H-NMR spectrum with spin-spin coupling¶

Logfile: AMStail¶

You can follow the progress of the calculation by clicking SCM → Logfile (AMStail). The chemical shifts will be calculated first, followed by the couplings constants for the perturbing and responding atoms.

When the calculation has finished the end of your logfile will look something like this:

View the ¹H-NMR spectrum¶

Use AMSspectra to visualize the calculated spectrum

The NMR chemical shift \(\delta_i\) is defined as the difference of the isotropic NMR chemical shielding of a reference compound \(\sigma_{ref}\) minus that of the interesting compound \(\sigma_i\):

The NMR program calculates NMR chemical shieldings \(\sigma_i\). AMSspectra uses as ¹H reference (TMS) shielding \(\sigma_{ref}\) = 31.7 ppm.

By default only the chemical shifts are visualized using plain singlets. To switch on the visualization of couplings:

This switches on the spin-spin coupling (default machine frequency at 200 Mhz). An additional section in the table will appear which displays additional information for any selected atom or peak in the spectrum. For example:

Average chemical shifts and couplings for equivalent atoms¶

As you may have noticed, there are a lot of splittings in the simulated spectrum. This is because all the chemical shifts and coupling constants are calculated at fixed geometry, which means that there is no rotation which would create magnetically equivalent groups.

You can resolve this issue by either flagging chemically equivalent atoms manually or have AMSspectra guess them for you. To manually supply the information:

Select groups of chemically equivalent atoms and

The atoms should now be surrounded by a colored sphere. Continue with all remaining atoms. Your spectrum should look as follows now:

Alternatively, to have AMSspectra guess chemically equivalent regions for you, go to

Comparison of calculated and experimental spectrum¶

A good source for experimental spectra is the SDBS database: http://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi

These are the peak data for the ¹ H-NMR spectrum of ethyl acetate (CAS = 141-78-6) from the SDBS database (the three columns are Hz, ppm and Intensity):

379.88   4.242     52
372.75   4.163    171
365.56   4.082    180
358.50   4.003     62
182.56   2.039   1000
119.69   1.337    245
112.63   1.258    560
105.38   1.177    214

Now we switch back to AMSspectra where we can paste the contents from the clipboard directly:

AMSspectra will ask which columns to use for X and Y, and offer to rescale the data:

You now should see both the experimental and the calculated spectrum in one graph. The experimental spectrum (at least the one used to create this tutorial) was measured at 89.56 MHz. So adjust the frequency (default at 200 MHz) of the calculated spectrum:

This should give you something like this:

Sometimes it makes sense to change one of the spectra to a stick spectrum, showing only sticks for the calculated positions and heights, not applying the broadening. As an example lets do that here for the calculated peak positions:

Now you should get the experimental spectrum as a broadened curve, with sticks for the calculated positions: