Bayesian analysis investigation of chemical exchange above and below the coalescence point

The coalescence temperature for the exchange of the N-methyl substituents in N,N-dimethylacetamide (DMA) was determined to be 97°C by Bayesian analysis, which is higher than any previously reported temperature. The FFT method is unable to determine that there are still two separate peaks for the N-methyl groups just below this temperature due to line broadening in the Fourier transformed spectrum. The decay rates for DMA over a temperature range 23-141°C were determined by the doublet frequency model of Bayesian analysis below the coalescence temperature, 97°C, and by the singlet frequency model at and above 97°C. These decay rates were used with the time domain spectral equations to determine the rate of exchange at each temperature. The Arrhenius plot of the rate constant as a function of temperature was linear over this temperature range and gave the following activation parameters: E_a = 19.37 ± 0.03 kcal/mol, In A = 31.10 ± 0.07, and ΔG^‡_298K = 18.40 ± 0.03 kcal/mol.