Hydroxyl Radical OH

The hydroxyl radical has a strong tendency to abstract a hydrogen atom whenever possible, forming the 

The temperature dependence is given in the form k = BT e c /1, where n is usually taken as 2, except for CH4. This fit procedure provides curvature in the Arrhenius plot similar to that which is observed. Donahue et al. (1998a) propose an alternate form of the 

TABLE 6.2 Rate Constants and Temperature Dependence 11 for Reaction of OH Radicals with Some Alkanes 

The first thing that stands out in Table 6.2 is that the OH-CH4 rate constant, 6.2 X 10 15 cm3 molecule 1 s-1, is much smaller than those for the higher alkanes, a factor of 40 below that for ethane. This relatively slow reaction between OH and CH4 is the reason that the focus is on non-methane hydrocarbons (NMHC) in terms of ozone control in urban areas. Thus, even at a typical peak OH concentration of 5 X 106 molecules cm 3, the calculated lifetime of CH4 at 298 K is 373 days, far too long to play a significant role on urban and even regional scales. Clearly, however, this reaction is important in the global troposphere (see Chapter 14.B.2b). 

Second, the room temperature rate constants increase with increasing size and complexity of the alkane and are of the order of 10-11 cm3 molecule-1 s-1 for the largest alkanes. To put this in perspective, a diffusion-controlled reaction, i.e., one that occurs on every collision of the reactants, is of the order of (3-5) X 10 10 cm3 molecule-1 s-1. Thus for the larger alkanes, reaction occurs in approximately one in 10 collisions, which is quite a fast process. 

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In many of the processes, it is believed that hydroxyl radicals, OH-, are formed and that some of these unite to form hydrogen peroxide ... 

One of the important consequences of neuronal stimulation is increased neuronal aerobic metabolism which produces reactive oxygen species (ROS). ROS can oxidize several biomoiecules (carbohydrates, DNA, lipids, and proteins). Thus, even oxygen, which is essential for aerobic life, may be potentially toxic to cells. Addition of one electron to molecular oxygen (O,) generates a free radical [O2)) the superoxide anion. This is converted through activation of an enzyme, superoxide dismurase, to hydrogen peroxide (H-iO,), which is, in turn, the source of the hydroxyl radical (OH). Usually catalase... 

The ion-pair complex formed by the interaction of hydroxobis(8-quinolyloxo) vanadium (V) [VOQ2OH] and /i-butyl amine is also effective in photoinitiation of polymerization of MMA in bulk and in solution [40]. The quantum yield of initiation and polymerization determined are equal to 0.166 and 35.0, respectively. Hydroxyl radical ( OH) is reported to be the initiating radical and the following photoreaction is suggested ... 

Planar images obtained with laser-based imaging techniques in a propane-fueled research engine showing how closely coupled hydroxyl radical (OH) concentration is with temperature. (From Orth, A., Sick, V., Wolfrum, J., Maly, R.R., and Zahn, M., Proc. Combust. Inst., 25,143,1994. With permission.)... 

Fenton chemistry comprises reactions of H2O2 in the presence of iron species to generate reactive species such as the hydroxyl radical OH. These radicals ( = 2.73 V) lead to a more eflident oxidation chemistry than H2O2 itself (E° = 1.80 V). 

Several powerful oxidants are produced during the course of metabolism, in both blood cells and most other cells of the body. These include superoxide (02 ), hydrogen peroxide (H2O2), peroxyl radicals (ROO ), and hydroxyl radicals (OH ). The last is a particularly reactive molecule and can react with proteins, nucleic acids, lipids, and other molecules to alter their structure and produce tissue damage. The reactions listed in Table 52-4 play an important role in forming these oxidants and in disposing of them each of these reactions will now be considered in turn. 

The mechanism(s) by which these photocatalyzed oxidations are initiated remain uncertain. Early proposals have included involvement of either the photo-produced holes (h+) arising directly from semiconductor photo-excitation, or the (presumed) derivative hydroxyl radical (OH) which was argued to arise from the hole oxidation of adsorbed hydroxyls (h+ + OH-—> OH ). Recent subambient studies [4] with physisorbed chloromethane and oxygen suggest the dioxygen anion (02 ) as a key active species, and the photocatalytic high efficiency chain destruction of TCE is argued to be initiated by chlorine radicals (Cl) [5]. The chlorine-enhanced photocatalytic destruction of air contaminants has been proposed [1, 2, 6] to depend upon reactions initiated by chlorine radicals. 

