Cationic species

By ab initio calculations at the MP4(SDTQ)/6-311G(2df,2pd)//MP2/6-31G(d,p) level of theory, the equilibrium geometries, dissociation energies, and vibrational frequencies for [BHe] were obtained. The calculations were performed for the electronic ground states and selected excited states. The theoretically predicted vibrational frequencies are for the X state ( n state in parentheses) v = 87 (450) cn the partial charges are q(He) = 0.01 (0.13), and the overlap populations p(He-B) = 0.003 (0.030) [14]. 

Properties of [BHe] and [BHe]2+ such as interatomic distances, bond strengths, and dissociation energies have been discussed in terms of donor-acceptor interactions between neutral helium as the donor and the cationic B or B + fragment as the acceptor. In addition, the mechanism of bonding was analyzed by utilizing the properties of the calculated electron density. [BHe] in its ground state is understood as a van der Waals complex stabilized by charge-induced dipole interactions [14]. 

The ground state of [BHe] does not dissociate into He + B + because the second ionization energy of boron (IE = 25.154 eV [24]) is slightly larger than the first ionization energy of helium [14]. 

The structures and stabilities of Ng and its possible first-row isoelectronic analogs (e.g., [BNe]+, [NNe] , or [FNe] ) were examined using ab initio molecular orbital theory. Equilibrium structures were obtained at a variety of theory levels including MP3/6-311 G(d) and ST4CCD/6- 

311 + G(2df), and dissociation energies were determined at the MP4/6-311 + G(3d2f) level. A potential energy curve for [BNe]+, obtained at the CASSCF/6-311 G(d) level, is given. Spectroscopic constants were also determined at this level [12]. 

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Lead ll) oxide, PbO, exists in two forms as orange-red litharge and yellow massicot. Made by oxidation of Pb followed by rapid cooling (to avoid formation of Pb304). Used in accumulators and also in ceramics, pigments and insecticides. A normal hydroxide is not known but hydrolysis of lead(II) oxyacid salts gives polymeric cationic species, e.g. [Pb OfOH) ] and plumbates are formed with excess base. 

Here again the simple formulation [Sb ] is used to represent all the cationic species present.) The hydrolysis is reversible and the precipitate dissolves in hydrochloric acid and the trichloride is reformed. This reaction is in sharp contrast to the reactions of phosphorus(III) chloride. 

Solutions of many antimony and bismuth salts hydrolyse when diluted the cationic species then present will usually form a precipitate with any anion present. Addition of the appropriate acid suppresses the hydrolysis, reverses the reaction and the precipitate dissolves. This reaction indicates the presence of a bismuth or an antimony salt. 

It is extensively used industrially as a catalyst, notably in the oxidation of sulphur dioxide to the trioxide in sulphuric acid manufacture. It is an essentially acidic oxide, dissolving in alkalis to give vanadates however, addition of acid converts the anionic vanadate species to cationic species, by processes which are very complex, but which overall amount to the following ... 

In concentrated sulphuric acid. The way in which the rate of nitration of some non-basic compounds depends upon acidity in the region above that of maximum rate ( 90% sulphuric acid) has been discussed ( 2.3.2). Cationic species behave similarly (table 2.4, fig. 2.1). 

The equation does not take into account such pertubation factors as steric effects, solvent effects, and ion-pair formation. These factors, however, may be neglected when experiments are carried out in the same solvent at the same temperature and concentration for an homogeneous set of substrates. So, for a given ambident nucleophile the rate ratio kj/kj will depend on A and B, which vary with (a) the attacked electrophilic center, (b) the solvent, and (c) the counterpart cationic species of the anion. The important point in this kind of study is to change only one parameter at a time. This simple rule has not always been followed, and little systematic work has been done in this field (12) stiH widely open after the discovery of the role played by single electron transfer mechanism in ambident reactivity (1689). 

Substitution at C-2 may either occur through a cationic species or by... 

In this compound, synthesized in the low temperature reaction between diborane and excess ammonia, the cationic boron is coordinatively saturated in a tetrahedral environment. More recendy, cations having boron in tricoordinate or dicoordinate environments have been observed. These cationic species, called borenium and borinum ions, respectively, have been reviewed (19,20). 

Relatively few data are available for protonated cationic species, but from what there are it appears that protonation has little effect on the position and intensity of the absorption. 

