Bonding geometry

Fig. XVin-5. HREELS spectra for NO2 on Pt(lll) adsorbed in different bonding geometries. [From M. E. Bartram and B. E. Koel, / Vac. Sci. Tech., A6, 782 (1988).]...

Hydrogen bond geometries may be reproduced or predicted fairly weH with reasonable, but sometimes underestimated heavy atom—heavy atom distances radial dependence of the hydrogen bond may be in error. 

Figure 4 Specular spectra in HREELS of NO2 adsorbed in three different bonding geometries. ...

Scheme 2.8. Stereoisomeric Alkenes and Related Molecules with the Double-Bond Geometry Named According to the Sequence Rule...

Annulene is a very imstable compound that undergoes cyclization to bicyclic isomers and can be kept only at very low temperature. The NMR spectrum has been studied at low temperature." Besides indicating the double-bond geometry shown in the stmcture below, the spectrum reveals a paramagnetic ring current, the opposite to what is observed for aromatic systems. This feature is quite characteristic of the [4 ]annulenes and has been useful in characterizing the aromaticity or lack of it in annulenes." ... 

A j 5 Coppe -Medloted 0 tlose ectlve Siibstitiition Reoctlo s Tab. A. Dependence on double bond geometry in 10. 

As in die case of die substitution reaction of compound 7, die absolute conbgu-radon of die product depends on die double bond geometry of die starting material, as sliown by die example in Tab. 8.1. 

The cis-trans naming system used in the previous section works only with disub-stituted alkenes—compounds that have two substituents other than hydrogen on the double bond. With trisubstituted and tetrasubstituted double bonds, a more general method is needed for describing double-bond geometry. (Tiisub-stitnted means three substituents other than hydrogen on the double bond tetrasubstituted means four substituents other than hydrogen.)... 

Figure 31.5 (a) Natural rubber is elastic and noncrystalline because of its cis double-bond geometry, but b) gutta-percha is nonelastic and crystalline because its geometry allows for better packing together of chains. 

It was anticipated all along that the vinylsilane residue could serve as a vinyl iodide surrogate. After protection of the C-14 secondary hydroxyl in 180 in the form of a triisopropylsilyl ether, the vinyltrimethylsilyl function can indeed be converted to the requisite vinyl iodide with AModosuccinimide (NIS) (see 180—>181, Scheme 43). Vinyl iodide 181 is produced stereospecifically with retention of the A17,18 double bond geometry. This transformation is stereospecific since the stereochemistry of the starting vinylsilane and the vinyl iodide product bear a definite relationship to each other.67b 75... 

In a recent study, simple diastereoselectivities in the reactions of 2-butenylsilanes with cyclo-hexenone were reported24 25. As shown in the table below, the double-bond geometry plays a principal role in the stereochemical outcome of the reaction. In general, the ( )-2-butenylsi-lanes gave better selectivities, whereas the effect of the silyl substituents was hard to correlate with the changes in selectivity. 

A detailed discussion about the functional form for f(v[r) can be found in Ref. [15]. The frequencies of molecular vibrations depend on the force constants which are themselves attributed to the bond geometry. It is then not surprising that useful information on bond deformation under stress can come from IR or Raman spectroscopy. 

When enantiomerically pure allyl p-tolyl sulfoxide is deprotonated and then treated with electrophilic 2-cyclopentenone, a conjugate addition occurs forming a new carbon-carbon bond with very high control of absolute stereochemistry (equation 25)65. See also Reference 48. Similarly, using more substituted enantiomerically pure allylic sulfoxides leads to virtually complete diastereocontrol, as exemplified by equations 26 and 27 the double bond geometry in the initial allylic sulfoxide governs the stereochemistry at the newly allylic carbon atom (compare equations 26 vs. 27)66. Haynes and associates67 rationalize this stereochemical result in terms of frontier molecular orbital considerations... 

However, an examination of the bonding geometries in a range of sulphones has produced some differences between those sulphones that are readily reduced and those that are not. These are summarized in Table 1. 

The protocol of the allylic alkylation, which proceeds most likely via a c-allyl-Fe-intermediate, could be further improved by replacing the phosphine ligand with an M-heterocyclic carbene (NHC) (Scheme 21) [66]. The addition of a ferf-butyl-substituted NHC ligand 86 allowed for full conversion in the exact stoichiometric reaction between allyl carbonate and pronucleophile. Various C-nucleophiles were allylated in good to excellent regioselectivities conserving the 71 bond geometry of enantiomerically enriched ( )- and (Z)-carbonates 87. Even chirality and prochirality transfer was observed (Scheme 21) [67]. 

DFT-GGA calculations are very useful for investigating plausible reaction pathways of various molecules on surfaces. The method provides detailed information on the bonding geometry, on bond energies as well on activation barriers, and transition states which are otherwise not accessible. Typical accuracies in such numbers amount to a few tenths of an eV, making the method particularly useful to investigate trends. 

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