Radical Termination Reactions Research Articles

Liquefaction process for a hydrothermally treated waste mixture containing plastics

Most landfilled plastic waste is a mixture or is in the form of composites with incombustible wastes such as glass, metals, and ceramics. After hydrothermal treatment, including a steam-explosion process, the separation of mixed waste (MW) into organic and inorganic substances becomes easy. However, the effect of hydrothermal pretreatment on the subsequent liquefaction of organic substances from MW is not obvious. In this study, the effects on the liquefaction of polystyrene and high-density polyethylene are discussed. Moreover, optimum conditions for the liquefaction of organic substances from hydrothermally treated MW are identified. By means of this hydrothermal pretreatment, including the steam-explosion process, polystyrene and high-density polyethylene can be significantly converted to oil by liquefaction at 300°–400°C. In comparison with liquefaction of hydrothermally pretreated mixed waste (HMW) at 300°–400°C with a batch type reactor, the yield of oil increases significantly on liquefaction using a semi-batch type reactor. It is considered that the radical chain and termination reactions among the radicals from HMW were inhibited in the semi-batch type reactor. On liquefaction of HMW in a semi-batch reactor, the conversion of HMW to oil was enhanced on increasing the liquefaction temperature to 350°C and the holding time to 60 min.

Comparison of the carbon bond and SAPRC photochemical mechanisms under conditions relevant to southeast Texas

Gridded, regional photochemical models use simplified photochemical reaction mechanisms, and two commonly used mechanisms are the SAPRC and the carbon bond (CB) mechanism. Versions of the mechanisms currently in use include SAPRC99 and the CB-IV mechanism. For most urban areas, the CB-IV and SAPRC mechanisms yield similar results, but for the modeling done of the summer of 2000 in southeast Texas, the SAPRC mechanism leads to concentrations of ozone that are 30–45 ppb higher than with CB-IV. The differences are due to differences in both reaction rate/stoichiometry parameters and condensation methods in the mechanisms. Major reasons for the differences are: (1) the products of the reactions of aromatics with hydroxyl radical, which are more reactive in the SAPRC formulation, (2) the overall balancing of radical generation and termination reactions, which lead to higher radical concentrations in the SAPRC formulation, and (3) the production of higher aldehydes, which is greater in the SAPRC formulation. The differences between the mechanisms is particularly relevant for evaluating attainment with the National Ambient Air Quality Standard (NAAQS) for ozone concentrations averaged over 8 h.

Curcumin, a natural colorant as initiator for photopolymerization of styrene: kinetics and mechanism

The photopolymerization of styrene in presence of an efficient, eco-friendly, and a cost-effective photoinitiator, curcumin, which is found in turmeric root, has been reported for the first time. The catalytic concentration (10−6 M) of curcumin is effective to photoinitiate the polymerization of styrene. The kinetic data, inhibiting effect of benzoquinone and electron spin resonance studies, indicate that the polymerization proceeds via a free radical mechanism. The system follows non-ideal kinetics (Rp ∝ [Cur]0.36 [Sty]1.04) due to both primary radical termination and degradative chain transfer reactions. The broad peaks due to methine and methylene protons in 1H-NMR (nuclear magnetic resonance [NMR]) spectrum and a band of resonances at 145–146 ppm in 13C-NMR indicate atactic nature of the polystyrene formed. The maximum conversion at 30 ± 0.2 °C in 17 h has been limited to 23% without gelation. The formation of radicals and mechanism of polymerization are also discussed.

A reinvestigation of the kinetics and the mechanism of the CH3C(O)O2+ HO2reaction using both experimental and theoretical approaches

