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

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.


APT, 13C-1D: wider ranges and new text

i600: air dryer

s400: now on anti-vibration legs (field without a single cable!)

FAQ 2003-01.1: I expected a doublet of doublets but I see many more lines, what is wrong?


APT, 13C-1D: wider ranges and new text
To accommodate users with needs for larger sweep widths in 13C NMR, the defaults in EZ NMR button-selected APT and 13C-1D experiments have been extended to cover a 250 to -20 ppm range. Referencing has been adjusted and other pertinent changes have been implemented as well.

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.

i600: air dryer
The installation of an air dryer for low temperature work created a most interesting and unexpected problem that has now been solved. Through different absorption of nitrogen vs. oxygen in the air dryer's desiccant, the gas exiting the air dryer has a periodically changing N2/O2 composition, resulting in a change of the magnetic susceptibility around the sample (O2 is paramagnetic!). This entailed large, unacceptable drops in the lock signal. The installation of a ballast tank after the air dryer to remix the air proved efficient to eliminate this problem.

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!

s400: now on anti-vibration legs
The s400 spectrometer in WB-13 has been equipped with anti-vibration legs. Tests show a substantial improvement in spectral quality (it suffered from vibration side-bands before). This should particularly benefit the more delicate experiments such a NOESY and TROESY.

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!

FAQ 2003-01.1: I expected a doublet of doublets but I see more lines, what is wrong?
Aside from the chemical shift, the appearance of proton NMR signals is largely the result of couplings to neighboring protons (mostly 2J and 3J coupling constants are responsible, but occasionally also 4J and 5J can create line splitting). The anticipated number of lines (four in a doublet of doublets, for example) is based on the expectation/hope that first order rules apply to the spectrum. Fortunately, this is happening most of the time when using spectrometers at 300 MHz and higher. Therefore, today's NMR users often are totally taken by surprise when more lines than expected appear in a multiplet, i.e. the first order rules no longer work (those of us, who had a chance to "enjoy" 60 MHz spectrometers, are typically far less surprised by such events!).

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.

A particularly severe case of a higher order H2 is shown on the left: the worst or most beautiful one, it depends on the perspective, the editor has ever seen. Hard to believe but this one was recorded at 600 MHz and 10 more lines than expected according to first order rules were observed for a grand total of 14 lines. Amazing!

Especially noteworthy is the nearly perfect symmetry of the signal around its center. Since the coupling between H2 and H4 is also zero in this case (the molecule is similar to the one shown in the first example), the two coupling constants creating the most intense lines can be extracted: J(H1H2) and J(H2H3).

 A signal like that is best called a higher order multiplet.

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