One-dimensional spectra

It is therefore apparent that the first series of Fourier transformations of the data (FIDs) with respect to yield the so called one-dimensional spectra, in which the amplitude of the peak oscillates (with frequency 1)2) as a function of t (Fig. 3.3) ... 

One-dimensional spectra obtained by projecting 2D spectra along a suitable direction often contain information that cannot be obtained directly from a conventional ID spectrum. They therefore provide chemical shift information of individual multiplets that may overlap with other multiplets in the corresponding ID spectra. The main difference between the projection spectrum and the ID spectrum in shift-correlated spectra is that the projection spectrum contains only the signals that are coupled with each other, whereas the ID H-NMR spectrum will display signals for all protons present in the molecule. 

Vosegaard and Massiot [144] showed that it is possible to create a high-resolution 2D spectrum correlating the chemical shifts with the second-order quadrupolar lineshapes from several spectra recorded at different magnetic field strengths using a projection-reconstruction method called chemical shift-quadrupolar projection-reconstruction of one-dimensional spectra (CQ-PRODI). 

Figure 5.3 displays a comparison of one-dimensional spectra of a neuropeptide in the presence of either DPC or SDS micelles at various pH values. Note that the signal at approx. 8.6 ppm due to HE1 of His vanishes in SDS at a much higher pH compared to DPC. In general, spectra in SDS still yield reasonable quality at neutral pH in contrast to those recorded in DPC. 

Fig. 5.3 Proton one-dimensional spectra displaying the region of amide resonances of 0.5 mM Ala31, Pro32-NPY recorded in 300 mM SDS (left) or 300 mM DPC (right) at various values of the pH.

Bodenhausen ° developed a pattern-recognition programme (MARCO POLO) in order to extract coupling pathways in COSY spectra. He subsequently described a recursive deconvolution technique for the measurement of couplings, but this seems to offer more benefits to the measurement of scalar couplings in two-dimensional spectra than in one-dimensional spectra. 

One-dimensional spectra obtained in these experiments can be compared to ID traces of nD NMR spectra but offering much better digital resolution and shorter acquisition times. On the negative side each trace needs to be acquired separately and thus, if several sites are to be inspected, a series of ID experiments must be performed. In practice, this exercise is preceded by careful inspection of standard two-dimensional COSY, TOCSY, NOESY or ROESY spectra and only the ambiguous assignments are tackled by combined ID techniques. 

A series of selective inversions was performed on the compound, using the simple 7r/2-r-7r/2 excitation sequence, in which the two pulses are hard. Our experience is that the quality of the data does not seem to depend as critically as we thought [48] on the choice of initial conditions. Most reasonable perturbations, so long as they are not completely non-selective (fig. 7), seem to give similar data. Figure 8 shows the reassuring quality of the fit between the observations and the model, which is available from only 13 one-dimensional spectra. The data were then analyzed to see if the concerted mechanism was appropriate, as indeed it was. 

Figure 8.2.15 One-dimensional spectra acquired with the four-coil probe. Each sample (250 mM in D2O) was loaded into the coil via the attached Teflon tubes 32 scans were acquired for each spectrum, with no delay between excitations of successive coils. Concurrent with the switch position being incremented, the spectral width was optimized for each compound 1 Hz line-broadening was applied before Fourier transformation and baseline correction. The spectral widths were (a) 600 Hz (galactose) (b) 1400 Hz (adenosine triphosphate) (c) 2000 Hz (chloroquine) (d) 500 Hz (fructose). 2048 complex data points were acquired for each spectrum, giving data acquisition times of approximately 1.7, 0.7, 0.5 and 2.0 s, respectively. The delay between successive 90 degree excitations was 4.9 s for each sample. Reprinted with permission From Li, Y., Walters, A., Malaway, P., Sweedler, J. V. and Webb, A. G., Anal. Chem.,l, 4815-4820 (1999). Copyright (1999) American Chemical Society...

An a-y coincidence experiment was performed using a cooled Si(Li) detector for the detection of photons and a Si detector for the detection of a-particles. Three parameter events were collected on tape and one dimensional spectra were later generated in coincidence with various gates. The spectra showed that the and a3Q are in prompt coincidence with L X-rays and the delay occurs at the 27.4 keV level. The analysis of the time spectrum between the group and the 27.4 keV photopeak gave a half-life of 38.3 - 0.3 ns, in agreement with previous measurements. 

