Determining Signs of Coupling Constants

McFarlane has come to the conclusion that all AT(X—H) are positive. However, relativistic calculations indicate that coupling of H to heavy (fifth or sixth row) atoms with lone pairs will be negative. 

The use of single resonance experiments for sign determination depends on the presence of second-order features which in favorable cases incidentally lead to relative sign information. This has been widely used in magnetically inequivalent systems such as AA XX. While the early relative sign data were obtained in this fashion, multiple resonance techniques are the preferred direct method. 

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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Population transfer experiments may be selective or nonselective. Selective population transfer experiments have found only limited use for signal multiplicity assignments (SSrensen et al, 1974) or for determining signs of coupling constants (Chalmers et al., 1974 Pachler and Wessels, 1973), since this is better done by employing distortionless enhancement by polarization transfer (DEPT) or Correlated Spectroscopy (COSY) experiments. However, nonselective population transfer experiments, such as INEPT or DEPT (presented later) have found wide application. 

Me3Pb)3N 1H, 15N and 207Pb NMR. The value and sign of coupling constants were determined ( Pb N) the largest value for this constant published so far, VC H). 153... 

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... 

For the determination of the sign of coupling constants involving tin, 2D correlation spectra in the form of iH- Sn HETC0R2 -29,136 HMQC O- or HMBC43.265 gj-e employed. Due to the good sensitivity, Sn-Sn COSY experiments can be conducted in a... 

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. 

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