Chemical exchange broadening

Gorenstein Unfortunately I do not know of any chemical shift anisotropy theoretical studies on ATP (or any phosphates in fact). Some recent advances in solid state, oriented crystal, P NNR of various nucleic acids have appeared in the literature. Large C.S.A. s are found for mononucLeoside monophosphates. Your suggestion for the line broadening in ATP is dertainly reasonable but possibly could also be explained by chemical exchange broadening ( ). 

NMR signals from the third and fourth site are not observed presumably due to substantial chemical exchange broadening. 

The water nucleus chosen also has an impact on relaxation data. Proton relaxation is affected by chemical exchange and cross-relaxation, 2H by chemical exchange, and lyO by proton-exchange broadening (scalar spin-spin coupling between lH and lyO nuclei affects T2, but not 7)) (Glasel,... 

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... 

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]. 

In many of these descriptions of lineshapes, chemical exchange line-shapes are treated as a unique phenomenon, rather than simply another example of relaxation effects on lineshapes. This is especially true for line-shapes in the intermediate time scale, where severe broadening or overlapping of lines may occur. The complete picture of exchange lineshapes can be somewhat simplified, following Reeves and Shaw [13], who showed that for two sites, the lineshape at coalescence can always be described by two NMR lines. This fact can be exploited to produce a clarified picture of exchange effects on lineshapes and to formulate a new method for the calculation of exchange lineshapes [16, 23]. This method makes use of the fact that lineshapes, even near coalescence, retain Lorentzian characteristics [13] (fig. 3). These lines, or coherences, are each defined by an intensity, phase, position, and linewidth, and for each line in the spectrum, the contribution of that particular line to the overall free induction decay (FID) or spectrum can be calculated. 

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. 

Note that on a 200-MHz instrument, Av is 30 Hz (0.15 ppm x 200 Hz/ppm) and the NMR timescale rc is 15 ms (l/(2.22 x 30)), but the shutter speed is faster as we go to higher field instruments because the chemical shift difference Av is measured in hertz, not ppm. Thus, moving to higher field shortens the shutter time rc in a way that is inversely proportional to B0. Figure 10.7 shows simulated spectra of the DMF sample at the same temperature that gives rex = 15 ms, analyzed on three different spectrometers with ywB0/27T = 60, 200, and 600 MHz. At 60 MHz (top), we have an exchange-broadened fast... 

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