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This tutorial introduces Direct Hard Modeling — a simple yet powerful approach within Spectral Hard Modeling.

In this tutorial, you learn how to:

1. Load Bruker NMR data
2. Apply Direct Hard Modeling
3. Define Custom Results
4. Analyze unknown spectra

If PEAXACT is installed on your computer, you may follow along immediately. If not, get a free trial now.

The NMR spectroscopic data, kindly provided by Prof. Thiele’s group at TU Darmstadt, originates from the thermal relaxation process of a UV-irradiated molecule containing three azobenzene units [1]. It reveals four distinct photoisomers, distinguished by their side-chain configurations. In this tutorial, you will construct Hard Models for each isomer and quantify them based on peak area conversions.

In the first step, you load Bruker 1r files into PEAXACT and complement them with corresponding titles.

• Open the File Explorer and navigate to DATA\Analysis. In the search bar, type 1r.
• Drag and drop all matching 1r files into the PEAXACT Samples Panel.

Since the file names are not distinctive, let’s load titles for each in the second step:

• Select all samples in the Sample Panel and choose Data > Data Inspector….
• In the Data Inspector window, press Ctrl+A to select all samples.
• Select Change Title > From File Reader to auto-load titles.
• Click Apply at the bottom.

By these steps, you define a special categorical feature {Title} that sets the display name for each sample. The display names are now shown in tables and plots instead of the filenames.

We're focusing on the H₇-Proton signals for each side chain (see Figure below). Let’s select the relevant spectral region:

• In the Data Inspector window, switch to the Data Plotter tab.
• At the bottom-right panel, apply the following pretreatments:
  • Global Range: 7.65 to 8.2
  • Excluded Ranges: 7.85 to 8.04
• Select Export to create a new model with these settings. The model will appear in the Model Tree Panel of the Main Window.

Next, we’ll model the mixture spectra. A Hard Model is ideal here since it handles overlapping and shifting peaks well. We'll use Direct Hard Modeling (DHM), as pure component spectra are unavailable. DHM requires all components that should be modeled to be present in a so-called representative spectrum. Start by manually selecting a spectrum containing all four isomers:

• In the Samples Panel, browse and select a suitable spectrum (e.g., 2816\1\relaxation reaction).
• Select Edit Model > Hard Model > Add & Autofit Peaks.
• Enter a name for the first component, set 12 peaks, and proceed.

These peaks correspond to the four photoisomers. Assign them accordingly:

• Hold Ctrl while clicking on all peaks for a specific isomer in the Plot Panel. Selected peaks turn bold cyan.
• Right click to open the context menu, select New Component..., and name the isomer.
• Repeat for each isomer.

So far, we’ve built a mixture model that can compute each component’s area. Thanks to NMR, these areas can easily be converted to molar fractions using Custom Results.

• Select the mixture model and choose Edit Model > Custom Results > Add...
• Use the dialog window to specify the result as follows:
  • Analysis = Component Fitting
  • Returns = Component Area
  • Of (component) = ttt
  • As (name) = Area ttt
  • Intermediate (Hidden) = enabled
• Repeat for components tcc, ttc, and ccc.

Now, define the sum of the areas:

• Add another Custom Result for the area sum:
  • Analysis = Custom
  • Returns = Custom Number
  • As (name) = Area Sum
  • Hidden = yes
• Use the editor in the Model Properties Panel and the References drop-down list to build the expression: Area ttt + Area tcc + Area ttc + Area ccc
• Important: Press Enter key to save the expression.

Finally, let's analyze the spectra.

• Select all samples in the Samples Panel and choose Analysis > Custom.

The results regarding area ratios defined in the previous step are displayed in the Report Window. Now, you can evaluate the results as a graph or as a table and export them by clicking the Export button. What you see is what you export!

That's the power of Direct Hard Modeling for NMR spectra! Still want more? Check out our other tutorials at the PEAXACT Quick Start page!

[1] J. Kind, L. Kaltschnee, M. Leyendecker, and C.M. Thiele (2016). Distinction of trans–cis photoisomers with comparable optical properties in multiple-state photochromic systems–examining a molecule with three azobenzenes via in situ irradiation NMR spectroscopy. Chemical Communications, Vol. 52 (84), pp. 12506–12509.