Abstract
Methoxyphenols (MPs) are a significant component of biomass burning emissions which mainly exists in our atmosphere in the gas phase where they contribute to the formation of secondary organic aerosols (SOAs). Rovibrational spectroscopy is a promising tool to monitor atmospheric MPs and infer their role in SOA formation. In this study, we bring a new perspective on the rovibrational analysis of MP isomers by taking advantage of two complementary devices combining jet-cooled environments and absorption spectroscopy: the Jet-AILES and the SPIRALES setups. Based on Q-branch frequency positions measured in the Jet-AILES Fourier-transform infrared (FTIR) spectra and guided by quantum chemistry calculations, we propose an extended vibrational and conformational analysis of the different MP isomers in their fingerprint region. Some modes such as far-IR out-of-plane -OH bending or mid-IR in-plane -CH bending allow us to assign individually all the stable conformers. Finally, using the SPIRALES setup with three different external cavity quantum cascade laser sources centered on the 930-990 cm-1 and the 1580-1690 cm-1 ranges, it was possible to proceed to the rovibrational analysis of the ν18 ring in-plane bending mode of the MP meta isomer providing a set of reliable excited state parameters, which confirms the correct assignment of two conformers. Interestingly, the observation of broad Q-branches without visible P- and R-branches in the region of the C-C ring stretching bands was interpreted as being probably due to a vibrational perturbation. These results highlight the complementarity of broadband FTIR and narrowband laser spectroscopic techniques to reveal the vibrational conformational signatures of atmospheric compounds over a large infrared spectral range.
Highlights
Methoxyphenols (H3CO–C6H4–OH, MP) are aromatic oxygenated compounds present in our atmosphere in their three isomeric forms: 2MP, 3MP, and 4MP, which designate the ortho, meta and para isomers
We investigate MPs rovibrational spectra at low temperature by taking advantage of two complementary supersonic jet devices associated to two different infrared spectroscopic methods: (i) the Jet-AILES apparatus, coupled to the AILES beamline at synchrotron SOLEIL, which was used for low resolution broadband Fourier-transform infrared (FTIR) scans of the three MP isomers in the fingerprint region (900-1700 cm−1); and (ii) the SPIRALES setup, with mid-IR external cavity quantum cascade laser (ECQCL) sources coupled to a pulsed molecular jet
The 3MP isomer spectrum has been investigated under various experimental conditions to optimize the signal-to-noise ratio (SNR) of the recorded MPs vibrational spectra
Summary
Methoxyphenols (H3CO–C6H4–OH, MP) are aromatic oxygenated compounds present in our atmosphere in their three isomeric forms: 2MP ( called guaiacol), 3MP, and 4MP ( called mequinol), which designate the ortho, meta and para isomers. Their measurements were performed using synchrotron-based roomtemperature Fourier-transform infrared (FTIR) absorption spectroscopy on the AILES beamline at the synchrotron SOLEIL [7] These cross-sections were determined at low resolution (0.5 cm−1) by integrating the rovibrational patterns but it was never possible to fully resolve the rotational structure even at the maximal resolution of the interferometer (0.001 cm−1) due to the huge density of states populated at ambient temperature. Millimeter-wave (mmw) spectroscopy allowed to fully resolve Doppler limited rotational structure at 300 K. as demonstrated recently with a full conformational analysis in the ground and in the lowest energy vibrational states of the three MP isomers using mmw absorption and emission spectroscopies [8, 9]. While the first group focused on the anharmonic coupling in the C-H stretch region, the second group provides MP conformational analyses in the electronically excited states
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