Relative Signs of Coupling Constants

In its basic (magnitude mode) version the length of PO is adjusted either to maximize the sensitivity (PO = 90° ) or to yield structured cross peaks (PO = 45° ). In the latter case information regarding the relative signs of coupling constants may be deduced. 

For geminal nuclei A and X, the Pauli principle favors a parallel precession (Fig. 1.11 (b)). A stabilization-destabilization pattern opposite to the directly bonded and vicinal nuclei arises (Fig. 1.11(a)), and the coupling constant is defined to be negative (Ax < 0). There are experimental techniques to determine the relative sign of coupling constants [5]. 

FIGURE 9. 29Si SPT NMR spectmm of [(CFF HSfFO determination of relative signs of coupling constants (acquisition time 4 s). Reprinted with permission from Reference 80. Copyright 1975 American Chemical Society... 

There has been some discussion (127,129,132) of the relative merits of such techniques as pseudo-INDOR, SPT, SPI, spectral analysis, and spin tickling for the determination of the relative signs of coupling constants. It appears to the present writers that much depends upon the particular circumstances, namely, the nature of the spin system (including the relative values of relaxation times), the available instrumentation, and the abilities of the spectroscopist. Certainly, when Tx > "/ A(e g.for C- H ) the effects of simultaneous spin population transfer can make the interpretation of spin tickling effects very tricky. 

When double resonance experiments are used to determine relative signs of coupling constants a set of unequivocally correlated couplings can be obtained. In the case of Me(,Pb2 a suggestion, (545) based on... 

In these types of experiment the cross and diagonal peak structure are tailored to correlate the relative signs of coupling constants and to suppress passive coupling or unwanted coherences. In addition from the appearance of the cross peaks it is possible to determine the coupling constants. 

Analysis of COSY spectra started with the paper by Bax and Freeman who discussed the potential of the Teener s experiment . A modification, later called COSY-45, is introduced for the observation of the relative sign of coupling constants in triangularly coupled spin systems. This kind of multiplet structure prompted the development of specific sequences in order to extract a certain kind of coupling constant information and coupling network. Another modification involves the use of selective pulses giving rise to spectra that are simplified in... 

Figure 2.31. AB portion of an ABX system, (a) Relative signs of coupling constants, Jax 2ind Jgx are the same, (b) Relative signs of coupling constants, J x and J x are different, (c) Relative signs of J x and Jbx triay be same or different.

Details of the interpretation of double resonance spectra in the determination of relative signs of coupling constants are found in several references.With polarization transfer techniques described in Chapter 2, it is possible to determine relative signs of coupling constants involving less sensitive nuclei such as N. ... 

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