Theoretical Predictions of Molecular Properties

When we discuss molecular properties, we are usually referring to the properties of a molecule when it is at equilibrium: i.e., when the nuclei are in their minimum-energy configuration on the potential energy surface. It is important to keep this in mind, since computational chemistry programs often report properties (energy, dipole moment, etc) for any geometry at which they are run. So, if you are trying to report the equilibrium properties, you need to first optimize the molecule to get the nuclei at the minimum on the PES. Only then should you attempt to determine the equilibrium molecular properties. In particular, be careful that you don't accidentally reset the geometry to the Hyperchem ``guess'' geometry by hitting ``Model Build'' after the geometry optimization. If you do this, your results will all be for the wrong geometry!

A special comment should be made about theoretically determined vibrational frequencies. Every computational chemistry program defaults to predicting the harmonic vibrational frequencies, which are the frequencies determined from the second-derivative of the potential energy surface according to the harmonic oscillator approximation:

$$\omega_i = \sqrt{\frac{k_i}{m}}$$ (2)

for each normal mode i. However, the potential energy surface is of course not exactly harmonic, and the true vibrational frequencies will have contributions from third, fourth, and higher derivatives of the PES. Thus, theoretical harmonic frequencies are almost always too high compared to experimentally measured fundamental frequencies.

That said, it is still sometimes possible to make correct comparisons between theory and experiment. One way is to compute the fundamental frequencies using theory. This is very involved and far beyond the scope of this lab. The other way is for experimentalists to do enough experiments to extract out the harmonic frequency. These are usually available for diatomic molecules and some polyatomic molecules. If experimentally deduced harmonic frequencies are available for your molecule, please report them. If not, please note that you have fundamental frequencies, and be aware that there will be a small discrepancy due to anharmonicity (often about 2-3%).