The visible excitation spectra of benzyl,benzyl-d7 and p-methylbenzyl in rigid solution at 77 K

The full low resolution fluorescence-excitation spectra of benzyl, benzyl-d₇ and p-methylbenzyl in rigid amorphous solution at 77 K are reported, together with the polarization spectrum of the visible excitation band of p-methylbenzyl.The three excitation spectra are roughly similar in their low energy region appearing like weakly allowed spectra having stronger “forbidden” components. The three spectra more or less agree with the mirror image of the benzyl and benzyl-d₇ fluorescences. The emission of p-methylbenzyl is strikingly different, showing a considerable relative strengthening of the allowed portion over the “forbidden” component and, with that, an apparent strong break with the absorption fluorescence mirror image rule.One interpretation anticipates an important effect of geometry change in pMB, due to electronic excitation, upon both the allowed and “forbidden” components of this well known weak transition. Alternatively, it may be that the elusive second excited state (²B₂) appears in the pMB excitation spectrum in such a coincident manner that the mirror image rule holds even while conferring upon the excitation spectrum a structure very much like the absorption and emissions of the two benzyl radicals. This second state, in any case, is supposed to lie very near tthe first excited (²A₂^?) state. The polarization results in pMB are consistent with either interpretation. Since theoretical predictions indicate that the ratio of ²B₂^? to ²A₂^? intensity should be much larger in p-methylbenzyl than in benzyl, it may well be that the ²B₂^? band is an important part of the excitation spectrum in the latter case. The necessary coincidence of energy and intensity required of the ²B₂^? transition with what should characterize the nontotally symmetric vibronic activity in the ²A₂^? transition is a complicating factor which these low resolution studies are not able to overcome in trying to resolve this question.     

Laser-induced fluorescence, dispersed fluorescence and lifetime measurements of jet-cooled chloro-substituted benzyl radicals

We measured the laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of jet-cooled -, o- and m-chlorobenzyl radicals after they were generated by the 193 nm photolysis of the corresponding parent molecules. The vibronically resolved spectra were obtained to analyze their D₁–D₀ transitions. The fluorescence lifetimes of -, o-, m- and p-chlorobenzyls in the zeroth vibrational levels of the D₁ states were measured to estimate the oscillator strengths of a series of benzyl derivatives. It was found that the -substitution is inefficient to break the ‘accidental forbiddenness' of the D₁–D₀ transition of benzyl, while the ring-substitution enhances the oscillator strength by 50%.     

Laser-induced fluorescence of benzyl radicals in the gas phase

The excitation spectrum of the laser-induced fluorescence of benzyl has been observed in the gas phase. Fluorescence lifetimes of 880 ± 10 ns at zero pressure were obtained for the s, t and 6a¹₀ bands of the (1 ²A₂—1 ²B₂, 2 ²B₂—1 ²B₂) transitions of benzyl-h₇. The fluorescence lifetime of the t band in the corresponding transition of benzyl-d₇ was 1340 ns.     

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.     

Rate constants for the termination of benzyl radicals in solution

The rate constant for the bimolecular combination of benzyl radicals in cyclohexane and toluene is determined as a function of temperature. Further, it is studied in cyclohexane–toluene mixtures of different compositions. In the entire range covered, 9.8 × 10⁸ ? 2k_t ? 9.0 × 10⁹M^?1·sec^?1, the data are very well described by the Smoluchowski equation for a diffusion-controlled reaction to ground-state products using a spin statistical factor of 1/4, a temperature- and solvent-independent reaction distance, and the known diffusion coefficient of toluene.     

Fluorescence lifetimes of benzyl and deuterium-substituted benzyl radicals

Fluorescence lifetimes of C₆H₅CH₂, o-, m-, p-DC₆H₅CH₂, C₆H₅CD₂, C₆D₅CH₂, and C₆D₅CD₂ have been measured at 77 K in rigid glasses. Very long decay times are observed for all the radicals. There is a dramatic deuterium effect upon ring deuteration; alkyl deuteration lengthens the lifetime if the phenyl ring is already deuterated, but otherwise exhibits a slight inverse deuterium effect. The oscillator strength of the ²A₂ ? ²B₂ transition is calculated to be no larger than 8 × 10^?4.     

