Absolute Rate Coefficient Determination and Reaction Mechanism Investigation for the Reaction of Cl Atoms with CH₂I₂ and the Oxidation Mechanism of CH₂I Radicals

Abstract

The gas-phase reaction of atomic chlorine with diiodomethane was studied over the temperature range 273-363 K with the very low-pressure reactor (VLPR) technique. The reaction takes place in a Knudsen reactor at pressures below 3 mTorr, where the steady-state concentration of both reactants and stable products is continuously measured by electron-impact mass spectrometry. The absolute rate coefficient as a function of temperature was given by k = (4.70 +/- 0.65) x 10-11 exp[-(241 +/- 33)/T] cm3molecule-1s-1, in the low-pressure regime. The quoted uncertainties are given at a 95% level of confidence (2sigma) and include systematic errors. The reaction occurs via two pathways: the abstraction of a hydrogen atom leading to HCl and the abstraction of an iodine atom leading to ICl. The HCl yield was measured to be ca. 55 +/- 10%. The results suggest that the reaction proceeds via the intermediate CH2I2-Cl adduct formation, with a I-Cl bond strength of 51.9 +/- 15 kJ mol-1, calculated at the B3P86/aug-cc-pVTZ-PP level of theory. Furthermore, the oxidation reactions of CHI2 and CH2I radicals were studied by introducing an excess of molecular oxygen in the Knudsen reactor. HCHO and HCOOH were the primary oxidation products indicating that the reactions with O2 proceed via the intermediate peroxy radical formation and the subsequent elimination of either IO radical or I atom. HCHO and HCOOH were also detected by FT-IR, as the reaction products of photolytically generated CH2I radicals with O2 in a static cell, which supports the proposed oxidation mechanism. Since the photolysis of CH2I2 is about 3 orders of magnitude faster than its reactive loss by Cl atoms, the title reaction does not constitute an important tropospheric sink for CH2I2.

A Multidimensional Study of the Reaction CH2I+O2: Products and Atmospheric Implications

“…The yield of IO and CH 2 O was estimated to be~30-40 % and~70 %, respectively, of the CH 2 I 2 that was photolysed. A recent study by Stefanopoulos et al [19] observed the formation of CH 2 O from the photolysis of CH 2 I 2 in the presence of O 2 , supported by ab initio calculations. The results suggested that CH 2 O forms via CH 2 IO 2 , but it was not determined if CH 2 IO 2 was a chemically bound or unbound species.…”

“…An alternative identity for species X is an isomeric form of CH 2 O 2 , for example, formic acid, but the electronic absorption spectrum of formic acid (HCOOH) does not extend beyond~260 nm. [34] Although strongly absorbing in the IR, formic acid was not reported in the FTIR study of Cotter et al [16] or in the MS investigation of Eskola et al [18] However, in the recent study by Stephanopoulos [19] formic acid was observed, but it will not contribute to the observed spectrum in Figure 5 above 260 nm. Other isomeric forms of CH 2 O 2 include formaldehyde carbonyl oxide (H 2 COO) and dioxirane (H 2 CO 2 ), in which the C and O atoms form a three-membered ring.…”

“…Stefanopoulos et al, [19] using a Knudsen cell, reported observations of CH 2 O and HCOOH products from reaction (R2).…”

Perspective: Spectroscopy and kinetics of small gaseous Criegee intermediates

“…Sunlight can efficiently induce the photolytic decomposition of CH 2 I 2 to form CH 2 I + I; the reaction of CH 2 I + O 2 eventually releases a second I atom; these I atoms might destroy ozone or become transformed to aerosol. 58,59 Even though the reaction of CH 2 I + O 2 has been investigated extensively, [52][53][54][55][56][60][61][62][63][64][65][66] a detailed mechanism of this reaction was established only recently. 67 The proposed mechanisms in the early studies were incomplete because only product channels for the formation of ICH 2 OO and H 2 CO + IO, not CH 2 OO + I, were considered.…”

CH3CO + O2 + M (M = He, N2) Reaction Rate Coefficient Measurements and Implications for the OH Radical Product Yield

“…"# ! , is determined experimentally in separate experiments by monitoring the first-order decay of a species mass spectrometric signal after halting the gas flow (see Stefanopoulos et al 22 for further details). !…”