Covalent bond rotation around

Since the peptide units are effectively rigid groups that are linked into a chain by covalent bonds at the Ca atoms, the only degrees of freedom they have are rotations around these bonds. Each unit can rotate around two such bonds the Ca-C and the N-Ca bonds (Figure 1.6). By convention the angle of rotation around the N-Ca bond is called phi (<[)) and the angle around the Ca-C bond from the same C atom is called psi (y). 

Figure 1.4 The characteristic tetrahedron building block of all SiC crystals. Four carbon atoms are covalently bonded with a silicon atom in the center. Two types exist. One is rotated 180° around the c-axis with respect to the other, as shown.

Atoms within a molecule move relative to one another hy rotation around single bonds. Such rotation of covalent bonds gives rise to different conformations of a compound. Each structure is called a conformer or conformational isomer. Generally, conformers rapidly interconvert at room temperature. 

That rotation around the Q—C2 bond of 61en is faster than that around the C2—C3 bond is consistent with the proposal that the transition state for rotation in allyllithium involves some increase in Cj—Li covalence, see 62, compared to the delocalized state. 

The activation energy for a torsional process around a covalent bond depends, among other factors, on the 71-electron density associated with this bond. It is of interest to investigate whether the energy barrier for rotation around a carbon-carbon double bond can be sufficiently reduced to allow the establishment of the dynamic equilibrium between the isomers 26 and 28 in the ground state of the system. The complete equilibrium must also include the transformations 26 29 and 28 30, which are associated with restricted rotation around the C—N bond (Scheme 1). 

The control of the rotational sense of axial-rotation-based machines can operate through covalent changes in the constitution of the initial station. In the cases discussed hereafter, this type of molecular machine consists of a part of molecule that performs an axial rotation around an axis that is a single covalent bond. 

Another example of covalent-ionic resonance is provided by VB studies of N-N dimers of HNO [61]. It has been calculated that the barrier to rotation around an N-N bond involves a substantial contribution from three-electron bond... 

Current methods take root in the early 1960s, when the conformational analysis of macromolecules became of general interest [29-30]. Anderson et al. [31] used model building and X-ray diffraction studies to determine the double helical structures of polysaccharides using crystalline structure data as an initial set of coordinates followed by computational sampling of new structures by rotation around selected covalent bonds. The details of these so-called hard-sphere calculations are described in Rees and Skerrett [32] and Rees and Smith [33]. This approach was also applied to carbohydrate conformations in the analysis of bacteria and polysaccharidic structures and linkages [34-35]. 

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