The kinetics and the mechanism of the reaction CH(3)C(O)O(2)+ HO(2) were reinvestigated at room temperature using two complementary approaches: one experimental, using flash photolysis/UV absorption technique and one theoretical, with quantum chemistry calculations performed using the density functional theory (DFT) method with the three-parameter hybrid functional B3LYP associated with the 6-31G(d,p) basis set. According to a recent paper reported by Hasson et al., [J. Phys. Chem., 2004, 108, 5979-5989] this reaction may proceed by three different channels: CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)OOH + O(2) (1a); CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)OH + O(3) (1b); CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)O + OH + O(2) (1c). In experiments, CH(3)C(O)O(2) and HO(2) radicals were generated using Cl-initiated oxidation of acetaldehyde and methanol, respectively, in the presence of oxygen. The addition of amounts of benzene in the system, forming hydroxycyclohexadienyl radicals in the presence of OH, allowed us to answer that channel (1c) is <10%. The rate constant k(1) of reaction (1) has been finally measured at (1.50 +/- 0.08) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, after having considered the combination of all the possible values for the branching ratios k(1a)/k(1,)k(1b)/k(1,)k(1c)/k(1) and has been compared to previous measurements. The branching ratio k(1b)/k(1), determined by measuring ozone in situ, was found to be equal to (20 +/- 1)%, a value consistent with the previous values reported in the literature. DFT calculations show that channel (1c) is also of minor importance: it was deduced unambiguously that the formation of CH(3)C(O)OOH + O(2) (X (3)Sigma(-)(g)) is the dominant product channel, followed by the second channel (1b) leading to CH(3)C(O)OH and singlet O(3) and, much less importantly, channel (1c) which corresponds to OH formation. These conclusions give a reliable explanation of the experimental observations of this work. In conclusion, the present study demonstrates that the CH(3)C(O)O(2)+ HO(2) is still predominantly a radical chain termination reaction in the tropospheric ozone chain formation processes.

Controlled/living radical polymerization

Until a little more than a decade ago, controlled/living radical polymerization (CRP) would have been an oxymoron. Full control over all aspects of radical polymerization was deemed well-nigh impossible because radical termination reactions occur at diffusion-controlled rates. However, there are now several procedures for controlling radical polymerization, and corporations are introducing products based on CRP into numerous high-value markets. This review briefly summarizes the evolution of CRP, describes some of the materials that can now be prepared, and highlights some of the commercialization efforts currently underway.

Chemical studies on antioxidant mechanisms and free radical scavenging properties of lignans

The antioxidant activity, in terms of radical scavenging capacity, of altogether 15 different lignans was measured by monitoring the scavenging of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). The effect of differences in skeletal arrangement or the degree of oxidation of the lignans was investigated in a structure-activity relationship study. A large variety in the radical scavenging capacities of the different lignans was observed and related to some structural features. Lignans with catechol (3,4-dihydroxyphenyl) moieties exhibited the highest radical scavenging capacity, while the corresponding guaiacyl (3-methoxy-4-hydroxyphenyl) lignans showed a slightly weaker scavenging capacity. In addition, the butanediol structure was found to enhance the activity, whereas a higher degree of oxidation at the benzylic positions decreased the activity. Additionally, the readily available lignans (-)-secoisolariciresinol, a mixture of hydroxymatairesinol epimers and (-)-matairesinol were studied in more detail, including kinetic measurements and identification of oxidation products in the reactions with DPPH and ABAP (2,2-azobis(2-methylpropionamidine) dihydrochloride. The identification of reaction products, by GC-MS, HPLC-MS and NMR spectroscopy, showed that dimerisation of the two aromatic moieties was the major radical termination reaction. Also, the formation of adducts was a predominant reaction in the experiments with ABAP. The kinetic data obtained from the reactions between the lignans and DPPH indicated a complex reaction mechanism.

Kinetics of the Benzoyl Peroxide/Amine Initiated Free-Radical Polymerization of Dental Dimethacrylate Monomers: Experimental Studies and Mathematical Modeling for TEGDMA and Bis-EMA

The kinetics of the benzoyl peroxide/amine initiated free-radical cross-linking polymerization of dimethacrylate monomers used as dental materials was studied. The monomers examined were triethylene glycol dimethacrylate (TEGDMA) and ethoxylated bisphenol A glycol dimethacrylate (Bis-EMA). As co-initiator to the benzoyl peroxide (BPO) the new amine 4-(N,N-dimethylamino)phenethyl alcohol was used. The polymerization rate vs time for several initial initiator concentrations was measured using differential scanning calorimetry. Differences in the polymerization kinetics of the two monomers are discussed in connection with the chemical structure of dimethacrylates Furthermore, a mathematical model was developed to simulate the BPO/amine initiated free-radical polymerization of dimethacrylate monomers. In the elementary reaction mechanism besides initiation, propagation, and termination reactions, also inhibition, primary radical termination, radical trapping, and secondary initiator chain transfer reactions w...

