Equatorial ionization

The axial C—H bonds are weaker flian the equatorial C—H bonds as can be demonstrated by a strongly shifted C—H stretching frequency in the IR spectrum. Axial C-2 and C-6 methyl groins lower the ionization potential of the lone-pair electrons on nitrogen substantially more than do equatorial C-2 or C-6 methyl groups. Ehscuss the relationship between these observations and provide a rationalization in terms of qualitative MO theory. 

In piperidine the electron lone-pair can occupy either an axial or an equatorial position in 1-methylpiperidine the axial orientation (lb) is favoured by 99 1 over the equatorial (la). PE spectra and ab initio calculations on methylpiperidines indicate that axial 2-methyl substituents lower the amine lone-pair ionization potential by about 0.26 eV, while equatorial 2-methyl substituents as well as methyl groups on carbon atoms 3 and 4 lower the lone-pair IP by less than 0.1 eV63. This establishes the mechanism of stabilization of the amine radical cation as hyperconjugative electron release, which is larger for CC bonds than for CH bonds. The anti-periplanar orientation of the nitrogen lone-pair and the vicinal C—Me bond (lc) is much more favourable for this type of interaction than the synclinal orientation (Id). 

The PE spectra of four 2-substituted 1,3-dimethylimidazolidines (58) have been recorded and analysed using AMI and PM3 quantum chemical calculations106. A single broad band is found for the two hn ionizations and the energies of the two mn orbitals are split by only 0.0-0.3 eV indicating little n/n interaction. This is consistent with envelope conformations of the five-membered ring of 58 and an axial-equatorial orientation of the two N-methyl groups. 

Clark and Hartwell have prepared the but-3-enyl complex (CH2= =CHCH2CH2)3PRhCl (52) in which the Rh is again pentacoordinate, an X-ray study showing that the olefin groups are parallel to the equatorial plane of the trigonal bipyramidal coordination sphere as in RhBr-(tvpp). The complex shows a conductivity in methanol appropriate for ionization to LRh MeOH)+ Cl. ... 

Photoelectron spectra and STO-3G calculations on methylpiperidines show that equatorial methyl substituents at the 2-, 3-, and 4-positions lower the ionization potential of the nitrogen lone pair by <0.1 eV. An axial methyl at C-2, however, lowers the ionization potential by 0.26 eV. These influences on ionization potentials are nearly identical in both piperidines and A-methylpiperidines, indicating that the lone-pair orientations in both systems are similar. The STO-3G calculations indicate a favoring of the equatorial A-H conformation of the piperidines by 1.0—1.9 kcal mol-1.174... 

Photoelectron spectroscopy (PES) has been shown to provide a convenient probe of metal ion effective nuclear charge and the nature of the metal-ligand bond via the energy of valence-electron photoionizations (16, 20, 22, 284, 285, 312, 332-334). Recently, PES spectroscopy has been employed in the study of oxo-molybdenum compounds of the type (L-A5)MoE(X,Y) [E = O, S, NO X, Y = halide, alkoxide, or thiolate] in order to evaluate the synergy between the axial (E) and equatorial (X,Y) donors in affecting the ionization energy of the HOMO localized on the Mo center (16, 284, 334). These studies have conclusively shown that equatorial dithiolene coordination electronically bulfers the Mo center in (L-A pMoEttdt) (Fig. 13) from the severe electronic perturbations associated with the enormous variation in the Ji-donor/acceptor properties... 

Ci-Cs hydrocarbons and chlorophyll a analyses were performed on approximately 400 near-surface water samples 2m below the surface) during a one and half month cruise to the Equatorial Pacific Ocean. The hydrocarbons were determined by a flame ionization gas chromatograph. Chlorophyll a was determined fluorometrically. The objective was to determine if a correlation existed between the C2-C3 hydrocarbons and chlorophyll a in open ocean environments. A slight correlation was found between chlorophyll a and ethylene and propylene (R = 0.56). No correlation xjoas found between chlorophyll a and methane, ethane, or propane. 

Fused ring C-glycosides. Ionization of glycosides in the presence of appropriate alkenes leads to C-C bond formation. When C-2 is protected by an acid- cleav-able group, formation of a tetrahydrofuran ring ensues. Since the alkene attacks from the axial direction, the C-2 oxygen function must be equatorial in order to form the new ring. 

For the k process the nonclassical ions 373, 374 and 376 initially formed upon ionization of the corresponding tosylates 370-372 can either be captured by a nucleophile with full stereospecificity yielding axial epimers, or rearrange into the classical ions 377, 318 and 352, respectively, which are captured, already non-stereo-specifically, by a nucleophile forming a mixture of axial and equatorial epimers. Comparison of the acetolysis and trifluoroacetolysis supports the schemes proposed for the k processes of these substrates. Trifluoroacetolysis, as compared to acetolysis, shows a sharp decrease (or complete absence) of equatorial epimers with axial epimers formed as nearly the only products. 

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