Results Obtained from Nuclear Magnetic Resonance

THE DETERMINATION OF IONIC STRUCTURES 2.1. Results Obtained from Nuclear Magnetic Resonance... 

In this present chapter, we will discuss how we can characterize microemulsion structures. A number of different experimental methods will be mentioned but we will mainly focus on results obtained by nuclear magnetic resonance (NMR) self-diffusion measurements since this is a straightforward approach to distinguish between droplet and bicontinuous structures. Examples from structural studies will mainly be taken from nonionic surfactant systems since such surfactants are capable of forming the whole spectrum of microemulsion structures without the addition of cosurfactant or salt. 

As indicated in the previous discussion, Mossbauer spectroscopy provides information that when coupled with results using other structural techniques assists in determining the structure of the complex under analysis. The relationships between the various techniques are summarized in Table II. The Mossbauer chemical shift provides information about the 4 electron contribution to the bond between the metal and the ligands in a complex. Similar estimates can be obtained from the results of measurements on the fine structure in the x-ray absorption edge and nuclear magnetic resonance data. The number of unpaired electrons can be evaluated from magnetic susceptibility data, electron spin resonance, and the temperature coeflScient of the Mossbauer quadrupole splitting (Pr). 

As part of a mechanistic and synthetic study of nucleophihc carbenes the spirocyclic 4(5/l)-oxazolone 18 has been obtained from benzoyl isocyanate (Scheme 6.1) Thermal extrusion of nitrogen from the 1,3,4-oxadiazoline 14 produced the carbonyl ylide 15 that fragmented via loss of acetone to the aminooxycarbene 16. Spectroscopic data [gas chromatography-mass spectrometry (GC-MS), infrared (IR), proton and C-13 nuclear magnetic resonance ( H and NMR)] of the crude thermolysate was consistent with 18. The formation of 18 was rationalized to result from nucleophihc addition of 16 to benzoyl isocyanate followed by cyclization of the dipolar intermediate 17. Thermolysis of 19 and 21 under similar reaction conditions afforded 20 and 22 respectively, also identified spectroscopically as the major products in the thermolysate. 

Fig. 5. The 13C nuclear magnetic resonance line widths of the (enriched) choline methyl resonances in dipalmitoylphosphatidylcholine (A) and in dielaidoylphosphatidylcholine (O), as a function of temperature. Spectra taken at 90.5 MHz similar results were also obtained at 25.2 MHz. Note that the higher-melting lipid, dipalmitoylphosphatidylcholine, shows a readily observable enhanced line broadening at temperatures TU 32°C, corresponding to the onset of the lateral phase separation. (Data from Ref. 4.) [Reprinted with permission from P. Brulet and H. M. McConnell, J. Am. Chem. Soc., 98, 1314 (1977). Copyright by American Chemical Society.]...

Electron spin resonance (ESR) measures the absorption spectra associated with the energy states produced from the ground state by interaction with the magnetic field. This review deals with the theory of these states, their description by a spin Hamiltonian and the transitions between these states induced by electromagnetic radiation. The dynamics of these transitions (spin-lattice relaxation times, etc.) are not considered. Also omitted are discussions of other methods of measuring spin Hamiltonian parameters such as nuclear magnetic resonance (NMR) and electron nuclear double resonance (ENDOR), although results obtained by these methods are included in Sec. VI. 

Comparisons of estimated diffusivity values on zeolites from sorption uptake measurements and those obtained from direct measurements by nuclear magnetic resonance field gradient techniques have indicated large discrepancies between the two for many systems [10]. In addition, the former method has often resulted in an adsorbate diffusivity directly proportional to the adsorbent crystal size [11]. This led some researchers to believe that the resistance to mass transfer may be confined in a skin at the surface of the adsorbent crystal or pellet (surface barrier) [10,11]. The isothermal surface barrier model, however, failed to describe experimental uptake data quantitatively [10,12]. 

In the series of microbubble experiments (ref. 394) included in this chapter, the actual film material, contained in compressed microbubble-surfactant monolayers, was collected for structural determinations using H-nuclear magnetic resonance (NMR) spectroscopy. The resulting spectrum is then compared to the H-NMR spectrum which was obtained beforehand from the partially purified, microbubble surfactant mixture prior to monolayer formation and compression. 

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