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Integrating the Cross-Peaks by Fitting the Volumes with Gaussian Bells

The cross-peaks manager allows you to compare the volume of bidimensional peaks measured in 3 different ways. You can choose a different method for each peak.

Standard Integration: the integral is the sum of the data points inside a rectangle that you specify.

Arbitrary Value: in rare cases you might want to set the peak volume by hand. For example, if you have a doublet peak where one component is fully resolved and the other one hidden, you could integrate the unoverlapped part and manually double the volume. Under the Cross Peak Manager, select, with the picker tool, a single assignment. In the next dialog, choose “Set the value of this Integral”. You can either change the numerical value or change the shape and volume of the simulated peak, graphically and interactivelly. You can also change the frequencies and the widths of the peak in the same ways.

Line Fitting: iNMR can find the gaussian bells that best approximate the shapes and the volumes of one or more selected cross-peaks. This is the method of choice for overlapping peaks. In the case of multiplets, you have to create an assignment for each peak (including each component, in case of multiplets). When you will generate lists, with the commands “Copy Peaks” and “Copy Integrals”, a single entry will be reported for the whole multiplet, if all the components have been assigned in the same way.

To Fit a Cross-Peak or a Group of Cross-Peaks:

Step 1

After processing the 2-D spectrum, create a Cross Peaks Manager and add your spectrum to its list of managed experiments. With the peak-picker tool, create a cross at the center of the peaks of interest. Assign those peaks to the respective atoms.

Step 2

From the “Actions” menu of the Cross Peaks Manager, choose “Fitting Options”. These options belongs to the manager in use, not to the application. You might create many managers, each one with different options. More realistically, you can change these options before fitting different cross-peaks. If the options are not self-explanatory, read the help tips. If in doubt, keep the default options. All the parameters are integer values. If you don't want the peaks to move during fitting, set Max Motion = 0 ppb (1000 ppb = 1 ppm).

Step 3

With the picker tool, select the cross-peaks to fit, then choose “Fit Volumes w. Gaussian Bells”. 2-D spectra are normally processed with strong weighting functions, therefore the natural (and uncomfortable) Lorentzian shape is rarely seen. You can select all the peaks of the spectrum, if you like. iNMR will split the calculation in parts, if the peaks are well separated.

Step 4

All you can see, at the end of the operation, is a movement (if any) of the cross symbols (the assignments). To verify the goodness of the fit, select the Cross-Hair tool and move it over the plot. iNMR will show the cross-sections of the experimental spectrum and the fitting gaussian bells (in red, or the same color used for the scale labels). The more the two lines overlap, the better the fit.

Step 5

If the first run does not satisfy you, try again, using the same or different options.

Step 6

Some times it's better to restart from different intensity values and widths. Select one or more assignments with the picker tool and choose the command “Reset the Values”. iNMR will guess new starting values for you when you'll try to fit again the same peaks.

Related Topics

1-D Curve Fitting