Tom Hoye's 2/18/05 Friday seminar contained a number of ideas that are valuable and are easy to do in practice. Here are some suggestions on reducing three of them to practice on the KU Bruker spectrometers.
1. Coupling constant analysis: Extracting the coupling constants is often aided by resolution enhancement after the spectrum is acquired. This does not affect the raw data, so you can try it as many ways as you like without losing anything from the original spectrum. A very quick no-adjustment enhancement can be done in XWIN-NMR on previously acquired data by typing
This is a fairly extreme enhancement and the signal to noise may be too low afterwards. A kinder, gentler approach is to experiment with a negative lb and a positive gb. This approach may benefit from experimenting with the values:
set lb to -.5
set gb to 0.5
More negative lb and gb closer to 1 provide more extreme enhancement. Putting lb back to 0.3 and typing efp will restore the original spectrum.
Once you have your J values, you need to know how to interpret them. The two PC's in room 3002 have a desktop link to a program called MestReJ (from the same people that wrote MestReC) that allows you to calculate J as a function of dihedral angle accounting for the effects of various functional groups. You can select from several popular methods of calculating the J (the Karplus method was the first but not necessarily the best). MestReJ is also available as a free download.
Molecular modeling is very valuable aid to interpreting both angles and distances from NMR--either Monte Carlo or molecular dynamics simulations. If you don't know how to do these things, ask Gerry Lushington for assistance.
2. Presenting spectra (e.g. in the supplementary material of a publication) as PDF files: a really good idea because the lines can be expanded and retain their sharpness. The two PC's in 3002 have Acrobat installed so you can create PDF from previously acquired spectra. Each computer has posted instructions about how to switch between printing to paper and to a PDF. Unfortunately, the SGI computers running XWIN-NMR will not create PDF files directly. Bruker's new software, TopSpin, will. This software is running on two instruments already. We plan to upgrade all the instruments over time, starting with the 500, at which point the autosampler software will be able to email you a PDF of your spectra automatically as soon as the data is acquired.
3. Spectra in protonated solvents: you can hand shim without a lock. Load the normal parameters and do an rga. Then type
This puts you in "go setup" mode and you can watch the signal in real time as you shim. The goal is to get the longest, smoothest exponential decay possible. For experienced users, this does indeed work fine but I recommend doing gradient shimming instead. In addition to practice, hand shimming takes concentration, patience, and time. Gradient shimming only takes pushing a software button that says "Start Gradient Shimming." Your login needs a one-time configuration to enable this--ask for assistance.
To run the spectrum unlocked, you should make sure the light in the "sweep" button on the BSMS panel is off (push the button once if it is on), otherwise the spectrum will be heavily distorted. You should be able to run unlocked experiments up to about 15 minutes duration without excessive line broadening from magnet drift.
Below are two spectra of a taxol analog on the bulletin board outside 3002 taken in CHCL3. (These are JPEGs for being embedded in the web page, so they don't expand nicely like a PDF.) The first one is taken "as is" after gradient shimming. The chloroform's carbon-13 satellites provide a sense of scale. Neat chloroform is 12.5 M and the satellite peaks are 182 times smaller than the main peak, so the satellite is effectively one proton at a concentration of 6.8 mM.
The second spectrum was taken with suppression of the solvent peak (something I also recommend) with a parameter set called AAWET (WET is a popular water suppression technique but it works on any solvent). You can do solvent suppression with no lock, but the advantage of having even 1 drop of deuterated solvent present IF you can is that if you have any lock, no matter how weak, you can dial in the solvent suppression by supplying only the chemical shift of the solvent (as o1p), and it works. (The spectra below were taken with a drop of CDCl3 in the tube.) Without a lock, it is necessary to take a spectrum first without solvent suppression to get the absolute frequency of the solvent peak, and then set it by hand. You do lose any peaks within about 0.1 ppm of the solvent peak, but these peaks are pretty much lost anyway when the solvent peak is covering them up. The rejection of the solvent peak can be improved by tweaking the settings, but the no-tweak spectrum is usually good enough for almost anything when there is just one solvent peak. WET can also be tweaked to reject multiple solvent peaks, and remove the 13C satellites from the spectrum, if desired.
Spectrum in CHCl3, no solvent suppression
Spectrum with WET suppression of the chloroform (rpar AAWET, o1p set to 7.26, rga, zg)
Dave Vander Velde 2/20/05