Coupling constant signs

Active Figure 13.20 A tree diagram for the C2 proton of frans-cinnamaldehyde shows how it is coupled to the C1 and C3 protons with different coupling constants. Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

See note 18(c), (d). The algebraic sign is predicted theoretically to be opposite for a and /3 couplings. Few coupling constant signs have been determined experimentally. 

The Selective Population Transfer (SPT) experiment is usually used in spin system analysis with a FT spectrometer. Normally the experimental SPT spectra are compared with calculated SPT spectra simulated using different combinations of coupling constant signs. In common with many textbooks the AMX spin system 2,3-dibromopropionic acid will be used to introduce the concepts behind the SPT experiment. The IH spin system parameters for 2,3-dibromopropionic acid are shown below. The only difference between Spin System A and Spin System B is the sign of the coupling constant J(H(2), H(3)), the results of SPT experiments will be used to distinguish between the two possible spin systems. 

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... 

The CIDNP spectrum is shown in figure B 1.16.1 from the introduction, top trace, while a dark spectrum is shown for comparison in figure B 1.16.1 bottom trace. Because the sign and magnitude of the hyperfine coupling constant can be a measure of the spin density on a carbon, Roth et aJ [10] were able to use the... 

Follow Enting s [ent77] observation that all of the coupling constants but one -h, say - can be eliminated by multiplying together the four equations in which h appears with a 4- sign. As the reader can easily show, we get the result that... 

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

The remainder of equation (38) describes the multiplet effect, and it can be seen that whether an individual line in the multiplet corresponds to emission or absor ption depends on the signs of the hyperfine coupling constants but is independent of Hq. The nature of the hyperfine field is such that the integral over the whole multiplet is zero if Ag = 0. 

However, the nuclear-nuclear coupling constant also has an influence, since changing its sign changes the assignment of the lines in the spectrum. 

Radical IV can be considered as a unique phosphorus radical species. Reduction of the parent macrocycle with sodium naphtalenide in THF at room temperature gave a purple solution. The FPR spectrum displayed a signal in a 1 2 1 pattern, with flp(2P)=0.38 mT. DFT calculations on radical IV models indicated a P-P distance of 2.763 A (P - P is3.256 A in the crystal structure of the parent compound and the average value of a single P-P bond is 2.2 A). According to the authors, the small coupling constant arises from the facts that the principal values of the hyperfine tensor are of opposite sign and that the a P P one electron bond results from overlap of two 3p orbitals [88]. 

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. 

This relation is only valid for a crystal with isotropic /-factor. The effect of crystal anisotropy will be treated in Sect. 4.6.2. The function h(6) describes the probability of finding an angle 6 between the direction of the z-axis and the y-ray propagation. In a powder sample, there is a random distribution of the principal axes system of the EFG, and with h 6) = 1, we expect the intensity ratio to be I2J li = I, that is, an asymmetric Mossbauer spectrum. In this case, it is not possible to determine the sign of the quadmpole coupling constant eQV. For a single crystal, where h ) = — 6o) 5 delta-function), the intensity ratio takes the form... 

W(VI) compounds with 5cP electronic configuration, the EFG is reasonably weU described by the lattice contribution only. The regular octahedral WOs structure yields zero EFG in agreement with experiment, while the deformed tetrahedral W04 strucmre results in significant quadrupole coupling constants of either sign, e Q > 0 and also <0, depending on the distortion of the O4 tetrahedron. 

Intramethyl couplings in ellipses. Upper and lower set of sign combinations fit spectrum equally well. (B) NMR spectra of AcAlaNHMe in 42%/wt CsPFO in D20. Frequency scale with 0 Hz = 0 ppm. (a) Isotropic spectrum at 47°C. (b) Oriented LX-NMR spectrum at 25°C. (c) Simulated spectrum using best-fit coupling constants shown in (A). From Poon et al., (2000)./. Am. Chem. Soc. 122, 5642-5643, 2000, Reprinted with permission from the American Chemical Society. 

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