Example 1: Oxalic Acid Dihydrate

In the first example, we look at a molecule with 1 distinct carbon environment and 2 oxygen environments. Firstly you’ll note that we see multiple carbon environments. The extra carbons can be assigned as adventitious carbon and used to energy correct the spectra. Check out the processed vamas file to see the component fits referred to as C1 and adventitious.

Regarding the O 1s spectrum, we can indeed fit a doublet relating to -OH and carbonyl groups (using area/fwhm constraints to keep the peaks at identical peak areas and widths). For simplicity however, it is often preferred to just treat this as a single peak; since resonance of the carbonyl bond makes these very close in energy.

Example 2: Polyvinylpyrrolidone

For the PVP example, we’ve chosen to use area constraint to force the peaks into the expected ratio. Look at the constraints sections of the processed file for help. This example is intended to show how prior knowledge of the sample can go a long way to helping describe the data we record!

Example 3: Sulfanilamide

Looking at the molecule we would initially be tempted to say (correctly), that we expect 3 distinct carbon environments. However if we actually look at our C 1s spectrum, we clearly only see 2. Rather than try and cram 3 peaks in, if we fit these with 2 we actually see that we see we observe 2 peaks in a 2:1 ratio. Though we might initially think that these groups not be equivalent (different electronegativities, amines are strong electron donating groups to aromatic rings while sulfamides are weak electron withdrawing groups) – we may potentially find an answer if the resonance structure. The resonance structure of sulfanilamide is thought to be dominated by the sulfamide, therefore reducing the capacity for the amine to act as an electron donating group and resulting in relatively equivalent electron densities on the para– carbons in the ring.

Another oddity is that of the N 1s region. Though we see some emission at the high BE side of the peak, there are not two distinct nitrogens as we may expect. This may again be explained by the resonance structure; where the local electron density around the nitrogen centre may be balanced by resonance (which may also give rise to the slight signal above the main peak).

This example is intended to show how it is important to think about the data both in terms of the environments we might expect to see – but also keep in mind it is important to work from what the data is showing us!

Example 4: Urea

Again – a simple example, remember the adventitious carbon!