1,3-Interactions, destabilization

An unfavorable methyl-methyl interaction destabilizes the s-trans conformation of 4-methyl-3-penten-2-one relative to the s-cis conformation, and the equilibrium favors the s-cis form. 

Fig. 3.6 Variation of the energy of a molecule with separation of nonbonded atoms or groups. Atoms/ groups A and B may be in the same molecule (as indicated here) or the interaction may be intermolecular. The minimum energy occurs at van der Waals contact. For small nonpolar atoms or groups the minimum energy point represents a drop of a few kJ mol-1 ( m in = —1.2 kJ mol-1 for CH4/CH4), but short distances can make nonbonded interactions destabilize a molecule by many kJ mol-1...

After adsorption one side of the protein molecule is oriented towards the sorbent surface, turned away from the aqueous solution. As a consequence, hydrophobic parts of the protein that are buried in the interior of the dissolved molecule may become exposed to the sorbent surface where they are still shielded from contact with water. Because hydrophobic interaction between apolar amino acid residues in the protein s interior support the formation of secondary structures as a-helices and P-sheets, a reduction of this interaction destabilizes such structures. Breakdown of the a-helices and/or P-sheets content is, indeed, expected to occur if peptide units released from these ordered structures can form hydrogen bonds with the sorbent surface. This is the case for polar surfaces such as oxides, e.g. silica and metal oxides, and with sorbent retaining residual water at their surfaces. Then the decrease in ordered secondary structures leads to an increased conformational entropy of the protein. This may favour the protein adsorption process considerably.13 It may be understood that proteins having an intrinsically low structural stability are more prone to undergo adsorption-induced structural changes. 

In moving from X to M, the d and dyz bands now become degenerate, with two antibonding interactions (destabilization = 4/3) per metal atom with the oxygen p orbitals. These two bands are lower in energy than the d y band, which has four antibonding interactions per metal atom and a destabilization of 8/3 (Eq. 5.38). Figure 5.9 also... 

Larger axial substituents create unfavorable 1,3-diaxial interactions, destabilizing a cyclohexane conformation. 

Diaxial interaction (Section 4.13A) A steric interaction between two axial substituents of the chair form of cyclohexane. Larger axial substituents create unfavorable 1,3-diaxial interactions, destabilizing a cyclohexane conformation. 

The most difficult rings to close are those that are of medium size (8-11) because torsional and transannular interactions destabilize the transition state. 

In the case of systems based on donor/acceptor interactions, destabilization of the original structure can be achieved by reduction of the electron-acceptor unit(s) or by oxidation of the electron-donor unit(s) as a consequence of chemical, electrochemical, or photochemical electron-transfer processes. The original CT interaction can usually be restored by an opposite redox process, which thus leads back to the original structure. In the case of systems based on metal complexes, the stability of the original structure is related to the compatibility between oxidation state of the metal ion and coordination environment. If the oxidation state of the metal ion is modified as a consequence of a redox process, the original structure is destabilized and the system evolves toward another, more stable structure. Again, an opposite redox process can lead back to the original structure. 

Thus, the rearrangement can in principal lead to four products through transition states 20 A-20D. Flowever, because the relative asymmetric induction is the primary determinant for the synjanti selectivity (vide infra), each rearrangement produces one major Claisen product with 97- 98% ee640,643. Incomplete internal asymmetric induction (incomplete chair/boat selectivity) accounts for the observed 85 15 product ratios. Substrate 19a, however, rearranges with increased chair selectivity (chair/boat 95 5) indicating that when R1 = R4 = alkyl, a nonbonding interaction destabilizes transition states 20C and 20D relative to transition states 20 A and 20B. 

Scheme 6.12. A low enantioselectivity may ensue in some instances, for example when a trans-annular interaction destabilizes the favored carbanion configuration [80].

As noted in (3), borane adds selectively to the less sterically hindered carbon of the alkene or alkyne, but mixtures of Markovnikov proddct (addition of boron to the most hindered carbon) and anti-Markovnikov (addition of boron to the less hindered carbon) product are usually observed. This selectivity is best understood by examining the four-center transition state required for the addition (see 4). When borane approaches the alkene unit of 2-methylpropene, two orientations are possible (8 and 9). In 8, the BH2 moiety is positioned over the less hindered carbon bearing the hydrogen atoms [ BH2 - H2C= ] because this minimizes the destabilizing steric interactions with the methyl groups on the alkene [ BH2 Me2C= ] found in the other orientation (9). In other words, this interaction destabilizes 9 and leads to selective delivery of boron to the less hindered carbon via 8. Table 5.1 2 shows the relative proportions of alkylborane formed by addition of diborane to several representative alkenes. 

Qin, S., Zhou, H.X. Do electrostatic interactions destabilize protein-nucleic acid binding Biopolymers 2007,86,112-8. 

The frontier molecular orbitals involved in reaction of tropone illustrate that a repulsive secondary orbital interaction destabilizes the endo transition state leading to the complete selectivity for exo products. The frontier orbitals have a repulsive interaction (wavy lines) between C-3 and C-4 on the tropone and C-2 on the diene (and between C-5 and C-6 on the tropone and C-3 on the diene) in the endo transition state (Figure 4.23). 

If the electrophile is added ortho or para, it is possible to draw a contributing structure that places the positive charge directly adjacent to the partially positive carbon atom of the carbonyl group. This interaction destabilizes the carbocation, which in turn disfavors ortho-para attack of the electrophile. 

Stabilized by hydrophobic interaction (destabilized at high temperature). Differences ... 

Cooperative, cooperativity Two or more sets of intermolecular interactions are said to be cooperative if they have a mutually stabilizing effect. Conversely two or more sets of interactions are anticooperative if one set of intermolecular interactions destabilizes another set. See also allostery. 

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