Singlet oxygen-mediated damage to proteins and its consequences

Proteins comprise approximately 68% of the dry weight of cells and tissues and are therefore potentially major targets for oxidative damage. Two major types of processes can occur during the exposure of proteins to UV or visible light. The first of these involves direct photo-oxidation arising from the absorption of UV radiation by the protein, or bound chromophore groups, thereby generating excited states (singlet or triplets) or radicals via photo-ionisation. The second major process involves indirect oxidation of the protein via the formation and subsequent reactions of singlet oxygen generated by the transfer of energy to ground state (triplet) molecular oxygen by either protein-bound, or other, chromophores. Singlet oxygen can also be generated by a range of other enzymatic and non-enzymatic reactions including processes mediated by heme proteins, lipoxygenases, and activated leukocytes, as well as radical termination reactions. This paper reviews the data available on singlet oxygen-mediated protein oxidation and concentrates primarily on the mechanisms by which this excited state species brings about changes to both the side-chains and backbone of amino acids, peptides, and proteins. Recent work on the identification of reactive peroxide intermediates formed on Tyr, His, and Trp residues is discussed. These peroxides may be important propagating species in protein oxidation as they can initiate further oxidation via both radical and non-radical reactions. Such processes can result in the transmittal of damage to other biological targets, and may play a significant role in bystander damage, or dark reactions, in systems where proteins are subjected to oxidation.

Effects of Diffusion‐Controlled Radical Reactions on RAFT Polymerization

AbstractThe ‘livingness’ of a controlled radical polymerization process such as reversible addition–fragmentation transfer polymerization (RAFT) depends on the rapid deactivation of propagating radicals (the radical addition reaction in RAFT) that suppresses radical termination reactions. However, at high monomer conversions when the polymerization system becomes viscous, polymer chains may experience diffusion limitations and the radical reactions (radical addition and termination) readily become diffusion controlled. The effects of the diffusion‐controlled reactions on the RAFT kinetics and molecular‐weight development are investigated in this work using a modeling approach. It is demonstrated that the diffusion‐controlled radical termination accelerates the polymerization rate and improves the control of polymer molecular weight, while the diffusion‐controlled radical addition also accelerates the rate but broadens the molecular‐weight distribution. This model elucidates the magnitudes and changes for various types of chains involved in the RAFT, i.e., propagating radical chain, adduct radical chain, dormant chain, and dead chain. Polydispersity vs conversion of diffusion‐controlled radical termination (left) and diffusion‐controlled radical addition (right).magnified imagePolydispersity vs conversion of diffusion‐controlled radical termination (left) and diffusion‐controlled radical addition (right).

ESR Study on Diffusion-Controlled Atom Transfer Radical Polymerization of Methyl Methacrylate and Ethylene Glycol Dimethacrylate

An electron spin resonance (ESR) spectrometer was applied on-line to the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) initiated by methyl α-bromophenylacetate (MBPA) with copper bromide (CuBr) and 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) as catalyst and ligand. The Cu(II) ESR signal was observable from the very beginning of the polymerization. The methacrylate radical spectrum appeared at a later stage due to the network formation of the system that imposed diffusion limitations on the fast radical deactivation and termination reactions. The radical and Cu(II) concentrations were measured and analyzed. The polymerization process appeared to have three definable stages. In the first stage, the Cu(II) concentration increased continuously but slowly. The methacrylate radical signal was not detectable because its concentration was below the sensitivity of the ESR machine. In the second stage, the Cu(II) concentration increased...

