We report on the vibrational mode dependence of the F19 and H1 hyperfine interaction constants in the Ã1A″ state of HCF, determined using polarization quantum beat spectroscopy. The nuclear spin/overall rotation coupling constants display a pronounced energy dependence and mode selectivity which can be traced to variations in both the A rotational constant and nuclear spin/electron orbital coupling constant a. In particular, modes containing C–F stretching excitation display significantly larger F19 spin-rotation constants, which is explained in terms of a decrease in back donation of electron density into the C 2pπ orbitals.