Fast exchange, nuclear magnetic resonance

Another mechanism is also possible. The slow step is an associative exchange between NH2 and HD, and it is followed by the exchange between NHD and NH, which is known to be very fast from nuclear magnetic resonance (15) ... 

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). 

In protein solutions the water protons may be considered to reside in two different environments, i.e. the bulk water, and the hydration spheres of the protein molecules. If there is fast exchange of protons between these environments a single proton nuclear magnetic resonance will be observed, which corresponds to the average of the resonances in the different environments. Following McConnell (74) the observed longitudinal relaxation time is to a good approximation... 

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... 

Fig. 3. Calculated NMR lineshapes for equally populated two-site exchange as a function of the dimensionless parameter a = nf rA. The abscissa is the dimensionless relative offset parameter, x = A/// (see Eq. (18)). (a) a = 4 (b) a = 2 (c)a=l (d) a = l/ /2 (e) a = 0.5 (f) a = 0.2. Spectra (a) and (f) are near the slow and fast exchange limits, respectively. Reproduced with permission from R. K. Harris, Nuclear Magnetic Resonance Spectroscopy A Physicochemical View, p. 124, Longman Scientific and Technical, Harlow, 1986.

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. 

We define the hydration number as the average number of water molecules in the first sphere about the metal ion. The residence time of these molecules is determined generally by the nature of the bonding to the metal ion. For the f-element cations, ion-dipole interactions result in fast exchange between the hydration layer and the bulk solvent. The techniques for studying the nature (number and/or structure) of the hydration shell can be classified as either direct or indirect methods. The direct methods include X-ray and neutron diffraction, luminescence and NMR (nuclear magnetic resonance) relaxation measurements. The indirect methods involve compressibility, NMR exchange and absorption spectroscopy measurements. 

An nuclear magnetic resonance (NMR) study of the same complex indicates that the observed hexacoordination in the crystal structure is combined with a fast exchange of the donor thioethers in solution as all carbon atoms of the side chains have equivalent signals. Even at temperatures of 183 K, the signals remain equivalent. This could lead to an extra stability, but despite the high thermodynamic stability the kinetic stability is still too low and slow exchange with serum proteins has been observed. 

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