Conformational analysis staggered

A conformational analysis of the methanesulfonyl radical indicates the staggered structure 1 as the most stable, with a barrier of rotation of ca. 3.8 kcal mol"17. The... 

Conformation and Conformational Analysis Conformation of Ethane Conformation of Propane Conformation of Butane Eclipsed and Staggered Eorms Ring Strains in Cycloalkanes Principles of Conformation 165... 

Influence of Flexible Bond Angle on Torsional Conformations, Hexamethyldisiloxane was used as a test compound for several procedures applied in conformational analysis. This compound is relatively simple, and structural data are available in the literature. The experimental value for the Si-O-Si bond angle in hexamethyldisiloxane is 148 3°, as measured by electron diffraction (12). The electron diffraction data indicate a staggered conformation 2 symmetry), although a model with twist angles around Si-O bonds of about 30° cannot be excluded. 

Conformation analysis is particularly important in aliphatic systems. Taking butane as an example, there is obviously an infinite number of possible dihedral angles that the two methyl groups can adopt with respect to each other. However, two of the conformers are of particular interest in that they represent the states of lowest and highest interaction between the hydrogens on the adjacent methyl groups these are the staggered and eclipsed conformers respectively. 

It is possible to apply the methods of conformational analysis to predict the relative yields of the cis- and metal atom to which the 2-butyl radical is attached may be regarded as equivalent to another methyl group, it is found that there are three possible staggered conformations (see Fig. 4). Conformation (I) would lead only to ci -2-butene, (II) only to [Pg.106]

Nearly all side-chain prediction methods depend on the concept of side-chain rotamers. From conformational analysis of organic molecules, it was predicted long ago [157, 158] that protein side chains should attain a limited number of conformations because of steric and dihedral strain within each side chain and between the side chain and the backbone (dihedral strain occurs because of Pauli exclusion between bonding molecular orbitals in eclipsed positions) [159]. For sp3-sp3 hybridized bonds, the energy minima for the dihedral are at the staggered positions that minimize dihedral strain at approximately 60°, 180°, and —60°. For sp3-sp2 bonds, the minima are usually narrowly distributed around +90° or —90° for aromatics and widely distributed around 0° or 180° for carboxylates and amides (e.g., Asn/Asp y2 and Glu/Gln /3). 

PDO, and EDO preparations (12) are shown in fig. 4. The poly-(MDI-BDO) pattern has been analyzed previously and indicates a triclinic unit cell in which the base plane is tilted, i.e. it is not perpendicular to the fiber axis, such that the chains are in a staggered hydrogen bonded array. The conformational analysis above shows that the polymer chain is fully extended, with the all-trans -(CH2)ij- unit coplanar with the urethane groups. 

The electrons in a C—H bond will repel the electrons in another C—H bond if the bonds get too close to each other. The staggered conformation, therefore, is the most stable conformation of ethane because the C—H bonds are as far away from each other as possible. The eclipsed conformation is the least stable conformation because in no other conformation are the C—H bonds as close to one another. The extra energy of the eclipsed conformation is called torsional strain. Torsional strain is the name given to the repulsion felt by the bonding electrons of one substituent as they pass close to the bonding electrons of another substituent. The investigation of the various conformations of a compound and their relative stabilities is called conformational analysis. 

Much use is made of molecular mechanics to explore rotational conformations, and it is easy to see that if each staggered conformation is a double minimum, minimization from an approximate structure, or an approach from a different staggered conformation by driving the dihedral angle, may arrive at one or other minimum. Unless the second local minimum is sought and evaluated explicitly, there can be no certainty that the conformational analysis is correctly based. An analysis of an unsymmetrical acyclic alkyl compound which does not discuss all six conformations about each bond, even if only to propose a simplification, is liable to be erroneous in a way which may be trivial in simple cases, but critically important when the two local minima are quite different. 

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