High-temperature thermal decomposition of benzyl radicals

The thermal decomposition of the benzyl radical was studied in shock tube experiments using ultraviolet laser absorption at 266 nm for detection of benzyl. Test gas mixtures of 50 and 100 ppm of benzyl iodide dilute in argon were heated in reflected shock waves to temperatures ranging from 1430 to 1730 K at total pressures around 1.5 bar. The temporal behavior of the 266 nm absorption allowed for determination of the benzyl absorption cross-section at 266 nm and the rate coefficient for benzyl decomposition, C6H5CH2 --> C7H6 + H. The rate coefficient for benzyl decomposition at 1.5 bar can be described using a two-parameter Arrhenius expression by k1(T) = 8.20 x 10(14) exp(-40 600 K/T) [s(-1)], and the benzyl absorption cross-section at 266 nm was determined to be sigma(benzyl) = 1.9 x 10(-17) cm2 molecule(-1) with no discernible temperature dependence over the temperature range of the experiments.     

Substituent effects on the lifetime and fluorescence of excited diphenylmethyl radicals in solution

The room-temperature lifetimes of ring-substituted excited diphenylmethyl radicals have been shown to correlate well with the energy gap between the fluorescent lowest excited doublet and the ground state. Attempts to correlate the lifetimes with Hammett constants were unsuccessful.     

Rearrangement with 1,2-migration of chlorine in reaction of p-methylbenzyl radicals with 3,3,3-trichloropropene

Conclusions The reaction of p-xylene with 3,3,3-trichloropropene, initiated by peroxides, leads to the formation of 1,1-dichloro-4-(p-tolyl)-1-butene, 1,1,2-trichloro-4-(p-tolyl)butane, and 1,1-dichloro-2-chloromethyl-3-(p-tolyl)propane, which testifies to a rearrangement of the CCl₃HCH₂CH₂C₆H₄CH₃ radicals to the C1₂CHCI-CH₂ CH₂C₆H₄CH₃ radicals with the 1,2-migration of chlorine.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2122–2124, September, 1976.     

Stationary nutations and CIDEP of benzyl radicals during photolysis of methyl benzyl ketone

Transient benzyl radicals are generated by photolysis of methyl benzyl ketone in toluene solution. Modulation of the rate of radical initiation at 86 kHz causes stationary nutations of the magnetization. They are detected by ESR spectroscopy and used to evaluate the relaxation times as well as the chemically induced dynamic electron polarization of the spin system.     

Electronic and geometric structure of fluorine-substituted benzyl radicals

According to the data from quantum-chemical calculations and theoretical analysis, the effect of fluorine atoms on the electronic and stereochemical structure of the benzyl radical depends substantially on their position. Fluorine atoms in the ring hinder the out-of-plane distortions of the CH₂ group, and substituted C₆F_nH_{5_n}-CH₂ radicals are accordingly planar radicals. On the other hand, the presence of -fluorine atoms assists pyramidal deformation of the methylene fragment, leading in the case of the ,-difluoro-substituted compounds to the formation of nonplanar pseudo-radical structures. Here the fluorine atoms in the ring at the p and o positions should increase somewhat while those at the m positions should reduce the capacity of the CF₂ group for deformation.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1069–1074, May, 1990.     

Two-photon laser-induced fluorescence studies of HS radicals,DS radicals,and I atoms

A two-photon laser-induced excitation and fluorescence technique has been used to study the A ²Σ⁺-X ²Π transition of HS and DS radicals and various high-lying ⁴P^o, ²D^o, and ⁴D^o states of the I atom. The two-photon excitation cross sections and detection sensitivity are discussed.     

Calculations on the electronic spectra of anilino,phenoxyl and benzyl radicals

The electronic spectra of benzyl, anilino and phenoxyl have been calculated, using two well known SCF-MO methods. Good agreement is found with experiment in all instances. However, the calculations still cannot explain the red shift produced by the addition of an extra electron to benzyl.

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Zesummenfassung Die Elektronenspektren des Benzyl-, Anilino- und Phenoxylradikals sind mit Hilfe zweier wohlbekannter SCF-MO-Methoden berechnet worden. Die Übereinstimmung mit den Experimenten ist überall gut. Die Rechnungen können jedoch nicht die Rotverschiebung erklären, die beim Hinzufügen eines Elektrons zum Benzylradikal erzeugt wird.

