Rotational Spectroscopic and Ab Initio Investigation of the Rotamer Geometries of 2‑Fluoroanisole and 3‑Fluoroanisole

The rotational spectra of 2-fluoroanisole and 3-fluoroanisole were investigated using Fourier transform microwave spectroscopy in the 4–26 GHz range. Assigned transitions correspond to the lowest energy rotamer for 2-fluoroanisole which has the O–CH₃ group directed away (anti) from the fluorine substituent whereas for 3-fluoroanisole, the spectrum is consistent with the presence of two rotamers arising from syn and anti orientations of the methoxy moiety relative to fluorine. Ab initio calculations at the MP2/cc-pVTZ level were used to estimate the equilibrium (r_e) geometries of the three observed rotamers. Their assignments were confirmed through the observation of the rotational transitions of eight minor isotopologues (¹³C and ¹⁸O) in natural abundance for each species. The mass dependence (r_m⁽¹⁾) structures derived using the experimentally determined rotational constants compare favorably with the ab initio estimates. The resulting sets of geometric parameters suggest that the aromatic ring backbone is distorted by the introduction of the angular methoxy substituent, with a tendency to induce bond length alternation around the ring, and by the electron withdrawing effects of fluorine.