Nuclear magnetic resonance fast exchange reactions

Physical Chemical Characterization. Thiamine, its derivatives, and its degradation products have been fully characterized by spectroscopic methods (9,10). The ultraviolet spectmm of thiamine shows pH-dependent maxima (11). H, and nuclear magnetic resonance spectra show protonation occurs at the 1-nitrogen, and not the 4-amino position (12—14). The H spectmm in D2O shows no resonance for the thiazole 2-hydrogen, as this is acidic and readily exchanged via formation of the thiazole yUd (13) an important intermediate in the biochemical functions of thiamine. Recent work has revised the piC values for the two ionization reactions to 4.8 and 18 respectively (9,10,15). The mass spectmm of thiamine hydrochloride shows no molecular ion under standard electron impact ionization conditions, but fast atom bombardment and chemical ionization allow observation of both an intense peak for the patent cation and its major fragmentation ion, the pyrimidinylmethyl cation (16). 

The term DNMR, Dynamic Nuclear Magnetic Resonance, refers to the process of recording and usually computer simulating exchange broadened NMR spectra at a number of temperatures in order to determine mechanistic and/or kinetic information. b The terms fast and slow are used rather than the more familiar labile and nonlabile because the latter imply an intermolecular reaction. 

The development of nuclear magnetic resonance spectroscopy for the measurement of the rates of fast reactions (preexchange lifetimes 1-0.001 second) has made it possible to study many alkyl-metal exchange processes which heretofore were experimentally inaccessible. A substantial number of papers dealing with the exchange reactions of Group I, II, and III... 

Nuclear magnetic resonance spectroscopy alkyls and aryls, 7, 168-170 fast exchange reactions, 8, 167-205 metallocenes, 10, 85-89, 102-104 nitrosyls, 7, 234... 

This method is closely related to the nonhydrolytic sol-gel method. For example, titania is prepared by etherolysis/condensation of TiCl4 by diisopropyl ether (Equation 2.4) or by direct condensation between TiCl4 and Ti(0- Pr)4 (Equation 2.5). Detailed chemistry of the reaction was examined by means of nuclear magnetic resonance (NMR), and it has been reported that the tme precursors are titanium chloroisopropoxides in equilibrium through fast ligand exchange reactions. A variety of metal oxides, " nomnetal oxides," multicomponent oxides" " were studied, and the nonhydrolytic sol-gel method was surveyed by Vioux and Leclercq. ... 

The dynamics of the so-called biological water molecules in the immediate vicinity of a protein have been studied using dielectric relaxation [18], proton and O NMR relaxation [19], reaction path calculation [20], and analytical statistical mechanical models [21]. While the dielectric relaxation time of ordinary water molecules is 10 ps [16], both the dielectric [18] and nuclear magnetic resonance (NMR) relaxation studies [19], indicate that near the protein surface the relaxation dynamics are bimodal with two components in the 10-ns and 10-ps time scale, respectively. The 10-ns relaxation time cannot be due to the motion of the peptide chains, which occurs in the 100-ns time scale. From the study of NMR relaxation times of " O at the protein surface, Halle et al. [19c,d] suggested dynamic exchange between the slowly rotating internal and the fast external water molecules. 

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