Broadening Due to Chemical Exchange

This aspect of NMR is sometimes called dynamic NMR, perhaps sweepingly in view of the plethora of other dynamic effects. Several useful books are available. The text by Sandstrom is particularly oriented towards practical problems, and that by Kaplan and Fraenkel presents the full theory. The earlier books edited by La Mar, Horrocks, and Holm and Jackman and Cotton are rich in examples from organometallic and coordination chemistry. 

Simple kinetic systems may be quantitatively analyzed using formulas derived by adding exchange terms to the Bloch equations. For example, the line shape v(m) in the case of exchange at rate between two sites with populations and at 

In the case where P/ = P, and hence kf = k, the merging bandshape is flat-topped when k = diojl-Jl, provided that 2 

Kinetic exchange processes can contribute to Ti as well as to and provided they are fast enough. Thus measurements have been used to measure free energy 

At the other kinetic extreme, reactions with A 10 may be investigated by flow or stopped-flow NMR (A 1). In this manner one may extend the temperature range of Arrhenius plots so as to reduce their all too prevalent errors. Observed line shapes in flow FT NMR can become complex. 

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Resonance line broadening due to chemical exchange and quadrupole-induced relaxation in the H and nB n.m.r. spectra of some boron-nitrogen adducts ArNMe2,BY3 (Y = halogen) has been observed and used to determine the mechanism of amine scrambling in these adducts.165 This is thought to occur via a unimolecular ionization rather than a B—N bond-rupture process. 

In spectra dominated by anisotropic interactions like quadrupolar interactions and chemical shifts, the concept of dynamic broadening due to chemical exchange can be applied to the detection of molecular processes the resonance frequency of a nucleus depends not only on its chemical identity (isotropic chemical shift) but also on its... 

For the HCOs" signal the T2 values as estimated from the line width are much larger than Tf. Since the equation for Tf , analogous to Equation (2.14), would predict similar Tj and T2 values,a sizeable broadening due to chemical exchange must be present. Indeed, unlike (Equation 2.11), Tip may be a complicated function of the exchange time Tm and of the isotropic shift, Acum,... 

Fig. 7. One-dimensional NMR spectra of the designed four-helix bundles SA-42 (lower trace) and GTD-43 (top two traces). The chemical shift dispersion of SA-42 in 90% H2O and 10% D2O at 323 K and pH 4.5 is poor and the resonances are severely broadened due to conformational exchange. The chemical shift dispersion of GTD-43 in the same solvent at 288 K and pH 3.0 is comparable to that of the naturally occurring four-helix bundle IL-4 and the resonances are not significantly affected by conformational exchange. Upon raising the temperature to 298 K line broadening is observed (top trace) which shows that GTD-43 is in slow exchange on the NMR time scale, unlike SA-42 where an increased temperature reduces the line width. These spectra are therefore diagnostic of structures with disordered (SA-42) and ordered (GTD-43) hydrophobic cores...

Scalar relaxation due to chemical exchange has the usual influence (broadening) on the appearance of Si NMR spectra. A consequence of such exchange processes, depending on the rate of exchange, is that PT techniques may not work very well or sometimes not at all. Scalar relaxation of the second kind affects the line widths by j. sc(29si) jf silicon atom is bonded to one or more quadrupolar nuclei as in... 

The broadening of P peaks in a-P4S3(NCS)2, a-P4S3(NCS)Cl, and a-P4S3(NCS)I has previously been observed, but it had not been clear if this was due to chemical exchange or a relaxation mechanism. Further work eliminates the chemical exchange possibility. The kinetics of reaction (25) in 1-chloronaphthalene have been studied,where q- -r lLs- -lLt>p. The reaction is postulated to proceed by steps (26)-(29). The sulfur may be formed by reactions of type (29). 

An even more useful property of supercritical fluids involves the near temperature-independence of the solvent viscosity and, consequently, of the line-widths of quadrupolar nuclei. In conventional solvents the line-widths of e. g. Co decrease with increasing temperature, due to the strong temperature-dependence of the viscosity of the liquid. These line-width variations often obscure chemical exchange processes. In supercritical fluids, chemical exchange processes are easily identified and measured [249]. As an example. Figure 1.45 shows Co line-widths of Co2(CO)g in SCCO2 for different temperatures. Above 160 °C, the line-broadening due to the dissociation of Co2(CO)g to Co(CO)4 can be easily discerned [249]. 

These phenolic hydroxyls have been observed in the proton NMR spectrum of N-acetyldaunomycin in chloroform solution (83) though they are broadened out due to rapid exchange with solvent in the spectrum of daunomycin in H2O solution. We can therefore compare the H-6 and H-ll ring B hydroxyl chemical shifts of 12.3 and 11.5 ppm for the Nuc/D = 5.9 daunomycin poly(dA-dT) complex in aqueous solution (Figure 25) with the values of 13.86 and 13.15 ppm observed for N-acetyldaunomycin in nonpolar solution (83). 

Almost any textbook on NMR describes the classical phenomenon of chemical exchange a spin system can exist in two or more different states with different resonance frequencies, giving rise to different lines in a liquid-like spectrum. A dynamic process that interconverts the two states affects the NMR spectmm if the rate of the process is of the order of the difference of the resonance frequencies, the spectrum broadens due to a loss of phase coherences of the two frequencies. Eventually, a single broad line forms... 

Another source of line broadening in solutions arises from the fluctuation in spectra parameters due to chemical reactions, e.g., solvent-ligand exchange, ion-pair formation, or complex formation. 

Figure B2.4.3. Proton NMR spectrum of the aldehyde proton in N-labelled fonnainide. This proton has couplings of 1.76 Hz and 13.55 Hz to the two amino protons, and a couplmg of 15.0 Hz to the nucleus. The outer lines in die spectrum remain sharp, since they represent the sum of the couplings, which is unaffected by the exchange. The iimer lines of the multiplet broaden and coalesce, as in figure B2.4.1. The other peaks in the 303 K spectrum are due to the NH2 protons, whose chemical shifts are even more temperature dependent than that of the aldehyde proton.

The NMR investigation is presented in two parts the first covers i3C 99rpc ancj i5N studies where protonations of the dioxo complexes induce chemical shift changes but where line-broadening effects due to exchange are not observed. The second part covers 170 studies which produce more complicated spectra due to protonation and simultaneous exchange with bulk water. 

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