⁹ and (2.1 ± 0.4) × 10⁹M^?1 · s^?1, respectively, in wet acetonitrile at 300 K.     

Reaction of methyl radicals with atomic oxygen

The reaction of hydrogen atoms with diazomethane was used as a source of methyl radicals to study the reaction of oxygen atoms with methyl radicals. This investigation verifies directly the earlier results that formaldehyde is a major product and that the rate constant is greater than 3×10^?11cm³ molecule^?1 sec^?1.     

A relative rate study of the reaction of chlorine atoms (Cl) and hydroxyl radicals (OH) with a series of ethers

Rate constants for the gas phase reactions of hydroxyl radicals and chlorine atoms with a number of ethers have been determined at 300 ± 3 K and at a total pressure of 1 atmosphere. Both OH radical and chlorine atom rate constants were determined using a relative rate technique. Values for the rate constants obtained are as follows.

Room temperature rate constants for the reaction of OH with selected chlorinated and oxygenated hydrocarbons

A study was conducted to measure the hydroxyl radical rate constants using a relative rate procedure in which the photolysis of methyl nitrite was the source of OH. During the course of this study, the OH rate constant was measured for a number of chlorinated solvents for which measurements have not previously been reported or for which there are few reliable measurements. Room temperature OH rate constants are presented for six chlorinated hydrocarbons (allyl chloride, benzyl chloride, chlorobenzene, epichlorohydrin, trichloroethylene, and vinylidene chloride) and four oxygenated hydrocarbons (acrolein, methacrolein, methyl ethyl ketone, and propylene oxide). Also included are OH rate constants for alkanes (ethane, propane, isobutane, and cyclohexane), alkenes (trans?2-butene and isoprene), and aromatic hydrocarbons (benzene, toluene, o^?, m^?, and p-xylene). Rate constants for compounds not previously reported include vinylidene chloride (1.49 ± 0.21 × 10^?11 cm³ molecule^?1 s^?1) and benzyl chloride (2.96 ± 0.15 × 10^?12 cm³ molecule^?1 s^?1). The analysis for chlorinated hydrocarbons included a correction for possible chlorine atom reactions.     

Kinetics and mechanism of the reaction of propylene oxide with chlorine atoms and hydroxy radicals

The kinetics and mechanism of gas‐phase propylene oxide (PPO) reactions were studied in a 142‐L reaction chamber by long‐path Fourier transform infrared spectroscopy at atmospheric pressure and 298 K. Rate coefficients for the reaction of PPO with ozone (O₃), chlorine atoms (Cl), and hydroxyl radicals (OH) were measured using the relative rate technique. Product yields of acetic acid, acetic formic anhydride, formic acid, and carbon monoxide were determined for the following reactions: PPO with Cl both in the presence and absence of NO, PPO with OH and NO, methyl acetate with Cl both in the presence and absence of NO, and ethyl formate with Cl both in the presence and absence of NO. The measured rate coefficients for PPO with O₃, Cl, and OH are <3.5 × 10^?21 cm³ molecule^?1 s^?1, (3.0 ± 0.7) × 10^?11 cm³ molecule^?1 s^?1, and (3.0 ± 1.0) × 10^?13 cm³ molecule^?1 s^?1, respectively. The carbon balance for the products measured ranged from 10% (for OH + PPO) to 100% (for Cl + methyl acetate in the absence of NO). The mechanistic and atmospheric implications of these measurements are discussed. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 507–521, 2011     

Modeling of swelling by the fast transient fluorescence technique in a polymeric gel

The time‐resolved, fast transient fluorescence technique, which uses a strobe master system, was employed for studying the swelling of a disc‐shaped poly(methyl methacrylate) gel. The disc‐shaped gel was prepared by free‐radical copolymerization of methyl methacrylate and ethylene glycol dimethacrylate. Pyrene (Py) was introduced as a fluorescence probe during polymerization. After this gel was dried, swelling and slow‐release experiments were performed in chloroform at room temperature. Lifetimes of Py were measured during in situ swelling and slow‐release experiments. An equation was derived for low‐quenching efficiencies to interpret the behavior of lifetimes in and out of the gel during swelling. Py lifetimes in the gel decreased as swelling proceeded, but Py lifetimes out of the gel stayed constant during the slow‐release experiments. The Li–Tanaka equation was used to determine the cooperative and mutual diffusion coefficients, which were around 10⁻⁵ and 10⁻⁷ cm²s⁻¹, respectively. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 739–746, 2000     

Double collision experiments

A variety of double collision experiments, whereby fast species undergo collisional interactions in two distinct regions of a mass spectrometer, are described. These include two-stage charge reversal of negative ions, two-stage double electron transfer from targets to cations, neutralization-reionization experiments as well as delayed analysis of organic cations formed in a one-step charge reversal of anions. Experiments have been performed on a number of systems of current interest in gas-phase ion chemistry. It is concluded that autoelectron detachment of benzyl anions leads to benzyl radicals, whereas the collisionally induced electron detachment produces a mixture of benzyl and tropyl radicals. By contrast, electron detachment from [H₃CNH]^? is not a metastable process and occurs only after excitation to produce H₃CNH˙ radicals, which do not rearrange into the thermodynamically more stable H₂CNH₂˙. It is shown that in the double electron transfer reactions H⁺ + Xe→H˙ + Xe⁺˙ and H˙ + Xe→H^? + Xe⁺˙, excited states are produced. From double collision experiments on methyl formate ions, it is concluded that the non-decomposing ions have undergone rearrangement on the time-scale of 10 μs into the distonic isomer, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm H} - \mathop {\rm C}\limits^ + ({\rm OH}){\rm O}\mathop {\rm C}\limits^. {\rm H}_2 $\end{document}. Finally, it is shown that short-lived (<0.2 μs) [H₂, C, N]⁺ ions generated by charge-reversal of [H₂CN]^? have the [H₂CN]⁺ structure, whereas most of the long-lived (10 μs) ions have the [HCNH]⁺ structure.     

