Electron Pulse Radiolysis Research Articles

Kinetic studies of the AOP radical-based oxidative and reductive destruction of pesticides and model compounds in water

Absolute second-order rate constants for hydroxyl radical (HO) reaction with four organophosphorus pesticides, malathion, parathion, fenthion and ethion, and a suite of model compounds of structure (EtO)2P(S)-X (where X = Cl, F, SH, SEt, OCH2CF3, OEt, NH2, and CH3) were measured using electron pulse radiolysis and transient absorption techniques. Specific values were determined for these four pesticides as k = (3.89 ± 0.28) x 109, (2.20 ± 0.15) x 109, (2.02 ± 0.15) x 109 and (2.93 ± 0.10) x 109 M−1 s−1, respectively, at 20 ± 2 °C. The corresponding Brönsted plot for all these compounds demonstrated that the HO oxidation reaction mechanism for the pesticides was consistent with the model compounds, attributed to initial HO-adduct formation at the P(S) moiety. For malathion, steady-state 60Co radiolysis and 31P NMR analyses showed that hydroxyl radical-induced oxidation produces the far more potent isomalathion, but only with an efficiency of 4.9 ± 0.3%. Analogous kinetic measurements for the hydrated electron induced reduction of these pesticides gave specific rate constants of k = (3.38 ± 0.14) x 109, (1.38 ± 0.10) x 109, (1.19 ± 0.12) x 109 and (1.20 ± 0.06) x 109 M−1 s−1, respectively, for malathion, parathion, fenthion and ethion. Model compound measurements again supported a single reduction reaction mechanism, proposed to be electron addition at the PS bond to form the radical anion. These results demonstrate, for the first time, that the radical-based treatment of organophosphorus contaminated waters may present a potential toxicological risk if advanced oxidative processes are used.

Reactivity of prehydrated electrons toward nucleobases and nucleotides in aqueous solution.

DNA damage induced via dissociative attachment by low-energy electrons (0 to 20 eV) is well studied in both gas and condensed phases. However, the reactivity of ultrashort-lived prehydrated electrons ([Formula: see text]) with DNA components in a biologically relevant environment has not been fully explored to date. The electron transfer processes of [Formula: see text] to the DNA nucleobases G, A, C, and T and to nucleosides/nucleotides were investigated by using 7-ps electron pulse radiolysis coupled with pump-probe transient absorption spectroscopy in aqueous solutions. In contrast to previous results, obtained by using femtosecond laser pump-probe spectroscopy, we show that G and A cannot scavenge [Formula: see text] at concentrations of ≤50 mM. Observation of a substantial decrease of the initial yield of hydrated electrons ([Formula: see text]) and formation of nucleobase/nucleotide anion radicals at increasing nucleobase/nucleotide concentrations present direct evidence for the earliest step in reductive DNA damage by ionizing radiation. Our results show that [Formula: see text] is more reactive with pyrimidine than purine nucleobases/nucleotides with a reactivity order of T > C > A > G. In addition, analyses of transient signals show that the signal due to formation of the resulting anion radical directly correlates with the loss of the initial [Formula: see text] signal. Therefore, our results do not agree with the previously proposed dissociation of transient negative ions in nucleobase/nucleotide solutions within the timescale of these experiments. Moreover, in a molecularly crowded medium (for example, in the presence of 6 M phosphate), the scavenging efficiency of [Formula: see text] by G is significantly enhanced. This finding implies that reductive DNA damage by ionizing radiation depends on the microenvironment around [Formula: see text].

Temperature dependence of hydroxyl radical reactions with chloramine species in aqueous solution

The absolute temperature-dependent kinetics for the reaction between hydroxyl radicals and the chloramine water disinfectant species monochloramine (NH2Cl), as well as dichloramine (NHCl2) and trichloramine (NCl3), have been determined using electron pulse radiolysis and transient absorption spectroscopy. These radical reaction rate constants were fast, with values of 6.06 × 108, 2.57 × 108, and 1.67 × 108 M−1 s−1 at 25 °C for NH2Cl, NHCl2, and NCl3, respectively. The corresponding temperature dependence of these reaction rate constants, measured over the range 10–40 °C, is well-described by the transformed Arrhenius equations:ln k (OH & NH2Cl) = (23.68 ± 0.23) – {(8570 ± 580)/(8.314*T/K)}ln k (OH & NHCl2) = (21.96 ± 0.56) – {(6110 ± 400)/(8.314*T/K)}ln k (OH & NCl3) = (21.26 ± 0.29) – {(5770 ± 720)/(8.314*T/K)}giving activation energies of 8.57 ± 0.58, 6.11 ± 0.40, and 5.77 ± 0.72 kJ mol−1 for these three chloramines, respectively. These data will aid water utilities in predicting hydroxyl radical partitioning and chemical contaminant removal efficiencies under real-world advanced oxidation process treatment conditions.

