Proton reference spectra

Fig. 13. Results obtained with 4 mm samples in a 500 MHz gradient inverse triple resonance cryogenic NMR probe, (a) Non-spinning resolution of the -methanol multiplet for a 30 mm solvent column in a 4 mm tube, (b) Non-spinning resolution of the -methanol multiplet for a 22 mm solvent column in a 4 mm tube. As expected from Fig. 12, the resolution is lower with a solvent column of this short (the optimal solvent column for a 4 mm tube is 30 mm) is shown in Panel A. (c) Resolution of the -methanol multiplet for a 22 mm solvent column in a 4 mm tube with the sample spinning at 20 Hz. For very scarce samples when it is necessary to resort to the shortest possible solvent column height to facilitate the acquisition of high-quality 2D-NMR data, it may be beneficial to spin the sample during the acquisition of the proton reference spectra.

The order in which various NMR data are acquired is largely one of user preference. Acquisition of the proton reference spectrum will invariably be undertaken first. Whether a user next seeks to establish homo- or heteronuclear shift correlations is where individual preferences come into play. Many spectro-scopists proceed from the proton reference spectrum to either a COSY or a TOCS Y spectrum next, while others may prefer to establish direct proton-carbon chemical shift correlations. This author s preference is for the latter approach. From a multiplicity-edited HSQC spectrum you obtain not only the carbon chemical shifts, which give an indication of the location of heteroatoms, the degree of unsaturation and the like, but also the number of directly attached protons, which eliminates the need for the acquisition of a DEPT spectrum [51, 52]. The statement in the prior sentence presupposes, of course, that there the sensitivity losses associated with the acquisition of multiplicity-edited HSQC data are tolerable. 

Fig. 1. HMQC spectrum of a 12 pig (0.05 pmol) sample of the indoloquinoline alkaloid cryptolepine (1) dissolved in 145 pL of d6-DMSO. The data were recorded overnight (16 h) at 500 MHz using a 3 mm micro-inverse-detection NMR probe.6 Both a proton reference spectrum (top trace) and the projection through the Fi frequency domain (bottom trace) are shown above the contour plot. (Reprinted with permission from Ref. 6. Copyright 1992, American Chemical Society and American Society of Pharmacognosy.)...

Fig. 8.37 Spectra recorded with a 2.9 gg sample of paclitaxel ( laxol , 10, 3.4 nanomol) dissolved in 165 gL CDCI3 [219]. A. Proton reference spectrum recorded in 32 transients. B. COSY spectrum recorded in 46 min as a 2048 X 128 point file accumulating 12 transients/ti increment. All of the expected correlations are ob-serverbable in the spectrum and are discernible from the noise. C. COSY spectrum recorded in 3 h 4 min as a 2048 X 192 point file accumulating 32 transients/ti increment. The spectrum is essentially noise-free.

Our study of cryptolepine, performed on 5 mg of material dissolved in 0.8 ml of DMSO-d6, highlights what will probably become an increasingly common protocol as inverse-detected 2D NMR methods become more widely implemented. Typically, we acquire a proton reference spectrum followed by COSY and HMQC spectra. On the sample in question here. 

This formulation is supported by the proton resonance spectrum of the trifluoromethyl compound 101 which shows that it exists in the CH form shownd However, strong electron-withdrawing groups in the 4-position apparently lead to enolization, and compound 102, for example, gives an intense color with ferric chloride, - Other 4-acylated oxazol-5-ones are often formulated as 103 (see, e.g, reference 113). Tautomerism of the type illustrated by the equilibrium 104 103 has been discussed (see reference 115 for further references). 

The proton NMR spectrum of griseofulvin (Figure 2) was obtained in DMSO-d, solution (cone, w/v = 10 mg/0.40 ml) containing TMS as internal reference utilizing the Varian Associates CFT-20 Spectrometer operating at a frequency of 79.5 MHz. The chemical shifts (<5, ppm) are with reference to TMS. The experimental conditions are ... 

Of the multitude of ID 13C NMR experiments that can be performed, the two most common experiments are a simple broadband proton-decoupled 13C reference spectrum, and a distortionless enhancement polarization transfer (DEPT) sequence of experiments [29]. The latter, through addition and subtraction of data subsets, allows the presentation of the data as a series of edited experiments containing only methine, methylene and methyl resonances as separate subspectra. Quaternary carbons are excluded in the DEPT experiment and can only be observed in the 13C reference spectrum or by using another editing sequence such as APT [30]. The individual DEPT subspectra for CH, CH2 and CH3 resonances of santonin (4) are presented in Fig. 10.9. 

FIGURE 3. 400 MHz proton NMR spectrum of 15 in CDCI3 (olefinic protons) with (a) no shift reagent, (b) racemic 15 with Eu(tfc)3 and Ag(fod), (c) 15 produced from piperylene with Ni(COD)2 and D-EPHOSNH, with Eu(tfc)3 and Ag(fod). Reproduced by permission of Elsevier Sequoia S.A. from Reference 24... 

