A Kinetic and Spectroscopic Study of the CH₃I−Cl and ICH₂I−Cl Adducts

Abstract

Laser-induced fluorescence from the CH3I-Cl and ICH2I-Cl adducts formed in association reactions between chlorine atoms and CH3I and CH2I2 has been observed for the first time. The LIF excitation and dispersed fluorescence spectra have been measured in the range 345-375 nm and 380-480 nm, respectively, at 204 and 296 K. The excitation spectra exhibit vibrational fine structure, and a semiquantitative analysis of the spectra yields a similar binding energy for both adducts of approximately 60 kJ mol(-1). The adduct fluorescence is efficiently quenched by N2 and exhibits a zero-pressure lifetime of approximately 25-30 ns. Using LIF excited from the CH3I-Cl and ICH2I-Cl adducts, the kinetics of the reactions of atomic chlorine with methyl iodide and diiodomethane have been investigated, the results showing that both reactions proceed via two independent channels, an association reaction to form the adduct and a bimolecular abstraction reaction. At T approximately 200 K, the association reaction is predominant, and CH3I-Cl formation is irreversible, with rate coefficients for adduct formation found to be pressure-dependent and in reasonable agreement with the literature. At approximately 200 K, removal of the adduct is dominated by reaction with radical species (CH3 and ClSO) and by self-reaction, which proceed at close to the gas kinetic limit. At 296 K, CH3I-Cl formation is reversible, and the equilibrium constant, K(p) = (70.9 +/- 27.4) x 10(3) atm(-1), was determined, which is in excellent agreement with the literature, and the adduct does not significantly react with CH3I. The uncertainty is at the 95% confidence level (2sigma) and includes systematic errors. At approximately 200 K, the ICH2I-Cl adduct is again stabilized, with pressure-dependent rate coefficients reaching the high pressure limit at lower pressures than for the Cl + CH3I reaction. At room temperature, the ICH2I-Cl adduct is removed via an additional unimolecular decomposition channel, which dominates over the reversible decomposition channel to reform Cl + CH2I2. Neither adduct was observed to undergo significant reaction with molecular oxygen at approximately 200 or 296 K, with an upper limit rate coefficient determined as k < 10(-16) cm(3) molecule(-1) s(-1).

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

“…By comparing the peak IO concentrations with the corresponding calculated initial CH 2 I concentrations it is found that, if we assumed that IO is generated from the reaction of CH 2 I + O 2 (and from this paper a central theme is that it is not), the yield of IO would be~30-40 %, in disagreement with the unity yield reported by Enami et al [1] However, more recently, Enami et al [2] reported a lower IO yield (< 30 %) from reaction (R2), which showed a dependence on pressure and temperature, however, the interpretation is complicated by the formation of Cl-CH 3 I adducts. [23] The rate of formation of IO is also strongly dependent on the concentration of CH 2 I 2 , as shown in Figure 2, implying that IO is not generated directly from the reaction of CH 2 I + O 2 , as the concentration of O 2 is at least 17 000 times that of CH 2 I and first-order reaction conditions are present. Regression analysis of the rate of formation of IO (k' IO ) versus [CH 2 I 2 ], see Figure 2, yields an effective bimolecular rate coefficient for a reaction of CH 2 I 2 + an unknown species to form IO of k = (3.7 AE 0. , approximately 15 times smaller than that determined by Enami et al, [1]~5 0 times smaller than the more recent determination by Enami et al [2] and almost 60 times smaller than the MS determinations of Masaki et al [15] and Eskola et al [18] for the reaction of CH 2 I + O 2 .…”

“…A noticeable difference in the work of Sehested et al [14] was their method of CH 2 I generation (F + CH 3 I!HF + CH 2 I). It is known that halogen atoms form stable adducts with the alkyl iodides [23,29] and the CH 3 I-Cl adduct absorbs strongly over the 345-375 nm region. [23] It is reasonable to expect the absorption spectrum of CH 3 I-F to be significantly blue shifted to that of CH 3 I-Cl, and the "CH 2 IO 2 " absorption spectrum of Sehested et al [14] is likely contaminated by CH 3 I-F absorption between 220-310 nm, consistent with their comment that the CH 2 IO 2 absorption spectrum is "an unusual peroxy spectrum".…”

A laser induced fluorescence study relating to physical properties of the iodine monoxide radical

“…The experimental apparatus employed in this study was very similar to that used in previous laboratory studies of IO by the Leeds group. 24,25 A notable exception was that the fluorescence cell had four horizontal axes (giving eight reaction cell arms). Two of these were used as the principal laser axes (arranged orthogonally), and a third (oriented 451 with respect to the laser axes) was used as the dispersed fluorescence axis.…”

Photochemistry in a dense manifold of electronic states: Photodissociation of CH2ClBr

“…Y where X and Y are either Cl or Br and the halogens make a weak bond with each other. In previous studies, complexes of this kind have been identified, many of them between closed-shell species and halogen atoms, [47][48][49][50][51] but also between the halogen atom and one bound halogen in the remaining radical (e.g., formation of iso-bromoform in the photolysis of bromoform in solution, 52 and formation of iso-chloroiodomethane in the photolysis of chloroiodomethane in cryogenic matrices). 53 We have identified the CH 2 X .…”