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1H NMR spin-lattice (T1) and spin-spin (T2) relaxation times have been measured for the residual CHCl3 component of deuterochloroform (99.8 atom% D) in neat solvent samples and in a series of isotropic solutions of the ionic surfactant n-octylammonium-n-octadecanoate and also in degassed/dried and aerated solvent systems at 298 K. Analysis of the proton relaxation times as composite values from various relaxation mechanisms suggests that the spin-lattice process is predominantly via spin-rotation while spin-spin relaxation principally involves 1H-Cl scalar coupling. Rotational correlation times from relaxation rates via the dipolar and spin-rotation mechanisms reveal similar fast picosecond molecular rotation of chloroform in the different samples. The presence of surfactant in the solution appreciably affects the 1H NMR spin-lattice relaxation time of CHCl3 and the chemical shift, which is concentration dependent owing to weak intermolecular interactions. Effects of sample condition and solvent degassing, drying, and aeration treatments on the NMR relaxation times and line shape properties of the residual CHCl3 isotopomers are also examined. [ABSTRACT FROM AUTHOR]

H NMR chemical shift, line width, indirect nuclear splitting value, peak area integration value, and spin-lattice and spin-spin relaxation times at 298 K are compared for low-concentration isotropic solutions of n-octylammonium n-octadecanoate prepared via different techniques and conditions using dried, distilled, and degassed deuterochloroform and the nontreated solvent containing tetramethylsilane. The nature of the variation of observed spectral parameters and relaxation/rotational behavior with chemical composition (presence of oxygen and other paramagnetic species, stabilizer, impurities, and degradation products) of the solvent, history of the solution, and sample containment are analyzed. Relaxation times are interpreted in terms of monomer structure and reorientation and internal rotation modes as a function of atomic position along the n-alkyl chains. Collectively, the relaxation behavior of the surfactant complies with the two-step model of fast picosecond internal rotations of different size segments containing methylene groups separated in timescale from slower large segment and overall molecular tumbling modes of the monomer. Fast motional phenomena do not appear to be appreciably influenced by the chemistry of the solvent in contrast to spectral parameters such as chemical shift and line width of the labile ammonium protons. A model is also presented to explain anomalous variation of the peak area integration value with chemical shift of the ammonium resonance peak. [ABSTRACT FROM AUTHOR]

Abstract: 1H NMR spin–lattice and spin–spin relaxation times of isotropic solutions of n-octylammonium n-octadecanoate in deuterochloroform containing ca. 0.1–0.2% tetramethylsilane (v/v) at ca. 292K are observed to vary with position of the functional group in the amphiphile ion-pair, and with solute concentration. The latter dependence can be satisfactorily described using either single or multiple equilibria models of association complex formation of the reverse micelle type above a critical concentration (ca. 0.13mol kg−1). Transverse relaxation is accounted for predominantly via reorientation/rotational diffusion of monomers and micellar aggregates, and is analyzed in terms of two component times, while longitudinal relaxation principally involves motions of different size segments around covalent bonds. Picosecond correlation times are attributed to intramolecular and monomer rotations, whereas multimer reorientation/tumbling processes are an order of magnitude longer. Results are analyzed for size of the reorienting species in terms of theoretical calculations for rotational diffusion of model cylindrical and ellipsoidal volumes of revolution, which above the critical micelle concentration correspond to association complexes of 2–4 monomers. Dilution shifts of the proton NMR peak positions also comply with the model of reverse micelle formation. The use of nonspinning liquid samples for the characterization of micellization phenomena via a nuclear magnetic resonance imaging spectrometer is investigated. [Copyright &y& Elsevier]