Contour spectra

Figure 1 Contour spectra of BuPBD and Poly (VBuPBD)...

Figure 3.49 COSY profile and contour spectra of sucrose in D2O. Note the strong contour connecting the diagonal peak for the proton at 5.41 ppm with the doublet of doublets at 3.55 ppm. In addition, the triplet at 4.05 ppm has strong off-diagonal contours indicating it is coupled to the doublet at 4.22 ppm. Compare with Fig. 3.25. (Modified from Petersheim, used with permission.)...

Rotational contours spectra in fig.3 are of great help in assigning the A bands to a parallel transition and the B bands to a perpendicular transition. In the quasi diatomic limit the A-X transition correlates in Hg NH3 to a parallel transition 0" - 0+ while the B-X transition to a perpendicular one, 1 - 0". ... 

Use the stored ID spectra as projections and verify the correct calibration of the 2D spectrum. The selected peak(s) in the ID projection spectrum should appear at the same position (ppm) as the corresponding cross peak in the 2D contour spectrum. 

This general behaviour is characteristic of type A, B and C bands and is further illustrated in Figure 6.34. This shows part of the infrared spectrum of fluorobenzene, a prolate asymmetric rotor. The bands at about 1156 cm, 1067 cm and 893 cm are type A, B and C bands, respectively. They show less resolved rotational stmcture than those of ethylene. The reason for this is that the molecule is much larger, resulting in far greater congestion of rotational transitions. Nevertheless, it is clear that observation of such rotational contours, and the consequent identification of the direction of the vibrational transition moment, is very useful in fhe assignmenf of vibrational modes. 

The pulse sequence which is used to record CH COSY Involves the H- C polarisation transfer which is the basis of the DEPT sequence and which Increases the sensitivity by a factor of up to four. Consequently, a CH COSY experiment does not require any more sample than a H broadband decoupled C NMR spectrum. The result is a two-dimensional CH correlation, in which the C shift is mapped on to the abscissa and the H shift is mapped on to the ordinate (or vice versa). The C and //shifts of the //and C nuclei which are bonded to one another are read as coordinates of the cross signal as shown in the CH COSY stacked plot (Fig. 2.14b) and the associated contour plots of the a-plnene (Fig. 2.14a and c). To evaluate them, one need only read off the coordinates of the correlation signals. In Fig. 2.14c, for example, the protons with shifts Sh= 1.16 (proton A) and 2.34 (proton B of an AB system) are bonded to the C atom at c = 31.5. Formula 1 shows all of the C//connectivities (C//bonds) of a-pinene which can be read from Fig. 2.14. 

From the HH COSY contour plot 12a it can be established which cycloadduct has been produced from l-(iV,iV-dimethylamino)-2-methylbuta-1,3-diene and tra 5-P-nitrostyrene. The Jhh coupling constant in the one-dimensional H NMR spectrum 12b can be used to deduce the relative configuration of the adduct. 

From which compound were the INADEQUATE contour plot and C NMR spectra 21 obtained Conditions (CD3)2CO, 95 % v/v, 25 °C, 100 MHz. (a) Symmetrised INADEQUATE contour plot with C NMR spectra (b) H broadband decoupled spectrum (c) NOE enhanced coupled spectrum (gated decoupling) (d) expansion of multiplets of (c). 

Conditions CDCI3, 25 °C, 200 MHz ( //), 50 MHz ( C). (a) CH COSY (shaded contours) and CH COLOC diagrams (unshaded contours) in one diagram with enlarged section (b), and with expanded methoxy quartets (c) (d) sections of C NMR spectra, each with //broadband decoupled spectrum below and NOE enhanced coupled spectrum (gated decoupling) above. 

Fig. 0.3. Raman spectrum of liquid oxygen [6]. The positions of the free rotator s. /-components are shown by vertical lines and the isotropic scattering contour is presented by the dashed line.

Chapter 3 is devoted to pressure transformation of the unresolved isotropic Raman scattering spectrum which consists of a single Q-branch much narrower than other branches (shaded in Fig. 0.2(a)). Therefore rotational collapse of the Q-branch is accomplished much earlier than that of the IR spectrum as a whole (e.g. in the gas phase). Attention is concentrated on the isotropic Q-branch of N2, which is significantly narrowed before the broadening produced by weak vibrational dephasing becomes dominant. It is remarkable that isotropic Q-branch collapse is indifferent to orientational relaxation. It is affected solely by rotational energy relaxation. This is an exceptional case of pure frequency modulation similar to the Dicke effect in atomic spectroscopy [13]. The only difference is that the frequency in the Q-branch is quadratic in J whereas in the Doppler contour it is linear in translational velocity v. Consequently the rotational frequency modulation is not Gaussian but is still Markovian and therefore subject to the impact theory. The Keilson-... 

