Studies of the Perchlorate Ion

Studies of the perchlorate ion have already been briefly mentioned in previous chapters where the natural division between covalent and ionic environments have been made. The perchlorate ion (as also the perbromate and periodate ions) does, however, not fit well into this scheme for the present treatment The chlorine atom is participating in covalent bonds but the field of applications is more closely related to that of halide ions. Since the Cl NMR properties of the perchlorate ion are in some important respects rather different from those of both covalent organic compounds and halide ions, it was found most convenient to discuss these studies separately. 

Perchlorates of several metals are known and they are as a rule readily soluble in water. The tetrahedral perchlorate ion is known to have a particularly small tendency to form complexes with metal ions and perchlorates are, therefore, widely used in connection with studies of complex formation when an inert anion is required. Per-bromates and periodates are also known but BrO has, because of its low stability, not been much studied. Periodate solutions may in addition to the tetrahedral lO ion contain several hydrated species [489]. 

Although a successful chemical application of Cl NMR of the perchlorate ion was presented more than 20 years ago [280], subsequent studies have been very few. This is somewhat surprising in view of the chemical interest in the interaction of the CIO ion with other species. Furthermore, the relaxation time is generally within a range easily accessible for measurements. 

That the shielding of the perchlorate ion is very much different from 

Several authors [210 257 280 490 491] have considered the possibility of using the Cl NMR shielding to measure the degree of dissociation of perchloric acid in aqueous solutions. The underlying principles were discussed first by Masuda and Kanda [280] and more recently by Akitt et at. [491]. For an equilibrium 

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For convenience the material has been divided up in different ways. On the one hand we have discussed separately the different para-meters that characterize the halogen NMR spectra and on the other we have treated separately halide ions and covalent halogen compounds, Such a sub-division, however, has its limitations - the need for a separate section for studies of the perchlorate ion exemplifies this. 

Introductory Aspects. Relaxation in Molecules or Ions with Covalently Bonded Halogens. Shielding Effects in Covalent Halogen Compounds. Scalar Spin Couplings. Relaxation of Chloride, Bromide and Iodide Ions. Shielding of Halide Ions. Quadru-pole Splittings in Liquid Crystals. Halide Ions in Biological Systems. Studies of the Perchlorate Ion. 

Hisatsune and Linnehan [299] have used infrared measurements to study the decomposition of C104 in a KC1 matrix. Despite the differences in the environment of the perchlorate ion, the kinetics of reaction were similar to those reported by Cordes and Smith [845] for pure KC104. The reaction was second order and E was 185 kJ mole-1. Comparable behaviour was observed for CIOJ in KC1, except that E was lower ( 125 kJ mole-1) and when both ions (CIOJ and C104) were present the reaction was approximately first order. 

X-Ray diffraction studies of aqueous CdS04 solutions have clearly demonstrated the formation of O-sulfato complexes.668-670 These are of the general form [Cd(H20)6- (0S03) ](2-2")+, and appear to be the major species present in concentrated solutions. These results are fully in accord with tensimetric studies of the cadmium and magnesium sulfate aqueous systems, which indicate an increased interionic attraction at elevated temperatures.671 In contrast, X-ray diffraction studies on cadmium perchlorate solutions show no evidence for the coordination of the perchlorate ion to the metal, the only cationic species present being [Cd(H20)6]2+(Cd—O, 2.292 A, cf. Cd—O, 2.292 A in solid [Cd(H20)6][C104]2),670... 

Cl relaxation of the perchlorate ion in the presence of various proteins is presently being studied in our laboratory. Of the results obtained so far may be mentioned the demonstration of an effect of CIO on the molecular mobility at the active site anion binding region of fructose-1,6-diphosphate aldolase [1Z6]. Thus addition of fructose-1,6-diphosphate is accompanied by a decreased Cl relaxation rate -6f both Cl and CIO and comparisons of T and... 

