Methyl interaction

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. 

Conformations in which there is a 1,3-diaxial interaction between substituent groups larger than Iqidrogen are destabilized by van der Waals repulsion. Equilibration of mixtures of cis- and /ran5-l,l,3,5-tetramethylcyclohexane reveals that the cis isomer is favored by 3.7 kcal/mol. This provides a value for a 1,3-diaxial methyl interaction that is 1.9 kcal/mol higher than that for the l,3-methyl-4iydrogen interaction. 

The permethylated cyclopolysilanes are colourless, crystalline solids. The hexamer (Me2Si)e adopts a chair conformation with negligible methyl-methyl interactions in the solid state, whereas the larger rings (Me2Si) ( = 13 or 16) are distorted in response to endocyclic interactions between methyl groups (see Figure 6.1). In solution, the permethylated cyclopolysilanes show only sharp... 

For a methylene or methine carbon with an axial y-methyl interaction subtract 5-8 ppm for a methyl with n-axial y-methylene proton interactions subtract n x 3-6 ppm. 

The observed 5-exo-trig cyclization in the above reaction is attributed to the formation of more stabilized malononitrile radical intermediate compared to the trialkyl-substituted radical. The stereochemical outcome of these transformations with regards to the malonodinitrile group is consistently P-sclcctivc this suggests least steric interactions between these substituents as compared to the alternative methyl-methyl interaction in the transition state. 

The product of this reaction is the result of a sequence of equilibrium processes. As illustrated below, initial protonation of the civ-olefin allows transient formation of single-bond character. This single-bond character then allows for rotation around the central carbon-carbon bond. Final deprotonation liberates the trans-olefin. The overall process is driven by the reduced steric interactions present in the trans-olefin compared to the cis. Specifically, the civ-olefin possesses methyl-methyl interactions that are not present in the trans. 

The eclipsed coiiformer of propane has three interactions— two ethane-type hydrogen-hydrogen interactions and one additional hydrogen-methyl interaction. Since each eclipsing He—interaction is the same as that in ethane and thus has an energy "cost" of 4.0 kJ/mol, we can assign a value of 14 (2 X 4.0) =... 

Like surface tension, viscosity is a manifestation of the physical interaction between molecules. The uniquely shallow slope of the viscosity-temperature curve of PDMS (20) is due in part to low intermolecular forces. The interfacial viscosity of PDMS ( 10 xg/s) is the lowest known for a polymer film (9) and is also partly the result of low methyl-methyl interactions dominating polymer-polymer interactions in PDMS. 

Derivation (1) Synthetic tryptophan can be made by the conversion of indole to gramine followed by methylation, interaction with acetylaminomalonic ester and hydrolysis, (2) hydrolysis of proteins. 

These results coupled with the observed significant increase in the volume of the solution due to the formation of solvated electrons in hexamethylphosphotriamide, as reported by Gremmo and Randles , enable us to assume that the electron is in the cavity formed by the solvent s molecules oriented with their methyl groups toward the cavity. These data fit the accepted structure of hexamethylphosphotriamide molecule where the positive charge sited mainly on phosphorus is well shielded. Indeed, at the positive pole of the molecule the methyl substituents at the amino group face outward these methyls interact directly with the electron. 

The compound [Li(Tp )(Hpz )]-Hpz is the unique reported lithium compound, structurally characterized, containing a homoscorpionate ligand. It shows a UN4 distorted tetrahedral geometry with a triden-tate Tp (Fig. 2.1). The crystal packing is dominated by hydrophobic methyl-methyl interactions.1... 

This syndiotactic model resembles the isotactic one in that it also is based on an octahedral complex that has an alkyl ligand as the growth site, and a vacant octahedral position through which propylene is complexed. In this homogeneous model for polymerization, methyl-methyl interactions force the propylene to be complexed in opposite configuration at each consecutive growth step. Copolymerization studies (85) with soluble syndiotactic-specific catalysts support this view. 

Methyl-methyl interactions are reduced during cyclization of the molecule on the right. This sort of steric acceleration is different from the acceleration induced by holding two functionalities together. In fact, we take great care to avoid reactions where classical steric accelerations muddle the issue. Consider the following the intramolecular nucleophilic displacements (17) ... 

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