Coupling constant relative signs

For more complex spin systems, a computer program PHIP+ has been developed [13, 45] which allows the expected PHIP spectra to be calculated from the chemical shifts and coupling constants of the products. Depending upon which proton pair in the product molecule stems from p-H2, different - but characteristic - polarization patterns result [14]. The patterns also depend on the sign of the coupling constants. Simple sign rules governing the relative sequence of the emission and absorption lines in the PHIP spectra (i.e., their phase ) can be formulated in similar manner to the Kaptein Rules of chemically induced dynamic nuclear polarization (CIDNP) [15]. 

Determination of relative signs of isotropic hyperfine coupling constants J. Chem. Rhys. 63 3515-22... 

The radical cation of 1 (T ) is produced by a photo-induced electron transfer reaction with an excited electron acceptor, chloranil. The major product observed in the CIDNP spectrum is the regenerated electron donor, 1. The parameters for Kaptein s net effect rule in this case are that the RP is from a triplet precursor (p. is +), the recombination product is that which is under consideration (e is +) and Ag is negative. This leaves the sign of the hyperfine coupling constant as the only unknown in the expression for the polarization phase. Roth et aJ [10] used the phase and intensity of each signal to detemiine the relative signs and magnitudes of the... 

From a study (29) of the relative signs of various H, H coupling constants of carbohydrate derivatives it is evident that the couplings... 

The polyerystalline spectrum of N02 on MgO is somewhat complex, but it yields an unambiguous g and hyperfine tensor which can be checked by comparison with data for NO2 in single crystals. For N02 on MgO, principal values of the hyperfine tensor are m = 53.0, 21 = 49.0, and a31 = 67.0 G (29). It should be noted here that neither the signs of the coupling constants nor their directions relative to the molecular coordinates... 

In this section analytical expressions for ENDOR transition frequencies and intensities will be given, which allow an adequate description of ENDOR spectra of transition metal complexes. The formalism is based on operator transforms of the spin Hamiltonian under the most general symmetry conditions. The transparent first and second order formulae are expressed as compact quadratic and bilinear forms of simple equations. Second order contributions, and in particular cross-terms between hf interactions of different nuclei, will be discussed for spin systems possessing different symmetries. Finally, methods to determine relative and absolute signs of hf and quadrupole coupling constants will be summarized. 

To compare measured magnetic coupling constants with values obtained from theoretical calculations, the signs of the magnetic parameters should be known. In the following, different approaches for the determination of absolute and relative signs of the principal values of hf and quadrupole tensors will be discussed. 

Proton ENDOR and DOUBLE ENDOR data on VO(acac)2 in frozen solutions have been reported by van Willigen2895. Since the CH-protons exchange with deuterons in CD3OD they may easily be discriminated from the methyl protons. From the single crystal-like ENDOR spectrum for B0 along g and the two-dimensional powder spectrum at gi, the hf coupling constants Ap 5 = 0.7 MHz and AfHj = 0.7 MHz, A 3 = 1.5 MHz, respectively, have been found. The relative signs of the principal... 

Crepaux, D., Lehn, J. M. Nuclear Spin-Spin Interactions Part IX. Relative Signs of the Geminal Nitrogen-Hydrogen Coupling Constants for Doubly Bonded Nitrogen. Mol. 

From the previous discussion, we can conclude that when the observed value of /(H-Si) is relatively large, say, in the range 70-160 Hz, it is safe to think that the Si H bond is involved in nonclassical bonding with metal. In contrast, any conclusion on the presence or absence of a Si-H interaction on the basis of a small value of silicon-proton coupling constant in the absence of an independent evidence can be erroneous. In this case, the measurement or calculation of the sign of /(Si-H) can help to identify a direct Si H interaction. 

More recently, single and double resonance techniques on both " N and N compounds are being employed for accurate shift and coupling constant measurements including relative signs. Use of enrichment with N has grown because of its favorable spectral characteristics in proton magnetic resonance studies. 

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. 

ABX, and AMX. The AMX case is three nonequivalent nuclei with all spin-spin couplings much less than the corresponding chemical-shift differences. The appearance of a three-spin spectrum depends, in general, on the relative (but not the absolute) signs of the coupling constants. 

In the absence of X-ray crystallographic data, 31P and 195Pt NMR spectroscopy is a useful method to investigate the nature of the products in the solution. In addition to earlier references to these topics, the Pople-Santry theory has been used to calculate the signs and relative magnitudes of the coupling constants for the type of phosphorus-bridged complexes covered in this section.1502... 

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