Wow more magnetic fields (anisotrophy), so any anisotrophic effects will inform you if the system is pi or not. Can you determine anything else about the compound from anisotrophic magnetic effects? It looks like there's an awful lot to learn here, I imagine it takes quite a bit of practice before you can look at the resonance lines of a variety of organic compounds and accurately describe their atomic structure.
Sort of. When looking at a spectra of a pure compound, there are several things that you might look at:
- Chemical shift. This would give an idea of what sort of functional groups are involved, or allow you to identify your peaks if you already know (or have an idea of) the structure. Anisotropic effects play into this a lot, and we use this to explain the fact that, for example, aromatic protons occur at such a relatively high chemical shift. Tables such as this are very useful.
- Integration. We discussed this earlier.
- Splitting pattern. I didn't mention this before. Take the 1H NMR spectrum of methyl propanoate:
The simplest peak you will see in an NMR is represented by the methyl peak at ~ 3.7 ppm. We would call this a singlet peak, since it appears as only one peak. The remaining two peaks look a bit funny by comparison, but there is an explanation for this. The protons labelled A are all what we call magnetically equivalent; that is, they are in the same magnetic environment as one another and thus converge as the same peak (I say peak in the singular, but I am referring to the entire triplet). The protons on the neighbouring carbon, B, are not the same as A. They experience a different magnetic environment, and hence occur at a different chemical shift. My very basic understanding of how this plays into splitting is that because B can be either spin up or spin down, A will experience three different magnetic environments (which I guess comes from the 3 possible combinations of spin states on the two B protons), and therefore appear as three peaks (a triplet). Similarly, the protons on B experience 4 different environments, and is hence a quartet. The general rule is that a given proton will split into n+1 number of peaks, where n is the number of protons on neighbouring carbons (that is, protons attached to carbons directly next door). With this information in hand, we can construct an idea of which peaks are next door to each other based on this and the integration. If I have a triplet peak with an integration of 3 and a quartet peak with an integration of 2 (as in the above example), I could reasonably expect that these two proton sets are right next to each other in the molecule.
The ratios of the peak heights is also important, and can help identify the splitting pattern. This gets pretty complicated pretty quickly (you can, for example, get very complex splitting such as a doublet of doublet of doublets), and there are exceptions, but you should have no trouble if the compounds you are dealing with are as simple as the iodo compounds you mention. Certain systems, notably aromatics, can experience coupling through multiple bonds. You can also get fine coupling through space. One other thing to note is that you can also get coupling to other types of atoms, depending on their nuclei. Phosphorus is one example that comes to mind. For more: http://www.chemguide.../splitting.html and https://www.khanacademy.org/science/organic-chemistry/spectroscopy-jay/proton-nmr/v/complex-splitting
- Coupling constants, or J coupling. These are the distance, in hertz, between the split peaks in a signal. You calculate it by calculate the difference in chemical shift between two peaks, and multiply by the applied magnetic field power in MHz. The best way to visualise them is to construct a splitting tree, such as these:
For reference, both of these peaks are doublets of doublets, which arise when a proton (A) is coupled to two other sets of protons that are not magnetically equivalent and do not have the same coupling constant (B and C). B and C are different in this case because there is no rotation around the double bond, forcing B and C to exist in different environments.
J couplings are quite useful, and can tell you a lot of information if you know what you're looking at. You'll notice in the above example that the Jab coupling in the peak for Ha is the same as the one for Hb. This is true for all such couplings, since the coupling that A experiences to B should always be equal in magnitude to the coupling experienced by B to A. We can therefore relate peaks to one another in this way by looking at their coupling constants, and begin to get an idea about which peaks are next to one another (with help from the splitting pattern).
The other thing you can glean from this comes about from this wonderful diagram:
The dihedral angle between neighbouring protons has a huge effect on the magnitude of the coupling constant. As you can see in the example, this is extremely useful when you are discerning between non-equivalent protons on a single carbon such as those on a cyclohexane ring or a sugar. They are also very handy when looking at cis and trans double bonds, since the dihedral angles between the protons either side of the double bond are very different.
I have been provided with chemical drawings for different molecular compounds and I have to be able to determine what their peaks would look like, also I am supposed to be able to describe what the compounds look like based on their names. I'm studying physics, so chemistry notation etc is not my forte. Looking at the scope of the subject, I'm not sure we have been given enough time (about 3 weeks). Perhaps I've misunderstood though, perhaps my tutor was referring to chemists or other users of NMR and not me, when she said 'alkane, alkene, alkyne, alcohol, amide, nitro & carboxylic acid are important? Or is it quite possible that I should be understanding the atomic structure of these function groups?
Thank you very much, you have been very helpful. Do you know of any papers, journal articles on basic NMR particularly to do with 1-iodopropane & 2-iodopropane or something similar. I have looked at a couple:
Unfortunately they either seem to be very complicated, far too complex for what I am doing or I can't get access with my athens?
You should look into some basic organic chemistry text books. Organic Chemistry by McMurry was a great starting point for me as I recall, and there are heaps of examples and chapter questions there to help you learn. I assume you are at college, in which case I would go to your library and see what they have. Clayden is another great one, though I can't speak for how well they go into NMR. The basic compounds you describe are also well characterised. You could very easily google their NMR spectra, or look it up on SDBS (or Reaxys / SciFinder if you have access to those). I am also happy to go through any specific spectra you are looking at.
I couldn't comment on what is expected of you in terms of learning, but given the timeframe I'd say you're not going to be going into huge amounts of depth. Much of what I've described is well beyond the scope of what you would encounter when you initially come into NMR. In fact, most of it I didn't learn until the third year of my chemistry degree. I only bring it up in so much detail because I find it interesting.