Ion geometries

The so-called Coulumb explosion technique, in which ions with translational energies in the MeV regions are denuded of electrons by a thin carbon foil, also promises to provide information on structures and on the symmetries of polyatomic ions [325, 326, 327], as has already 

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The first step in the addition of an electrophile such as HBr to an alkyne involves protonation and subsequent formation of an intermediate vinyl cation. Where does propyne protonate Compare energies of 1-methylvinyl and 2-methylvinyl cations. Which is more stable Why Measure CC bond distance in the more stable cation. Does the cation incorporate a full triple bond (as in propyne) or a double bond (as in propene). Examine atomic charges and electrostatic potential maps to locate the positive charge in the two cations. Is the more stable ion the one in which the charge is better delocalized Use the charges together with information about the ions geometry to draw Lewis structures (or a series of Lewis structures) for 1-methylvinyl and 2-methylvinyl cations. 

The potential surfaces of the different states of a reagent may be related in three ways (Scheme 6.6) (a) the three surfaces have minima at closely related geometries (b) the radical ion geometry is related to that of the parent, but the triplet state or biradical has a different geometry (c) in systems giving rise to bifunctional-distonic radical ions (see above) triplet states or biradicals may have structures related to the radical ions. ... 

The discussion of crystals and ion geometry leads to a question. If the mineral crystals in the igneous rocks can largely be accounted for by Si-O tetrahedra plus the major elements, how do the trace elements occur The answer lies in the natural impurity of the minerals. As magma cools and the minerals appropriate to a particular fluid composition and temperature - pressure combination form, then other elements in the magma are incorporated into the growing crystals, but this is not a haphazard affair and involves isomorphous substitution, ie, substitution by an ion of similar size. 

Calculation of a rate coefficient, k(E), on the basis of eqn. (1) demands that values of vibrational frequencies be available for the reactant ion and the transition state. Vibrational frequencies of gaseous polyatomic ions have been measured in only a very limited number of cases and the ion geometries are almost invariably not known. The term ion structure , therefore, in mass spectrometry is rather a loose expression and is concerned with qualitative structure. The concern is more with which atom can be said to be bonded to which others, rather than with precise bond lengths and bond angles. The vibrational frequencies of the reactant ion... 

On the other hand, if the primary beam is illuminating the complete sample surface (case 2), the absorption factor for symmetrical-ttfiec ion geometry becomes... 

Addition of 5-substituted 2-pyrrolidinones to methyl glyoxylate produces useful precursors (190) to exocyclic A-acyliminium ions (equations 132). The preferred iminium ion geometry is most probably ( ) as depicted in (191) for steric reasons. This geometry explains the stereochemical relationship between the ring junction hydrogen atom and the ester function in the products (192) and (193). Best yields are achieved by using the thermal cyclization of intermediate mesylates, and the major products have the ester and vinyl function trans. ... 

Crystal structures have also been obtained for the corresponding chloronium and iodonium ions and for the bromonium ion with a triflate counterion. Each of these structures is somewhat unsymmetrical, as shown by the dimensions below. The significance of this asymmetry is not entirely clear. It has been suggested that the bromonium ion geometry is affected by the counterion and it can be noted that the triflate salt is more symmetrical than the tribromide. On the other hand, the dimensions of the unsymmetrical chloronium ion, where the difference is considerably larger, has been taken as evidence that the bridging is inherently unsymmetrical. Note that the C- C bond lengthens considerably from the double-bond distance of 1.35 A. 

ESDIAD ESD-lon Angular Distribution (See ESD) (See ESD) Directional dependence of emitted ions - - Geometries of adsorbed species (atoms or molecules) 22... 

In the determination of ions geometries, the comparison with experimental data, underlines, in some case, the importance of the employed functional in determining reliable structures. 

The stereochemical interaction of the classical ion 159 with the solvent should not be determined by the ion geometry since it is symmetrical. The participation of the solvent and the shielding by the leaving group should lead to some net inversion of the configuration if the solvolysis proceeds via the classical ion 159. 

Beyond its influence on IMS separation parameters for a fixed geometry, the gas temperature affects ion geometries. As ion-molecule collisions must be sufficientiy frequent for a steady drift, ions are thermalized their internal, rotational, and translational modes are equilibrated at a single temperature. At low E/N where the ion drift is much slower than the Brownian motion of gas molecules, relative ion-molecule velocities conform to the Maxwell-Boltzmann distribution and ion temperature equals T of the gas. 

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