Mesolysis Processes with Benzylic Carbon–Oxygen Bond Cleavage in Radical Anions of Aryl Benzyl Ethers Studied by Electron Pulse Radiolysis in DMF

Abstract Based on transient absorption measurements obtained upon electron-pulse radiolysis of aromatic ethers in N,N-dimethylformamide (DMF), mesolysis processes of the radical anions were investigated. We observed transient absorption spectral change due to the formation of the radical anions in the temperature range (220–295 K). The transient absorption of the radical anions decreased with first-order kinetics along with the growth of the absorption spectrum of the corresponding benzylic radical. Arrhenius analysis of the decay rates of the radical anions provided the activation energies and the frequency factors for the mesolysis. These observations indicated that the mesolysis of the radical anions occurs via a stepwise mechanism, resulting in the formation of benzylic radical and phenolate anion due to the C–O bond cleavage.

A Comparison of the γ-Radiolysis of TODGA and T(EH)DGA Using UHPLC-ESI-MS Analysis

Solutions of TODGA and T(EH)DGA in n-dodecane were subjected to γ-irradiation in the presence and absence of an aqueous nitric acid phase and analyzed using UHPLC-ESI-MS to determine the rates of radiolytic decay of the two extractants, as well as to identify radiolysis products. The DGA concentrations decreased exponentially with increasing dose, and the measured degradation rate constants were uninfluenced by the presence or absence of an acidic aqueous phase, or by chemical variations in the alkyl side-chains. The DGA degradation was attributed to reactions of the dodecane radical cation, whose kinetics were measured for TODGA using picosecond electron pulse radiolysis to be k2 = (9.72 ± 1.10) × 109 M−1 s−1. The identified radiolysis products suggest that the bonds most vulnerable to radiolytic attack are those in the diglycolamide center of these molecules and not on the side-chains.

Hidden chemistry of substituted aniline radical cations in water: a mechanistic study

Absolute rate constants for hydroxyl radical, azide radical, and hydrated electron reactions with a sulfa drug 4,4'‐diamino diphenyl sulfone (dapsone) in water have been evaluated using electron pulse radiolysis technique. Absolute rate constants for hydroxyl radical and azide radical were determined as (8.4 ± 0.3) × 109 and (5.6 ± 0.5) × 109 M−1 s−1, respectively. The reduction of dapsone with the hydrated electron occurred with rate constant of (9.2 ± 0.1) × 109 M−1 s−1. Hydroxyl radical reactions result in the synchronous formation of adduct as well as anilino radical. The interesting observation is that the yield of the anilino radical increases with increase in pH. Contrary to this, the yield of the adduct decreases with pH. We propose that hydroxyl radical adds predominantly to the aniline. In contrast, the reaction of azide radical with the dapsone suggests that the reaction occurs at the –NH2 moiety of the aniline ring. The free radical electron transfer from dapsone to parent radical cation of non‐polar solvent also results in the formation of anilino radical only suggesting that the radical cation of dapsone has a short lifetime. The reaction of hydrated electrons with the dapsone suggests that the reaction occurs at different reaction site. The experimental results supported by theoretical calculations of this study provide fundamental mechanistic parameters that probably decide the fate of the radical cation of aniline derivatives. Copyright © 2014 John Wiley & Sons, Ltd.

