Coupling constants formates

Another parameter that may be very relevant to the structural characterization of polyhydrido complexes in both solution and solid is the deuterium quadrupole coupling constant Formation of a chemical bond creates a strong... 

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). 

Two isomeric structures can be obtained for these products ( -113 and/or Z-113). The stereochemistry was conveniently elucidated on the basis of NMR data which showed coupling constant values 7(H,CO) consistent with the -isomers only. The formation of 113 was explained to occur via the enolate salts (86JHC199). Catalytic... 

The formation of alkyl shifted products H and 14 can be explained in terms of the formation of endo-intermediate 21 formed by endo attack of bromine to 2 (Scheme 4). The determined endo-configuration of the bromine atom at the bridge carbon is also in agreement with endo-attack. Endo-Intermediate 21 is probably also responsible for the formation of cyclopropane products 12 and 15. The existence of cyclopropane ring in 12 and 15 has been determined by and 13c NMR chemical shifts and especially by analysis of cyclopropane J cH coupling constants (168 and 181 Hz). On the basis of the symmetry in the molecule 12 we have distinguished easily between isomers 12 and 15. Aryl and alkyl shift products IQ, H, and 14 contain benzylic and allylic bromine atoms which can be hydrolized easily on column material. 

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...

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. 

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