In the figure below, the mass of the terminal hydrogen atoms on the acetylene is hypothetically varied from zero to infinity. The response of the symmetric stretching (3374 cm–1 ) and triple-bond stretching (1974 cm–1 ) modes to the change in mass is shown.
(a) Calculate the expected deuterium isotope shift for the symmetric stretching mode. Is the hypothetical value close to the calculated value? Explain.
(b) Explain why the triple-bond stretching frequency is approximately 100 cm
1 higher for high-mass terminal isotopes (>100) than for the low-mass terminal isotopes ( <>