Kinetics laser reactions

In the 1988-1999 period, almost all absolute kinetic studies of carbenic reactions employed LFP with UV detection. Carbenes that contain a UV chromophore (e.g., PhCCl) are easily observed, and their decay kinetics during reaction can be readily followed by LFP.11 However, alkyl, alkylhalo, and alkylacyloxycarbenes are generally transparent in the most useful UV region. To follow their kinetics, Jackson et al. made use of the ylide method, 12 in which the laser-generated carbene (2) is competitively captured by (e.g.) pyridine, forming a chromophoric ylide (3, cf. Scheme 1). The observed pseudo first order rate constants (kobs) for the growth of ylide 3 at various concentrations of pyridine are monitored by UV spectroscopy, and obey Eq. 1. 

The kinetics of reactions involving the tributylstannyl radical have been refined by laser flash time-resolved UV spectroscopy. The measured extinction coefficient of the BujSn- radical in benzene was 1620 40 M 1cm 1 at 400 nm, the rate constant of the reaction of the /-butoxyl radical with Bu3SnH was (3.5 0.3) x 108M 1 s 1, and the rate constant for the self-reaction of the Bu3Sn radical was (3.6 0.3) x 109 M s1. S29... 

The kinetic data reported in this chapter have been determined either by direct measurements, using for example kinetic EPR spectroscopy and laser flash photolysis techniques or by competitive kinetics like the radical clock methodology (see below). The method for each given rate constant will be indicated as well as the solvent used. An extensive compilation of the kinetics of reaction of Group 14 hydrides (RsSiH, RsGeH and RsSnH) with radicals is available [1]. 

There are a number of non-electrochemical techniques that have proven invaluable in combination with electrochemical results in understanding the chemistry and the kinetics. Laser flash photolysis (LFP) is a well-established technique for the study of the transient spectroscopy and kinetics of reactive intermediates. The technique is valuable for the studying of the kinetics of the reactions of radical anions, particularly those that undergo rapid stepwise dissociative processes. The kinetics of fragmentation of radical anions can be determined using this method if (i) the radical anion of interest can be formed in a process initiated by a laser pulse, (ii) it has a characteristic absorption spectrum with a suitable extinction coefficient, and (iii) the rate of decay of the absorption of the radical anion falls within the kinetic window of the LFP technique typically this is in the order of 1 x 10" s to 1 X 10 s . 

It is believed that the electrochemical reductive of aliphatic halides [58], benzyl halides and aryldialkylsulfonium salts [89] are concerted, i.e., electron acceptance is concomitant with bond cleavage, due in part to the a nature of the LUMO as well as the instability of the anion-radical species and stability of the products. If the anion-radical is not a discrete chemical entity back ET cannot take place. Therefore, the efficiency of PET bond cleavage reactions would be expected to be greater for the reasons mentioned above. However, due to the localized nature of the a molecular orbitals the probability for intermolecular and intramolecular ET, for example, to a a MO may be quite low. However, the overall efficiency of PET concerted bond cleavage reactions may approach unity provided that ET to the This topic clearly requires further consideration and research using fast kinetic laser spectrophotometric techniques to go beyond the qualitative discussion provided here. 

Generally, ROH or B is an indicator, so that RO-HB+ absorbs strongly in the visible region of the spectrum, and its concentration may be determined spectrophotometrically. The kinetics of reactions of this type have been observed by the temperature-jump method with microwave or laser heating. 

Kinetics of reactions of Fe(CO), (x = 2, 3, 4) with CO. 547 Co-ordinatively unsaturated intn carbonyls generated by exclmer laser flash photolysis of FelCO) and monitored using CW CO laser resonance absorption Competition between photofragmentation and photo- 548 ionization of jet-cooled FefCO) used to obtain excited-state lifetimes in the wavelength range 290—310 nm. 

The methodology for studying M-Ng complexes in the gas phase is essentially the same as the TRIR method for liquified noble gases a pump pulse photolyzes a metal carbonyl ion and the fragment is detected with the aid of a continuous IR laser. In these experiments helium is utilized as the standard buffer gas. A xenon complex may be detected by alteration in the spectrum and kinetics on addition of xenon. Since the spectra are free of solvent effects, the effect of coordination should be more easily discerned than in the liquid phase. This method has been used to study M(CO)sXe (M = Cr, Mo, W) and W(CO)sKr. Metal-xenon bond energies of ca. 35 kJmol are deduced from the kinetics of reaction with CO. The variation between metals in comparable to the error bars, about 4 kJ mol . The W-Kr bond energy is estimated to be less than 25 kJmol . ... 

Laser flash photolysis study of the UV spectrum and kinetics of reactions of HOCH2CH2O2 radicals... 

