[dave37]

Hi, during a lecture on IR spectroscopy, our professor gave us an oscillation frequency for Hcl of 2885.9 wavenumbers. He asked us to estimate what it would be for cl2. Could anyone give me a start in the right direction?
 Thanks

[Enthalpy]

It won't be better than an estimation.

Suppose the stiffness of the bond is the same for H-Cl and Cl-Cl, deduce the frequency ratio by the mass. Don't forget than both atoms move in each case.

[dipesh747]

E = hv (where v is frequency)
E = hc / λ
E = mc²

This is a very 'waffly' way of looking at it, but here we go.

Energy is proportional to mass and frequency. Therefore mass is proportional to frequency. As mass is increased frequency is increased. As v = c/λ, v is proportional to 1/λ. 1/λ = wavenumber. Therefore as mass is prop. freq which is prop. wavenumber, then as mass is increased then wavenumber also increases.

If you want numbers, I would suggest using reduced mass for the mass part of the equation (A1A2/A1+A2)

I might be wrong here, so anyone feel free to pick up if I made a mistake.

[Enthalpy]

Yes, you're wrong here, sorry. This is a mechanical oscillation, it doesn't convert the atoms' mass into energy.