On the Isotopic Shift for Large-Amplitude Vibrations

The isotopic frequency ratio ${\nu }_{\mathrm{H}}{\mathrm{/\nu }}_{\mathrm{D}}$ is calculated for a linear molecular model $\mathrm{O–H–O}$ where the hydrogen atom moves in a double‐minimum potential. The results show that the isotopic ratio deviates considerably from the harmonic value but never becomes less than unity.