Halogenation, nuclear

In Sect. 2, it was indicated that changes AXap(X) and A/ / (Y) in halogen nuclear quadrupole coupling constants xap X) and Xafs(Y) of a dihalogen... 

Fig. 5. Sigma orbital populations of the halogen atoms in Group IV tetrahalides obtained from the halogen nuclear quadrupole coupling constants by use of Equation (1) with an = 2.0...

Table 3. Halogen nuclear qadrupole resonance frequencies of some transition metal complexes of the type A MX at room temperature...

Table 7. Halogen nuclear quadrupole resonance frequencies of some complex tons...

Since quadrupole interaction constitutes such an effective relaxation mechanism it is only rarely that other types of interactions have to be considered for relaxation of covalent chlorine, bromine or iodine. However, it can be estimated that in paramagnetic molecules the interactions between the halogen nuclear spin and the unpaired electron briefly mentioned in sub-Section 1.3.2 may give significant contributions to the relaxation rate. For VCl modulation of the electron-... 

With the exception of the nuclear amination of 4-methylthiazole by sodium amide (341, 346) the main reactions of nucleophiles with thiazole and its simple alkyl or aryl derivatives involve the abstraction of a ring or substituent proton by a strongly basic nucleophile followed by the addition of an electrophile to the intermediate. Nucleophilic substitution of halogens is discussed in Chapter V. 

The cyanothiazoles are not obtained by replacement of a nuclear halogen with cyanide. 

J. C. Million, C. W. Weber, and P. R. Kuehn, Gas Chromatography of Some Corrosive Halogen-Containing Gases, Report No. K-1639, Union Carbide Corp., Nuclear Division, New York, 1966. 

At temperatures near the critical temperature, many organic degradation reactions are rapid. Halogenated hydrocarbons loose the halogen in minutes at 375°C (38). At temperatures typical of nuclear steam generators (271°C (520°F)), the decomposition of amines to alcohols and acids is well known (39). The pressure limits for the treatment of boiler waters using organic polymers reflect the rate of decomposition. 

Nuclear halogen atoms also show many of the reactions typical of aryl halogens, (i) They can be replaced with hydrogen atoms by catalytic (Pd, Ni, etc.) or chemical reduction (HI or Zn/H2S04). For example, halogenopyrazoles with HI and red phosphorus at 150 °C... 

Polymers have been prepared from nuclear substituted di-(4-hydroxyphenyl)-alkanes, of which the halogenated materials have been of particular interest. The symmetrical tetrachlorobis-phenol A yields a polymer with a glass transition temperature of 180°C and melting range of 250-260°C but soluble in a variety of solvents. 

Nuclear halogenation of acetophenone depends on formation of the aluminum chloride complex. If less than one equivalent of aluminum chloride is used, side-chain halogenation occurs. 3-Bromoacetophenone has been prepared from 3-aminoaceto-phenone by the Sandmeyer reaction. The synthesis described here has been taken from work of the submitters, who have used it to prepare many 3-bromo- and 3-chloroacetophenones and benzaldehydes, as well as more highly halogenated ones (Notes 7 and 8). 

If chlorine and bromine are allowed to act upon an aromatic hydrocarbon like toluene, which has a side-chain, substitution may occur in the nucleus or the side-chain, according to the conditions. Generally speaking, in the cold and in presence of a halogen carrier, nuclear substitution occurs, Irut at a high temperatuie the halogen passes into the side-chain (see Piep. 

The nuclear spin of the stable isotopes of the halogens has been exploited in nmr spectroscopy. The use of in particular, with its 100% abundance, convenient spin of j and excellent sensitivity, has resulted in a vast and continually expanding literature since chemical shifts were first observed in 1950. The resonances for Cl and Cl were also first observed in 1950. Appropriate nuclear parameters are in Table 17.6. From this it is clear that the F resonance can be observed with high receptivity... 

Table 17.6 Nuclear magnetic resonance parameters for the halogen isotopes...

The molecular and bulk properties of the halogens, as distinct from their atomic and nuclear properties, were summarized in Table 17.4 and have to some extent already been briefly discussed. The high volatility and relatively low enthalpy of vaporization reflect the diatomic molecular structure of these elements. In the solid state the molecules align to give a layer lattice p2 has two modifications (a low-temperature, a-form and a higher-temperature, yS-form) neither of which resembles the orthorhombic layer lattice of the isostructural CI2, Br2 and I2. The layer lattice is illustrated below for I2 the I-I distance of 271.5 pm is appreciably longer than in gaseous I2 (266.6 pm) and the closest interatomic approach between the molecules is 350 pm within the layer and 427 pm between layers (cf the van der Waals radius of 215 pm). These values are... 

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. 

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