Multiscale modelling of the radical-induced chemistry of acetohydroxamic acid in aqueous solution

Acetohydroxamic acid (AHA) is a small organic acid with a wide variety of industrial, biological, and pharmacological applications. A deep fundamental molecular level understanding of the mechanisms responsible for the radical-induced reactions of AHA in these environments is necessary to predict and control their behaviour and elucidate their interplay with other attendant chemical species, for example, the oxidative degradation products of AHA. To this end, we present a comprehensive, multiscale computer model for interrogating the radical-induced degradation of AHA in acidic aqueous solutions. Model predictions were critically evaluated by a systematic experimental radiation chemistry investigation, leveraging time-resolved electron pulse irradiation techniques for the measurement of new radical reaction rate coefficients, and steady-state gamma irradiations for the identification and quantification of AHA degradation products: acetic acid, hydroxylamine, nitrous oxide, and molecular hydrogen, with formic acid and methane as minor products. Excellent agreement was achieved between calculation and experiment, indicating that this fundamental model can accurately predict the degradation pathways of AHA under irradiation in acidic aqueous solutions.

The derivatization method developed by Frear and Burrell was used to determine the concentration of HA: 2 1 mL of 1% 8-quinolinol in ethanol and 1 mL of 1.0 M K 2 CO 3 were added to 2 mL of sample and heated to 95 °C for 1 minute in closed test tubes. After being cooled for 15 minutes, the sample absorbance was measured at λ max = 710 nm. The molar extinction coefficient was found to be 14213 ± 396 M -1 cm -1 (see Fig. S2), consistent with the reported value of ε 705nm = 14485 ± 364 M -1 cm -1 . 2 This method was determined to be accurate to within ± 10 %, with a limit of quantification of 17 µM HA. A modified version of the Shinn method was used to quantify the concentration of NO 2 -/HNO 2 . 3 This method involved the sequential addition of 80 µL each of 58 mM sulfanilamide in 1.2 M HCl and 1.92 mM N-(1-naphthyl)ethylenediamine dihydrochloride to 3.84 mL of sample. The UV-visible spectrum for standard NaNO 2 solutions at λ max = 542 nm exhibited an extinction coefficient of ε 542nm = 49500 ± 300 M -1 cm -1 (see Fig. S3), consistent with the reported literature value of ε 543nm = 48000 M -1 cm -1 . 3 This spectrophotometric method was determined to be accurate to within ± 4 %, and had a limit of quantification of 0.3 µM NO 2 -/HNO 2 .

Ion Chromatography (IC)
A Thermo Scientific (Waltham, MA, USA) Dionex ICS-5000 + EG ion chromatograph was used to measure the concentrations of acetate, NO 3 -/HNO 3 , NO 2 -/HNO 2 , and formate ions present in the AHA solutions post-irradiation, using a modified version of a standard organic acid detection method. 4 The eluent was a solution of KOH made from a EGC 500 KOH eluent generator cartridge and degassed, deionized water. The eluent was set to an initial concentration of 1 mM for 8 minutes, increased gradually to 27 mM over 12 minutes, ramped quickly to 60 mM over 0.5 minutes where it was then held for 2 minutes to flush the column, and finally decreased back to 1 mM and held for a total run time of 30 minutes per injection. The detector was a conductivity detector with eluent conductivity suppressor. The column was an IonPac AS11-HC analytical column with an AG-11-HC guard column. The column and compartment temperatures were 25 °C and the pump flow rate was 0.4 mL min -1 . All injections were repeated in triplicate and the results were averaged. Calibration curves for six standard solutions of 0.125 -1.5 mM acetate, NO 3 -, NO 2and 0.025 -0.5 mM formate were prepared freshly and run before every set of IC measurements. For all solutions measured, the concentrations of NO 2 -/HNO 2 and formate were below the method limit of quantification. This method was determined to be accurate to within ± 10 %.

Proton Nuclear Magnetic Resonance ( 1 H-NMR) Spectroscopy
1 H-NMR was used to confirm the concentrations of HA, formate, and acetate detected using UV-Vis and IC for selected doses. NMR spectra were acquired using a Bruker Avance III spectrometer at a field strength of 9.4 T (ν 1 H = 400.03 MHz). An aliquot of the irradiated samples was transferred to a clean, thinwalled 5 mm NMR tube (SP Wilmad-Labglass, Vineland, NJ, USA), where d 6 -DMSO (Cambridge Isotope Labs, Andover, MA, USA) inside a coaxial insert acted as both the lock solvent and chemical shift standard. The standard Bruker "zgesgp" pulse program was used, where a pulse field gradient double-echo was used to achieve water-suppression. 5 1 H spectra were acquired with a hard π/2 pulse of 8.5 μsec at 29 W, a selective square-shaped π pulse of 2 msec at 1.1 mW in power, and smooth-square gradient pulses of 1 msec in duration. 16 transients were acquired after 4 dummy scans, with a sweep-width of 20 ppm and a relaxation delay of 30 sec. Data was phased and analyzed using MestReNova 12 (Mestrelab Research, Compostela, Spain).

