Nmr Line Broadening

The same nucleus (say methyl protons) in different chemical environments A and B will generally have nuclear magnetic resonances at different frequencies. If the exchange of pro- 

This behavior is well typified in Fig. 3.13 involving the exchange  

The width W is related to the transverse relaxation time T2 by the expression 

Now is the required kinetic information, since it represents the mean lifetime of the nucleus (for example, a proton) in the environment A. 

The values of and Xg and the manner in which they vary with the concentrations of A and B yield information on the rate law and rate constants for the exchange. (Prob. 11.) 

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NMR line broadening occurred more pronouncedly for copolymers with aromatic hydrophobic units than for those with aliphatic ones [31]. This implies that polycyclic aromatic rings tend to be more tightly packed when self-aggregating than do aliphatic groups. 

Transition state theory is presented with an emphasis on solution reactions and the Marcus approach. Indeed, to allow for this, I have largely eliminated the small amount of material on gas-phase reactions that appeared in the First Edition. Several treatments have been expanded, including linear free-energy relations, NMR line broadening, and pulse radiolytic and flash photolytic methods for picosecond and femtosecond transients. 

Fe(Et2dtc)3]BF4 has the advantage that the product is completely free from by-products and is isolable in yields of 90%. The relationship between Ei,2 values and electronic structures has been described in terms of Ai - T2 crossover equilibrium (273). The rate of e transfer between [Fe(Me2dtc)3] and [Fe(Me2dtc>3]BF4 has been measured by H-NMR, line-broadening experiments (275). Mossbauer-spectral data have been published (276) for a series of (R2dtc) complexes of iron(IV). 

NMR line broadening is a suitable kinetic method for determining activation parameters for Co—C bond homolysis, and gave A//- values in the range 18-22.5 keal mol for a selection of Co(Por)R complexes containing secondary or tertiary alkyl groups.Bond dissociation enthalpies and entropies for several... 

For the phenanthroline and 4,7- and 5,6-dimethylphenanthroline complexes a further estimate of A > 3x 10 l.mole. sec has been proposed, on the basis of NMR line broadening measurements, by Larsen and Wahl". 

Isomerization sets in at higher temperatures than 273 K as apparent from NMR line broadening. The reason for a preference for the particular (cis O, N1) isomer is not obvious neither the O/N alternative for organosilyl migration nor the position of the cis/trans equilibrium with respect to the N-C(=X) bonds is easily predictable. 

In a general way, the ions in the first two classes would be considered labile while those in the last two classes would be considered inert. Labile complexes are regarded as those in which the reaction is complete on a time scale that would be comparable to the time necessary to mix the solutions of the reacting species. Such reactions can be studied by flow techniques or by NMR line broadening. Inert complexes are those that can be followed by conventional kinetic techniques. 

Similar approaches to those described herein are already underway with [2Fe-2S] ferredoxins. From NMR line-broadening studies Markley has demonstrated that redox inactive Cr(1 3)53+ preferentially associates at a specific site close to Tyr 83 (13,27). This particular group is a relatively long way from the X2Fe-2S] cluster and the result is surprising since conserved or partially conserved negative patches at 67-69 and 94-96 are nearer to the active site and might have been expected... 

A non-dissociative mechanism for water-catalysed tautomerisation of uracil monoanion and of imidazole has been suggested on the basis of nmr line broadening measurements (Chang and Grunwald, 1976). However, doubts have been expressed about these conclusions (Bensaude et al., 1978). The mechanisms deduced from nmr measurements are shown in (108) and (109) and involve water behaving as a bifunctional catalyst. Rate coefficients... 

In conclusion, oxygen-17 NMR line-broadening provides the unique opportunity to study very fast proton transfer reactions on these metal oxocyano complexes by lowering the concentration of the reacting species through pH manipulation. 

The exchange of the coordinated aqua ligand of the W(IV) aqua oxo species was qualitatively studied by NMR line-broadening as a function of temperature based on Eq. (26), where the transverse relaxation time of the bound oxygen-17 nucleus is given by 1/T2b. The l/T2Qb represents the quadrupolar relaxation rate and kmi the chemical exchange rate constant... 

DR. SWADDLE We are measuring the volume of activation for solvent exchange in a region where it is in the NMR time frame in other words, where the NMR line broadening is exchange controlled, i.e., k = 1(T s. ... 

DR. SWADDLE The problem with entropies of activation is that they are obtained in conjunction with the enthalpies. Those of us who have worked in NMR line broadening can relate numerous stories about entropies of activation which range all over the place, depending on exactly how one analyzes the data [See, e.g., Newman, K. E. Meyer, F. K. Merbach, A. E. J. Am. Chem. 

