Cr(acac)3 works every well in organic solvents for speeding up T1 relaxation, but does not dissolve in water. A good choice of water soluble T1 relaxation agent is GdCl3. Recommended concentration is 0.1-0.3 mg/ml.
This method is also good for water-swellable solid samples – prepare a GdCl3 solution and swell your polymer sample in the solution, and you have finally found a solution to your challenging signal intensity problem!
1. Set up your solvent suppression standard file (you only need to do this once).
(a) create a blank file from any template. Name it something like “standard-watersup”
(b) type rpar, select “…/user” directory, then select “ZGPR1”. Click OK on the window that subsequently pops up.
(c) type getprosol to complete setup. (you could also complete steps (b) and (c) by checking appropriate boxes in the edc window).
2. Find the frequency of the solvent (most frequently water) peak. You will need to do this every time you run a solvent suppression experiment. Water suppression only works when the frequency of your pulses is exactly on the water peak.
(a) run a regular proton spectrum;
(b) zoom in your solvent peak on the screen;
(c) click this button (set RF by cursor):
(d) left click on solvent peak top, and the following window will pop up. Write down the frequency shown in the pink blank:
(e) click Cancel.
3. Run experiment.
(a) create a new file from your standard water suppression file;
(b) type o1 (o stands for offset), enter the value you recorded in Step 2.
(c) run your expt like you do any other NMR expt (rga, zg).
(d) use manual phase correction as apk often has a hard time dealing with the somewhat distorted water peak. phase it such that your solute peaks are correctly phased, which might mean that the solvent peak has to be left out of phase. This will be addressed in the next step.
4. Optimization
(a) display a spectrum range of ca. 1 ppm wide with the solvent peak roughly in the middle. Type dpl1. This defines the spectral range that will be displayed during parameter optimization.
(b) type paropt (which stands for parameter optimization). You will be asked several questions: (1) parameter to optimize. enter o1. (2) beginning value. e.g. if the o1 determined in the last step was 2350Hz, enter 2345. (3) increment. enter 1. (4) number of experiments. enter 11. This will run 11 experiments with o1 ranging from 2345 to 2355.
(c) tighten the paropt step size and find the best solvent suppression. When the suppression is best, the out-of-phase water peak problem will be minimized. You might have to use 0.01 Hz step size to find the best suppression.
Molecule: 2-alkyl-benzimidazole
Red spectrum: regular 13C
Blue spectrum: 13C with Cr(acac)3 using the setup procedure described in this blog entry.
Both were run at ca. -24degC. Both took ca. 45 min.
Cr(acac)3 is especially powerful in speeding up the T1 relaxation of unprotonated 13C.
Sample: ca. 3 wt% regio-random P3HT in CDCl3.
Red spectrum is regular 13C. experiment time = 28 min.
Blue spectrum is DEPT135. experiment time =18 min. S/N is about 3 times better, with about 1/2 of experiment time compared to regular 13C. Also clarifies assignment of protonated vs non-protonated carbon peaks.
It is straightforward to setup and run the DEPT experiment. Refer to the “Training Materials” section of this blog to find out.
Graphic outputs are advantageous over printouts in many respects. They are easier to store and file. They have higher resolution and are easier to incorporate into papers or reports.
There are two ways to generate graphic outputs in Topspin:
1. In xwp, choose Print and select Print to File. It will generate a .ps file which is of high resolution and can be easily processed in standard graphic software such as Adobe Illustrator.
2. In main menu, click File and select Export. you will need to add an extension name to specify the graphic format. Popular choices are .png and .jpg. The resolution of these files is not as high as the .ps file generated in xwp.
These files are saved in your home folder (/home/…) rather than the Bruker data file folder (/opt/topspin…). You will need to delete those files in you home folder frequently since they reside on a small disc partition, which gets full quickly.
NOE effect is a powerful tool to probe spatial relationship between atoms in a molecule. 2D NOESY is a relative easy experiment to set up. However, samples with small concentration have challenging signal sensitivity for 2D NOESY. In such cases, 1D NOE Difference Spectroscopy is an excellent alternative.
Assuming you are dealing with two peaks, each from a different molecule (A and B), and you want to figure out their molar ratio and weight ratio.
First, measure the signal area of the two peaks, area(A) and area(B).
Second, count number of protons (or other nuclei in question) contributing to the peak, N(A) and N(B).
Third, find out the molecular weight of the molecules, MW(A) and MW(B).
The molar ratio is:
mol(A)/mol(B) = [area(A)/N(A)]/[area(B)/N(B)]
The weight ratio is:
wt(A)/wt(B) = [mol(A) * mw(A)]/[mol(B) * mw(B)]
You can also calculate the concentration of one sample using another sample as a reference with known structure and concentration. First, run NMR for the two samples. They will have to be run with the same NMR techniques, same parameters, and with the same rg (receiver gain). Second, integrate the peaks in the reference sample. Third, integrate the peaks in the sample in which you want to find the concentration, then right click on the integration and select “Use last scale for calibration”. Then you will be able to find the area ratio between the two samples. This ratio, normalized by number of scans, can be used to further determine mole ratio, weight ratio, or concentration ratio.
Same principles can be applied when calculating moles and weights of interested molecules in solid-state NMR. For two samples A and B, the weight ratio for the molecules contributing to the peaks of interest is:
wt(A)/wt(B) ={ [area(A)*mw(A)]/[N(A)*NS(A)] } / { [area(B)*mw(B)]/[(N(B)*NS(B)] }
First, you will need to create a standard file. Create an empty file by typing edc. Then type rpar, which brings up a big window. Look for the nuclei of interest, eg., P31, Si29, B11, etc., and click on the choice, then click OK. Then you must type getprosol to complete the standard file setup.
Once you have the standard file, the rest is similar to running a 1H or 13C experiment. On 400, you will need to run atma.
Some nuclei have several options in the rpar window. For example, P31CPD is P31 with proton decoupling, while P31 is without the decoupling. You can run both and compare the spectra and look for the difference.
F19 only works on DPX300 (B622 Conte). You can only select F19 (not F19CPD) in rpar window. 31P and 13C work on both DPX300 and Avance400 (LGRT room 075). All other nuclei can only be done on Avance400.