Coupling signs

Inversion of the coupling signs has been found in the anionic sp>ecies [(Me3Sn)3Sn] with J( Sn," Sn)=-5200 Hz [98], the negative sign of... 

S. Zhao, G. Bondo, J. Zajicek, and A. S. Serianni, Two-bond 13C-13C spin-coupling constants in carbohydrates New measurements of coupling signs, Carbohydr. Res., 309 (1998) 145-152. 

A. S. Serianni, P. B. Bondo, and J. Zajicek, Verification of the projection resultant method for two-bond 13C-13C coupling sign determinations in carbohydrates,./. Magn. Reson., B, 112 (1996) 69-74. 

Except for thephosphane adducts of tin(IV) halides, the experimental evidence points towards a positive sign of i/( Sn, P) ( K(" Sn, P) < 0 ) in all tin-phosphorus(III) compounds, a consequence of the dominant influence of the lone pair of electrons at the phosphorus atom. Even for transition metal complexes of stannylphosphanes, or in borane complexes of stannylphosphanes, the coupling sign remains positive. 

Fig. 3. Basic pulse sequences for 2D- X,"T H correlations. Tbe same notation as in Hg. 1 is used. Minimum phase cycles for selection of correlation signals are given, more elaborate schemes for quadrature detection in FI and phase-sensitive spectra may be applied following standard rules. (a) HETCOR (without 180° pulses)/INEPT (with 180° pulses), the refocusing delays A are optional in both experiments setting the mixing pulses 8 to 45°/135° instead of 90° allows to determine coupling signs in ABX-type spectra. (b), HSQC. (c), HMQC the refocusing delay A2 is optional.

Due to the high receptivity of F, E nf correlation spectroscopy became available very early in the history of NMR spectroscopy. Using the INDOR technique on a cw-spectrometer, McFarlane and coworkers gave the first report whatsoever on the use of "X, "Y correlated spectroscopy for indirect observation via of the resonances in complexes of the type WF6 0R and WOp4L. Beside the first chemical shift determination for a low-7 transition metal, which was at that time not feasible by any other technique, further the indirect determination of y( W, P), and the assignment of relative coupling signs, was reported. 

The most convenient techniques for the determination of coupling signs involving ll9Sn have been reviewed,109 and the analogous techniques can be applied in the case of 207Pb. Numerous coupling constants are already listed in previous reviews.3-5 Therefore, this section will deal mainly with some results that have been published in the last decade. 

Fig. 14. Contour plot of the 125.8 MHz 2D 13C/1H HETCOR experiment for Pb(OC-Me)4 in C6D6 (polarization transfer was based on 2 7(I3C, H) = 7 Hz which gives approximately the same signal-to-noise ratio for both alkynyl l3C nuclei).131 The tilt of the pairs of cross peaks for the 207Pb satellites compares the coupling signs of J(207Pb,l3C=) (>0) with 4./(207Pb, H) (<0), and of 2y(207Pb,= 3C) (>0) with VpTbJH) (<0).

All known coupling constants /( Sn, Si) appear to possess a positive sign. Some typical examples are given in Scheme 19. However, only a limited data set is available and therefore, it is well possible that changes in the coupling signs will be found. In principle, data /( Sn, Si) can be readily determined either from Si or from Sn NMR spectra by observation of the respective satellite signals. ... 

A new homonuclear coupling sign edited (HCSE) experiment has been designed by Blechta et alfl It can detect signed homonuclear couplings between low abundant nuclei such as C, N and Si in linear spin... 

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