Y-X bonds

(boldface = recommended, The broken bonds i, x Methods data reference in parentheses), (boldfarp= (reference m dissociated fragment) kcal/mol kj/mol parentheses) References 

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The bond between the bridgehead carbons of bicyclo[1.1.0]butanes can be either normal or inverted [35]. The normal bond is stabilized by the high s-character (Scheme 16a). In this case, the hybrids on the bridgeheads Y have low s-character for the bridging Y-X bonds. Electronegative atoms X or substituents R at Y stabilize... 

DY-x e dissociation energy of Y X bond probability of formed free radical pair to escape the cage of solvent or polymer kJ moU1... 

The process of the interconversion of an open-chain tautomer into a cyclic form formally involves three stages (1) Q —X bond cleavage, (2) Q —Y or Q —Z bond formation (ring closure), and (3) Z —X or Y —X bond formation. The sequence of these stages may vary. Most commonly, the migrating particle X is a proton. 

The atom X, still in an sp hybridization state of a heterocyclic system, is at one end of one unsaturated Y=X bond in a nonaromatic ring. The same conformational profile as in case 1 is expected if reference is made to vinyl ketones or a-diketones, which are corresponding acyclic derivatives. 

The position of the insertion equilibrium depends upon the strengths of the M-X, M-Y, M-(YX) bonds and M(Y-X) bonds. 

The electron density distribution around the Y-X bond described above indicates that the electropositive crown available for interaction with a lone pair of electrons should be greater for the more polarizable halogens so that putative N- - -I interactions shonld be stronger than their N- - -Cl connterparts. This expectation is borne out experimentally. Fnrthermore, it is also possible to moderate the magnitnde of N- - -X by systematically varying the nature of the halocarbon acid. ... 

A convenient dividing line is 1600 cm where bands in the region 1800-1600 cm" are due to X=Y double bond stretching vibrations, and bands in the region 1600-1500 cm" are due to the asymmetric stretching of the X—Y X bond-and-a-half bonds such as in carboxyl salts, nitro groups, and aromatic rings. 

Gavezzotti A and Filippini G 1994 Geometry of the intermolecular XH.. . Y (X,Y = N,0) hydrogen bond and the calibration of empirical hydrogen-bond potentials J. Phys. Chem. 98 4831... 

We ai e free to pick a tefei ence poitit of energy once, but otily otice, for each system, l,et us choose the reference point t.. We have obtained the energy eigenvalues of the x bond in ethylene as one [f greater than y. 011)11 bunding) and one p lower than y ( bunding) (Fig, 6-3),... 

FIGURE 2.7. (a) Three active pz orbitals that are used in the quantum treatment of the X + CH3-Y— X-CH3 + Y Sw2 reaction, (b) Valence-bond diagrams for the six possible valence-bond states for four electrons in three active orbitals, (c) Relative approximate energy levels of the valence-bond states in the gas phase (see Table 2.4 for the estimation of these energies). 

Sn2 stands for substitution nucleophilic bimolecular. The lUPAC designation (p. 384) is AnDn- In this mechanism there is backside attack The nucleophile approaches the substrate from a position 180° away from the leaving group. The reaction is a one-step process with no intermediate (see, however, pp. 392-393 and 400). The C—Y bond is formed as the C—X bond is broken ... 

A cyclic conjugate molecule composed of n bonds, A, B,..., X, and Y interacts at A with a reactant Z (Scheme 12). When the molecule is an electron donor (Scheme 12a), electrons delocalize from a to z. The resulting electron hole in a is supplied with an electron by the neighboring b. Similar delocalization sequentially follows from c to b, from d to c and so on. This is also the case with the opposite side Y, X,. It follows that the cyclic orbital interaction of a, b,.x, andy is important in the conjugated molecule. The orbitals are all electron-donating orbitals. When each neighboring pair of orbitals is combined out of phase, the interaction of the cyclic... 

C09-0101. Shorter bonds are usually stronger bonds, but this is not always the case. Using Tables 9 and, find and list any X—bonds, for X and Y both Row 2 elements, that are (a) shorter but weaker and (b) longer but stronger than the corresponding X—X bond. 

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