Sample: 5% dimethylformamide in C₂D₂Cl₄. Warning: Do not overheat the sample!

Connect your instrument for high temperature measurements, adjust a reasonable nitrogen flow, set the control to room temperature, and let the sample equilibrate for at least 5 minutes. The spectra should be recorded with homonuclear decoupling of the aldehyde proton in order to simplify the evaluation. Record a normal ¹H NMR spectrum, note the position of the aldehyde proton resonance, and set the decoupler offset accordingly. Run the spectrum again under decoupling conditions. The signals of the methyl groups should now have equal heights. Change the temperature in 10 K steps at first and within the actual exchange region in 5 K steps until the signals of the methyl groups coalesce, or with high field instruments to a maximum of 430 K. Perform the experiments in reverse order and check for reproducibility. Adjust the decoupler position for the aldehyde proton before every measurement. You have to set:

decoupler attenuation corresponding to γB₂ = 10 Hz
td	32 k
sw	12 ppm
o1	middle of ¹H NMR spectrum
o2	on resonance of the aldehyde proton
p1	45° ¹H transmitter pulse
d1	300 s to equilibrate temperature
stable gas flow for temperature regulation
ns	8

5. Processing

Use standard processing as described in Experiment 3.1, and for each temperature run an expanded plot of the signals of the methyl groups. The comparison of theoretically calculated line-shapes with the experimental ones is performed by simulation programs, such as DNMR3 or MEXICO, which are now implemented in the SpinWorks PC software [7]. Note for each temperature the line-width of the residual proton signal of the solvent.

6. Result

Displayed on the next page are four typical spectra taken on an AM-400 spectrometer at 400–430 K. From the complete series the result ΔG^‡(298) = 21.4 kcal/mol was obtained; ΔS^‡ was essentially zero.

7. Comments

It would be beyond the scope of this book to describe the theory of line-shape investigations in NMR spectroscopy. The interested reader is therefore referred to the fundamental review articles [1–3]. For our purpose here it is sufficient to know that two exchanging sets of nuclei can only be separately observed if the rate constant of the exchange is considerably smaller than their chemical shift difference in Hz (NMR time-scale). The goal of the experiment is to derive a table of rate constant vs. temperature.