Tunnelling splittings in the rotational spectrum of 3-fluoro-benzylalcohol

► μc-Type transitions of 3-fluoro benzyl alcohol (3FBA) are split into two evenly spaced component lines separated by ≈1.64 MHz, showing that the CH2OH group undergoes a tunnelling motion which connects two equivalent minima above and below the aromatic ring. ► The μc-doubling of rotational transitions is not observed in the related molecules benzyl alcohol (BA, prototype) and 2-fluoro benzyl alcohol (2FBA). ► The observed tunnelling splitting is interpreted in terms of a smaller inversion path and of a narrower barrier width of 3FBA with respect to BA and 2FBA. ► The barrier to inversion and the potential energy function to the CH2OH group internal rotation have been estimated from the measured splitting by applying a numerical flexible model.