Modeling the internal rotation tunnelling in benzyl alcohol by ring fluorination: The rotational spectrum of 3,5-difluorobenzyl alcohol

The rotational spectra of the –OH and –OD isotopologues of 3,5-difluorobenzyl alcohol have been measured by pulsed jet Fourier transform microwave spectroscopy. The main structural evidence is that the OCα-C1C2 dihedral angle, approximately 20° is ca. 40° smaller than in benzyl alcohol. This difference is reflected in the features of the rotational spectrum: μc-type (instead of μb-) transitions are split into two evenly spaced component lines (by 14.367(1) × 2 MHz) showing that the CH2OH group undergoes a tunnelling motion which connects two equivalent minima above and below the aromatic ring. The barrier for the interconversion of the two mirror form has been estimated from the experimental tunnelling splitting to be 380 cm−1.