High-resolution sub-Doppler infrared spectroscopy of atmospherically relevant Criegee precursor CH₂ I radicals: CH₂ stretch vibrations and “charge-sloshing” dynamics

Journal Article · Fri May 04 00:00:00 EDT 2018 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.5028287· OSTI ID:1540202

Kortyna, A. ^[1]; Lesko, D. M. B. ^[2]; Nesbitt, D. J. ^[1]

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1. National Inst. of Standards and Technology (NIST), Boulder, CO (United States); Univ. of Colorado, Boulder, CO (United States)
2. Univ. of Colorado, Boulder, CO (United States)

The combination of a pulsed supersonic slit-discharge source and single-mode difference frequency direct absorption infrared spectroscopy permit first high resolution infrared study of the iodomethyl (CH₂I) radical, with the CH₂I radical species generated in a slit jet Ne/He discharge and cooled to 16 K in the supersonic expansion. Dual laser beam detection and collisional collimation in the slit expansion yield sub-Doppler linewidths (60 MHz), an absolute frequency calibration of 13 MHz, and absorbance sensitivities within a factor of two of the shot-noise limit. Fully rovibrationally resolved direct absorption spectra of the CH₂ symmetric stretch mode (ν₂) are obtained and fitted to a Watson asymmetric top Hamiltonian with electron spin-rotation coupling, providing precision rotational constants and spin-rotation tensor elements for the vibrationally excited state. Assessment of the asymmetric top rotational constants confirms a vibrationally averaged planar geometry in both the ground- and first-excited vibrational levels. Sub-Doppler resolution permits additional nuclear spin hyperfine structures to be observed, with splittings in excellent agreement with microwave measurements on the ground state. Spectroscopic data on CH₂I facilitate systematic comparison with previous studies of halogen-substituted methyl radicals, with the periodic trends strongly correlated with the electronegativity of the halogen atom. Surprisingly, we do not observe any asymmetric CH₂ stretch transitions, despite S/N ≈ 25:1 on strongest lines in the corresponding symmetric CH₂ stretch manifold. This dramatic reversal of the more typical 3:1 antisymmetric/symmetric CH₂ stretch intensity ratio signals a vibrational transition moment poorly described by simple “bond-dipole” models. Instead, the data suggest that this anomalous intensity ratio arises from “charge sloshing” dynamics in the highly polar carbon-iodine bond, as supported by ab initio electron differential density plots and indeed consistent with observations in other halomethyl radicals and protonated cluster ions.

• Research Organization: Univ. of Colorado, Boulder, CO (United States)
• Sponsoring Organization: USDOE Office of Science (SC); National Science Foundation (NSF)
• Grant/Contract Number: SC0002123; FG02-09ER16021
• OSTI ID: 1540202
• Alternate ID(s): OSTI ID: 1436014; OSTI ID: 1924535
• Journal Information: Journal of Chemical Physics, Vol. 148, Issue 17; ISSN 0021-9606
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY