Pressure effects flash photolysis

The effect of wavelength upon the relative proportion of the products is very marked. When the 2537 A. line from a low-pressure mercury arc is used, the yield of all three ethanes is drastically reduced, while the yield of both halogenated methanes is enhanced. The 1,3-dichlorohexafluoropropane can no longer be detected. This effect is unlikely to be due to alternative primary processes, such as mercury photosensitization, since it is also observed under flash photolysis conditions. As before, it is considered that the additional energy given to the radicals at shorter wavelengths, increases the rate of abstraction and decreases the rate of combination because of the third-body restriction. Some evidence for this... 

Unfortunately, preparative experiments of Iwaoka and Kondo (35) are of no direct relation to mechanistic investigations the use of a low pressure mercury lamp provides no selectivity as far as excitation of substrate or products is concerned. However, the fact that photolysis in strong acid solution decreased the bleaching rate would indicate the absence of an anchimeric effect and the results of their investigations by flash photolysis are in agreement with the electron (Equation 2) and energy transfer (Equation 4) reactions upon direct excitation. 

Pulse Radiolysis A technique related to flash photolysis pulse radiolysis uses very short (nanosecond) intense pulses of ionizing radiation to generate transient high concentrations of reactive species. See Salmon, G. A. and Sykes, A.G., Pulse radiolysis, Methods Enzymol. 227, 522-534, 1993 Maleknia, S.D., Kieselar, J.G., and Downard, K.M., Hydroxyl radical probe of the surface of lysozyme by synchrotron radiolysis and mass spectrometry. Rapid Commun. Mass Spectrom. 16, 53-61, 2002 Nakuna, B.N., Sun, G., and Anderson, V.E., Hydroxyl radical oxidation of cytochrome c by aerobic radiolysis, Free Radic. Biol. Med. 37, 1203-1213, 2004 BataiUe, C., Baldacchino, G., Cosson, R.P. et al., Effect of pressure on pulse radiolysis reduction of proteins, Biochim. Biophys. Acta 1724, 432-439, 2005. 

Lesclaux etal. [86] used flash-photolysis u.v. absorption to follow reactions (47) and (48) between 373 and 573 K. In particular, the presence of polar gases did not affect the derived rate constants and no pressure effect was apparent in contrast to behaviour observed for the HO2 -H HO2 reaction at these temperatures. They reported (/c 47a + 4 b) = 1-3 x 10 exp(365/T) cm molecule" s with = 45 exp(-1465/T), so that /c47a dominates above about 500 K. They also reported /c48 = 4.3 X 10 exp(780/T)... 

A paper by Suppan draws attention to electrostatic interaction effects on condensed phase photoinduced electron transfer and the need to take account of the fact that solvent is not in reality a uniform dielectric material. Pressure effects on exciplex formation has been exemplified in the pyrene-p-cyanobenzene system. Ternary electron donor acceptor complexes are formed and in the case of anthracene-tetracyanoethylene gives rise to (DO ) dimer radical cations. Laser flash photolysis shows that perylene in acetonitrile undergoes three distinct electron transfer processes, (i) gives pt + MeCNT, (ii) gives... 

CT term is lowest in energy followed by a LF term lying about 200 cm above. Orbital schemes are proposed for both compounds.The temperature dependence of the luminescence of the hexanuclear Mo(II) chloride cluster [Mo Cl, ] has been investigated over the rauige 1.4-300 K and analysed in terms of the emission from several Boltzmeuin populated triplet sublevels. The lowest triplet state was identified as t, and is due to the tjg to t y orbital excitation.A flash photolysis study of the quenching of [Mo Cl, ] by [IrCl ] shows it to occur by a one electron transfer process from the cluster medium euid pressure effects have been noted.An investigation of the... 

The photochemistry of ethyl esters of 2-oxo-carboxylic acids has the participation of both singlet and triplet excited states o. Triplet state lifetimes have been measured and the occurrence of Norrish type II splits in these molecules established. Other flash photolysis studies reported deal with the photoinduced tautomerism of 2-hydroxyphenazine °, 3-methylisoxazolo[5,4-b]pyridine °, and the photolysis of 4,4 -biphenylbiazide °. The tautomerism and phototautomerization of 4(3H)-pyridinethione has been examined theoretically by the infra red isolation technique . The effect of pressure on the photoinduced abstraction reaction of azanaphthalenes in mixed crystals of durene has also been studied. Photosolvolysis of arylmethanols also occurs in aqueous solutions of sulphuric acid . 

