The aim of this work is to present a full set of 34 empirically determined harmonic force constants F-i,F-k for benzene (in symmetrized Whiffen's coordinates), as well as the corresponding set of 20 harmonic normal mode (NM) frequencies for all four D-6h isotopomers: C6H6, C6D6, (C6H6)-C-13, (C6D6)-C-13. The reliability of the obtained harmonic force constant values is reinforced by their ability to reproduce satisfactorily the experimentally measured fundamental vibrational frequencies of the four D-6h benzene isotopomers. A specific combined LM (local mode)/SM (symmetrized modes) complex symmetrized nonperturbative vibrational model, developed in our previous work, has been employed for calculations on the vibrational energy levels in benzene, using the harmonic force constants F-i,F-k and a few diagonal anharmonic parameters, as input data. A set of local (valence) harmonic force constants has also been derived from the empirically determined symmetrized force constants F-i,F-k, and their physical meaning was discussed. The set of 20 harmonic NM frequencies omega(k) for all four D-6h benzene isotoporners, calculated in the present work using the empirically determined set of F-i,F-k values, have been analyzed and compared to previous empirical and ab initio determinations by other authors.