In the segment on bimolecular reactions, the concepts of kinetics and reaction dynamics are developed further in particular, the ideas of three-dimensional (3D) collision dynamics and technologies (e.g. molecular beam techniques) and the idea of state-to-state reactivity are outlined (Chapters 20 and 21). The preparation of reagents and the probing of reaction products by laser techniques are extensively discussed in Chapters 22 and 23. 

In most experiments, ultraviolet or infrared absorption, resonance fluorescence, or laser-induced fluorescence (LIF) is used to follow how transient concentrations change after the photolysis pulse. These optical techniques vary considerably in their sensitivity and hence to the extent to which they isolate the primary reaction. LIF is extremely sensitive, enabling one to follow decays of concentrations from an initial value of 10 ° cm , but its use is restricted to species with a bound-bound electronic transition within the range of tunable dye lasers. LIF has been used to follow the kinetics of reactions of, inter alia, the radicals OH [12-14], CN [15] and CH3O [16,17]. It is more difficult to apply to radical atoms vihich usually have allowed electronic transitions only in the vacuum ultraviolet. Some LIF measurements utilising two-photon excitation of atoms have been reported [18]. 

Bencsura et al. [12] in 1992 produced the most recent experimental data on the reaction kinetics of the propyl radical, based on previous work. Radicals were produced by pulsed laser photolysis and their unimolecular decay subsequently studied by photoionization mass spectrometry. Tsang [13] in 1988 produced a compilation of revised and evaluated data on the kinetics of reactions involving propane and the propyl radical, among other species. The general mechanism for the decomposition of propane was initially determined by Papic and Laidler [14, 15] who experimentally identified most of the products which are consistently predicted in the mechanism proposed in this paper. We followed their rationale for the development of our own model. 

Recently, the photochemistry of azide 42 was studied by laser flash photolysis (Aex = 266nm) techniques in Fieon-113 (CF2CICFCI2) at room temperature. The formation of at least two intermediates, viz., triplet nitrene M3 (Aa,ax = 4(X)nm, lifetime 1.5 fis) and ethoxycarbonyl radical 44 (2 = 333 nm, lifetime 0.4/is), was observed (Scheme 11.22). The singlet nitrene M3 was deduced to have a lifetime between 2 and 10 ns in Freon-113 at ambient temperature. The kinetics of reactions of M3 with tetramethyleth-ylene and triethylsilane were also measured. ... 

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

Another variation is the mode-locked dye laser, often referred to as an ultrafast laser. Such lasers offer pulses having durations as short as a few hundred femtoseconds (10 s). These have been used to study the dynamics of chemical reactions with very high temporal resolution (see Kinetic LffiASURELffiNTS). 

Because of the tunabiUty, dye lasers have been widely used in both chemical and biological appHcations. The wavelength of the dye laser can be tuned to the resonant wavelength of an atomic or molecular system and can be used to study molecular stmcture as well as the kinetics of a chemical reaction. If tunabiHty is not required, a dye laser is not the preferred instmment, however, because a dye laser requires pumping with another laser and a loss of overall system efficiency results. 

The availability of lasers having pulse durations in the picosecond or femtosecond range offers many possibiUties for investigation of chemical kinetics. Spectroscopy can be performed on an extremely short time scale, and transient events can be monitored. For example, the growth and decay of intermediate products in a fast chemical reaction can be followed (see Kinetic measurements). 

The examples given above represent only a few of the many demonstrated photochemical appHcations of lasers. To summarize the situation regarding laser photochemistry as of the early 1990s, it is an extremely versatile tool for research and diagnosis, providing information about reaction kinetics and the dynamics of chemical reactions. It remains difficult, however, to identify specific processes of practical economic importance in which lasers have been appHed in chemical processing. The widespread use of laser technology for chemical synthesis and the selective control of chemical reactions remains to be realized in the future. 

The rates of these reactions bodr in the gas phase and on the condensed phase are usually increased as the temperature of die process is increased, but a substantially greater effect on the rate cati often be achieved when the reactants are adsorbed on die surface of a solid, or if intense beams of radiation of suitable wavelength and particles, such as electrons and gaseous ions with sufficient kinetic energies, can be used to bring about molecular decomposition. It follows drat the development of lasers and plasmas has considerably increased die scope and utility of drese thermochemical processes. These topics will be considered in the later chapters. 

There is a great deal of flexibility in the choice of laser radiation in the production of thin Aims by photochemical decomposition, and many routes for achieving the same objective can be explored. In most reactions of indusuial interest the reaction path is via tire formation of free radicals as intermediates, and the complete details of the reaction patlrs are not adequately defined. However, it may be anticipated that the success of the photochemical production of new materials in tlrin fllms and in fine powder form will lead to considerably greater effort in the elucidation of these kinetics. 

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