Ligand Substitution Reactions. Flash photolysis has been adopted to study the substitution behavior of reactive intermediates in organometallic chemistry. Irradiation of M(CO)6 (M = Cr, Mo, W) in a coordinating solvent (S) produces intermediates of the type M(CO)sS [135], which can undergo rapid solvent displacement by a nucleophile (L) to produce M(CO)5L as shown in Eq. (46). The effect of pressure on such substitution reactions has been studied for a series of M, S, and L, for which the results are summarized in Table 11. 

Fig. 6.16 A plot of ln( foi,s) against hydrostatic pressure for data from six runs, investigating the effect of pressure on the reaction of CO with the intermediate BHm formed by nanosecond laser-flash photolysis of BHm—CO in a mineral oil/toluene solution 90/10 to 95/5 (v/v). The experimental conditions were [heme] = 1 x 10 m, [CO] = 7.5 x 10 m, and T = 25°C. The curve is a mathematical fit to these data based on the model described. ...

The pressure effects on spin relaxation dynamics for these iron(II) complexes have been examined using laser flash photolysis techniques. For Fe(pyim) the two spin states are in equilibrium with a K = 0.56 in 298 K acetone with a partial molar volume difference AV = +8.1 cm mol [34]. Photoexcitation (2ex = 532 nm) leads to transient bleaching of the low spin isomer s MLCT bands followed by first order relaxation to the original spectrum with a 45-ns lifetime. Transient bleaching and subsequent return of the MLCT absorption was attributed to formation of the HS isomer and subsequent spin relaxation. The pressure dependence of the relaxation lifetimes was used to determine the activation volumes of the spin relaxation rates for a variety of FeL in different solvents. It was found that AV j fell into a remarkably narrow range of values (-5.5 + 1 cm mol ) and it was concluded that the spin crossover for these species follows a common mechanism via a transition state located midway between the high and low spin states [33]. 

Croup VI Carbonyl complexes In the case of substitution of neutral ligands by neutral ligands, pressure effects can be better correlated with the intrinsic volume changes associated with the mechanism. One such study dealt with the photosubstitution reactions of the hexacarbonyls M(CO)6 (M = Cr, Mo, W) to give M(CO)sL (Eq. 6.22) and M(CO)4L2, where L is a ligand such as pyridine [61]. For each M, Oco decreased with increasing pressure. Under the risky assumption that kn is independent of P, the pressure dependence of laser flash photolysis techniques have shown that CO loss to form the 5-coordinate intermediate M(CO)s occurs in less than 1 ps. For this reason, one cannot treat the ligand substitution pathway from the reactive ES in terms of mechanisms elucidated for bound excited states. Instead the positive... 

The situation is considerably different for the iron(III) analog (TPPS)Fe "(H20)2 which is hexacoordinate in aqueous solution. In this case the reaction with NO is considerably slower (kon = 4.5 x 10 s ) and kojr (500 s ) is sufficiently large to be measurable by the flash photolysis method as well [92]. Temperature and hydrostatic pressure effects were probed and AH, AS and AV values of 69 + 3 kj mol , 95 10 J mol K and +9+1 cm mol were determined for the "orf reaction and 76 + 6 kJ mol , 60 + 11 ) mol and 18 + 2 cm mol respectively, for the off reaction [93j. The activation parameters for the on reaction compare very favorably with those measured for exchange between coordinated and solvent water for aqueous solutions of (TPPS)Fe "(H20)2 [94] and indicate that kinetics for the reaction of NO with this complex are dominated by the lability of the coordinated water. Furthermore, the large and positive values of AVj and ASj point to a dissociative substitution mechanism as described in Eq. (6.43). 

Pressure effects on the reaction of NO with metmyoglobin (Eq. 6.45) has also been studied by laser flash photolysis and by high pressure stopped flow reactions [%]. 

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