The intensity at the periphery of the line ( Ageneral rule (2.62) [20, 104]. However, the most valuable advantage of general formula (3.34) is its ability to describe continuously the spectral transformation from a static contour to that narrowed by motion (Fig. 3.1). In the process of the spectrum s transformation its maximum is gradually shifted, the asymmetry disappears and it takes the form established by perturbation theory. 

Although from a mathematical point of view formulae (3.34) and (3.40) have little in common, the spectral transformation described by them proceeds in a similar way (Fig. 3.2). Just as with strong collisions, the contour is gradually symmetrized and its centre is shifted to the average frequency coq with an increase in the density. When the spectrum is narrowed (at T 1), its central part ( Aco] < 1/tj) takes the following form ... 

The quasi-classical description of the Q-branch becomes valid as soon as its rotational structure is washed out. There is no doubt that at this point its contour is close to a static one, and, consequently, asymmetric to a large extent. It is also established [136] that after narrowing of the contour its shape in the liquid is Lorentzian even in the far wings where the intensity is four orders less than in the centre (see Fig. 3.3). In this case it is more convenient to compare observed contours with calculated ones by their characteristic parameters. These are the half width at half height Aa)i/2 and the shift of the spectrum maximum ftW—< > = 5a>+A, which is usually assumed to be a sum of the rotational shift 5larger scale A determined by vibrational dephasing. 

In the pioneering work the same information was extracted from the extremum position assuming it is independent of y [143]. This is actually the case when isotropic scattering is studied by the CARS spectroscopy method [134]. The characteristic feature of the method is that it measures o(ico) 2 not the real part of Ko(icu), as conventional Raman scattering does. This is insignificant for symmetric Lorentzian contours, but not for the asymmetric spectra observed in rarefied gas. These CARS spectra are different from Raman ones both in shape and width until the spectrum collapses and its asymmetry disappears. In particular, it turns out that... 

Two-dimensional NMR spectra are normally presented as contour plots (Fig. 3.11a), in which the peaks appear as contours. Although the peaks can be readily visualized by such an overhead view, the relative intensities of the signals and the structures of the multiplets are less readily perceived. Such information can be easily obtained by plotting slices (cross-sections) across rows or columns at different points along the Fi or axes. Stacked plots (Fig. 3.11b) are pleasing esthetically, since they provide a pseudo-3D representation of the spectrum. But except for providing information about noise and artifacts, they offer no advantage over contour plots. Finally, the projection spectra mentioned in the previous section may also be recorded. 

Heteronuclear two-dimensional /-resolved spectra contain the chemical shift information of one nuclear species (e.g., C) along one axis, and its coupling information with another type of nucleus (say, H) along the other axis. 2D /-resolved spectra are therefore often referred to as /,8-spectra. The heteronuclear 2D /-resolved spectrum of stricticine, a new alkaloid isolated by one of the authors from Rhazya stricta, is shown in Fig. 5.1. On the extreme left is the broadband H-decoupled C-NMR spectrum, in the center is the 2D /-resolved spectrum recorded as a stacked plot, and on the right is the con tour plot, the most common way to present such spectra. The multiplicity of each carbon can be seen clearly in the contour plot. 

Figure 5.2 Presentation of 2D /-resolved spectra. In the ID plot (i), both 8 and / appeared along the same axis, but in the 2D /-resolved spectrum (ii), the multiplets are rotated by 90° at their respective chemical shifts to generate a 2D plot with the chemical shifts (8) and coupling constants (/) lying along two different axes, (iii) The 2D /-resolved spectrum as a contour plot.

Figure 5.22 Artifact signals appear due to strongly coupled nuclei, as shown by the vertical lines of contours at about 6 7.16 in the spectrum.

Figure 5.47 Two-dimensional exchange spectrum of N,Af-dimethylacetamide and its generation, (a) The first set of spectra results from the first series of Fourier transformations with respect to The modulation of signals as a function of t is observed, (b) The second set of spectra is obtained by the second series of Fourier transformations. The unmodulated signals appear on the diagonal at (v, v ), (vx, Vx), and (v, v ), whereas the modulations due to exchange show up as crosspeaks on either side of the diagonal at (vx, Vx) and (vx, Vx). (c) A contour plot representation of (b). (Reprinted from Science 232, A. Bax, et ai, 960, copyright (1986), with permission from Science-AAAS, c/o Direct Partners Int., P.O. Box 599, 1200 AN Hilversum, The Netherlands)...

Figure 8. 2D NIR/IR correlation spectrum. Only contours for correlation coefficients with numerical values above 0.5 are shown.

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