Unimolecular decomposition rates of a methyl-substituted Criegee intermediate syn-CH3CHOO

Criegee intermediates play important roles in atmospheric chemistry. Methyl Criegee intermediate, CH3CHOO, has two conformers, syn- and anti-conformers. Syn-CH3CHOO would undergo fast unimolecular decomposition to form OH radical via 1,4 H-atom transfer. In this work, unimolecular decomposition of syn-CH3CHOO was probed in real time with UV absorption spectroscopy at 278–318 K and 100–700 torr. We used water vapor as the scavenger of anti-CH3CHOO to distinguish the absorption signals of the two conformers. After removing the contributions from reactions with radical byproducts, reaction with water vapor and wall loss, we obtained the unimolecular reaction rate coefficient of syn-CH3CHOO (at 300 torr), which increases from (67 ± 15) s−1 at 278 K, (146 ± 31) s−1 at 298 K, to (288 ± 81) s−1 at 318 K with an Arrhenius activation energy of ca. 6.4 kcal mol−1 and a weak pressure dependence for 100–700 torr. Compared to previous studies, this work provides temperature dependent unimolecular rates of syn-CH3CHOO at higher pressures, which are more relevant to atmospheric conditions.

We found the reaction of syn-CH 3 CHOO with water vapor has a weak pressure dependence for 100 to 700 Torr.To ensure the consistency of k w (P,T), we normalized the values of k w obtained from Figure S4 to be consistent with the value obtained from the Arrhenius plot at 298 K and 300 Torr (Figure S3).For k w at 500 Torr, we used the average value of k w at 300 and 700 Torr.The solution of (Eq 3):

Effect of second-order reactions
The solution of (Eq 4):

Modelling of diffusion and wall loss (i) Estimation of the diffusion coefficient of CI
The diffusion coefficient of CI, which has not been reported, is estimated with the approach proposed by Fuller et al.: Where D CI-A is the binary diffusion coefficient in cm 2 /s, T is the temperature in K, P is the pressure in atm, and M CI and M A is the molar mass of CI and species A, respectively.The dimensionless diffusion volume,Σ, of various gases have been reported elsewhere, whereas that of CI is estimated by summing up the contributions of every atoms of CI. 1 Since there are only trace amounts of CI in the system, the diffusion constant of CI in a homogeneous gas mixture is estimated with the Blanc's law: 2 The diffusion volumes of various gases and CI, and the diffusion constants of various reaction conditions are summarized in Tables S5 and S6.

(ii) Simulation of the diffusion loss
The diffusion and wall loss of CI is estimated with the approximation of Fick's law.Considering the cylindrical symmetry, the governing equation can be simplified as: Where C is the concentration of targeted molecules, r is the radius, and D CI-mix is the diffusion coefficient of CI in the gas mixture.The governing equation of diffusion was solved with the buildin partial differential equation solver of MATLAB to obtained the time dependent concentration distribution in radial direction of the targeted molecule, C(r,t).S6 (Sim# 1-4).

Simulation of diffusion loss for the experiments of Zhou et al.
The experimental setup has been mentioned in their previous publication. 4The inner diameter of the reaction cell is 5.30 cm, whereas the diameter of the photolysis laser beam is 0.6 cm.Hence, the boundary condition is set as: (2.65, ) 0 , (0, ) 0 and the initial condition is set as: The probe beam, whose diameter 0.4 cm, is perpendicular to the photolysis laser.Hence, the calculated C(r,t) is weighted with the differential area at different photolysis beam radius and at different probe beam radius.The simulated decay curve (Figure S7) shows that the diffusion loss is quite significant.The main reasons are (i) their photolysis beam size is much smaller than that of ours and (ii) their pressures are lower.This result indicates that diffusion loss is not negligible under the experiment conditions of Zhou et al.

Figure S1 .Figure S2 .Figure S3 .Figure S4 .
Figure S1.Fitted parameters of the double exponential model (Eq 1) for Exp.W2-3 at 308 K. (a) Pre-exponential factors and the offset plotted as functions of [H 2 O].(b) The observed decay rate coefficient of anti-CH 3 CHOO (k obs,anti ) as a function of [H 2 O].


A and  B are the mole fractions of gas species A and B, and D CI-A and D CI-B are the diffusion constants of CI in gases A and B, respectively.

Figure S8 .Figure S9 .S19Figure S10 .
Figure S8.The simulated diffusion loss for the experiment conditions of Zhou et al.The reaction condition is described in Table S6 (Sim 5-9).

Table S1 .
Summary of experimental conditions for the reaction of syn-CH 3 CHOO + H 2 O a Error bar is one standard deviation obtained from the linear fitting of k obs against [H 2 O].
a Error bar is one standard deviation, which will be discussed in Error Estimation.

Table S3 .
Summary of experimental conditions for syn-CH 3 CHOO thermal decomposition at various Error bar is one standard deviation, which will be discussed in Error Estimation. a

Table S4 .
Summary of experimental conditions for wall loss estimation a Error bar is one standard deviation obtained from the linear fitting of k obs against [CH 2 OO] 0 .[H 2 O] / 10 17 cm 3

Table S5 .
The molecule weight and diffusion volume of various species 1 Diffusion volume is calculated by summing up the contributions of the atoms.1

Table S6 .
Experimental condition for the simulation and the calculated diffusion coefficient of various Criegee intermediate (CI).