We have studied the isotope effect on the superconducting transition t
emperature T-c of d-wave pairing instabilities due to antiferromagneti
c spin fluctuation exchange. We found that models with conventional co
upling to harmonic phonon degrees of freedom and reasonable limits on
coupling strength and phonon spectra, give values for the isotope expo
nent alpha which are at least one order of magnitude smaller than typi
cal observed values of alpha in the cuprates. Models with coupling to
non-linear tunneling modes in a strongly anharmonic multi-well lattice
potential and realistic coupling strengths have effectively larger is
otopic mass dependences and may therefore account for the observed mag
nitudes of alpha.