Solvent water suppression

Key Words Nuclear magnetic resonance spectroscopy, Solvent, Water suppression. Radiation damping. Demagnetization field. Bulk susceptibility effect. Presaturation, Watergate, Purge, Metabonomics. 

Kemp et al., 1978). The rate is slowest in an aqueous solution and is enhanced in aprotic and/or dipolar solvents. The rate augmentation of 106—108 is attainable in dipolar aprotic solvents such as dimethyl sulfoxide and hexamethylphosphoramide (HMPA). Interestingly, the decarboxylation rate of 4-hydroxybenzisoxazole-3-carboxylate [53], a substance which contains its own protic environment, is very slow and hardly subject to a solvent effect (1.3 x 10-6 s-1 in water and 8.9 x 10-6 s-1 in dimethylformamide Kemp et al., 1975). The result is consistent with the fact that hydrogen-bonding with solvent molecules suppresses the decarboxylation. 

Since water protons are not bound to or nuclei, the water signal is also suppressed by the spin-lock purge pulse. In practice, the suppression of the water signal is sufficient to record HSQC spectra of protein samples dissolved in mixtures of 95% H20/5% D2O without any further water suppression scheme [12]. For optimum water suppression the carrier frequency must be at the frequency of the water resonance. On resonance, the phase of the water magnetization is not affected by imperfections of the first 180°(ff) pulse, so that no solvent magnetization ends up along the axis of the spin-lock purge pulse. 

The situation in LC-NMR is different. A solvent mixture is used and in most cases two or more solvents are suppressed. The solvents which are delivered from the LC pump are not completely isolated from the humidity of the ambient air. Additional water is brought into the system via the acetonitrile, the sample itself and any additives such as ammonium acetate which are used in the undeuterated form. Therefore, the HDO signal is relatively strong and has to be suppressed as well. 

Typically, reactions are carried out open to air, at room temperature (rt), using a mixture of tBuOH/water (9 1) or MeCN/water (9 1) as solvent and sodium or potassium hydroxide as base. The presence of water suppresses undesired side reactions (e.g., Cannizzaro reaction, Williamson alkylation, solvent reactivity) [37]. Benzyl bromides are most commonly reported as the halide component. 

The reactivity of 02 - with alkyl halides in aprotic solvents occurs via nucleophilic substitution. Kinetic studies confirm that the reaction order is primary > secondary > tertiary and I > Br > Cl > F for alkyl hahdes, and that the attack by 02 - results in inversion of configuration (Sn2). Superoxide ion also reacts with CCI4, Br(CH2)2Br, CeCle, and esters in aprotic media. The reactions are via nucleophilic attack by 02 on carbon, or on chlorine with a concerted reductive displacement of chloride ion or alkoxide ion. As with all oxyanions, water suppresses the nucleophilicity of 02 (hydration energy, lOOkcalmoL ) and promotes its rapid hydrolysis and disproportionation. The reaction pathways for these compounds produce peroxy radical and peroxide ion intermediates (ROO and ROO ). 

Although NMR is probably the most widely used NMR technique in chemical applications, it has only been used sparingly in studies of humic substances. Much of the literature in which this technique is used was published prior to 1987 and is well covered in the review by Wilson.(5) Use of NMR is mostly limited to solutions, and it is thus necessary to dissolve humic substances in aqueous solutions. Of course, one must reduce the amount of H s from solvent molecules by use of deuterated water, but it is often difficult to reduce these background signals to negligible amounts. Wilson(5) described the application of water suppression techniques which have been necessary to obtain H NMR spectra of humic isolates from soil. In the case of solid-state NMR of whole soils and humic substances, the development of a new technique called CRAMPS (combined rotation and multiple pulse spectroscopy) has not received the attention it has in the field of coal science, primarily because of the lack of resolution. Thus, recent applications of either solution or solids NMR has been very limited in humic substance science since Wilson s(5) review. 

Figure 6-42 (a) Water suppression with excitation sculpting on 2-mM sucrose in 9 1 H2O/D2O. (b) The residual solvent peak has been eliminated by further processing. (Reproduced from T. D. W. Claridge, High-Resolution NMR Techniques in Organic Chemistry, Pergamon Press, Amsterdam, 1999, p. 365.)... 

The above introduction to solvent suppression serves to elucidate some of the complications imposed by the solvent water signal and common measures for ameliorating them. Note that many modern pulse... 

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