Effect of Mass on Group Vibrations. Part 1: Carbon Dioxide, O=C=O.
Molecules of the type X=C=X or X=C=Y have two strong stretching vibrations, a symmetric and an asymmetric stretch. In the symmetric stretching vibration, the atoms at the ends both move away or toward the C-atom at the same time. In contrast, in the asymmetric stretching vibration, one of the bonds contracts while the other one is elongated and vice versa. The frequencies of these vibrations depend on the masses of the atoms X and Y. The heavier the atom, the lower the frequency. The series of molecules O=C=O, S=C=S, and O=C=S nicely exemplify the effect of mass. Look for the frequencies in the output files behind the links "Freqs".