Reply

[1] We thank Stockwell and Goliff [2002] for their comments on, and interest in, the paper by Srivastava et al. [2001]. Seldom do atmospheric chemists give so much interest to a paper dealing with a new transport algorithm. Their comments about the chemical mechanism being simplified are quite correct, and we caution the readers directly in the above paper that the mechanism is not considered realistic. That caveat alone should keep anyone from considering the mechanism for use in an engineering model. The use of a simplified mechanism is to identify problems with a numerical transport algorithm that may result from nonlinear chemical interactions between species. For that purpose, the mechanism provided works fine, and even more simplified versions have been used successfully for this purpose in the past [e.g., Hov et al., 1989; Odman and Russell, 1991; Chock and Winkler, 1994]. Srivastava et al. [2001] added an additional reaction to provide greater realism than past efforts. Specifically, the mechanism was augmented with a HO2 + HO2 → H2O2 + O2 radical termination reaction to ensure that concentrations of NO during simulations do not drop to very low and unrealistic values. The reason one uses simplified mechanisms, not increasingly complex ones, is that the solution of the chemical dynamics starts taking a bulk of the computational effort and hinders the advancement and testing of the horizontal transport solver, which is the focus of Srivastava et al. [2001]. Also, the greater the complexity of the chemical mechanism, the more difficult it is to track down which aspects of the transport algorithm may be causing problems. After one has fully tested the transport algorithm and identified/fixed any problems, then the transport algorithm is used in an actual application, where it is linked with a comprehensive chemical mechanism. Likewise, when atmospheric chemists test their mechanisms, they do not do so by embedding their mechanism in a full three-dimensional model. They do so in zero dimensions. If they tested their mechanism in a full three-dimensional model at first, it would take much longer and also obscure problems. [2] Indeed, the mechanism proposed by Stockwell and Goliff [2002] probably is chemically more comprehensive and more closely depicts the actual chemistry in the atmosphere. It does this at the expense of increasing by about 50% the number of reactions; the rate constants become more complex, etc. The need to consider the pressure dependence when testing a horizontal transport algorithm is misplaced: This is not a model application. There is nothing in the comment that suggests that the simplified mechanism used by Srivastava et al. [2001] is not suitable for testing horizontal transport algorithms and that it does not capture the nonlinear interactions between species. Indeed, prior tests show that it does. It is not meant to be, and again, the reader is directly warned against, using the mechanism for atmospheric simulations. [3] More comprehensive mechanisms can be employed as one is willing to devote more time to solving the chemistry and less to actually testing the horizontal advection algorithm, which obviously is not the point of the work discussed here. Indeed, the mechanism proposed, since it is still relatively simple, may be useful for future testing of advection routines. The concern of Stockwell and Goliff [2002] that our simplified chemical schemes, which are useful for testing model transport, may ultimately end up in an air quality model could also be directed at their mechanism. Consequently, we should be striving for even greater chemical fidelity in air quality models.

Benzoyl peroxide–p‐acetylbenzylidenetriphenyl arsoniumylide initiated copolymerization of citronellol and styrene

AbstractAlternating copolymers, containing styrene and citronellol sequences, have been synthesized by radical polymerization using benzoylperoxide (BPO)–p‐acetylbenzylidenetriphenyl arsoniumylide (pABTAY) as initiator, in xylene at 80 ± 1 °C for 3 h under inert atmosphere. The kinetic expression is Rp ∝ [BPO]0.88 [citronellol]0.68 [styrene]0.56 with BPO and Rp ∝ [pABTAY]0.27 [citronellol]0.76 [styrene]0.63 with pABTAY, ie the system follows non‐ideal kinetics in both cases, because of primary radical termination and degradative chain transfer reactions. The activation energy with BPO and pABTAY is 94 kJ mol−1 and 134 kJ mol−1, respectively. Different spectral techniques, such as IR, FTIR, 1H NMR and 13C NMR, have been used to characterize the copolymer, demonstrating the presence of alcoholic and phenyl groups of citronellol and styrene. The alternating nature of the copolymer is shown by the product of reactivity ratios r1 (Sty) = 0.81 and r2 (Citro) = 0.015 using BPO and r1 (Sty) = 0.37 and r2 (Citro) = 0.01 using (pABTAY), which are calculated by the Finemann–Ross method. A mechanism of copolymerization is proposed.© 2001 Society of Chemical Industry

From Chemical Kinetics to Streamer Corona Reactor and Voltage Pulse Generator

This paper discusses the global chemical kinetics of corona plasma-induced chemical reactions for pollution control. If there are no significant radical termination reactions, the pollution removal linearly depends on the corona energy density and/or the energy yield is a constant. If linear radical termination reactions play a dominant role, the removal rate shows experimental functions in terms of the corona energy density. If the radical concentration is significantly affected by nonlinear termination reactions, the removal rate depends on the square root of the corona energy density. These characteristics are also discussed with examples of VOCs and NOx removal and multiple processing. Moreover, this paper also discusses how to match a corona plasma reactor with a voltage pulse generator in order to increase the total energy efficiency. For a given corona reactor, a minimum peak voltage is found for matching a voltage pulse generator. Optimized relationship between the voltage rise time, the output impedance of a voltage pulse generator, and the stray capacitance of a corona reactor is presented. As an example, the paper discusses a 5.0-kW hybrid corona nonthermal plasma system for NOx removal from exhaust gases.

