Coupling constant transfer

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... 

DET calculations on the hyperfine coupling constants of ethyl imidazole as a model for histidine support experimental results that the preferred histidine radical is formed by OH addition at the C5 position [00JPC(A)9144]. The reaction mechanism of compound I formation in heme peroxidases has been investigated at the B3-LYP level [99JA10178]. The reaction starts with a proton transfer from the peroxide to the distal histidine and a subsequent proton back donation from the histidine to the second oxygen of the peroxide (Scheme 8). 

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. 

Two-dimensional spectroscopy has two broad classes of experiments (a) 2D /-resolved spectra (Mtlller et ai, 1975 Aue et ai, 1976), in which no coherence transfer or mixing process normally occurs, and chemical shift and coupling constant frequencies are spread along two different axes. 

A 90° Gaussian pulse is employed as an excitation pulse. In the case of a simple AX spin system, the delay t between the first, soft 90° excitation pulse and the final, hard 90° detection pulse is adjusted to correspond to the coupling constant JJ x (Fig- 7.2). If the excitation frequency corresponds to the chemical shift frequency of nucleus A, then the doublet of nucleus A will disappear and the total transfer of magnetization to nucleus X will produce an antiphase doublet (Fig. 7.3). The antiphase structure of the multiplets can be removed by employing a refocused ID COSY experiment (Hore, 1983). 

We assume that, on formation of B- XY, a fraction 5j (i = intermolecular) of an electronic charge is transferred from the electron donor atom of Z of the Lewis base B to the npz orbital of X and that similarly a fraction 5p (p = polarisation) of an electronic charge is transferred from npz of X to n pz of Y, where z is the XY internuclear axis and n and n are the valence-shell principal quantum numbers of X and Y. Within the approximations of the Townes-Dailey model [187], the nuclear quadrupole coupling constants at X and Y in the hypothetical equilibrium state of B- -XY can be shown [178] to be given by ... 

Fig. 20 Variation of the fraction <5 of an electronic charge transferred from B to XY on formation of B- XY with the ionisation energy 7b of B for the series XY = 02, BrO and IO. See text for the method of determination of Si from observed XY nuclear quadrupole coupling constants. The solid curves are the functions <5 = A exp(- al ) that best fit the points for each series B- XY. Data for B- -B are nearly coincident with those of B- BrO and have been excluded for the sake of clarity...

The values of 3/(NH,H) coupling constant observed for imine proton can be helpful in detection of the proton transfer processes and determination of mole fractions of tautomers in equilibrium. For NH-form, this value is close to 13 Hz, lower values usually indicate the presence of tautomeric equilibrium. It should be mentioned that the values below 2.4 Hz have not been reported. The chemical shift of C—OH (C-2 for imines, derivatives of aromatic ortho-hydroxyaldehydes or C-7 for gossypol derivatives) carbon to some extent can be informative, however, this value depends on type of substituents and should be interpreted with caution. 

The second drawback of the basic HMBC experiment is due to the actual range of /CH spin coupling constants, which vary from 1 to 50 Hz. Usually HMBC spectra are recorded with a compromise by setting the corresponding delay in the sequence to about 70 ms to allow an optimum transfer for correlation signals where the spin coupling constant is about 7 Hz. Under these conditions, several important connectivities may yield... 

A detailed study of the C02- species on MgO has been carried out by Lunsford and Jayne 26). Electrons trapped at surface defects during UV irradiation of the sample are transferred to the CO2 molecule upon adsorption. By using 13C02 the hyperfine structure was obtained. The coupling constants are axx - 184, am = 184, and a = 230 G. An analysis of the data, similar to that carried out in Section II.B.2 for N02, indicates that the unpaired electron has 18% 2s character and 47% 2p character on the carbon atom. An OCO bond angle of 125° may be compared with an angle of 128° for CO2- in sodium formate. 

We have not as yet however treated the charge-transfer data available for complexes of the 5 d series. For these latter species though the effective spin-orbit coupling constants are often of the order of 3 kK. or more, as compared with only about 1 kK. for Ad systems, and smaller values still for the 3d elements. Consequently, as for the d—d transitions it is often necessary explicitly to consider relativistic effects in the interpretation of charge-transfer spectra, and in particular to make allowance for the changes in spin-orbit contributions which may accompany a given di transition. In fact one of us has shown (18) that these changes are... 

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