Isotope Effects on Molecular Vibrations

This example should work well for physical chemistry in the discussion of molecular vibrations. Optimize the structure of HCl at the HF/3-21G(*) level and calculate the vibrational frequency. Translational modes are eliminated so you should see a single vibration at 3151 cm-1. This number is 5.4% above the experimental value of 2989 cm-1. Which atom moves when you animate the vibration?

What happens to the vibrational frequency when hydrogen is changed to deuterium or tritium? Each version of the program provides a way to make this change. For example, in Spartan PC Pro, this feature is provided in the Display-Properties menu when you click on an atom. Create a new model for D-Cl in which the H has been changed to D and submit this for optimization and frequency calculation. Create a third model for T-Cl and submit this. Each job should give the same electronic energy and geometry but different vibrational frequencies.

HOD makes another excellent example. Change one hydrogen in water to a deuterium and then submit the job for optimization and frequency analysis at the HF/3-21G level. The C-H and C-D vibrational modes now have quite different frequencies!