Solvents residual signals

Of course, you don t have to use either of the above standards at all. In the case of samples run in deutero chloroform/methanol and dimethyl sulfoxide, it is perfectly acceptable, and arguably preferable, to reference your spectra to the residual solvent signal (e.g., CD2HOH) which is unavoidable and always present in your spectrum (see Table 2.2). These signals are perfectly solid in terms of their shifts (in pure solvent systems) though the same cannot be said for the residual HOD signal in D2O and for this reason, we would advise adhering to TSP for all samples run in D20. 

FIGURE 7.6 Extracted NMR spectra (600 MHz, 1.5 xL solenoidal microprobe) of the tocopherol homologues at the corresponding peak maxima residual solvent signals. (Reproduced with permission from Ref. 30. Copyright (2004) American Chemical Society.)... 

An example of a spectrum with automated solvent suppression is given in Figure 2.6. Normally, one has to deal with a mixture of two solvents, where often one of the solvents is water and is replaced by D2O. This leads to one or two intense solvent lines which have to be suppressed. In this figure, a worst-case scenario is presented. The mixture consists of two non-deuterated solvents (acetonitrile and methanol) which show, in total, three intense solvent lines. The solvent suppression parameters were adjusted automatically, and an additional digital filtering to remove the residual solvent signals was also carried out. 

Fig. 12, Example of multiple suppression. (A) Reference spectrum of 4 mM Sandos-tatin at pH 7.6 in 250 /xl HjO, 250 /xl /is-DMSO (50 /xl HjO). (B) Spectrum recorded with the pulse sequence given in Fig. IIB using 16 scans. The length of the ttslp pulses was 2.9 ms. The asterisks denote residual solvent signal. (From Dalvit with permission.)...

Figure 9.30. Solvent suppression with the excitation sculpting scheme using the approach of Fig. 9.28b with 4.1 ms 90° Gaussian pulses and gradients of 0.1 0.1 0.03 0.03 T m . The sample is 2 mM sucrose in 9 1 H2O D2O. In (b) the small residual solvent signal has been completely removed through additional processing of the FID (see text).

Residual solvent signal amplitude is only one of the many criteria for evaluating suppression techniques. As spectroscopists, we are certainly interested in any baseline corruption that may result, as this will deter the ability to properly integrate and automatically interpret spectra. We also do not want the inclusion of spurious peaks, or the removal of actual information close to the solvent signal. 

TABLE 21. RESIDUAL SOLVENT SIGNALS OF COMMON NMR SOLVENTS... 

Fig. 2 H-NMR of poly(gfycidyf methacrylate) before and after fiinctionalizatirai. Typically, the epoxide proton resonances are seen at 2.6, 2.8, and 3.2 ppm. The ring opening reaction results in the disappearance of these proton resonances. Residual solvent signals are marked with an asterisk...

MHz. Chemical shifts were reported in S (ppm) relative to tetramethylsilane (TMS) or residual solvent signals as the internal standard (CHCI3, S = 7.26, DMSO-ds, S = 2.50). Spectra data were presented as follows Chemical shifts (S ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), coupling constants (Hz), integration, and assignment. 

Spectral data from the literature for chemical shift matches. Many researchers use NMR solvents that do not contain TMS and thus reference their chemical shift to the residual solvent signal, which we have just seen can vary. One should be sure to reference spectra in the same way as the literature data. When making such comparisons, it is not at all uncommon to have consistent chemical shift mismatches across a spectrum, with all of the resonances 0.06 ppm higher (or lower) than the literature data, for example. 

---