Isotopic shifts due to heavy and very heavy atom substitution: a new sum rule and on the sensitivity of force constants to very small frequency shifts

IR and Raman spectra of dihydro-9,10 anthracene and dihydroanthracene-D₄(9,10) have been investigated in the 3100 to 100 cm^?1 region; an assignment is given. Spectra are interpreted in terms of a molecule composed of two ortho disubstituted benzene rings very similar to that of o-xylene. The symmetry of the dihydro-9,10 anthracene molecule in solution is discussed. The differences with respect to anthracene spectra are correlated with the electronic structure of the latter compound.