How inclusion of mass of nucleus explains different isotopes of an element produce lines in the spectrum at slightly different wavenumber ?

For isotopic particles, i.e. particles that contrast just by the mass of either of the cores 

in any case, not by their nuclear number (for instance 1H35Cl and 1H37Cl), the vibrational frequencies 

are clearly unique the division of relating vibrational levels in two isotopic particles are to some degree moved (and subsequently unearthly lines are additionally moved). The levels of the lighter isotope dependably lie higher than those of the heavier isotope.