Solvents in NMR spectroscopy

Explain why water is generally not used as a solvent in NMR spectroscopy. 

There should be a proper selection for the choice of a solvent in NMR measurements. The solvent should be such that it does not give its own absorption and it should dissolve at least 10% of the substance under investigation. The commonly used solvents in NMR spectroscopy are ... 

An overview of the apphcation of NMR to the field of liquid crystals over the past 50 years has been presented and the important aspects of proton and deuterium NMR have been delineated. The use of liquid crystals as solvents in NMR spectroscopy has been reviewed." Reviews on NMR studies of orientational order with 20 references, rotational diffusion of liquid crystals in the nematic phase and NMR spectroscopy in liquid crystals and membranes are available. A review on the application of spectroscopic methods to liquid crystalline phases has appeared during the period under report and it includes a discussion of the NMR methods. NMR and other methods used to determine the order parameters of nematics have been reviewed. ... 

At present the applicability of nematic liquid crystals as anisotropic solvents in nmr spectroscopy seems to be limited to relatively simple and symmetric molecules which do not have more than 8 protons [40, 44]. Beyond this limit the spectra become prohibitively complicated and extensive overlapping of the lines occurs. Thus, a simple molecule as cyclohexane yields only a very broad spectrum [44]. 

The simplest of all electrophiles is H+, but exchanging one proton for another is not very interesting So we use D+ so that we can see what has happened. In concentrated D2SO4, we can readily exchange H and D, and this is the method used to prepare QDg to be used in mechanistic studies such as the one mentioned earlier, or as a solvent in NMR spectroscopy (Figure 12.8). The mechanism is just as before (Figure 12.8), and the reaction is driven to completion by a mass action effect in the presence of an excess of DjSO. ... 

On-line coupling of separation techniques to NMR has recently been reviewed [459,651-653], and solvent suppression methods in NMR spectroscopy in particular [654],... 

The separation of interactions by 2D spectroscopy can be compared with 2D chromatography. In a onedimensional thin layer or paper chromatogram, the separation of the constituents by elution with a given solvent is often incomplete. Elution with a second solvent in a perpendicular direction may then achieve full separation. In NMR spectroscopy, the choice of two solvents is replaced by the choice of two suitable (effective) Hamiltonians for the evolution and detection periods which allow unique characterisation of each line. 

Because most common solvents, including water, contain protons, and most NMR analyses involve the measurement of protons, a solvent without protons is generally used in NMR spectroscopy. Commonly, solvents in which the hydrogen atoms are replaced with deuterium (i.e., solvents that have been deuterated) are used, the most common being deuterochloroform. In addition, an internal standard, most commonly tetramethylsilane (TMS), is added to the sample in the NMR sample tube (see Figure 14.3, D) and all absorption features are recorded relative to the absorption due to TMS. 

Solvents for NMR Spectroscopy, NMR spectra are almost invariably obtained in... 

None of these terms appears to be completely satisfactory. The concept of "mobility in NMR spectroscopy is quite different from that in the field of separation science, where mobility generally requires a measurable degree of solubility and/or distillability in liquid or gaseous media, respectively. For example, polymethylene-like moieties, such as found in some coal components (0, are highly "mobile in NMR terms (1), without necessarily being extractable by solvents or distillable by nondestructive heating. 

There are currently three different approaches for carrying out ASC-PCM calculations [1,3]. In the original method, called dielectric D-PCM [18], the magnitude of the point charges is determined on the basis of the dielectric constant of the solvent. The second approach is C-PCM by Cossi and Barone [24], in which the surrounding medium is modelled as a conductor instead of a dielectric. The third, IEF-PCM method (Integral Equation Formalism) by Cances et al the most recently developed [16], uses a molecular-shaped cavity to define the boundary between solute and dielectric solvent. We have to mention also the COSMO method (COnductorlike Screening MOdel), a modification of the C-PCM method by Klamt and coworkers [26-28], In the latter part of the review we will restrict our discussion to the methods that actually are used to model solute-solvent interactions in NMR spectroscopy. 

