We have studied the d-wave superconducting transition temperature T-c
and the isotope effect in the spin fluctuation models. We found that t
he isotope exponent alpha is at least one order order of magnitude sma
ller than typical observed values in the cuprates with models coupling
to harmonic phonon degrees of freedom and reasonable limits on coupli
ng strength and phonon spectra. Models with coupling to non-linear tun
neling modes in a strongly anharmonic multi-well lattice potential and
realistic coupling strengths have effectively larger isotopic mass de
pendences and may therefore account for the observed magnitudes of alp
ha.