Signs of Coupling Constants

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... 

The ENDOR technique refers to electron-nuclear double resonance. This consists of the effect on a partially saturated ESR line of simultaneously irradiating the sample with a radiofrequency to induce nuclear resonance transitions of hyperfine coupled nuclei. It may enable one to obtain information about signs of coupling constants. ELDOR is the technique corresponding to electron-electron double resonance. Such techniques, coupled with TRIPLE resonance, have been utilized and well described in a discussion of pyridine and 4,4-bipyridyl radical anion ESR spectra measured in sodium/liquid ammonia (80JMR<41)17). 

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. 

The nuclear magnetic resonance (NMR) spectrum of chrom-3-ene (1) has been measured13 and the sign of coupling constants between protons on the hetero ring obtained from a study of double quantum transitions.13 An inter-ring coupling (J4 8) has been detected.14-16 In... 

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]. 

Analogously to isotope shift asterisk denotes heavier isotopomer. The absolute signs of coupling constants are usually unknown and absolute values are compared. 

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... 

TRIPLE was first demonstrated as a CW technique applied to organic radicals in fluid solution, for which the signs of coupling constants could provide important information on the nature of the SOMO. For bioinorganic systems, their inherently broad linewidths mean that maximum resolution is needed, thus the most meaningful form of TRIPLE is as a pulsed technique. A Davies pulse sequence is shown for TRIPLE in Figure 9. All aspects of the pulse sequence are the same as for a standard (i.e., single ENDOR)... 

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... 

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