Luminescence and triplet decay in quinoxaline vapors

The effects of the gas pressure on the quinoxaline triplet state kinetics and luminescence yields were studied in the 10^?3 ?10 torr pressure range. The non-exponential character of the fluorescence decay and long fluorescence lifetimes, previously reported, are confirmed. The collision-free lifetime of the “hot” triplet state measured by triplet—triplet absorption kinetics and by phosphorescence collisional induction is of the order of 100 μs, much longer than the long-fluorescence decay times (0.5–70 μs). The results are discussed on the basis of a model involving anharmonic coupling between triplet levels strongly and weakly coupled to the singlet.     

Polymer reactions—Part VII: Thermal pyrolysis of polypropylene

Polypropylene has been pyrolysed in a carrier stream of helium from 388° to 900°C in both the programmed heating and flash pyrolysis modes. The products were on-line identified and quantitatively analysed by an interfaced GC peak identification system. The first order rate constants for pyrolysis are 3·7 × 10^?4 sec^?1 and 4·0 × 10^?4 sec^?1, respectively, for atactic and isotactic polypropylene at 388°C; the corresponding overall activation energies are 56 ± 6 and 51 ± 5 kcal mole^?1. The main products in decreasing yields are 2,4-dimethyl-1-heptene, 2-pentene, propylene, 2 methyl-1-pentene and 2,4,6-trimethyl-1-nonene. Also isolated, but in much smaller quantities, are: ethane, isobutylene, 4,6-dimethyl-2-nonene, 2,4,6-trimethyl-1-heptene, 3-methyl-3,5-hexadiene and methane. Propylene is the product of an unzipping reaction. Most of the other products can be accounted for by a mechanism involving first, random scission of carbon-carbon bonds to produce methyl, primary and secondary alkyl radicals, followed by intramolecular hydrogen transfer processes. Methane and ethane are formed from the methyl radicals. All the products found in high yields are derived from the secondary alkyl radicals.     

Kinetics of the metathetical reaction between carbon tetrachloride and toluene

A metathetical reaction between carbon tetrachloride and toluene to give benzyl chloride and chloroform occurs at temperatures above 200°C (k = 4.8 × 10¹⁰ e^?32,900/RT cc mole^?1sec^?1). The reaction does not involve free radicals, as is shown by the kinetic behavior of the system, by the lack of effect of added free-radical chain inhibitors, and by the absence of the expected chain termination product, hexachloroethane. The reaction is one of a general type between carbon tetrachloride and alkanes or alkylaromatics, but at the temperatures required it is often obscured by dehydrohalogenation of the product to the highly reactive olefin. At high temperatures, benzyl chloride reacts with toluene to give bibenzyl and hydrogen chloride, apparently also by a metathetical reaction. The transition state is postulated to be four-center, in which the carbon-chlorine and carbon–hydrogen bonds are broken and reformed: The experimental preexponential factor is in good agreement with that calculated from transition-state theory.     

Rate constants for the bimolecular self-reaction of cyano-substituted alkyl radicals in solution

Cyano-substituted methyl radicals (cyanomethyl–hand 2-cyano-2-propyl radicals) and syn-and anti-1-cyano-allyl radicals were generated, and their recombination kinetics in solution were investigated between ?50 and +50°C by time-resolved electron-spin-resonance spectroscopy. The comparison of the activation energies for recombination with the activation energies of the solution viscosities proves that the dimerizations of the radicals are diffusion controlled with rate constants on the order of 10⁸–10⁹M^?1·s^?1. In the case of cyanomethyl radicals an additional pseudo-first-order process, hydrogen abstraction, was detected and analyzed kinetically. Product analyses support the kinetic measurements.     

Thermodynamic and kinetic characteristics of intermediates of electrode reactions. Comparative laser photoemission study of the kinetics of electron transfer for certain alkylaryl and alkylhalide radicals

Using the laser photoemission technique, a comparative study of thermodynamic and energy characteristics of electron transfer (ET) is carried out for a number of alkylaryl intermediates (IM) (benzyl, benzhydryl radicals) and alkyl halides (chloromethyl and dichloromethyl radicals). It is found that the standard free energies of activation of radicals under study are 0.34–0.38 eV and the preexpoential factors are between 10⁹ and 10¹⁰s^?1 and weakly depend on the solution nature. The reorganization energies of the medium for these IM are estimated in terms of the classical Marcus theory. The results are compared with the literature data on the dissociative ET in monohalogenomethanes CH₃Hal/CH₃Hal^?·, polychloromethanes CH_nHal_{4 ? n} /CH_nHal _4?n ^?· , and some other systems. Possible reasons for the different probabilities of observing reversible ET for IM under study are discussed.