The reactivity of these compounds is somewhat similar to that of the azolonium ions, particularly when the cationic species is involved. However, although the typical reaction is with nucleophiles, the intermediate (20) can lose the iV-oxide group to give the simple a-substituted azole (21). Benzimidazole 3-oxides are readily converted into 2-chloroben-... 

Ring expansion of five- to six-membered rings such as oxazole —> pyridine derivatives via a Diels-Alder reaction is a well-established procedure. However, the conversion of a six-membered heterocycle into a five-membered ring system has not been exploited to any great extent, and those systems that have been studied usually involve a cationic species. 

Binary Electrolyte Mixtures When electrolytes are added to a solvent, they dissociate to a certain degree. It would appear that the solution contains at least three components solvent, anions, and cations, if the solution is to remain neutral in charge at each point (assuming the absence of any applied electric potential field), the anions and cations diffuse effectively as a single component, as for molecular diffusion. The diffusion or the anionic and cationic species in the solvent can thus be treated as a binary mixture. 

In all of these oxide phases it is possible that departures from the simple stoichiometric composition occur dirough variation of the charges of some of the cationic species. Furthermore, if a cation is raised to a higher oxidation state, by the addition of oxygen to tire lattice, a conesponding number of vacant cation sites must be formed to compensate tire structure. Thus in nickel oxide NiO, which at stoichiomen ic composition has only Ni + cations, oxidation leads to Ni + ion formation to counterbalance the addition of extra oxide ions. At the same time vacant sites must be added to the cation lattice to retain dre NaCl sUmcture. This balanced process can be described by a normal chemical equation thus... 

Ca +, Fe + etc. which are the cationic species in the slag phase). Fellner and Krohn (1969) have shown that the removal of phosphorus from iron-calcium silicate slags is accurately described by the Flood-Grjotheim equation widr... 

For information on ionization (pK) see Chapter 1, p. 7, and Chapter 4, p. 80. In order to avoid repetition, the literature (or predicted) pK values of anionic and/or cationic species are usually reported at least once, and in several cases is entered for the free acid or free base, e.g. Na2S04 will have a pK value for Na" " at the entry for NaOH and the pK values for S04 at the entries for H2SO4. When the pK values of the organic counter-ions are not given in this Chapter, as in case of sodium benzoate, the reader is referred to the value(s) in Chapter 4, e.g. of benzoic acid. ... 

Homoaromaticity is a term used to describe systems in which a stabilized cyclic conjugated system is formed by bypassing one saturated atom. The resulting stabilization would, in general, be expected to be reduced because of poorer overlap of the orbitals. The properties of several such cationic species, however, suggest that substantial stabilization does exist. The cyclooctatrienyl cation is an example ... 

The name lariat ethers has recently been suggested by Gokel and coworkers to describe a class of compounds which are based on crown ethers and have secondary donor groups bound to the macrormg by flexible arms (see Sect. 1.3 5). The name lariat is suggested by the analogy to the lassoes used to rope and tie animals since it is hoped that the more successful molecules of this class will be able to do likewise with sundry cationic species. 

The resulting radical and/or cationic species (11) and (12) can then lead to rearrangements which have found interesting applications in the steroid field. 

FIGURE 4.6 The ionic forms of the amino acids, shown without consideration of any ionizations on the side chain. The cationic form is the low pH form, and the titration of the cationic species with base yields the zwitterion and finally the anionic form. (Irving Geis)... 

Nitrogen forms more than 20 binaiy compounds with hydrogen of which ammonia (NH3, p. 420), hydrazine (N2H4, p. 427) and hydrogen azide (N3H, p. 432) are by far the most important. Hydroxylamine, NH2(OH), is closely related in structure and properties to both ammonia, NH2(H), and hydrazine, NH2(NH2) and it will be convenient to discuss this compound in the present section also (p. 431). Several protonated cationic species such as NH4+, N2H5+, etc, and deprotonated anionic species such as NH2 , N2H3 , etc. also exist but ammonium hydride, NH5, is unknown. Among... 

In all the cluster compounds discussed above there are sufficient electrons to form 2-centre 2-electron bonds between each pair of adjacent atoms. Such is not the case, however, for the cationic bismuth species now to be discussed and these must be considered as electron deficient . The unparalleled ability of Bi/BiCb to form numerous low oxidation-state compounds in the presence of suitable complex anions has already been mentioned (p. 564) and the cationic species shown in Table 13.12 have been unequivocally identified. 

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