Stabilizing interactions

The issue of resonance in carboxamides has recently been reexamined. Most of the structural consequences predicted by resonance are consistent with a stabilizing interaction which results in a net shift of electron density from nitrogen to carbon and oxygen A. J. Bennet, Y Somayaji, R. S. Brown, and B. D. Santarsiero, J. Am. Chem. Soc. 113 7563 (1991) A. Greenberg, T. D. Thomas, C. R. Bevilacqua, M. Coville, D. Ji, J.-C. Tsai, andG. Wu,Jl Org. Chem. 57 7093 (1992). 

Display electrostatic potential maps for both anti and gauche conformers of 1,2-ethanediol. Do you see any examples of destabilizing interactions (between like charges) or stabilizing interactions (between unlike charges) in either conformer Are you able to explain the observed conformational preference ... 

The stability order of alkenes is due to a combination of two factors. One is a stabilizing interaction between the C=C tr bond and adjacent C-H a bonds on substituents. In valence-bond language, the interaction is called hyperconjugation. In a molecular orbital description, there is a bonding MO that extends over the four-atom C=C—< -H grouping, as shown in Figure 6.6. The more substituents that are present on the double bond, the more hyperconjugation there is and the more stable the alkene. 

Figure 6.6 Hyperconjugation is a stabilizing interaction between an unfilled w orbital and a neighboring filled C-H <7 bond on a substituent. The more substituents there are, the greater the stabilization of the alkene.

Since 1973, several authors have proved that there is a relationship between thermostability of collagen and the extent of hydroxylation of the proline residues31,34). Equilibrium measurements of the peptides al-CB 2 of rat tail and rat skin revealed a higher rm, for al-CB 2 (rat skin)157). The sequence of both peptides is identical except that in the peptide obtained from rat skin, the hydroxylation of the proline residues in position 3 has occurred to a higher extent than in the case of al-CB 2 (rat tail). Thus, a mere difference of 1.8 hydroxy residues per chain causes a ATm of 26 K. Obviously, there are different stabilizing interactions in the triple-helical state, that means al-CB 2 (rat skin) forms more exothermic bonds than al-CB 2 (rat tail) in the coil triple-helix transition. This leads to an additional gain of enthalpy which overcompensates the meanwhile occurring losses of entropy. 

An obvious refinement of the simple theory for cobalt and nickel and their alloys can be made which leads to a significant increase in the calculated value of the Curie temperature. The foregoing calculation for nickel, for example, is based upon the assumption that the uncoupled valence electrons spend equal amounts of time on the nickel atoms with / = 1 and the nickel atoms with J = 0. However, the stabilizing interaction of the spins of the valence electrons and the parallel atomic moments would cause an increase in the wave function for the valence electrons in the neighborhood of the atoms with / = 1 and the parallel orientation. This effect also produces a change in the shape of the curve of saturation magnetization as a function of temperature. The details of this refined theory will be published later. 

In the corresponding -alkene, where this factor is not present, the cyclization is much less stereoselective. A stabilizing interaction between the siloxy oxygen and the Hg2+ center has also been suggested.108... 

Incidently, the observed solvent effect may be supportive evidence against incursion in nonpolar solvents of a second reactive 3n-n state.129. 130) The zn-n state would be expected to be less selective for the head-to head dimer since one of the principle stabilizing interactions is missing. 

The question of the identity of the reactive excited state was left open, although n-n excited state was considered to be more probable.140 The interaction diagram, Fig. 7, shows that a n-n state would have an additional stabilizing interaction Ri(tt) ->-R2(7r) with orbital coefficients in phase for an all-suprafacial concerted reaction. The dominant reaction would theoretically depend upon the relative placements of the several levels, and since no experimental information for maleic and fumarate esters is presently available, a clear choice cannot be made. It is interesting that the reactive excited state could be inferred if both stereochemistry and a good molecular diagram were available. 

These are the favorable interactions that were discussed above. They work together to provide stabilizing interactions that hold the structure together. 

Stability Sugar side chains can potentially stabilize a glycoprotein in a number of ways, including enhancing its solubility, shielding hydrophobic patches on its surface, protection from proteolysis and by direct participation in intrachain stabilizing interactions... 

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