It was found previously that the proton-transfer reaction between trimethylammonium ion and trimethylamine in aqueous solution takes place via water. In the present investigation the number of water molecules involved in this reaction was determined. Proton exchange rates were determined from NMR measurements in trimethylamine—trimethylammonium chloride buffer solutions in O17-enriched water. It is concluded that one water molecule is involved in the transfer reaction. In the appendix, theoretical equations are derived for the dependence of the observed spin-echo decay rate on the 180° pulse rate for the case of fast exchange.

The splitting of signals in the NMR spectra originating from enantiotopic sites in prochiral molecules when dissolved in chiral solvents is referred to as spectral enantiotopic discrimination. This phenomenon is particularly noticeable in chiral liquid crystals (CLCs) due to the combined effect of the anisotropic magnetic interactions and the ordering of the solute in the mesophase. The enantiorecognition mechanisms are different for rigid and flexible solutes. For the former, discrimination results from symmetry breaking and is restricted to solutes whose point groups belong to one of the following four ("allowed") symmetries, Cs, C2v, D2d and S4. The nature of the symmetry breaking for each one of these groups is discussed and experimental examples, using mainly (2)H 1D/2D-NMR in chiral polypeptide lyotropic mesophases, are presented and analyzed. When flexible optically active solutes undergo fast racemization (on the NMR timescale) their spectrum corresponds to that of an average prochiral molecule and may exhibit enantiotopic sites. In CLCs, such sites will become discriminated, irrespective of their average (improper) symmetry. This enantiodiscrimination results mainly from the different ordering of the interchanging enantiomers. Several examples of such flexible molecules, including solutes with average axial and planar symmetries, are commented. Dynamic processes in solution that are not accompanied by the modulation of magnetic interactions remain "NMR blind". This is sometimes the case for interconversion of enantiomers (racemization) or exchange of enantiotopic sites in isotropic solvents. The limitation can be lifted by using CLCs. In such solvents, non-equivalence between enantiomers or between enantiotopic sites is induced by the chiral environment, thus providing the necessary interactions to be modulated by the dynamic processes. Illustrative examples involving exchange of both, enantiotopic sites and enantiomers are examined. In this comprehensive review, various important aspects of enantiodiscrimination by NMR are presented. Thus the possibility to reveal enantiotopic recognition using residual dipolar couplings or to determine the absolute configuration of enantiotopic NMR signals is discussed. The various kinds of chiral mesophases able to reveal enantiotopic discrimination in guest prochiral molecules are also described and compared with each other. Finally to illustrate the high analytical potentialities of NMR in CLCs, several and various applications involving the enantiodiscrimination phenomenon are described. A strategy for assigning the NMR signals of meso compound in a meso-threo mixture of cyclic molecules is first discussed. This is followed by a description of advantages of the method for the determination of (D/H) natural isotopic fractionation in biocompounds.

The nuclear relaxation of the hydroxyl and methyl protons in methanol containing low concentrations of Co++ and Ni++ ions has been investigated between -SO°C and +80°C. This work complements previous studies of the NMR spectra of the solvation complexes in similar solutions. Over the temperature range studied, the relaxation due to the dissolved paramagnetic ions changes over two or three orders of magnitude. This dependence, as well as a corresponding temperature dependence of the chemical shift of the methanol peaks, can be quantitatively interpreted in terms of chemical exchange of methanol molecules between the coordination sphere of the paramagnetic ions and the bulk methanol. It is found that the dominant exchange process involves whole methanol molecules, i.e., proton transfer is relatively unimportant. The exchange rate is characterized by 1l1It=13.S kcal mole-1 and IlS t =+7.2 eu for Co++ ions; andlllI l =15.Skcal per mole and IlSl = +8.0 eu for Ni++ ions. The hyperfine interaction between the paramagnetic ion and a methanol molecule in the coordination sphere is for Co++:+4.1 X 10 5 cps for the CH3 protons, and +80X 10 5 cps for the OH proton; the corresponding numbers for Ni++ are: +5. 7X 10 5 cps and +6.8X 10 5 cps. Proton relaxation times of the methanol in the coordination sphere are also obtained. Z. Elektrochem. 69, 110 (1960).

The formation mechanism of the hexagonal, MCM-41, and the lamellar, MCM-50, mesoporous materials, prepared at room temperature with the surfactant cetyltrimethylammonium chloride (CTAC) and tetraethylorthosilicon (TEOS), was studied by in situ EPR spectroscopy using the spin probe 4-(N,N-dimethyl-N-hexadecyl)ammonium-2,2,6,6-tetramethylpiperidinyloxy iodide (CAT16). This probe has a structure similar to that of the surfactant molecules with the nitroxyl radical situated at the head group. Accordingly, it probes the interface between the organic and inorganic phases during the formation of M41S materials. The EPR spectrum of CAT16 in the reaction gel, prior to the addition of TEOS, consists of a superposition of two subspectra due to spin probe molecules in micelles and in the aqueous phase, respectively. For a gel composition which forms MCM-41, the addition of TEOS leads to a gradual transformation of the micellar subspectrum into a characteristic rigid limit spectrum. This observation provides direct evidence that micellar structures present in the initial reaction mixture serve as precursors for the final mesoporous product. The temporal evolution of the spectrum is characteristic of an isotropic system undergoing a gradual increase in the microviscosity. The isotropic nature of the spectrum is a consequence of the specific geometry of the CAT16 head group and its motion in the interface region. Comparison of the temporal evolution of the EPR spectrum with that of the X-ray diffraction pattern indicates that the hexagonal long-range order is formed already 5−8 min after mixing the reagents, whereas the formation of the inorganic phase, which is apparently responsible for the slowdown of the spin probe motion, is considerably slower (>1.5 h). The latter process begins only after a critical amount of TEOS is added to the mixture. These results are consistent with a mechanism whereby the addition of TEOS initially forms clusters of rodlike micelles coated with silicate anions, followed by the condensation of the silicate anions at the interface to yield the final product. By monitoring the peak height of the central EPR line, phenomenological kinetic profiles of the reaction were obtained. These curves were quite different for MCM-41 and MCM-50 and they provide qualitative information regarding the sequence of transformations which occur during the reaction. Specifically, these curves show that while no intermediate phases occur during the formation of MCM-41, several phase transformations take place when MCM-50 is formed and the reaction is significantly slower.

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