Identifying the factors that influence the reactivity of effluent organic matter with hydroxyl radicals

Advanced oxidation processes (AOPs) are an effective treatment technology for the removal of a variety of organic pollutants in both water and wastewater treatment. However, many background constituents in water are highly reactive towards hydroxyl radicals (HO) and decrease the efficiency of the process towards contaminant oxidation. Up to 95% of the HO scavenging can come from dissolved organic matter (OM). In this study, 28 wastewater effluent samples were analyzed to find correlations between the reactivity of HO with wastewater-derived OM (known as effluent organic matter, EfOM), water quality parameters, treatment train characteristics, and fluorescence-derived data. Rate constants for the reaction between HO and EfOM (kEfOM-HO) were measured using a bench scale UV-based AOP system with methylene blue as an HO probe and confirmed using an electron pulse radiolysis method for a subset of the samples. The EfOM was characterized using a series of physicochemical parameters, including polarity, average molecular size and fluorescence. The kinetic data were analyzed with principal component analysis and Akaike Information Criterion. Four predictors were identified as dominant: chemical oxygen demand, retention onto NH2 extraction medium, fluorescence index, and total organic carbon. These four variables accounted for approximately 62% of the variability in the value of kEfOM-HO The average kEfOM-HO value for EfOM in this study was 2.5 × 108 MC−1 s−1, which is about 31% lower than the 3.6 × 108 MC−1 s−1 value determined for natural organic matter isolates and commonly used in AOP modeling.

Competition Reactions of H2O•+ Radical in Concentrated Cl– Aqueous Solutions: Picosecond Pulse Radiolysis Study

Picosecond pulse-probe radiolysis measurements of highly concentrated Cl(-) aqueous solutions are used to probe the oxidation mechanism of the Cl(-). The transient absorption spectra are measured from 340 to 710 nm in the picosecond range for the ultrafast electron pulse radiolysis of halide solutions at different concentrations up to 8 M. The amount of Cl(2)(•-) formation within the electron pulse increases notably with increasing Cl(-) concentration. Kinetic measurements reveal that the direct ionization of Cl(-) cannot solely explain the significant amount of fast Cl(2)(•-) formation within the electron pulse. The results suggest that Cl(-) reacts with the precursor of the OH(•) radical, i.e., H(2)O(•+) radical, to form Cl(•) atom within the electron pulse and the Cl(•) atom reacts subsequently with Cl(-) to form Cl(2)(•-) on very short time scales. The proton transfer reaction between H(2)O(•+) and the water molecule competes with the electron transfer reaction between Cl(-) and H(2)O(•+). Molecular dynamics simulations show that number of water molecules in close proximity decreases with increasing concentration of the salt (NaCl), confirming that for highly concentrated solutions the proton transfer reaction between H(2)O(•+) and a water molecule becomes less efficient. Diffusion-kinetic simulations of spur reactions including the direct ionization of Cl(-) and hole scavenging by Cl(-) show that up to 30% of the H(2)O(•+) produced by the irradiation could be scavenged for solutions containing 5.5 M Cl(-). This process decreases the yield of OH(•) radical in solution on the picosecond time scale. The experimental results for the same concentration of Cl(-) at a given absorbed dose show that the radiation energy absorbed by counterions is transferred to Cl(-) or water molecules and the effect of the countercation such as Li(+), K(+), Na(+), and Mg(2+) on the oxidation yield of Cl(-) is negligible.

Radical-based destruction of nitramines in water: kinetics and efficiencies of hydroxyl radical and hydrated electron reactions.

In support of the potential use of advanced oxidation and reduction process technologies for the removal of carcinogenic nitro-containing compounds in water reaction rate constants for the hydroxyl radical and hydrated electron with a series of low molecular weight nitramines (R(1)R(2)-NNO(2)) have been determined using a combination of electron pulse radiolysis and transient absorption spectroscopy. The hydroxyl radical reaction rate constant was fast, ranging from 0.54-4.35 × 10(9) M(-1) s(-1), and seen to increase with increasing complexity of the nitramine alkyl substituents suggesting that oxidation primarily occurs by hydrogen atom abstraction from the alkyl chains. In contrast, the rate constant for hydrated electron reaction was effectively independent of compound structure, (k(av) = (1.87 ± 0.25) × 10(10) M(-1) s(-1)) indicating that the reduction predominately occurred at the common nitramine moiety. Concomitant steady-state irradiation and product measurements under aerated conditions also showed a radical reaction efficiency dependence on compound structure, with the overall radical-based degradation becoming constant for nitramines containing more than four methylene groups. The quantitative evaluation of these efficiency data suggest that some (~40%) hydrated electron reduction also results in quantitative nitramine destruction, in contrast to previously reported electron paramagnetic measurements on these compounds that proposed that this reduction only produced a transient anion adduct that would transfer its excess electron to regenerate the parent molecule.