Nitric Oxide Reaction with Lipid Peroxyl Radicals Spares α-Tocopherol during Lipid Peroxidation: GREATER OXIDANT PROTECTION FROM THE PAIR NITRIC OXIDE/α-TOCOPHEROL THAN α-TOCOPHEROL/ASCORBATE

The reactions of nitric oxide ((.)NO) and alpha-tocopherol (alpha-TH) during membrane lipid oxidation were examined and compared with the pair alpha-TH/ascorbate. Nitric oxide serves as a more potent inhibitor of lipid peroxidation propagation reactions than alpha-TH and protects alpha-TH from oxidation. Mass spectrometry, oxygen and (.)NO consumption, conjugated diene analyses, and alpha-TH fluorescence determinations all demonstrated that (.)NO preferentially reacts with lipid radical species, with alpha-TH consumption not occurring until (.)NO concentrations fell below a critical level. In addition, alpha-TH and (.)NO cooperatively inhibit lipid peroxidation, exhibiting greater antioxidant capacity than the pair alpha-TH/ascorbate. Pulse radiolysis analysis showed no direct reaction between (.)NO and alpha-tocopheroxyl radical (alpha-T(.)), inferring that peroxyl radical termination reactions are the principal lipid-protective mechanism mediated by (.)NO. These observations support the concept that (.)NO is a potent chain breaking antioxidant toward peroxidizing lipids, due to facile radical-radical termination reactions with lipid radical species, thus preventing alpha-TH loss. The reduction of alpha-T(.) by ascorbate was a comparatively less efficient mechanism for preserving alpha-TH than (.)NO-mediated termination of peroxyl radicals, due to slower reaction kinetics and limited transfer of reducing equivalents from the aqueous phase. Thus, the high lipid/water partition coefficient of (.)NO, its capacity to diffuse and concentrate in lipophilic milieu, and a potent reactivity toward lipid radical species reveal how (.)NO can play a critical role in regulating membrane and lipoprotein lipid oxidation reactions.

Investigation of Oxidative Degradation in Polymers Using17O NMR Spectroscopy

The thermal oxidation of pentacontane (C50H102), and of the homopolymer polyisoprene, has been investigated using 17O NMR spectroscopy. By performing the oxidation using 17O-labeled O2 gas, it is possible to easily identify nonvolatile degradation products, even at relatively low concentrations. It is demonstrated that details of the degradation mechanism can be obtained from analysis of the 17O NMR spectra as a function of total oxidation. Pentacontane reveals the widest variety of reaction products and exhibits changes in the relative product distributions with increasing O2 consumption. At low levels of oxygen incorporation, peroxides are the major oxidation product, while at later stages of degradation these species are replaced by increasing concentrations of ketones, alcohols, carboxylic acids, and esters. Analyzing the product distribution can help in identification of the different free-radical decomposition pathways of hydroperoxides, including recombination, proton abstraction, and chain scission, as well as secondary reactions. The 17O NMR spectra of thermally oxidized polyisoprene reveal fewer degradation functionalities but exhibit an increased complexity in the type of observed degradation species due to structural features such as unsaturation and methyl branching. Alcohols and ethers formed from hydrogen abstraction and free radical termination reactions are the dominant oxidation products. In polyisoprene, the formation of esters and carboxylic acids is relatively minor, distinctly different from the oxidation of pentacontane. An approximately linear increase in these degradation functionalities is observed with increasing oxidation levels. These results demonstrate the promise of 17O NMR as a new technique for detailed investigation of oxidative polymer degradation.

Synthesis of arsenic containing polymethylmethacrylate using p-acetyl benzylidene triphenyl arsonium ylide as photoinitiator

Arsenic containing syndiotactic polymethylmethacrylate (PMMA) using p-ABTAY (p-acetyl benzylidene triphenyl arsonium ylide)in benzene as the photoinitiator at 30 ± 0.2°C has been synthesized. The system follows non-ideal kinetics ( R p ∝ [ I] 0.25 [M]) due to both primary radical termination and degradative chain transfer reaction. The glass transition temperature of PMMA is 145°C. Analysis of the polymer by SEM technique shows the presence of arsenic in it. The polymer has been characterized by Infra-red spectroscopy.

