Rate and product measurements for the reactions of hydroxyl with molecular iodine and iodine chloride at 298 K: separation of gas-phase and surface reaction components

Abstract

The kinetic behavior of OH with I2 and IC1 is reported within the context of a series of radical-halogen reactions to investigate the mechanism of such reactions through studies of reactivity trends. We report 298 K rates and product channels for OH + I2 -I + HOI (kla) and OH + I2 -IO + HI (klb, AH = 25 kcal mol-') of kl = kla + k1b = 1.6 X lo-'' cm3 molecule-' kcal mol-'), OH + IC1 -C10 + HI (kzc, A H = 16.9 kcal mol-'), and OH + IC1 -IO + HC1 (k2d, A H = 7.5 kcal mol-') of k2 = 2.0 X lo-'' cm3 molecule-' s-', where k2a + k2c + k2d < 0.01k2b. Atomic I and c1 products of the title reactions are measured by resolved resonance fluorescence vapor lamps. IO and C10 are detected by chemically converting them, using NO, to I and C1, respectively. A factor of 2 uncertainty must be associated with these values because of the inherent difficulty of, and uncertainty arising from, the separation of the homogeneous and heterogeneous reaction components. In addition, the rate constant of the reaction H + IC1 has been measured to be (5 f 2) X lo-" cm3 molecule-' s-I at 298 K; its only product channel is I + HC1. Observed products of the OH-halogen reactions suggest that -15 < AHf(HOI) < -1 kcal mol-'. Magnitudes of the rate constants of these OH reactions substantiate the reactivity trends in the OH-halogen system based on an electron-transfer mechanism from the highest occupied molecular orbital of the halogen to the lowest unoccupied orbital of the hydroxyl. The rapidity of the OH + I2 reaction makes it a possible source of HOI for photochemical studies.S-', with klb/kla < 0.01, and for OH + IC1 -CI + HOI (kza, AH unknown), OH + IC1 -I + HOC1 (k2b, A H = -6.1

Reactions of OH radicals with inorganic compounds in the gas phase

“…There is agreement among the reported values of the room temperature rate constant for OH+C12 from four studies [180,181,220,262], of which two [180,181] were obtained in connection with the measurement of the rate constant of the reaction OH+C10. Three reported values for the reaction with Br2 [220,221,262] are in fair agreement, and so are the two values for the reaction with I2 [222,223], within the large quoted uncertainties. From kinetic measurements in the temperature range 253-330K Boodaghians et al [262] report activation energies of (1.8+0.7) and (0+1) kcal/mol for the reactions with Cl2 and Br2 respectively.…”

“…The branching ratio of the reaction paths, has also been determined by Loewenstein and Anderson [220,223] from the halogen atom yields (measured by resonance fluorescence) and computer modelling of the reaction. A value of ka/(ka+kb) = 1.0+0.1 was obtained for Cl=, Br2 and Is.…”

Pathways for the OH + Cl2 → HOCl + Cl and HOCl + Cl → HCl + ClO Reactions

“…3 Rate coefficients and products have been reported experimentally by Ravishankara, Eisele, and Wine, 4 by Loewenstein and Anderson, 5,6 and by Boudaghians, Hall, and Wayne. 7 However, the reported experimental rate coefficients and activation energies have notable uncertainties (see Grills, Burkholder, and Ravishankara 8 ).…”

In situ detection of atomic and molecular iodine using Resonance and Off-Resonance Fluorescence by Lamp Excitation: ROFLEX

“…Oscillator strength measurements (see Table 2) indicate that the 178.276 nm transition is ∼20 times stronger than the 183.083 nm line (Clyne and Townsend, 1974;Spietz et al, 2001). However, such factor is not observed in emission spectra from iodine lamps (Hikida et al, 1983;Loewenstein and Anderson, 1985;Aleksandrov et al, 1985) as a result of the relative population of the two states of the 2 [2] J multiplet. Assuming equilibrium at the kinetic temperature of the gas (T ∼ 900 K, Spietz et al, 2001), the relative population of the J = 3/2 and J = 5/2 states would be 1:15, which partially balances the ratio of oscillator strengths.…”

“…Brune et al, 1989). Radio-frequency discharge coils have several advantages over microwave cavities (Brewer and Tellinghuisen, 1971;Loewenstein and Anderson, 1985;Aleksandrov et al, 1985): lower cost, lighter and smaller generator, lower power consumption and narrower atomic line width. stages.…”