Determination of %C-13 in Natural Carbon

Use sample 6-06. Use ns=1 (rg=30?) and obtain a normal proton spectrum for this sample. Check to see if the chemical shift scale is right by processing it with NUTS. You should see the H attached to acetone showing up at 2.04 ppm. This is due to the fact that only 99.9% of the hydrogens on the acetone are heavy hydrogen so that about 0.1% of them are ordinary hydrogen. Proton NMR is so sensitive that these hydrogens show up in the spectrum. This solvent peak, a 1:2:3:2:1 pentet, should be centered at 2.04 ppm. (If not, you can use fo on pnmr to set it there the next time a spectrum is taken.). You should also see a very large peak at about 7.3 ppm for the chloroform peak. This peak is due to the H of chloroform, CHCl₃, - 99% of which is attached to C-12. Look very closely and see if you can see the H attached C-13. These will be very small peaks about 1.8 ppm on either side of the C-12 H peak. Plot out this spectrum and interpret the peaks. If you can see the C-13 peaks measure the distance between them in ppm and use the conversion factor 1 ppm =60 Hz to calculate the CH one bond coupling constant, , J¹_CH.

Now go back to pnmr and set ns=32 and zg to obtain a better spectrum. Got to NUTS and use control F2 to process the data. If you look at the C-12 proton peak of chloroform, CHCl₃, you will see around it some peaks about 30 Hz (.5 ppm) to either side of it. These are spinning side bands and the frequency is equal to the frequency that the sample is being spun at. Further off to the side at 110 Hz or (1.8 ppm) you should see small peaks due to the C-13 proton peaks for chcl3. These are the peaks of interest in this experiment. Plot out the entire spectrum and label the peaks.

Enter the zoom subroutine (see p 18 in the NUTS manual) by double clicking the left mouse button. Now zoom the chloroform peaks so that you are looking at a region from 5 to 11 ppm. This is done by moving the mouse to 11 ppm and hold the left mouse button down and drag through cursor through to 5 ppm so that this region is highlighted. Right click once to zoom the region. Use the scroll bar on the right raise the vertical scale sufficiently that the c-13 satellite peaks are clearly visible.

If anything goes wrong or to exit the zoom region hit enter. If your display is not correct hit enter to go to the main menu and under the view menu choose view all reals.

Now phase the spectrum- ap and do a baseline correction bc.

Enter the integration subroutine.(see p 27) id. On the left will be a scroll bar to scale the integration and on the right a scroll bar to scale the spectrum peaks. Choose a region around the C-13 satellite peaks by clicking the left mouse button once which will bring a cursor on the screen click it again to the left of the peak and the move it to the right of the peak and click it again. Do the same around the C-12 peak. Use bc to base line correct. Scale the C-12 peak to 100 by placing the cursor in the middle of the peak and give the command v. p will plot the spectrum. From the integration calculate the %C-13 in natural carbon. For example if your integrations are 1.04, 100, .59 ; then the %C13 = 2*.82+/-.22 or 1.64 +/- .44 %. Show your calculation on your spectrum. Compare your results to the actual % of 1.11%.

On your spectrum indicate the C-13 peaks and the spinning side bands. Calculate the rate at which the sample is spinning in the instrument. Calculate the one bond C-H coupling constant, J¹_CH, of the C-13 to H bond in chloroform in Hz.

Write a summary of your results and tell how this relates to the relative difficulty of obtaining C-13 spectra relative to the difficulty of obtaining H-1 spectra.

If you wish you can expand the spectrum around the CHD₂ peak of the acetone and calculate the 2 bond coupling constant between deuterium and hydrogen and explain why this is a 1:2:3:2:1 pentet.