Cross signals

Figure 2.12. Two-dimensional (2D-)INADEQUATE diagram of 1-butanol (12) [ CDajzCO, 95% v/v, 25°C, 50 MHz, 128 scans, 128 experiments], (a) Contour plot with the AB systems of bonded C atoms on the horizontal axis (b) plots of the three AB systems of the molecule obtained from (a) (c) contour plot of the symmetrised INADEQUATE experiment showing the AB or AX systems of bonded C atoms in the HH COSY format (cross signals on the axes perpendicular to the diagonal)...

Figure 2.13. Symmetrised two-dimensional INADEQUATE experiment with isopinocampheol (2) [ CDshCO, 250 mg in 0.3 ml, 25 °C, 50 MHz, 256 scans and exp.], (a) Stacked plot of the section between 8c = 20.9 and 48.2 (b) complete contour plot with cross signal pairs labelled a-k for the 11 CC bonds of the molecule to facilitate the assignments sketched in formula 2...

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. 

Figure 2.21. HFI NOE difference spectra (b, c) and FIFI NOESY diagram (d) of a-pinene (1) with /-/ NMR spectrum (a) for comparison [(CD3)2CO, 10% v/v, 25 °C, 200 MHz, section from <5 = 0.85 to 2.34 ]. Vertical arrows in (b) and (c) indicate the irradiation frequencies in the HH NOESY plot (d), cross-signals linked by a dotted line show the NOE detected in (c)...

The cross signals in the INADEQUATE plot show the CC bonds for two part structures A and B. Taking the C signal at 8c = 174.1 as the starting point the hydrocarbon skeleton A and additional C3 chain B result. 

The structure of the dimer can be derived simply by evaluation of the cross signals in the HH COSY plot. The cycloalkene protons form two AB systems with such small shift differences that the cross signals lie within the contours of the diagonal signals. 

Carbon atoms and protons are assigned by means of the proton-carbon connectivities as identified in the HC HSQC and HMBC experiment (b and c). The latter also permits the derivation of the connection of the ethyl groups to the porphyrin ring. The cross signals in the relevant part a of the HH COSY plot (a) are used to connect the methyl and methylene subunits to the ethyl groups. 

The equivalence of the inner NH protons (Sm = - 2.35) as well as correlation signals with the pyrrolic carbons only (5c = 143.8, 141.6, 139.8 and 136.7) provide evidence for the diaza[18]-annulene tautomer 4a. Two separate NT/ proton signals and cross signals with the a-carbon atoms (5c = 159.3) of the triazole ring are expected, in contrast, for the tetraaza[18]annulene tautomer 4b. 

Well separated cross signals of the HH COSY plot demonstrate... 

Taking these methylene groups into account, interpretation of the HH COSY plot leads directly to the HH relationships C even if the protons at Sh = 2.34 and 4.58 do not show the expected cross signals because their intensity is spread over the many multiplet lines of these signals. 

CH COSY Correlation via one-bond CH coupling, also referred to as HETCOR (heteronuclear shift correlation), provides carbon-13- and proton shifts of nuclei in C//bonds as cross signals in a 5c versus 8h diagram, assigns all C//bonds of the sample... 

Figure 6.4 Three-dimensional spectrum of a three-spin system showing peak types appearing in a three-dimensional space. Three diagonal peaks, six (wi = Wj) and six (wj = w,) cross-signal peaks, six back-transfer peaks, and six cross-peaks are present in the cube, (a) The cubes (b-d) represent three planes in which crossdiagonal peaks and the back-transfer peaks appear on their respective (atj = 0)2), u>2 = cof), and ( >i = Wj) planes. (Reprinted from J. Mag. Reson. 84, C. Griesinger, et al., 14, copyright (1989), with permission from Academic Press, Inc.)...

Kotlikov, E.N. and Kondratjeva, V.A. (1980). Effect of a strong electromagnetic field on the shape of the crossing signals in zero magnetic field, Opt. Spectrosc. (USSR), 48, 367-371. 

An HH COSY diagram can be shown in perspective as a stacked plot (Fig. 2.10a). Interpretation of this neat, three-dimensional representation, where the signal intensity gives the third dimension, can prove difficult because of distortions in the perspective. The contour plot can be interpreted more easily. This shows the signal intensity at various cross-sections (contour plots, Fig. 2.10b). However the choice of the plane of the cross-section affects the information provided by an HH COSY diagram if the plane of the cross-section is too high then the cross signals which are weak... 

An examination of the cross signals of the HH COSY diagram leads to the proton connectivities shown in A starting from the alkene proton at Sh = 5.67. 

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