Kinetic model for the radical degradation of tri-halonitromethane disinfection byproducts in water

The halonitromethanes (HNMs) are byproducts of the ozonation and chlorine/chloramine treatment of drinking waters. Although typically occurring at low concentrations HNMs have high cytotoxicity and mutagenicity, and may therefore represent a significant human health hazard. In this study, we have investigated the radical based mineralization of fully-halogenated HNMs in water using the congeners bromodichloronitromethane and chlorodibromonitromethane. We have combined absolute reaction rate constants for their reactions with the hydroxyl radical and the hydrated electron as measured by electron pulse radiolysis and analytical measurements of stable product concentrations obtained by 60Co steady-state radiolysis with a kinetic computer model that includes water radiolysis reactions and halide/nitrogen oxide radical chemistry to fully elucidate the reaction pathways of these HNMs. These results are compared to our previous similar study of the fully chlorinated HNM chloropicrin. The full optimized computer model, suitable for predicting the behavior of this class of compounds in irradiated drinking water, is provided.

Investigation of Dynamics of Radiolytic Formation of CdSe Nanoparticles in Aqueous Solutions

The formation of cadmium selenide, CdSe, nanoparticles in aqueous solutions containing equimolar ammoniated cadmium sulfate, [Cd(NH(3))](4)SO(4) and sodium selenosulfate, Na(2)SeSO(3) as the starting materials, has been investigated by electron pulse radiolysis coupled with kinetic spectrometry. The formation of CdSe nanoparticles was found to proceed through the generation of short-lived transient intermediate species having an absorption peak at 520 nm, which is formed only upon the reaction of hydrated electrons, e(aq) with the precursor ions under deaerated conditions. The transient intermediate species decays with a weighted average rate constant, 1.2 × 10(7) s(-1). The transient intermediate species formed in the case of individual precursors did not match with the transients formed when both the precursors are taken together in the solutions under the present experimental conditions. The reaction rate constants between the precursor ions, [Cd(NH(3))(4)](2+) and the transient intermediate species formed from [SeSO(3)](2-) was 1.9 × 10(10) M(-1) s(-1). Similarly, the reaction rate constants between the precursor ions, [SeSO(3)](2-) and the transient intermediate species formed from [Cd(NH(3))(4)](2+) was 5.5 × 10(10) M(-1) s(-1). This clearly indicates that the formation of CdSe nanoparticles occurs through both reaction channels. However, the major reaction channel is through the reaction of e(aq) with the [Cd(NH(3))(4)](2+) ions (k = 3.1 × 10 (10) M(-1) s(-1)), as its rate constant is one order higher than that of the reaction of e(aq) with the [SeSO(3)](2-) ions (k = 2.3 × 10(9) M(-1) s(-1)).

Temperature Dependence of the Reaction between the Hydroxyl Radical and Organic Matter

The temperature-dependent bimolecular rate constants for the reaction of the hydroxyl radical (HO(•)) with organic matter (OM) (k(OM-HO(•))) have been measured for three natural organic matter (NOM) isolates and three bulk effluent organic matter (EfOM) samples using electron pulse radiolysis and thiocyanate competition kinetics. The range of values for the room temperature k(OM-HO(•)) was 1.21-9.37 × 10(8) M(C)(-1)s(-1), with NOM isolates generally reacting slower than EfOM samples. The NOM isolates had an average apparent activation energy of 19.8 kJ mol(-1), while the EfOM samples had an average value slightly lower (14.3 kJ mol(-1)), although one NOM isolate (Elliot Soil Humic Acid, 29.9 kJ mol(-1)) was a factor of 2 times greater than other samples studied. These apparent activation energies are the first determined for OM and HO(•), and the Arrhenius plots obtained for NOM isolates (lowest R(2) > 0.993) suggest that no significant structural changes are occurring over the temperature range 8-41 °C. In contrast, the greater scatter (lowest R(2) > 0.903) observed for the EfOM samples suggests that some structural changes may be occurring. These results provide a deeper fundamental understanding of the reaction between OM and HO(•) and will be useful in quantifying HO(•) reactions in natural and engineered systems.

