We report on the vibrational mode dependence of the F-19 and H-1 hyperfine interaction constants in the A(1)A(') 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 F-19 spin-rotation constants, which is explained in terms of a decrease in back donation of electron density into the C 2p(pi) orbitals. (C) 2004 American Institute of Physics.