Full Conformational Landscape of 3‐Methoxyphenol Revealed by Room Temperature mm‐wave Rotational Spectroscopy Supported by Quantum Chemical Calculations

Abstract

Room temperature millimeter-wave rotational spectroscopy supported by high level of theory calculations have been employed to fully characterise the conformational landscape of 3-Methoxyphenol, a semi-volatile polar oxygenated aromatic compound precursor of secondary organic aerosols in the atmosphere arising from biomass combustion. While previous rotationally-resolved spectroscopic studies in the microwave and in the UV domains failed to observe the complete conformational landscape, the 70-330 GHz rotational spectrum measured in this study reveals the ground state rotational signatures of the four stable conformations theoretically predicted. Moreover, rotational transitions in the lowest energy vibrationally excited states were assigned for two conformers. While the inertial defect of methoxyphenol does not significantly change between conformers and isomers, the excitation of the methoxy out-of-plane bending is the main contribution to the non-planarity of the molecule.

Jet-cooled rovibrational spectroscopy of methoxyphenols using two complementary FTIR and QCL based spectrometers

“…The 3MP isomer exhibits the richest conformational landscape with four stable conformers at room temperature, namely 3MP-C1 to 3MP-C4 (see Fig. 3a), as evidenced by previous rotationally resolved spectroscopic studies [8,11]. In the far-IR, the ν 6 out-of-plane OH bending mode has a very strong band intensity (I harm > 90 km/mol) compared to the other low-frequency vibrations (I harm < 10 km/mol) which are most probably below our detection limit.…”

“…High level of theory quantum chemical calculations on MPs were performed using the Gaussian 16 package [21] and have already been described elsewhere. [8,9] Briefly, optimized equilibrium structures and vibrational frequencies were calculated using the B3LYP functional and Dunning correlation consistent basis set cc-pVTZ [22]. This functional/basis set combination has proven to be extremely reliable in the computational study on the molecular conformations of phenolic compounds [23].…”

“…The latter setup allowed to fully resolve the ν 18 IR band of 3MP centered at 950 cm −1 , to fit the rotational constants in this in-plane ring bending mode and to assess the conformational landscape previously determined. [8] Moreover, using a second QCL source centered around 1610 cm −1 , strong rovibrational broadenings have been observed for the ν 36 and ν 37 modes, highlighting a likely vibrational perturbation. In the following, a brief description of experimental and theoretical methods is presented in a first section.…”

“…Furthermore, the different conformers of each isomer could not be distinguished. Only 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]. These studies emphasized the congestion of lines in MPs spectra due to rather small rotational constants and the presence of several conformers and pure rotational transitions within the low-lying vibrational excited states.…”

Monitoring of food spoilage by high resolution THz analysis

“…Many small polar molecules display a strong rotational spectral fingerprint in the THz domain. The analysis of a material by THz spectroscopy is particularly attractive as it has great potential for the discrimination between different chemical species, isomers [18], conformers [19], and isotopologues [20]. Such a degree of selectivity is obtained for the analysis of gas phase samples and especially at low pressure when the line widths are not broadened by pressure effects.…”