Curve a in Figure 5.1147 shows an nmr spectrum taken during the course of the reaction. Curve b is a reference spectrum of ethyl iodide (CH3 protons at S = 1.85 CH protons at... 

Fig. 3.9 Expan.sion of the ID NOE Spectra of peracetylated glucose Top trace -Unperturbed reference spectrum. Bottom trace - Difference spectrum. Proton H-C(6) with resonance at 4.3 ppm has been selectively saturated.

Adjust the vertical scale and the vertical offset for the main trace spectrum to obtain the best display of the signals for the ring protons. Click on the File Param. button and set the plot limits to 6.0 ppm and 3.5 ppm. Click on both the F1/F2 for all and the Y for all buttons to transfer the plotting information from the reference spectrum to the other six spectra. Click on the OK button to close the dialog box. Do not click on the Return button as this will lose all the Separate Plot plotting parameters that have just been set. From the Output pull-down menu choose the Page Layout option and select the same parameters, options, colors, fonts and the printer setup as used in the multiple display above. Use the Preview option for a final inspection before plotting this Separate Plot layout. 

Fig. 12. Proton NMR spectrum at 220 Me of cyanoferrideuteroporphyrin IX dimethylester. The letters refer to the resonance assignments of Fig. 9. The distinction between the resonances c and d is arbitrary (Reproduced from ref. (114))...

O NMR spectroscopic studies in HF-SbF5 with O-enriched hydronium ion have indicated strong 170-1 H coupling (Figure 4.1). In the proton-coupled spectrum, a quartet is observed at 8170 9 0.2 (with reference to S02 at 505 ppm) with i7o—1H = 106 1.5 Hz. 

According to the Forster cycle, if the longest wavelength electronic transition of the deprotonated form is of lower energy compared to that of the protonated form (red-shifted electronic absorption or emission spectrum of the deprotonated form with reference to the protonated-form spectrum), the molecule has enhanced excited-state acidity (i.e., the pK a of the molecule is lower than pKa). Equation (1) provides a quick and effective method for evaluating a molecule for its ESPT behavior. 

The proton NMR spectrum was recorded in DMSO-dg containing tetramethylsilane as internal reference and with the use of a Bruker WM-300 spectrometer at frequency 300.13 MHz. The spectrum is presented in Figure 2 and the spectral assignments are summarized in Table II (11). The chemical shifts roughly agree with those reported for sulfadiazine (12). The change in chemical shifts (to high field) for silver sulfadiazine compared to sulfadiazine is 0.3 ppm (NH2) or less (11). 

Fig. 10. Schematic representation of the creation of the STD or difference spectrum. In the off resonance or reference spectrum, all protons have normal intensity. Depending on the vicinity of the respective proton to the receptor surface, the resonance signal intensity is attenuated in the on resonance spectrum. The difference spectrum is created by subtraction of the on resonance from the off resonance spectrum and displays only resonances of signals of protons in vicinity to the receptor. This gives indications on the binding mode and can be employed for screening purposes, as nonbinding molecules do not display and intensities in the difference spectrum.

The proton decoupled 29Si spectrum of tetram-ethylsilane (TMS) is shown at the top of Figure 6.9 with the proton coupled spectrum for comparison as an inset. TMS is the obvious choice for a 29Si reference compound and we set it at zero ppm. The proton-coupled spectrum is quite interesting because the 29Si nucleus is coupled to 12 equivalent protons in TMS. First order rules predict a multiplet with 13 peaks. There are 9 peaks clearly visible and 11 with a little imagination we do not see the full 13 peaks because the outer ones are too weak and are lost in the noise. 

Fig. 18.11. Spectra collected as a function of time immediately before and after the admission of 1.0 M H,S04 to the cell. The reference spectrum was taken prior to the admission of the 1.0 M H2S04, when the cell contain 0.1 M H2S04. The spectrum close to the baseline is obtained before the exchange of electrolyte, and the four other spectra every minute after the exchange of the electrolyte, giving rise to stronger and stronger absorptions. The loss band at 1640 cm 1 is due to loss of free water, whereas the broad band between 1680 cm-1 and ca. 2000 cm- is due to water of solvation of the additional protons.

In the 1 H-NMR spectrum (270 MHz) the methine and the two methylene protons of methylsuccinic add gave rise to a well-resolved ABC system. In the two enantiomers 38 and 39 the H atoms HR<,R/HSlS and H5(R/HWe.v are pair-wise reflection equivalent, i.e., identical in the NMR spectrum, whereas the diastereotopic geminal protons can be distinguished by their different chemical shifts. For the assignment of the signals to the diastereotopic protons, reference compounds of known configuration were synthesized by treating mesaconic and citraconic acids (40 and 42) with deuterated diimide. The known syn-addition of deuterium [35,36] afforded the racemic but stereospecifically dideuterated methylsuccinic acids (41 and 43) (Fig. 25). 

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