In the absence of TCE and chlorine, the possible active species are holes (h+), anion vacancies, or anions (02 ), and hydroxyl radicals (OH ). At constant illumination and oxygen concentration, we may expect h+, and O2 concentrations to be approximately constant, and the dark adsorption to be a dominant variable. If kh+, or ko2- does not vary appreciably with the contaminant structure, the rate would depend clearly on the contaminant coverage as shown in Figme 2a, and the reaction would therefore occur via Langmuir-Hinshelwood mechanism. (Note only rates with conversions below 95% are correlated here (filled circles), as the 100% conversion data contains no kinetic information). This rate vs. d>r LH plot is smoother than those for koH or koH suggesting that non-OH species (holes, anion vacancies, or O2 ) are the active species reacting with an adsorbed contaminant. 

Hydroxyl radical (OH) is a key reactive intermediate in combustion and atmospheric chemistry, and it also serves as a prototypic open-shell diatomic system for investigating photodissociation involving multiple potential energy curves and nonadiabatic interactions. Previous theoretical and experimental studies have focused on electronic structures and spectroscopy of OH, especially the A2T,+-X2n band system and the predissociation of rovibrational levels of the M2S+ state,84-93 while there was no experimental work on the photodissociation dynamics to characterize the atomic products. The M2S+ state [asymptotically correlating with the excited-state products 0(1 D) + H(2S)] crosses with three repulsive states [4>J, 2E-, and 4n, correlating with the ground-state fragments 0(3Pj) + H(2S)[ in... 

Ferrous iron, by its reaction with hydrogen peroxide in the Fenton reaction, can yield the toxic hydroxyl radical, OH, which will further potentiate oxygen toxicity. 

The principal pathway leading to degradation of acrylonitrile in air is believed to be photooxidation, mainly by reaction with hydroxyl radicals (OH). The rate constant for acrylonitrile reaction with OH has been measured as 4.1 x 10" cm /molecule/second (Harris et al. 1981). This would correspond to an atmospheric half-life of about 5 to 50 hours. This is consistent with a value of 9 to 10 hours measured in a smog chamber (Suta 1979). 

A family of organic compounds obtained by removing one or more -H atoms from a paraffin and substituting the hydroxyl radical -OH. The best-known alcohols are ethyl alcohol (ethanol) and methyl alcohol (methanol). Glycerine is a trihydric alcohol. 

Hydroxyl radical OH Photolysis of hydroxo or other Fe3+ complexes, of N03", N02", photolysis of H202... 

Extensive research has been conducted into the atmospheric chemistry of organic chemicals because of air quality concerns. Recently, Atkinson and coworkers (1984, 1985, 1987, 1988, 1989, 1990, 1991), Altshuller (1980, 1991) and Sabljic and Glisten (1990) have reviewed the photochemistry of many organic chemicals of environmental interest for their gas phase reactions with hydroxyl radicals (OH), ozone (03) and nitrate radicals (N03) and have provided detailed information on reaction rate constants and experimental conditions, which allowed the estimation of atmospheric lifetimes. Klopffer (1991) has estimated the atmospheric lifetimes for the reaction with OH radicals to range from 1 hour to 130 years, based on these reaction rate constants and an assumed constant concentration of OH... 

Ionizing radiations (a, ft and y) react unselectively with all molecules and hence in the case of solutions they react mainly with the solvent. The changes induced in the solute due to radiolysis are consequences of the reactions of the solute with the intermediates formed by the radiolysis of the solvent. Radiolysis of water leads to formation of stable molecules H2 and H2O2, which mostly do not take part in further reactions, and to very reactive radicals the hydrated electron eaq, hydrogen atom H" and the hydroxyl radical OH" (equation 2). The first two radicals are reductants while the third one is an oxidant. However there are some reactions in which H atom reacts similarly to OH radical rather than to eaq, as e.g. abstraction of an hydrogen atom from alcohols, addition to a benzene ring or to an olefinic double bond, etc. 

NO may react with superoxide to yield the highly reactive peroxynitrite, ONOO-. Superoxide may also be converted into H202 and the reactive hydroxyl radical, OH. In this way excessive activation of glutamate receptors leads to oxidative damage. The calcium influx has a major effect on mitochondria and causes them to depolarize and swell. This leads to a pore being formed in the outer mitochondrial membrane, which allows the escape of cytochrome c and procaspases from the mitochondria into the cytosol. Cytochrome c activates the caspase cascade, which leads to apoptotic cell death (Ch. 35). 

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