Dynamic NMR: Field dependence of the coalescence temperature and temperature dependence of T₂ Relaxation times in N,N‐dimethyltrichloroacetamide

Abstract

The field dependence of the 'H and 13C coalescence temperatnres in N,N-dimethyltrichloroacetamide has been used to determine the most reliable activation parameters for internal rotation of the amide moiety. The results were found to be in excellent agreement with those obtained earlier by the double-fitting procedure and demonstrate'its validity. Direct measurements of the Tz relaxation times of the methyl protons show that Tz depends exponentially on temperature.

Cross‐Peaks in Simple Two‐Dimensional NMR Experiments from Chemical Exchange of Transverse Magnetisation

“…The HMQC experiment was selected, as this was established above to be the most sensitive experiment for observing phase distortions. Exchange rates were determined from 2D fitting of individual spectra, and varied from 19.0±0.6 s −1 at 278 K to 482±4 s −1 at 313 K. NOESY and HMQC results both fitted well to the Eyring equation (Figure ), fully consistent with previous measurements (Δ H ≠ =67.4±3.9 kJ mol −1 and Δ S ≠ =15±13 kJ mol −1 K −1 ) . We note that at higher temperatures, 2D lineshape analysis of the NOESY experiments, acquired with very short mixing times, was also required in order to characterise the exchange process (Figure S4).…”

“…Having established a conceptual basis for these unexpected exchange cross‐peaks, we sought to fit the observed spectra quantitatively, in order to verify our understanding of the process and to characterise the kinetics of the underlying exchange process. 2D lineshape fitting was performed using TITAN [which in this case reduces to the numerical integration of Equation (1)], and fits of NOESY spectra with both zero and non‐zero mixing times reproduced the observed spectra very closely (Figure C,D), with fitted exchange rates (indicated in the figures) consistent with published results . We note that while in principle cross‐peaks formed at zero mixing times are dispersive and integrate to zero, in practice the long dispersive tails render this impractical and lineshape fitting is therefore recommended.…”

“…As part of an effort to validate the accuracy of 2D lineshape analysis, we conducted a series of measurements of the small molecule N , N ‐dimethyl‐trichloroacetamide (DMTCA) (Figure A,B), the two methyl groups of which undergo exchange by rotation about the amide bond with a rate of 125 s −1 at 298 K . DMTCA is a simple molecule, with no resolved homonuclear scalar couplings, but serves to illustrate fundamental principles that will be equally applicable to more complex molecules and exchange processes.…”

Combined use of complete lineshape analysis of 1D spectra subjected to reference deconvolution and linear prediction, 2D-EXSY spectra and a double fitting method for the study of chemical exchange. Application to an eight-site exchange system

“…In the slow exchange regions, where the signals are not very sensitive to exchange effects, their linewidths depend mainly on the rate of spin-spin relaxation, R 2 0 Ð R 2 0 depends exponentially on the temperature at which the spectrum is measured. 4,5 The apparent relaxation rates, R 2 Ł , which are frequently used instead of R 2 0 in the CLSA, are unknown functions of the temperature, however. In the double fit approach (see later), one needs to use the R 2 0 values.…”

Cross-Peaks in Simple 2D NMR Experiments from Chemical Exchange of Transverse Magnetization

“…[9] This approach, and the associated TITAN analysis software, has since found applications to av ariety of biomolecular interactions. [10,11] As part of an effort to validate the accuracy of 2D lineshape analysis,weconducted aseries of measurements of the small molecule N,N-dimethyl-trichloroacetamide (DMTCA)( Figure 1A,B), the two methyl groups of which undergo exchange by rotation about the amide bond with arate of 125 s À1 at 298 K. [12][13][14] DMTCAisasimple molecule, with no resolved homonuclear scalar couplings,but serves to illustrate fundamental principles that will be equally applicable to more complex molecules and exchange processes.…”