Spectral Editing with Pulses and 2D NMR.

A. Attached Proton Test APT

An example of how pulses are uses in more sophisticated ways is the attached proton test. This sequence is as follows:

On the carbon 13 frequency the pulse sequence starts with 90 degree pulse on the x-axis, 90_x. A delay is put in then of or about .007 seconds. Analysis of what happens during this time period is then the key to understanding the resulting spectra.

A schematic of the simplified pulse sequences are:

Below the pulse sequences are pictures of the magnetic vectors of a C-13 nucleus with various hydrogens attached at various times in the pulse sequence. In each case the pulse is assumed to be a frequency in the center of the C-13 peak. For a CH₁ peak for example the pulse would be in the center of the doublet with the two peaks on either side- one at +70 Hz or J¹_CH/2 and one at -70 Hz . For a CH₂ peak for example the pulse would be in the center of the triplet with the main peak at the pulse frequency (0 Hz on the rotating frame) and the two peaks on either side- one at +140 Hz or J¹_CH and one at -140 Hz . The axis of the circles are x-axis down and y axis to the right. The time of the vector is labeled as B,C, D etc as indicated at the top of the diagram. The key to understanding what happens is: i. Examine one row at a time and ii. Focus on what takes place between B and C!! Before B you have the ordinary pulse spectra. After C no development of the spin system takes place because the hydrogens that are coupled to the C are being decoupled.

The resulting spectrum is one in which the carbons attached to even numbers of hydrogen show up as positive peaks and the carbons attached to odd hydrogens show up as negative peaks.

Run an APT spectrum of neat ethyl benzene 1-99 on the Unity + 500 NMR. First run a normal C-13 spectrum (qc with x as solvent and ns=2) and then an apt spectrum (use the apt command). Be sure to interpret the spectrum.

B. Het2dj -2 dimensional NMR.

The above pulse sequence can be used to illustrate 2 dimensional NMR. If instead of the delay times of 1/J used in APT the time between B and C, the delay time is varied and second Fourier transform is done on this time coordinate to give the second axis in a two d experiment. The resulting plot give the ordinary Fourier transform along the y-axis of the C-13 chemical shifts and along the x-axis the coupled CH peaks.

Below is an example of a Het2dj spectrum. The series of peaks at 60 ppm indictes a C-13 attached to 2 hydrogens (split into a triplet) whereas the peaks at 135 ppm indicate a C-13 attachd to just 1 hydrogen (split into a doublet).

Obtain a het2dj spectrum of neat ethyl benzene, 1-99. First obtain a decoupled C-13 1 D spectrum first. (qc solvent = x and ns=2).

Then simply command the instrument: het2dj . When you have collected adequate data - about 10 fids, you can check the data by doing the 2d processing (do2d or process). You may need to massage the display by increasing the vertical scale (vs) and or changing the threshold (th) etc. The screen may be refreshed with the dconi command or the redraw button. This spectra should be symmetrical around the y axis and this can be forced by using the foldj command. Data can be plotted by using the plot2d or plot command.

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On a cover page to these spectra summarize your results.