Spin coupling constants halogens

The spin-spin interaction of the protons of the CHj group in 4-RO-benzenium ions and that of the protons occupying the 2- and positions (see the spectrum of the 4-methoxybenzenium ion in Fig. 3) is fairly large. For methylbenzenium ions it is markedly weaker and does not display itself in the spectra. The spin-spin coupling constants of protons located at sp -hybridized carbons of hydroxybenzenium ions and their O-derivatives are also somewhat higher than those for methylbenzenium ions their values are close to those recorded for the respective cyclohexadienones Summarized in Table 12 are the PMR data on a number of halogen-substituted hydroxy- and methoxybenzenium ions. 

Fig. 2.4. Proton relaxation of HBr and HCl through scalar interaction. Plot used to obtain halogen relaxation time and scalar spin coupling constant [cf. Eq. (1.26)]. Tjp is the relaxation time in the rotating frame and where is the...

Table 4.1. Values of the spin dipolar contribution to the reduced spin coupling constant (k( )) in halogen bonds, calculated using = 1 and AE =...

In determinations of scalar spin coupling constants to bromine and chlorine, the presence of two magnetic isotopes of these halogens should be taken into account. In an appreciable number of cases this fact has not been considered, however. Since the Larmor frequencies and the quadrupole moments within the two pairs of halogen isotopes are reasonably similar, the differences may be disregarded to a first approximation. 

The only compounds where direct NMR observations have been employed to deduce a halogen spin coupling constant are FCIO, CIF ,... 

A second possibility of obtaining scalar halogen spin coupling constants is through measurements of the relaxation rate of the I nucleus in the rotating frame, Tj p (53). Examples of the use of this technique are found in references 54 and 55. 

Spin-orbit coupling has been shown to be particularly important for the chemical shifts of neighboring atoms of heavy halogen substituents, e.g for the shifts in iodomethanes (one speaks of a heavy-atom effect ). Early relativistic extended-Huckel calculations have indicated that even the shift of the heavy atom itself may be influenced by spin-orbit coupling (a heavy-atom effect on the heavy atom ). Thus, depending on the bonding situation of the molecule and on the position of the constituent atoms within the periodic table, spin-orbit corrections to chemical shifts have to be considered (the same holds for spin-spin coupling constants, a point we will not consider further here). 

Quadrupole coupling constants for molecules are usually determined from the hyperfine structure of pure rotational spectra or from electric-beam and magnetic-beam resonance spectroscopies. Nuclear magnetic resonance, electron spin resonance and Mossbauer spectroscopies are also routes to the property. There is a large amount of experimental data for and halogen-substituted molecules. Less data is available for deuterium because the nuclear quadrupole is small. 

From measurements of this type Thomaz and Stevens found a linear relationship for a graph of log(kJrPC,2) vs. where n is the number of halogen atoms in the molecule, is the spin-orbit coupling constant, and Em is the polarographic half-wave reduction potential of the heavy-atom quencher (Figure 5.16). This correlation suggests that an exciplex is formed by partial... 

The 127I NQR spectra of an extensive series of iodonium compounds, [R2I]+BF, have been reported52,53. In addition to the fact that iodine is the only halogen that forms an extensive series of halonium compounds, the study of 127I has the advantage that, since its spin is equal to 5/2, there are two transitions and therefore both the coupling constant and the asymmetry parameter may be obtained. This is particularly important for a polycoordinated nucleus, since we may anticipate that the asymmetry parameter will be far from negligible. A selection of the NQR parameters for these compounds is shown in Table 9. Also studied were the iodonium halides, whose structures differ from those of the... 

Significant changes are also observed for the proton and halogen constants. Thus, the 11 isotropic coupling constant which is negative in the planar methyl radical (spin polarization, Aiso = -23 G), becomes positive when protons are substituted by fluorines133 (Table 15) and a monotonic increase in the 19F isotropic coupling constant is also caused by the... 

---