Background:
Summary of Methods Used
Simple Example:
Theory
The Vibrations
Research Topics:
Fatty Acid Methyl Esters (FAMEs)
School of Chemistry
The Queen's University of Belfast
A Simple Example
Lets start off by looking at the Carbon Dioxide* molecule again.
CO2 has a simple linear structure and, because it has more than two atoms, it can vibrate in several different ways. These different types of motion occur at different frequencies.
The frequencies of these motions can be calculated using a variety of commercially available programmes based upon the mass of the atoms and the strength of the bonds (as I explained earlier, I use Gaussian 98). The animations that I present here are based upon this type of calculation. This means that they are not just animations generated from my ideas of how I think the atoms are moving, they are the vibrations as calculated by Gaussian 98.
The experimental I.R. spectrum of CO2 is shown below top, whilst the simulated spectrum generated from the calculated frequencies is shown bottom.
Hmm...I know that the quality of these spectra isn't very good so you can download better ones here:
The Experimental Spectrum (5kb) | The Calculated Spectrum (35kb)
Right away you can begin to see how useful the calculations can be. Assigning the vibrational bands of CO2 isn't a hard task, but as the molecules get bigger it becomes more and more difficult to make accurate assignments. With this very simple molecule there is very good agreement between the two spectra, of course the calculated one has no 'noise' on it and there are no combinations or overtones as can be seen on the real spectrum between 3500 - 3900 cm-1. Even with large molecules the agreement between real and calculated spectra is good enough to provide secure and unambiguous assignments of the observed infrared (and Raman) bands.
It's important to say at this point that the vibrational frequencies obtained from DFT calculations tend to be higher than those observed experimentally, but they are closer to the experimental frequencies than those calculated by semi-empirical and Hartree-Fock methods. However, this overestimation of vibrational frequencies is, generally, found to be uniform so that scaling factors can be used throughout a series of homologous molecules to give good agreement with the experimentally obtained vibrations. The determination of appropriate scaling factors for estimating experimental frequencies has received considerable attention in the literature [5-7].
So - what vibrations give rise to these peaks and why are there only two peaks (due to fundamental vibrations) to be found in the spectra? I'll answer both these questions on the next page. The files on the next page will take a short time to download, it shouldn't be more than about 30secs though so please be patient.