Bottomley PA (1987) Spatial localization in NMR spectroscopy in vivo. Ann NY Acad Sci 508 333-348 Bottomley PA, Edelstein WA, Foster TH, Adams WA (1985) In vivo solvent-suppressed localized hydrogen nuclear magnetic resonance spectroscopy a window to metabolism Proc Natl Acad Sci USA 82 2148-2152 Brown TR, Kincaid BM, Ugurbil K (1982) NMR chemical shift imaging in three dimensions. Proc Natl Acad Sci USA 79 3523-3526... 

The ring/chain tautomeric equilibrium between (2-hydroxyphenylimino)phos-phorane (21a) and 1,3,2-benzoxazaphospholine (21b) has been studied in thirteen solvents by NMR spectroscopy [80b]. This equilibrium is shifted towards the ring-form (21b) in hydrogen-bond acceptor solvents e.g. tris- -propylamine, dimethyl sulfoxide), compared to inert solvents such as benzene or acetone. Therefore, depending on substituents and solvents, it is possible to prepare either iminophosphoranes or benzox-azaphospholines [80b]. 

Dracinsk, M. Bouf, P. Computational analysis of solvent effects in NMR spectroscopy, J. Chem. Theor. Comput. 2009, 6, 288-299. 

A considerable amount of research has been concerned with the nature of the electrophiles that are involved in Friedel-Crafts acylation reactions. We will summarize the main points. Acyl halides and carboxylic acid anhydrides have been known, for many years, to form stable complexes with a variety of acid catalysts. A well-defined product is formed between acetyl fluoride and boron trifluoride at low temperatures. Analytical and conductivity data characterized the material as acetylium tetrafluoroborate, and this was further confirmed by IR measurements. In the system acetyl chloride-aluminum chloride the acetylium ion can be differentiated from the donor-acceptor complex involving the carbonyl group by means of their IR carbonyl stetching frequencies. A number of other acyl fluorides have been shown to form well-defined acylium salts by interaction with a number of metal fluorides. Acylium salts can also be prepared from acyl chlorides by means of metathetical reactions involving anhydrous salts such as silver hexafluoroantimonate. As well as characterization by means of IR spectroscopy, acylium salts have been studied in non-nucleophilic solvents by NMR spectroscopy. The NMR data for the ben-... 

The sample used to obtain an NMR spectrum is made by dissolving the compound in an appropriate solvent. Solvents with protons cannot be used since the signals for solvent protons would be very intense because there is more solvent than compound in a solution. Therefore, deuterated solvents such as CDCI3 and D2O are commonly used in NMR spectroscopy. 

Jonas and co-workers have pointed out that often a compromise must be found between sensitivity and resolution in NMR spectroscopy [76]. Line narrowing is optimum in regions of low supercritical fluid density (where the viscosity is low), but then the solubility of compounds is also low. Sometimes, admixtures with small amounts of low-viscosity solvents such as acetone may be tried to obtain a reasonable concentration of the compounds studied, i.e. coordination compounds such as (R-N=CH)2Mo(CO)4. However, line widths for this compound decrease by a factor of about four to six when comparing benzene-dg solutions to supercritical CO2 (with 8% acetone-de). The dispersion of the nitrogen chemical shift ensures identification of coordinated ligands by using NMR, in the above molybdenum complex, where A5( n) = -36 ppm) [75]. 

Usable FT-spectra of 0.1 mg of Baygon in 0.5 ml CDCI3 were obtained after 2,750 pulsed scans (46 minutes)—the same number of scans using CW-NMR at 250 and 500 seconds per scan would have taken 8 or 16 days, respectively. At this sensitivity, weak sample peaks can be observed, but often peaks from impiuities in the solvent are magnified also, and they may obscure some of these sample peaks. Accumulation of the same number of pulsed scans on a solvent blank and modification of the computer program to subtract this background from the sample data would eliminate this problem. More scans and use of a microcell should enable one to obtain spectra on 0.01 mg of sample or less. This enhanced sensitivity should do more to widen the use of NMR for pesticide residue analysis than any other innovation in NMR spectroscopy. Perhaps even GC-FT-NMR will become a reality in the future. 

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