A Cl line width study of the association of the perchlorate ion with Mg ions (and to some extent Li ) in mixed solvents has been presented by Berman et al. [528]. The CIO ion in contact ion pairs... 

The Brazilian workers have also studied complexes formed with the diamides, diacetamide (Airoldi and Gushikem, 1972), di-n-butyramide (Gushikem et al., 1973), N,N-dimethylacetoacetamide (Perrier and Vicentini, 1974), N,N,N, N -tetramethylmalonamide (Vicentini, 1972 Vicentini et al., 1974a), and N,N,N, N -tetramethyladipamide (Vicentini and Isuyama, 1975). The perchlorates of each of these have the formula R(diamide)4(C 104)3 with the exception of N,N,N, N -tetramethyladipamide for which the complexes of samarium to lutetium have only three molecules of the diamide. In all these complexes the Td symmetry of the perchlorate ion is preserved which implies that it is not coordinated. The complexes with other anions show a reduction in the number of coordinated organic ligands. 

The l,2,3-tri-/-butylcyclopropenium cation is so stable that the perchlorate salt can be recrystallized from water. An X-ray study of triphenylcyclopropenium perchlorate has verified the existence of the carbocation as a discrete species. Quantitative estimation of the stability of the unsubstituted ion can be made in terms of its pXn+ value of —7.4, which is intermediate between those of such highly stabilized ions as triphenylmethyl... 

It is known that tropylium may be prepared from tropylidene via hydride abstraction by PhgC or MegC carbonium ions therefore, it is very likely that here too the dehydrogenation is a hydride transfer from the 1,5-dione to an acceptor. A similar dehydrogenation of chromanones to chromones, with triphenylmethyl perchlorate was reported. A study of the electrooxidation of 1,5-diones on a rotating platinum electrode showed that 1,5-diaryl-substituted diones afford pyrylium salts in these conditions and that the half-wave potentials correlate with yields in chemical dehydrogenations. 

Kinetics studies of acid-catalysed chlorination by hypochlorous acid in aqueous acetic acid have been carried out, and the mechanism of the reactions depends upon the strength of the acetic acid an<( the reactivity of the aromatic. Different groups of workers have also obtained different kinetic results. Stanley and Shorter207 studied the chlorination of anisic acid by hypochlorous acid in 70 % aqueous acetic acid at 20 °C, and found the reaction rate to be apparently independent of the hydrogen ion concentration because added perchloric acid and sodium perchlorate of similar molar concentration (below 0.05 M, however) both produced similar and small rate increases. The kinetics were complicated, initial rates being proportional to aromatic concentration up to 0.01 M, but less so thereafter, and described by... 

In 1961, Roig and Dodson carried out a further study of the exchange in perchlorate media under identical conditions (25 °C, fi = 3.0 M) to those in the Tl(III) hydrolysis studyThe isotope was used, with a separation procedure based on extracting TI(III) from reaction mixtures with either methyl isobutyl ketone or diethyl ether. The exchange was examined in the absence of light, and a correction procedure to eliminate the catalytic effects of traces of chloride ions was used since Tl(III) concentrations of 10 M were necessary at the very low acidities employed. Using the known values of the first and second hydrolysis constants of Tl(III) (K2 and K3)... 

Four recent kinetic studies have been made of the exchange in the presence of the sulphate ion in perchlorate media, some measure of agreement being observed between the results of the various workers. 

The trigonal-bipyramidal pentaaquo zinc species has been structurally characterized332 IR spectroscopy has been utilized in studies of the hydration of zinc in aqueous solution and in the hydrated perchlorate salt aqua ions Zn(H20)62. 9... 

Studies of the kinetics of the polymerisation of 1,3 dioxolan and of the depolymerisation of its polymer by perchloric acid in methylene dichloride have been accompanied by measurements of the electrical conductivity of the reaction mixtures [23]. These have shown that ions are present during the reactions and the evidence strongly suggests that they are essentially concerned in these. This is one of the very few direct demonstrations of the participation of ions. 

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