Gas Chromatography (GC)
Radiolytically produced H 2 , N 2 O, and CH 4 were quantified by headspace GC measurements using a Shimadzu Co. (Kyoto, Japan) Nexus GC-2030 gas chromatograph equipped with both a barrier discharge ionization detector (BID) and a thermal conductivity detector (TCD). The H 2 and CH 4 analyses were performed using the TCD, while the N 2 O analysis was done using the BID. Both detectors were at 200 °C. The injection port was set to 150 °C with a split ratio of 15. The carrier gas was helium (He) with a linear velocity of 50.0 cm s -1 . A Restek #19722 molecular sieve 5 Å column was used with a column oven temperature profile of 40 °C for 2 minutes, followed by a ramp to 70 °C over 2 minutes, and finally a 1minute hold, for a total run time of 5 minutes per injection. All injections were repeated in triplicate and the results were averaged. The method was determined to be accurate to within ± 10%. Quality control checks were performed daily to confirm known concentrations relative to calibration curves measured for each of the gases.
Solutions to be irradiated and analyzed by GC were frozen in liquid nitrogen and flame-sealed inside evacuated glass ampoules prior to irradiation. Post-irradiation, a crush-tube method was employed [6][7][8] whereby ampoules were cracked inside a length of Nalgene 8005 braided PVC tubing that was closed on one end and fitted with a septum on the other end. The headspace of the tubing was then sampled by taking 100 µL with a Hamilton Model 1810 RN gas-tight syringe and injecting into the GC. The ampoule and tubing headspace volumes were determined by filling with water and weighing before and after cracking the ampoules. The pressure in the tubing headspace was estimated by using the ideal gas law with the calculated gas yields, the measured headspace volumes for the tubing and the ampoule, and the known pressure at which the ampoule was sealed.

CH 3 CONHOH + e aq -
The rate coefficient determined for the reaction of the hydrated electron with acetohydroxamic acid was k(CH 3 CONHOH + e aq -) = (6.02 ± 0.86) x 10 8 M -1 s -1 , which is within the experimental uncertainty of value of (6.4 ± 0.2) x 10 8 M -1 s -1 reported by Samuni and Goldstein 33 .  S4. Determination of the second order rate coefficient for the reaction of the hydrated electron with acetohydroxamic acid at pH 4.0 using pseudo-first-order rate coefficients found from exponential growth fits to raw kinetic data.

CH 3 CONHOH + • OH
The rate constant for the reaction between the hydroxyl radical and acetohydroxamic acid was determined using the thiocyanate anion as a competitor for the hydroxyl radical using k(SCN -+ • OH) = 1.1 x 10 10 M -1 s -1 . 34 The change in the dose-corrected absorption of a 103.85 μM solution of KSCN with the addition of different concentrations of AHA was fitted with a linear function to determine the ratio of the rate coefficients for their reactions with • OH. This yielded a second order rate constant of k(CH 3 CONHOH + • OH) = (4.75 ± 0.76) x 10 8 M -1 s -1 . Samuni and Goldstein 33 found a value of k(CH 3 CONHOH + • OH) = (2.7 ± 0.2) x 10 8 M -1 s -1 , using competition kinetics with ABTS 2-.

CH 3 CONHO -+ • OH
The rate constant for the reaction between the hydroxyl radical and deprotonated acetohydroxamic acid was determined using the thiocyanate anion as a competitor for • OH. The ratio of the rate coefficients for SCNand CH 3 CONHOreacting with • OH was fitted, yielding an "effective rate coefficient" of k eff = (3.47 ± 0.10) x 10 9 M -1 s -1 . At pH 9.8, 10.7 % of the AHA remains as protonated CH 3 CONHOH, and therefore, to isolate the rate constant for just CH 3 CONHO -, the following relationship is used: The resulting rate coefficient is k(CH 3 CONHO -+ • OH) = (3.86 ± 0.11) x 10 9 M -1 s -1 , which is within uncertainty of the value (4.0 ± 0.1) x 10 9 M -1 s -1 reported by Samuni and Goldstein 33 at pH 11 from competition kinetics with ABTS 2-. As expected from the electrophilic nature of • OH, the reaction with unprotonated AHA is significantly faster than with the protonated species.

CH 3 CONHOH + • H
The second order rate constant for the reaction between the hydrogen atom and acetohydroxamic acid was found to be k(CH 3 CONHOH + • H) = (5.43 ± 0.56) x 10 6 M -1 s -1 . Previous authors 33 were able to determine that the rate of this reaction was < 1 x 10 7 M -1 s -1 by using competition kinetics with phenol, consistent with our result.  S6. Determination of the second order rate coefficient for the reaction of the hydrogen atom with acetohydroxamic acid using pseudo-first-order rate coefficients found from exponential growth fits to raw kinetic data of competitor p-chlorobenzoic acid adduct.

NH 3 OH + + e aq -
The reaction between protonated hydroxylamine and the hydrated electron, although originally predicted to yield the amino radical, • NH 3 + , and hydroxide 21 , has since been proven 35 to take the form: The rate coefficient for this reaction was determined to be k(NH 3 OH + + e aq -) = (2.55 ± 0.10) x 10 9 M -1 s -1 at pH 4.0. This rate constant is lower than the value of (1.2 ± 0.1) x 10 10 M -1 s -1 reported in the original study by Simic and Hayon 21 .

NH 3 OH + + • H
The rate constant for the reaction between the hydrogen atom and hydroxylamine was determined as k(NH 3 OH + + • H) = (4.01 ± 0.43) x 10 5 M -1 s -1 . This is the first time that this rate constant has been reported.  S7. Determination of the second order rate coefficient for the reaction of the hydrogen atom with hydroxylamine in 3 M HClO 4 using pseudo-first-order rate coefficients found from exponential growth fits to raw kinetic data.

NH 3 OH + + • OH
The second order rate constant for the reaction between hydroxylamine and the hydroxyl radical was determined using competition kinetics with a 100.2 μM solution of KSCN at pH 3.98. The value found in this work, k(NH 3 OH + + • OH) = (1.59 ± 0.06) x 10 9 M -1 s -1 , is consistent with the results from Simic and Hayon 21 obtained using the same method. Although they reported a value of (5 ± 0.5) x 10 8 M -1 s -1 , these authors suggested that the actual rate coefficient may be somewhat higher than the one that they observed.