Gaseous complexes

Since the first theoretical ab-initio prediction of the existence of a stable gaseous complex between HC1 and NH3, NH3 + HC1 NH4CI [56], the interest has been focused on the nature of the stabilization. A number of theoretical and experimental studies have been carried out in order to establish the geometry of the equilibrium structure (or structures) of the complex and hence to define the character of the interaction as either ion-pair-like or H-bonded-like [56-65],... 

During the past decade, the study of photoinitiated reactive and inelastic processes within weakly bound gaseous complexes has evolved into an active area of research in the field of chemical physics. Such specialized microscopic environments offer a number of unique opportunities which enable scientists to examine regiospecific interactions at a level of detail and precision that invites rigorous comparisons between experiment and theory. Specifically, many issues that lie at the heart of physical chemistry, such as reaction probabilities, chemical branching ratios, rates and dynamics of elementary chemical processes, curve crossings, caging, recombination, vibrational redistribution and predissociation, etc., can be studied at the state-to-state level and in real time. 

Figure 3.9 shows a schemalic representation of the chemical filing process. The first step of the process involves the chlorination of the surface of the pyrochlore-based ceria-zirconia sample. The extent of the chlorination can be controlled by the concentration of the chlorine gas and/or chlorination time and the cerium and zirconium chlorides partially formed on the surface are vaporized and transported by the formation of gaseous complexes with aluminum chloride. This chemical filing process is carried out at 1273 K to stabilize the surface modification effects at high temperatures. A similar effect can also be achieved by chlorination with ammonium chloride followed by dominant vaporization of formed zirconium chloride. ... 

In the first step the metal ion is introduced into the zeolite pores by ion exchange or adsorption of a labile metal complex. In the second step the intermediate material is reacted with gaseous complexing ligands, such as 1,2-dicyanobenzene, to form a complex inside the pores that is too large to difiuse out Alternatively metal complexes can be directly encapsulated inside the zeolite cavities during hydrothermal synthesis, as has been shown for FePc, CoPc, NiPc and CuPc in zeolite X [224],... 

The basic idea behind the photochemical studies described in this review can be extended to cover a broad range of issues within the unique microcosms of small gaseous complexes. Although hybrid, being neither gas, condensed-phase nor interface, they offer the sanctuary of vacuum to species emerging from the maelstrom invited by photoexcitation. State-specific and/or time-resolved interrogation is natural in such environments and will continue to play a leading role in future experimental work. 

At 600 K, Al2Cl6(g) reacts with CrCl2 to give a gaseous complex CrAl2Cl8.492 The electronic spectrum of this leads to the suggestion that the Cr has distorted octahedral co-ordination (by Cl s). 

The stoichiometry of the gaseous complex can vary for any particular element, depending on the partial pressure of AI2CI6 and on the temperature. The complexes have been studied quite exhaustibly, though the only application in radiochemistry has yet been the spectacular IC interseparation of lanthanoids [118] and actinoids [119] see also Sect. 1.5.2. 

Particularly interesting are the intermetallie species such as Cs2lnSb3 or CsSnBiSb2 observed in the ternary systems Cs-Sb with In or Sn and the aforementioned quarternary system (ef. Table 3). They are first examples of gaseous complex Zintl compounds with alkali atoms as donor partners [193]. 

In the field of metals or alloys gaseous complex Zintl compounds were identified for the first time. The combination of the results of all electron ab initio computations and Knudsen effusion mass spectrometry have made the determination of improved values for bond energies possible. Moreover, bond energies for transition metal trimers were obtained for the first time. 

Spectrophotometric measurements showed the presence of the gaseous complexes V"Al3Cln, V "Al2Cl9, and V" A1C16 in mixtures of VCI2 or VCI3 with AICI3."" ... 

Fig. 4. The fast-mixing nozzle used to observe the rotational spectrum of the gaseous complex H3N—F2 [reproduced with permission from (33), p. 112].

As a pyrometallurgical approach, Ozaki et al. (1999) studied the appUcatimi of the chemical vapor transport method described in Section 2.3.1. Used polishing powder was chlorinated by chlorine gas at 1000 °C, and transported along the temperature gradient via gaseous complex with aluminum chloride. The rare-earth chlorides were mainly crmdensed over the temperature range of 457-947 C. The purity of the rare-earth chlorides in this temperature range was about 95%. 

Problem 16.5 Consider the structures of the two complexes in Fig. 16.2. Are the NBF valence angles in the gaseous complexes in agreement with the VSEPR model How would you explain the changes of these angles when the complex is studied in the crystalline phase ... 

Formation of solid ammonium chloride from the gaseous complexes may be divided into three steps. In the first, endothermic, step the complexes are dissociated to form neutral NH3 and HCl molecules in the second, undoubtedly also endothermic, protons are transferred from HCl to NH3 molecules to form the separated NH and Cl ions in the third, exothermic step the ions are combined to form a crystal which is stabilized by sttong Coulomb attfactions between cations and anions. The system NHz HCl, then, provides another demonstration of how intermolecular interactions is the solid phase may lead to more polar or ionic suiicture than in the gas phase. 

AHj was obtained calorimetrically from measurements of the heats of solution in water of the species implied by the reaction in the solid state. 4D [M-X] represents the enthalpy of dissociation of the gaseous complex ion. The lattice enthalpies AHi of the simple chlorides were calculated using the Born-Haber cycle. 

Zvarova and Zvara [73, 74] have demonstrated that at moderate temperatures (under 250 °C) lanthanide chlorides, actinide chlorides, and other chlorides can be separated by gas chromatography if use is made of an inert gas and aluminum chloride vapor as components of the carrier gas. The method relies on aluminum chloride vapor forming gaseous complexes with rare-earth chlorides, which are then transported by the carrier gas. The excess of aluminum chloride inhibits dissociation of the... 

Figure 12.3 Simplified model for the various processes governing the Cu and Zn isotopic composition collected in the Talbot area of Canada [33], The ore zone is redox-active, with interactions between microbes and fluids on the sulfide ores mobilizing Cu and Zn as aqueous and gaseous complexes into the overlying sandstone and carbonate, which are about 100 m thick in total. Migration of these components occurs through micro- and macrofractures and then they are trapped in the near-surface environment on clay minerals in the till or incorporated into the biosphere, including trees, moss, and organic matter in the soils.

It is likely that the decomposition of these compounds occurred in the open tubular column injection accessory rather than in the column. Contact of the gaseous complexes with heated stainless steel is prolonged in the injection block. Decomposition on the column is unlikely because the retention times were reproducible under a given set of conditions, and because the column was internally coated with powdered quartz and stationary phase. 

Theoretical simulations of IR spectra of different hydrogen-bonded crystals and gaseous complexes, presented in Sections III.A-III.F, based on the quantum-mechanical model presented in Section II, show that this model quantitatively explains details of the fine structure of the IR spectra and reproduces well deu-teration and temperature effects. The model takes into account the following mechanisms ... 

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