Cation geometry

Figure 12. Ab initio optimized cation geometries at UHF/6-31G level of theory, with a constrained to 90°. Bond lengths in A and bond angles in degrees.

TABLE 4. Change in cr and A/v between neutral and cation geometry for a series of selected non-conjugated diene radical cations... 

Single-point MP2/6-31G(d) calculation for the neutral species at the cation geometry. 

FIGURE 23. First-order Jahn-Teller distortion of Dn symmetrical cyclopropyl radical cation. Geometries [UMP2/6-31G(d) calculations] and relative energy [CISD/6-31G(d)] from Reference 231... 

The model has been tested for a wide variety of gas-zeolite combinations. Gases of increasing complexity were considered Ar(non-polar), (quadrupole moment, no dipole moment), NgO (quad-rupole moment, small dipole moment), and NHg (large dipole moment, small quadrupole moment). The zeolites tested were all in the synthetic faujasite family however, they ranged from the cation-rich zeolite X to the cation-poor zeolite Y. Cation geometries considered in the tests were those typical of the dehydrated zeolite form and those typical of the hydrated geometry (associated with NHg adsorption). Two forms of representative cations were considered, Li and Na+. 

The numbers are hartrees, and represent (other than the HOMO energies) for the neutrals and the cations at the cation geometry, uncorrected ab initio energy, ZPE, and corrected ab initio energy. The ZPEs shown have been multiplied [80] by 0.9135 (HF) or 0.9670 (MP2(fc)). For the cations at the neutral geometry, ZPE was not used and is not shown. Adiabatic IE = A(cation) — A(neutral), both corrected for ZPE. Vertical IE = (cation) — f.(neutral), both without ZPE. Hartrees were converted to eV in Table 5.17 by multiplying by 27.2116. 

There are several obstacles to precise correlations of geometry with reactivity in the various model systems, of which the most important is that none of the cation geometries is experimentally determined, and in only a few cases are the geometries of the precursors... 

Antimony(V) chloride also forms a 1 1 complex with PhjSbClj, but here the increased Lewis acidity of antimony pentachloride leads to strong interaction with one of the axial chlorines and the product, [Ph3SbCl] [SbClg], is basically ionic. There is a residual Sb---Cl cation-anion interaction at 3.231 A and the cation geometry is intermediate between the trigonal bipyramidal and tetrahedral extremes. 

Early observations of stable carbocations in solution relied heavily upon H NMR spectroscopy. Subsequently, C NMR spectroscopy has proved to be an even more useful technique. C NMR permits also the direct observation of the cationic center and the observed chemical shifts and coupling constants can be correlated to the cation geometry and hybridization. 

Ordered anion vacancies are partly responsible for some important solid state phenomena, such as superconductivity, as well as unusual cation geometries. For example, the superconductor YBa2Cu307 is built from three oxygen-deficient blocks of the perovskite (ABO3) structure (Figure 6.10). In the parent structure, B is in an octahedral hole, coordinated to six oxygens. 

The naphthalene cation has been studied by absorption spectroscopy in solutions [154,155], glasses [156-159], and matrices [160]. Gas-phase data stem from PES [161,162] and multiphoton dissociation [163,164] spectra. Recently, the 180- to 900-nm absorption spectrum of naphthalene formed by photoionization in a neon matrix has been reinvestigated [165]. These experiments show seven progressions, with peak maxima at 1.84, 2.72, 3.29, 4.06, 4.49, 5.07, and 5.57 eV. The excitation energies calculated by Schutz et al. for the cation geometry are shown in Table XIV. They are in agreement with experiment. The largest deviation amounts to 0.13 eV (2 B2g). Table XIV also shows the predicted PES, which matches experiment. 

The bicyclo[2,2,2]octyl cation, as shown by calculations, is less stable than the 2-nor-bornyl one both in the gas phase (by about 6 kcal/mole) and in solutions At the same time the angle strain in the 2-norbomyl ion is higher, and the torsional and long-range nonbonded effects are comparable with those for the bicyclo[2,2,2]-octyl ion the difference seems to be due to the norbomyl cation geometry more favourable for a-delocalization as compared with the bicyclo[2,2,2]octyl ion. 

Structure factor for small single crystals of C-type rare earth oxides of Y2O3, DyaOs, and H02O3 was investigated from the synchrotron X-radiation point of view [30]. Approximate symmetry in the deformation electron density (Ap) around a rare earth atom with pseudo-octahedral oxygen coordination is similar to the cation geometry. Interactions appeared between heavy rare earth atoms show a pronounced effect on the Ap map. The electron-density symmetry around second rare earth atom is also influenced appreciably by cation-anion interactions and the oxides magnetic properties also reflect this complexity. 

Ethane is an instructive example of the inherent limitations of our EBO approach, which this simple model shares with more realistic SCF procedures, e.g. the calculation of ethane CMOs by extensively parametrized semi-empirical models, or by ab initio procedures, such as the floating Gaussian orbital modeF" T Obviously, as shown by Richartz and his coworkersconfiguration interaction methods yielding reliable cation state energies, including their dependence on the cation geometry, are required. 

Figure 3. The effect of the cation geometry on cLathrate formation in the series a) RBU3NF-H2O [21] b) Bu N(CH2) NBU3F2-H2O [2 ], Melting point and hydrate numbers (in brackets) are gi en above the Liquidus Line.

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