Toluene nitration in irradiated nitric acid and nitrite solutions

The kinetics, mechanisms, and stable products produced for the nitration of aryl alkyl mild ortho-para director toluene in irradiated nitric acid and neutral nitrite solutions were investigated using γ and pulse radiolysis. Electron pulse radiolysis was used to determine the bimolecular rate constants for the reaction of toluene with different transient species produced by irradiation. HPLC with UV detection, GC-MS and LC-MS, were used to assess the stable reaction products. Free-radical based nitration reaction products were found in irradiated acidic and neutral media. In 6.0M HNO3, ring substitution, side chain substitution, and oxidation, produced different nitrated toluene products. For ring substitution, nitrogen oxide radicals were added mainly to cyclohexadienyl radicals, whereas for side chain substitution, these radicals were added to the carbon-centered benzyl radical produced by H-atom abstraction. In neutral nitrite solutions, radiolytically-induced ring nitration products approached a statistically random distribution, suggesting a direct free-radical reaction involving addition of the NO2 radical.

Distance Dependence of the Reaction Rate for the Reduction of Metal Cations by Solvated Electrons: A Picosecond Pulse Radiolysis Study

The decay of the solvated electron generated by picosecond electron pulse radiolysis is studied by broad-band transient absorption measurements in ethylene glycol solutions containing decimolar concentrations of Cu(2+), Ni(2+), and Pb(2+) metal cations. Analysis of the nonexponential kinetics of the decays reveals molecular parameters of the electron transfer reaction. It is found that the reaction occurs at long distance for Cu(2+) solutions and is only limited to contact distance in the case of Ni(2+) solutions. The distribution of reaction distance strongly depends on the free enthalpy change of the reactions.

Temperature and density effects on the absorption maximum of solvated electrons in sub- and super-critical methanol

The optical absorption spectra of the solvated electron ([Formula: see text]) in sub- and super-critical methanol are measured by both electron pulse radiolysis and laser photolysis techniques, at temperatures in the range 220–270 °C. Over the density range studied (~0.45–0.59 g/cm3), the position of the absorption maximum ([Formula: see text]) of [Formula: see text] is found to shift only slightly to the red with decreasing density. In agreement with our previous work in water, at a fixed pressure, [Formula: see text] decreases monotonically with increasing temperature in passing through the phase transition at Tc (239.5 °C). By contrast, at a fixed density, [Formula: see text] exhibits a minimum as the solvent passes above the critical point into the supercritical state. These behaviors are discussed in terms of microscopic arguments based on the changes that occur in the methanol properties and methanol structure in the sub- and super-critical regimes. The effect of the addition of a small amount of water to the alcohol on the optical absorption energy of [Formula: see text] is also investigated.

Hydroxyl-Radical-Induced Degradative Oxidation of β-Lactam Antibiotics in Water: Absolute Rate Constant Measurements

The beta-lactam antibiotics are some of the most prevalent pharmaceutical contaminants currently being detected in aquatic environments. Because the presence of any trace level of antibiotic in water may adversely affect aquatic ecosystems and contribute to the production of antibiotic-resistant bacteria, active removal by additional water treatments, such as using advanced oxidation and reduction processes (AO/RPs), may be required. However, to ensure that any AOP treatment process occurs efficiently and quantitatively, a full understanding of the kinetics and mechanisms of all of the chemical reactions involved under the conditions of use is necessary. In this study, we report on our kinetic measurements for the hydroxyl-radical-induced oxidation of 11 beta-lactam antibiotics obtained using electron pulse radiolysis techniques. For the 5-member ring species, an average reaction rate constant of (7.9 +/- 0.8) x 10(9) M(-1) s(-1) was obtained, slightly faster than for the analogous 6-member ring containing antibiotics, (6.6 +/- 1.2) x 10(9) M(-1) s(-1). The consistency of these rate constants for each group infers a common reaction mechanism, consisting of the partitioning of the hydroxyl radical between addition to peripheral aromatic rings and reaction with the central double-ring core of these antibiotics.

