Molecular-Level Insights of Interaction between Bromomethane and Water: Infrared Matrix-Isolation and Theoretical Studies

Abstract

Heteroaggregates of bromomethane and water were studied experimentally via matrix-isolation IR spectroscopy and theoretically using the ωB97X-D functional with the aug-cc-pVTZ basis set for all atoms except for bromine, for which the aug-ccpVTZ-PP basis set was employed. Bromomethane and water mixtures at various ratios were diluted in argon and deposited onto a window cooled to 4−10 K. The resulting matrices were studied by FTIR spectroscopy both before and after annealing to 30−35 K. The 1:1 CH3Br:H2O complex was clearly identified in the FTIR spectra, even in matrices containing only traces of water. The lowest-energy 1:2 CH3Br:H2O heteroaggregate was also observed in the FTIR spectra of the mixtures. Despite the high water concentration, no clear evidence has been found for the presence of heteroaggregates containing larger numbers of water molecules. However, our results demonstrate that homoaggregates of CH3Br are no longer formed once water molecules are present. To aid in the interpretation of the results in terms of heteroaggregates, Armatrices containing only CH3Br were also studied to investigate homoaggregate formation. The experimental IR findings were compared with simulations of theoretical IR spectra for dimers and trimers. In addition to identifying the previously reported lowenergy head-to-tail isomer of (CH3Br)2, further spectral features were attributed to a bromomethane trimer. IR bands corresponding to this species were detected in the more concentrated matrix and grew upon annealing. The homoaggregates of chloromethane and the CH3Cl:H2O heteroaggregates were also studied for comparison. This work highlights the affinity of halomethanes for water whatever the halogen (i.e., Cl, Br, and I), implying a probable modification of halo methane reactivity in the atmosphere that should be considered in the atmospheric studies.