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Resume On calcule les spectres électroniques des radicaux benzyle, anilino et phénoxyle par deux méthodes SCF-MO bien connues. L'accord avec l'expérience est bon partout. Cependant, ces calculs n'expliquent pas l'effet bathocrome de l'addition d'un électron au benzyle.

     

Generation of chemisorbed benzyl radicals on silica nanoparticles

Functionalized silica nanoparticles (NP) were obtained by esterification of the silanol groups of fumed silica nanoparticles with benzyl alcohol. These particles were characterized by Fourier transform infrared spectroscopy, ¹³C and ²⁹Si NMR spectroscopy, thermogravimetry, total organic carbon, Brunauer–Emmett–Teller analysis, UV-visible spectroscopy, and transmission electron microscopy. NP suspensions in water/acetonitrile mixtures were used as quenchers of benzophenone (BP) phosphorescence in time-resolved experiments at the excitation wavelength of 266 nm. The phosphorescence signals obtained in the presence of the nanoparticles were fitted to biexponential decays. Both decays were accelerated in the presence of increasing amounts of NP. A model, including the reversible adsorption of BP on the NP, which was supported by computer simulations accounts for the observed results. Laser flash-photolysis experiments with excitation at 266 nm of NP suspensions in water/acetonitrile in the presence of BP generated benzyl radicals that were attached to the silica surface. These radicals were detected at their absorption maxima (320 nm) by transient optical techniques.     

Relative rates of nucleophilic reactions of benzyl carbocation formed in photolysis of benzyl chloride and benzyl acetate in aqueous solution

Benzyl chloride and benzyl acetate were photolyzed in 30% methanol–water mixtures (V/V) at 0°C. The photolysis produces benzyl carbocations that react with nucleophiles. The reaction products were analyzed by gas chromatography or liquid chromatography. From the amounts of products the relative values of rate constants of reactions of benzyl carbocation with nucleophiles N and water k(N)/k(H₂O) were calculated. Benzyl carbocation reacts with I^?, Br^?, Cl^?, and Ac^? ions with approximately diffusion-controlled rate. A value of 2.4 × 10⁷ dm³ mol^?1 s^?1 for the rate constant k(H₂O) and a lifetime of 0.7 ns were estimated for benzyl carbocation in the aqueous solution.     

Radiolysis of benzyl alcohol in aqueous solution

Radiolysis of 0.05% aqueous solution of benzyl alcohol with 50Co gamma-rays ranging from 1 X 10(4) to 7 X 10(5) rad was investigated, in order to presume the change of it contained in radiopharmaceuticals. For both O2 free and oxygenated solutions, an approximately linear relationship holds between the retaining benzyl alcohol and dose in the range from 1 X 10(5) to 7 X 10(5) rads. The G(-M) values of benzyl alcohol calculated from the relation were 2.34 in the absence and 1.92 in presence of oxygen. In the presence of oxygen, a main product was benzaldehyde and its G value was 0.87. In the absence of oxygen, the main products of the radiolysis were dibenzyl, benzyl phenylcalbinol and hydrobenzoin, which were regarded as the radicao-reaction products of PhCH2 and PhCHOH, and the yield of benzaldehyde was negligible. Irrespective of the presence of oxygen, o- and p-hhdroxylated products of benzyl alcohol were found only in small quantity.     

Optical emission spectroscopy of benzyl radicals produced in a radio-frequency plasma

The benzyl radical was studied by optical emission spectroscopy in gas phase. This radical was produced in a radio-frequency (RF, 13.56 MHz) discharge, using benzyl alcohol (?CH(2)OH) as a precursor. The fluorescence from the first excited electronic state 1(2)A(2) to ground state 1(2)B(2) (450 nm) was studied as a function of several external parameters (pressure, RF power, electrodes and mixtures of the inert gases Ar, Ne, He, N(2), with the precursor). We also used a DC discharge to produce this radical but, in this case, the decomposition was fast. We observed changes in the electronic transitions of this radical, and found the best conditions to study it by optogalvanic spectroscopy.