Inverse halogen dependence

For example, the observation of hormal or inverse halogen dependence (NHD vs IHD [78]) of NMR chemical shifts throughout the periodic table may be understood to a large extent from this rule. Halogen-substituted main group compounds experience particularly large SO effects, provided that the NMR atom is in its maximum formal oxidation state (e.g. B , Al , Ga , In, Tl ,... 

Whereas the Cl, Br, and I compounds of other atoms span at least 50% of the total shielding range, these halides of phos-phorusQlI) are separated by only 45 ppm, barely 6% of the total p range of 720 ppm. Even more frustrating is the fact that there is no halogen sequence that can be used to establish either normal or inverse halogen dependence. 

Paralleling the inverse chalcogen dependence, an inverse halogen dependence of Mo shielding has been noted in octahedral complexes of the composition [Mo02(L)X] [L = HB(3,5-Me2-pyrazolyl)3- X = Br , C, NCS, OMe" shielding increases in this order]. " ... 

The rate of step 4 involving elimination of halosilane also depends on the nature of the halogen atom but in the inverse order F < < Br. For Hal=Br, intermediates (32) are impossible to detect. By contrast, for Hal=F, elimination of Si F is the rate-determining step, which requires a special procedure, for example, refluxing of intermediate (32) (Hal=F) in acetonitrile for 1 h. (Preliminary data show that the microwave technology makes it possible to sharply decrease the time required for completion of step 4 (90)). A decrease in the elimination rate in the series F < < Br is evidence that this reaction is not... 

It may be concluded that the Walden inversion is limited to the reactions between nitrosyl bromide and the amino group and between silver oxide and halogen fatty acid, and is dependent upon the presence of the carboxyl group. 

A systematic study of the alkylation of benzene with optically active 1,2-, 1,3-, 1,4-, and 1,5-dihaloalkanes showed that the stereochemical outcome depends on the halogens and the chain length.142 Retention of configuration in alkylation with (S)-1,2-dichloropropane was explained by invoking double inversion and the involvement of a bridged cation such as 27. (S)-l-Bromo-2-chloropropane, in contrast,... 

This mechanism is rather probable 02 is an electron donor, and organyl halogenides can, in principle, accept one electron. With the one-electron mechanism, the inversion of configuration is determined by 02 attack on the side, which is opposite to the halogen atom in RHal (Morkovnik Okhlobystin 1979). The direction of this reaction depends on the nature of the solvent. In pyridine, benzene, and DMF, the main product is alkyl peroxide. In DMSO, an alkyl carbinol is the main product (Sawyer Gibian 1979). Obviously, the aforementioned intermediary product ROO- reacts faster with the solvent Me2SO than with the substrate RHal, Scheme 1-78 ... 

Nucleophilic substitution may occur by two mechanisms. The SN2 mechanism is a one-step process. Its rate depends on the concentrations of substrate and nucleophile. If the halogen-bearing carbon is stereogenic, substitution occurs with inversion of configuration. The reaction is fastest for primary halides and slowest for tertiary halides. 

The anisotropic coupling of the halogen nuclei arises essentially from the p-character of the wave-function. The axial symmetry of p-electrons then implies that the maximum value of the anisotropic coupling is observed when the magnetic field is parallel to the direction of the p-orbital. It depends on the average value of the inverse cube of the radial wave-function and can be shown to have the value ... 

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