Polymerisation kinetics of allyl monomers at low conversions

Free radical polymerisation kinetics of several mono- and di-allyl monomers were studied using FT-NIR, ESR and GPC techniques to measure the monomer conversion, nature and the concentration of the radicals and the polymer molecular weight, respectively. By manipulating the modulation amplitude of the ESR, the allyl radical and the propagating radical concentrations were estimated and these values were used to calculate the propagation, transfer, re-initiation and termination rate constants. The activation energy for the allyl radical termination reaction was found to be higher than that for the termination of other vinyl monomers. The percentage cyclization for the di-allyl monomers was estimated and for CR39 the value was found to be as high as 25%. A gel with a higher modulus was found to form in CR39 when the reaction rate was kept low.

Examination of the Photochemical Curing and Degradation of Oil Paints by Laser Raman Spectroscopy

In the present study, the curing and the degradation of oil paints by irradiation with ultraviolet light was investigated by laser Raman spectroscopy. At present, the curing reaction of oil paints is interpreted by several reaction schemes, although none of them is considered to be based on definite experimental evidence. The most typical examples are as follows: one is the radical termination reaction of the crosslinking type; the other, the polymerization reaction of the Diels—Alder reaction (diene reaction) type. In the present investigation, oil paint film (ZnO White and TiO2 White) was irradiated with a Xe lamp, and the time dependence of the Raman spectra of the films was examined. It is shown that the curing reaction cannot be explained by either of the above-mentioned reaction schemes. A new reaction mechanism for the curing and the degradation of paint films is proposed. It is concluded that the microscopic approach to the elucidation of the degradation mechanism of oil paint films based on Raman spectroscopy is very useful for the improvement of oil painting techniques.

A spreading model for the oxidation of polypropylene

AbstractA model for the heterogeneous oxidation of polypropylene (PP) is proposed in which it is considered that there is a small initial fraction, po, of oxidizing centres which have a high local rate of oxidation. Within these zones there is a free radical chain reaction producing secondary oxidation products, volatiles and chemiluminescence (CL) from peroxy radical termination reactions. These zones progressively spread (rate coefficient b/s−1) and the free radical reactions die away within the volume of the original zones, producing a measurable concentration of oxidation products (rate coefficient α/s−1). Analysis of the CL‐time curve as representing the instantaneous infectious, fraction, pi, in the spreading model enables the parameters po, b and α to be determined and profiles of the remaining fraction (pr) and dead or oxidized fraction (pd) constructed. Analysis of CL curves from 120°C to 150°C gives an activation energy for spreading in PP particles of 96kJ/mol. Both single particles and groups of particles of different types of PP have been examined and evidence is presented of rapid surface spreading of oxidation from particle to particle.

Transient conductivity OF 1,3-dimethyluracil, uridine and 3-methyluridine in aqueous solution following 20-ns laser excitation at 248 nm

Uridine, 3-methyluridine and 1,3-dimethyluracil in aqueous solution were studied by timeresolved conductimetry after excitation at 248 nm by 20-ns laser pulses. The conductivity signal increases to the maximum value at the pulse end (Δκ m) and decreases then with time, depending on the saturating gas (Ar, N 2O, O 2 or McCI) and pH. The Δκ m signal is suggested to originate from hydrated electrons (e aq −) and protons, the latter resulting from radical cations after rapid reaction with water. Biphotonic photoionization occurs in the whole pH range 3–11 with a quantum yield of 0.016 or smaller for laser intensities of ⩽ 8 MW/cm 2. The reaction of eaq with uncharged bases in Ar-saturated solution at pH 5–8 generates radical anions which are subsequently protonated. The neutralization reaction kinetics of the uracil derivatives depend essentially on the transient proton concentration and lead to the disappearance of most of the conductivity (>90%) within a few microseconds or less in neutral or acidic solution, respectively. For 3-methyluridine and uridine after neutralization, the presence of a long-lived species with acidic properties was observed (in small yield) upon biphotonic (but not monophotonic) excitation. The time-resolved conductivity pattern in the alkaline pH range is different for each of the three pyrimidines, depending essentially on the generation or consumption of OH − in the radical termination reactions.