PGSE NMR diffusion study of the self‐association of N‐methylacetamide in carbon tetrachloride

Abstract

N-Methylacetamide (NMA) is known to self-associate in solution through the formation of hydrogen bonds. This behavior, and the presence in the molecule of an amide bond, make this an interesting model for study since the oligomer can be considered to be a protein analogue. The aims of the present study were to estimate the thermodynamic parameters of the self-association process, through the measurement of the diffusion coefficient of NMA in carbon tetrachloride (CCl 4 ), and to predict the population distribution of oligomers as a function of the concentration of NMA. Diffusion coefficients were measured using pulsed field gradient spin-echo (PGSE) NMR spectroscopy. A computer model based on the Kirkwood-Riseman theory of macromolecular diffusion and an attenuative model of self-association were used to fit the experimental data and to derive estimates of the thermodynamic association constants and bond length, and these values were, in turn, used to estimate the oligomer distribution. In addition, the temperature dependence of the diffusion coefficient was measured at three different concentrations in order to calculate the apparent Arrhenius activation energies so as to provide additional insight into the self-association process. Finally, the concentration dependence of the viscosity of NMA in CCl 4 was measured to characterize further the hydrodynamic behavior of the polydisperse system.

Supramolecular Polymerization

“…In both studies it was found that initial dimerization was less favorable compared to subsequent elongation, indicating a cooperative supramolecular polymerization. Further thermodynamic studies on the self-association of N-methylacetamide in apolar solvents probed using dielectric spectroscopy, 347 FT-IR, 348 and PGSE NMR diffusion 349 measurements have shown the generality of the results obtained by Davies, Thomas, and LaPlanche. On the basis of the values of the dimerization and elongation equilibrium constant that were reported in these studies, σ values between 10 -1 and 10 -3 can be calculated for systems that polymerize via amide type hydrogen bonds.…”

Properties and Catalytic Activities of New Easily‐Made Amphiphilic Phosphanes for Aqueous Organometallic Catalysis

“…In a solution in which the solute does not associate, it is expected that the selfdiffusion coefficient will be independent of the concentration. [9] As it can be seen from at the highest concentration of 2 and 4, respectively. This behaviour confirms that an aggregation phenomenon occurs.…”

Pulsed Gradient Spin Echo (PGSE) Diffusion Measurements as a Tool for the Elucidation of a New Type of Hydrogen‐Bonded Bicapsular Aggregate

“…The PGSE NMR diffusion measurements were carried out by using the stimulated echo pulse sequence,18b as has been explained elsewhere 17c. 28 All the experiments were performed on a 500 MHz Bruker AVANCE spectrometer, equipped with a microprocessor‐controlled gradient unit and a multinuclear inverse probe with an actively shielded Z‐gradient coil.…”