ULTRASONICS I LASER PHOTOLYSIS PULSE RADIOLYSIS I NMR LINE BROADENING I... 

Consider a mixture of AX and BX at chemical equilibrium. When, for example, radioisotopes are used as tracers, they are injected into the equilibrium mixture in the form of a very small amount of B X. At various times, either (BX + B X) or (AX + A X) is separated from the mixture and analyzed. When nmr line broadening is used to monitor the exchange the tracer is already present e.g. H or O (or an additional amount can be added) and the exchange is monitored in situ and assessed from the shape of the nmr signals (Sec. 3.9.6). If the concentration of (AX + A X) is a and the concentration of (BX + B X) is b, and the fraction of exchange at time t is F, it is not difficult to show that the gross or overall rate of X transfer between AX and BX, (M s ) is given by ... 

Fig. 2.3 Arrhenius ( ) and Eyring ( ) plots of data for the exchange of a single dmso molecule with Ga(dmso) + in CD3NO2. Nmr line-broadening was used for the higher temperatures and stopped-flow nmr experiments for the lower temperatures. For the Arrhenius plot, the slope and intercept (at T = 0) are -9.2 x 10 and +31,4 respectively, leading to = 76 kJ mol" and logA = 13.6. For the Eyring plot, the slope and intercept are —8.77 x 10 and +24.4 respeetively, leading to A// = 73 kJ mol" and AS = 5 J K" mol" .

The method has been particulary useful for studying fairly rapid isotopie exchange reactions (Table 3.3). Some of the reactions have also been studied without the necessity of a separation procedure, and therefore more conveniently, by nmr line-broadening methods (Sec. 3.9.6). The latter still require fairly high reactant concentrations, and with these conditions subtle medium... 

Conventional, flow, temperature-jump, ultrasonic absorption, electric-field jump and nmr line broadening have all been used to measure the rates. UV-vis spectrophotometry and conductivity are the monitoring methods of choice. A variety of solvents have been used. The focus has been often on the dissociation since the dissociation rate constant appears in general to be the main controller of the overall stability. 

A number of complexes of the type M(AA)2X2 are neutral and soluble in organic solvents. They are stereochemically non-rigid on an nmr time scale, that is they display nmr line broadening or collapse in accessible temperature ranges. Concommitant solvolysis problems can usually be avoided. The behavior shown is typified by the complexes Ti(P-diketone)2X2. The cis-configuration is invariably the stable one in solution, e.g. 6... 

The aqua ion is not easily reduced nor oxidized. It is the slowest reacting of the bivalent transition metal ions with e " k = 7.7 X 10 M s ) and the product Mn+q is very reactive. However Mn(CNR)5 (R = a variety of alkyl and aryl groups) is stable and the selfexchange in the Mn(l,ll) hexakis(isocyanide) system has been studied by Mn and H nmr line broadening. The effects of solvent, temperature, pressure and ligand have been thoroughly explored. 

As the TieS lengthen and l/Tie (s ) approaches the size of the electron—nuclear interaction, considerable NMR line broadening can occur, and it may not be possible to acquire high-resolution NMR spectra under these conditions. The effect of Tie on the nuclear relaxation times is discussed in more detail in ref 22. Large hyperfine constants are observed (of many MHz) when the nuclear and electronic spins are on the same atom. For example, a hyperfine constant A/h of —324 MHz was measured by electron... 

Ichikawa, K. and Matsumoto, T., An aluminium-27 NMR study of chemical exchange and NMR line broadening in molten butylpyridinium chloride + AlClg ll,/. Magn. Reson., 63,445,1985. 

Examination of Electron Transfer Self-Exchange Rates Using NMR Line-Broadening Techniques 63... 

Equilibrium constants have been measured for (41)-(45) using a variety of techniques spectrophotometry 49), conductivity, 170-NMR line broadening (50), isopiestic measurements, potentiometry, and others 51). From these data, speciation distribution estimates have been calculated 52). As the equilibrium constants for (41)—(45) are quite sensitive to ionic strength, so are the speciation distributions. Thus, for pH = 1, Cr(VI)T = 10-2 M and T = 25°C, the [H2Cr04] is calculated to be 29%at/ = 0 and 1% at I = 1.0M(52). Nevertheless, a considerable proportion ( 50%) of Cr(VI) in the pH range 1-6 is estimated to be in the HCr04 form. 

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