Department of Chemistry,
University of Alberta March 2003
NMR News 2003-01
News and tips for users of the Varian NMR systems in the Department
Editor: Albin.Otter@ualberta.ca http://nmr.chem.ualberta.ca
There are no fixed publishing dates for this newsletter; its appearance solely depends on whether there is a need to present information to the users of the spectrometers or not.
Other content of this NMR News is no longer meaningful and has been removed April 2010.
13C-1D: wider ranges and new text
The different phase of C/CH2 and CH/CH3 signals is a key feature of APT experiments. What signals point up and what down depends entirely on the phasing of the spectrum and is therefore reversible (however, CH2 and CH3, for example, will always be of opposite sign relative to each other). Consequently, the terms "positive" and "negative" or "up" and "down" to distinguish between the two groups of signals are not all that meaningful. It is better to refer to their phase relative to the solvent signal. APT text now reads that C/CH2 are on the same side as the solvent, whereas CH/CH3 are on the opposite side. Of course, in D2O such a distinction is, unfortunately, not possible.
Credit to Glen Bigam who solved this problem in a most cost-effective manner by installing a pressurized dispenser tank used previously in soft-drink dispensers!
now on anti-vibration legs
To install the new support structure, the magnet was lifted off the ground while at field. As this is not exactly an everyday event, a picture was taken and is shown here. It is at field no doubt, as evidenced by the pull on the large nut at the end of a string. Often visitors can hardly accept the fact that the many cables around the magnet do not provide the field (as in conventional electromagnets). Here is the proof: not a single cable and still field!
2003-01.1: I expected a doublet of doublets but I see more lines, what
Of the higher order spectra the so-called AB system is quite well understood. The close spectral proximity in chemical shift of the peaks under question makes it conceptually easy to understand that something "strange" is happening to them. Much more difficult is the so-called ABX case whereby X is nowhere close to A and B and therefore should show simple first order behavior (the nomenclature of spin systems follows the logic of the alphabet: X is far away from the letters A and B). This is not the case. As the example below shows, the X part can be very complicated. Strictly speaking this is an ABXY system with the Y part being of little consequence to what is happening: H3 and H4 form the AB part, H2 is X and H1 is Y.
At 600 MHz the separation between H3 and H4 (AB) is 27 Hz and the coupling constant between them is 9.5 Hz (resulting in a quotient 'delta chemical shift'/'coupling constant' of 27/9.5 = 2.8). Nothing unusual can be seen in the H2 expansion with regard to the doublet of doublet pattern.
At 300 MHz, the above mentioned quotient is reduced to 1.4 (the chemical shift difference is proportional to the field but the coupling constant is not, so 13.5 Hz divided by 9.5 Hz) and four new lines with very different intensities appear: two inside and two outside the expected d x d signal. The NMR theory stipulates that for values below 10 for this quotient higher order conditions apply. This example shows, and confirms many other observations, that it often takes a much smaller value than 10 to really make anything associated with higher order visible. Even 2.8 does not do it but 1.4 sure does!
In this case, the two coupling constants associated with H2 can still be extracted in the usual way but only because the coupling between H2 and H4 is zero (in general terms either JAX or JBX must be zero for this to hold true). If this were not the case, the signal would deteriorate further and instead of true couplings, sums and differences of coupling constants would appear in the signal. When this happens, only a computer-based simulation of the spectrum can reveal the correct coupling constants.
The phenomenon of extra lines in an NMR signal is also called virtual coupling. It is not a particularly good expression. It provides little in terms of explanation of what is going on, instead it just gives it a new name. Depending on the AB chemical shift difference (H3/H4 in this case) and the size of the coupling constants involved, the X (H2) signal can even be much more complicated than shown above.
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