| Summary: | Monomers of <i>meta</i>-fluorophenol (<i>m</i>FP) were trapped from the gas phase into cryogenic argon and nitrogen matrices. The estimated relative energies of the two conformers are very close, and in the gas phase they have nearly equal populations. Due to the similarity of their structures (they only differ in the orientation of the OH group), the two conformers have also similar predicted vibrational signatures, which makes the vibrational characterization of the individual rotamers challenging. In the present work, it has been established that in an argon matrix only the most stable <i>trans</i> conformer of <i>m</i>FP exists (the OH group pointing away from the fluorine atom). On the other hand, the IR spectrum of <i>m</i>FP in a nitrogen matrix testifies to the simultaneous presence in this matrix of both the <i>trans</i> conformer and of the higher-energy <i>cis</i> conformer (the OH group pointing toward the fluorine atom), which is stabilized by interaction with the matrix gas host. We found that the exposition of the cryogenic N<sub>2</sub> matrix to the Globar source of the infrared spectrometer affects the conformational populations. By collecting experimental spectra, either in the full mid-infrared range or only in the range below 2200 cm<sup>−1</sup>, we were able to reliably distinguish two sets of experimental bands originating from individual conformers. A comparison of the two sets of experimental bands with computed infrared spectra of the conformers allowed, for the first time, the unequivocal vibrational identification of each of them. The joint implementation of computational vibrational spectroscopy and matrix-isolation infrared spectroscopy proved to be a very accurate method of structural analysis. Some mechanistic insights into conformational isomerism (the quantum tunneling of hydrogen atom and vibrationally-induced conformational transformations) have been addressed. Finally, we also subjected matrix-isolated <i>m</i>FP to irradiations with UV light, and the phototransformations observed in these experiments are also described.
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