Ion structure and energetics

IV. ORGANOZINC ION STRUCTURE AND ENERGETICS A. Energetics and Photodissociation Studies... 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

To better understand the catalytic mechanism of DHFR and to use this information for the design of potent DHFR-specific inhibitors, we evaluated the proton and hydride transfers using an integrated ab initio Quantum Mechanics/Molecular Mechanics (QM/MM) approach in combination with FEP technology. The combinatorial application of these methods enabled us to propose a precise path along which the proton and hydride ion are transferred and to address the key structural and energetic changes associated with catalysis. 

One may consider a series of physical states ranging from the crystalline, where molecular aggregation and orientation are large, to the dilute gaseous state, where there are no significant orientational limits. States of intermediate order are represented by micelles, liquid crystals, monolayers, ion pairs, and dipole-dipole complexes. In the crystalline state, the differences between pure enantiomers, racemic modifications, and diastereomeric complexes are clearly defined both structurally and energetically (32,33). At the other extreme, stereospecific interactions between diastereomerically related solvents and solutes, ion pairs, and other partially oriented systems are much less clearly resolved. 

Anions (9), (10a) and (11) derived from cycloctatetraene have been studied using the selected ion flow tube technique and MO calculations, as a continuation of previous work on the structures and energetics of eight-membered-ring species, CgH and... 

I thoroughly believe that these contributions cover important advances in inorganic and bioinorganic chemistry, since information on the dynamics of water and on the molecules/ions to which water is bound is obtained. Sometimes, structural and energetic parameters are also obtained. These are precious and general data which are uniquely obtained by this approach. Applications in MRI are common. I trust that the inorganic and bioinorganic chemistry community will benefit from this thematic volume. 

Computational chemistry procedures describing the geometry and thermochemistry of gaseous ions are often used to predict and support experimental data. This approach has also been employed to investigate the structure and energetics of some organozinc ions at different levels of theory. Cations and anions having a Zn—C bond which were studied in silico are listed in Tables 6 and 7, respectively. 

Krestov, G.A. Thermodynamics of Solvation. Solutions and Dissolution Ions and Solvents Structure and Energetics, Ellis Horwood, Chichester, 1991. 

Since Bjerrum s [7] introduction of the concept of ion pairing in 1926, a variety of analytical methods has been employed to study the structure and energetics of ion pairs. Szwarc s Book deals with the development of the ideas up to 1972 [3a]. It is also a guiding reference for the different spectroscopic methods that have been employed in the examination of ion pairs. 

Using time-resolved CIDNP [56], the products of the fast geminate reaction may be detected, too. Obviously, the CIDNP effect is predetermined as an analytical tool for the examination of the structure and energetics of radical ions [57], less for ion pairs. 

The structure and energetics of [C, 3H, X]+ ions (isomers 14-16) were also investigated, both theoretically and experimentally, particularly for the case X = Cl. The ylidions... 

The structures and energetics of ions [C2H4X]+ with X = F, Cl, Br and I have been the subject of numerous experimental and computational investigations. Structures 20-22 were considered, with particular emphasis on the cyclic halonium species 21no. 

The particular chemical form in which an element exists in water is its speciation. For example, an element can be present as a simple hydrated ion, as a molecule, as a complex with another ion or molecule, and so forth. From what was said previously, bare ions or bare polar molecules do not exist in water. At the least, they would be solvated species. Species of an element are distinguishable from one another stoichibmetrically, structurally, and energetically. In addition to aqueous species, one can distinguish elements in different phases, for example, as gaseous species, as solid phases, or in adsorbed states, and on the basis of particle sizes. In the atmosphere, for example, speciation extends over liquid, gas, and aerosol phases (Seinfeld, 1986). The notion of chemical speciation is central to equilibrium and kinetic aspects of aquatic chemistry, as will be evident throughout this book. 

In 1979 the results of ab initio calculations at the 4-3IG and 6-3IG level on the same complex as well as a formaldehyde/H+ complex were reported. Structural and energetic ctxnparisons of the two complexes showed that while Li+ prefers a linear geometry for electrostatic ion-dipole bonding, the proton coordinates to the carbonyl through a largely covalent bond, resulting in a bent structure (Cs symmetry)... 

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