Free Radical-Induced Redox Chemistry of Nedaplatin and Satraplatin under Physiological Conditions

Temperature-dependent kinetics for the reactions of hydroxyl radicals and hydrated electrons with the anti-cancer drug nedaplatin have been determined using a combination of electron pulse radiolysis and absorption spectroscopy. Under physiological pH and chloride concentrations, the kinetics was well described by the equations [Formula: see text]and [Formula: see text]corresponding to Arrhenius activation energies of 15.88 +/- 1.16 and 14.14 +/- 1.41 kJ mol(-1) for hydroxyl radical oxidation and hydrated electron reduction, respectively. Through a comparison of spectral and kinetic literature it is believed that the oxidation reaction gives predominantly an intermediate Pt(III) species, whereas reduction gives a Pt(I) moiety. Analogous hydrated electron measurements for the Pt(IV) drug satraplatin showed multiple-component decays at higher temperatures (>20 degrees C), indicating that significant thermal degradation of this chemical occurs. From double-exponential curve fitting, the satraplatin reduction kinetics was found to be well described by the equation [Formula: see text]giving an activation energy of 22.78 +/- 1.78 kJ mol(-1) for this reaction. This measured temperature dependence was consistent with several model Pt(IV) compounds also investigated in this study, with all these data suggesting that the metal ion reduction to give Pt(III) was the dominant reaction occurring.

Pulse radiolytic studies on cis-dichlorobis(1,10-phenanthroline-5,6-dione)Cobalt(III) complex

Reactions of four reducing radical species namely e aq − , H-atoms, (CH3)2 •COH and •CO 2 − radicals with [Co(III)(phendione)2Cl2]Cl (complex) have been studied by electron pulse radiolysis. Time resolved transient absorption spectra for all the four species show two peaks which match with those of phendione anion radical produced by the reaction of e aq − with phendione. However, there are some differences in the spectral and kinetic features in the reactions of H-atoms and •CO 2 − radicals with the complex which are elaborated. Steady state absorption spectra on irradiation under reducing conditions indicate breakdown of the complex. The anion radical initially produced has fairly long lifetime of few 100 microseconds. Spectral studies show that initial site of attack for electron transfer is the phendione moiety. Subsequently the electron is transferred to other sites resulting in aquation/decomposition to metal ion. Conductometric studies show that conductance substantially increases on irradiation and could be attributed to aquation/de-ligation of the complex by intramolecular electron transfer.

Bimolecular Rate Constant Determination for the Reaction of Hydroxyl Radicals with Domoic and Kainic Acid in Aqueous Solution

Bimolecular rate constants for the hydroxyl radical, .OH, reaction with domoic acid, kainic acid, and several model compounds were determined using electron pulse radiolysis and transient absorption spectroscopy. These oxidation rate constants were determined as (9.22 +/- 0.60) x 10(9) M(-1) s(-1) and (2.46 +/- 0.19) x 10(9) M(-1) s(-1), for domoic acid and kainic acid, respectively. Model compound rate constant measurements suggested that the conjugated double bond of the C4 substituent of domoic acid, and the double bond in kainic acid, were the preferential initial site of .OH reaction, which would destroy the conjugation and/or molecular conformation of these toxins. These reaction rate constants were also used to determine potential persistence in natural water systems, however the calculated half-life for domoic acid of approximately 34 days implies that significant photodegradation via .OH reaction mechanisms is unlikely.

Formation of semiquinone radical in the reaction of embelin (2,5-dihydroxy-3-undecyl-1,4-benzoquinone) with reductants as well as oxidants. Characterization by pulse radiolysis and structure investigation by quantum chemical study

Embelin, 2,5-dihydroxy-3-undecyl-1,4-benzoquinone, has two intra-molecular hydrogen bonds between the quinone and hydroquinone groups present on the same ring. Therefore, it is expected to produce semiquinone radical in the oxidation as well reduction reactions. To investigate this aspect, kinetics and mechanism of reduction reactions of embelin have been studied using nanosecond electron pulse radiolysis technique and compared with the oxidation process. Two distinct transients observed in the reduction ( λ max = 450 nm) and the oxidation ( λ max = 410 nm) processes with various free radicals have been characterized and assigned to two different embelin semiquinone radicals on the basis of pulse radiolysis studies. The structures of these transients have been further ascertained using density functional theory with B3LYP functional and 6-31+G(d,p) basis set. Reduction reaction of embelin has been found to produce delocalized semiquinone anion radical. On the other hand, oxidation of embelin is expected to produce several transient species. However, the calculations suggest that oxidation of hydroquinone group is the first step, which is followed by transformation into carbon-centered radical.