[tulip11]

I was thinking about how well the harmonic oscillator approximation represents the real potential of a molecule and was wondering it there was any dependence on mass of the molecule, i.e. if anharmonicity would be expected to increase down a group?

Although I can't find any mass dependence in the equations it feels like anharmonicity would be bigger in Po₂ rather than in O₂. I can't find any grounds as to why this may be true, but was hoping someone could tell me if it is true either way! 

[Corribus]

Sorry you did not receive an answer yet. This is an interesting question that I'd have to think about.

[mjc123]

Couldn't find anything about Po₂, but got these data from NIST:
Molecule      ω_e (cm^-1)     x_eω_e (cm^-1)
O₂               1580.2          11.98
S₂                 725.7          2.844
Se₂               385.3          0.964
Te₂               247.1          0.515
Thus it seems for these molecules, relative to the fundamental frequency, the anharmonic correction is larger for the lighter molecules.

[tulip11]

Thankyou! I find this really interesting although the trend of anharmonicity to molecular weight appears to be the opposite way around to what I originally thought. Do you have any ideas as to what may cause greater deviations from harmonic potential in lighter molecules?

Could it be that in the case below it is due to
 1. Increasing size down the group tends to make the bond weaker and weaker therefore a quicker deviation from the harmonic potential as it tends to dissociation quicker
2. Smaller atoms have more repulsion, due to presence of lone pair causing there by at shorter bond lengths the repulsive forces are stronger again deviating from harmonic potential quicker?