INFRARED EMISSION FROM UV-IRRADIATED MIXTURES OF CH2I2 AND O2 PROBED WITH A STEP-SCAN FTIR SPECTROMETER

Abstract

The Criegee intermediates, carbonyl oxides proposed by Criegee in 1949 as key intermediates in the ozonolysis of alkenes, play important roles in organic chemistry and atmospheric chemistry. The simplest Criegee intermediate is \chem{CH_2OO}. In the reaction of \chem{O_3} with \chem{C_2H_4}, some \chem{CH_2OO} thus produced are internally excited so that they decompose to form \chem{OH}, \chem{CO}, \chem{CO_2}, and other compounds. Recently a new scheme for production of \chem{CH_2OO} in laboratories, ultraviolet (UV) irradiation of diiodomethane (\chem{CH_2I_2}) in \chem{O_2}, has enabled direct detection of \chem{CH_2OO} with various methods and stimulated active research on Criegee intermediates. Even though $\sim25 \%$ of \chem{CH_2OO} was reported to decompose at pressure smaller than 60 Torr,\footnote{W.-L. Ting, C.-H. Chang, Y.-F. Lee, H. Matsui, Y.-P. Lee, and J. J.-M. Lin, \textit{J. Chem. Phys.} 2014, \textbf{141}, 104308.} and infrared absorption of internally excited CO and \chem{CO_2} was reported,\footnote{Y. T. Su, H.-Y. Lin, R. Putikam, H. Matsui, M. C. Lin, and Y. P. Lee, \textit{Nat. Chem.} 2014, \textbf{6}, 477.} no investigation on the dynamics of the decomposition products in the reaction \chem{CH_2I} + \chem{O_2} has been reported.

We employed a step-scan Fourier-transform infrared (FTIR) spectrometer to record temporally resolved emission upon irradiation of mixtures of \chem{CH_2I_2}, \chem{O_2}, and Ar at 248 and 308 nm. IR emission of \chem{CO}, \chem{CO_2}, \chem{OH}, \chem{CH_2I}, and \chem{H_2CO} in the region $1860-4900$ \wn was recorded. At total pressure 8 Torr and irradiation wavelength 248 nm, rotationally resolved lines of \chem{CO} (v $\leq11$, J $\leq19$) in region $1860-2300$ \wn were observed; the rotational distribution is Boltzmann with temperature near 300 K, but the vibrational distribution is bimodal, with two components having averaged vibrational energies of 99 and 18 kJ mol$^{-1}$. Emission of \chem{OH} (v $\leq3$, J $\leq5.5$) in region $2980-3600$ \wn was observed with ambient rotational distribution and average vibrational energy of 41 kJ mol$^{-1}$. The branching ratio of \chem{CO} : \chem{OH} is 60:40. Emission of highly internally excited \chem{CO_2} was also observed; its average internal energy was estimated. The effects of pressure and irradiation wavelength on the emission of these species will be discussed.