Tab. 5.9 Peak intensity ratios obtained from one-dimensional spectra obtained at day 4 after transplantation of 105 cells/mL (mean SD). (Reprinted from Tab. 1 of ref. 101 with permission from the American Association for Cancer Research)...

Next, one should consider the line width of the one-dimensional spectra of both nuclei. The detected nucleus should be the one with the smaller linewidth, since a short T2 leads to signal losses during the pulse sequence. Compared with metal nuclei usually has the smaller linewidth. Because of the chemical shift anisotropy and the temperature dependence of the chemical shift of heavy nuclei, the measurement of is therefore favoured in these cases. For detection, in addition, the large NOE enhancement caused by proton irradiation has to be taken into account. Heteronuclei usually have no directly bound proton their NOE factors are therefore lower. To facilitate further discussion, the y values and the natural abundances of the nuclei used for C-X correlation are given in Table 1. 

Figure 27. Lower, three-dimensional presentation of the spectral region, 0.5-6.0 ppm of a 360 MHz spin-echo-correlated H-NMR spectrum of BPTI. The chemical shift, 5, on the horizontal axis corresponds to that in conventional, one-dimensional spectra. Ad on the vertical axis stands for the difference frequencies between correlated nuclei. Cross-peaks between J-coupled nuclei are at 0.5 Ad. The solvent singlet resonance is at 4.35 ppm. Upper, contour plot of the same spin-echo-correlated spectrum of the inhibitor.

The sensitivity of is inconveniently low, and in the conventional H-decoupled spectrum the excitation of the proton resonances increases the strength of the signal in addition to simplifying it. However, rather than simply applying rf power to the protons, a series of pulses to both protons and carbons results in Distortionless Enhancement by Polarisation Transfer (DEPT) and gives one-dimensional spectra with usefully higher sensitivity than from proton decoupling. Additionally, the one-dimensional spectrum... 

The approach to any structural or mechanistic problem will invariably start with the acquisition of one-dimensional spectra. Since these provide the foundations for further work, it is important that these are executed correctly and full use is made of the data they provide before more extensive and potentially time-consuming experiments are undertaken. This chapter describes the most widely used one-dimensional techniques in the chemistry laboratory, beginning with the simple single-pulse experiment and progressing to consider the various multipulse methods that enhance the information content of our spectra. The key characteristics of these are summarised briefly in Table 4.1. This chapter does not cover the wide selection of techniques that are strictly one-dimensional analogues of two-dimensional experiments, as these are more appropriately described in association with the parent experiment and are found throughout the following chapters. 

Before moving on, a comment on the presentation of 2D spectra is required. The spectra of Figs. 5.6 and 5.9 have been presented in the stacked-plot mode to emphasise the similarity with one-dimensional spectra, and the presence of two frequency axes and one intensity axis. Although these may look aesthetically impressive, this form of presentation is of little use in practice. The usual way to present 2D spectra is via contour plots , in which peak intensities are represented by contours, as a mountain range would be represented on a map. Fig. 5.10 shows the equivalent contour presentation of Figs. 5.6 and 5.9 and unless stated otherwise, all 2D spectra from now on will make use of this contour mode. 

Conventional NMR spectra (one-dimensional spectra) are plots of intensity vs. frequency in two-dimensional spectroscopy intensity is plotted as a function of two frequencies, usually called F1 and Fr There are various ways of representing such a spectrum on paper, but the one most usually used is to make a contour plot in which the intensity of the peaks is represented by contour lines drawn at suitable intervals, in the same way as a topographical map. The position of each peak is specified by two frequency co-ordinates corresponding to Fl and F2. Two-dimensional NMR spectra are always arranged so that the F2 co-ordinates of the peaks correspond to those found in the normal onedimensional spectrum, and this relation is often emphasized by plotting the onedimensional spectrum alongside the F2 axis. 

Generally, two-dimensional experiments produce amplitude modulation, indeed all of the experiments analysed in this chapter have produced either sine or cosine modulated data. Therefore most two-dimensional spectra are fundamentally not frequency discriminated in the F1 dimension. As explained above for one-dimensional spectra, the resulting confusion in the spectrum is not acceptable and steps have to be taken to introduce frequency discrimination. 

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