Rigid group constraints

A rigid group is a number of atoms in a given spatial arrangement, for example five atoms on a perfect pentagon (Cp ligand) or five atoms forming a SO4 tetrahedron. 

Special position Constraints on coordinates Constraints on Ijy values Constraints on occupancies 

Inversion Centre X, y, z fixed to lie on inversion centre None 0.5 

Mirror plane J. to y y is fixed to lie on the mirror plane 1/12 ij23 0.5 

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First there are the already mentioned rigid group constraints. Rigid groups are defined and constrained with the AFix instruction, which, in combination with two numbers, m and n, describes the geometry and mathematical treatment of the... 

This route to cyclic ketones was suggested by the rigid group principal .2 As expected from this rule, dialdehydes such as I, because of constraint imposed by the triple bond, undergo cycloolefination more efficiently than saturated dialdehydes. 

Bead-spring models without explicit solvent have also been used to simulate bilayers [40,145,146] and Langmuir monolayers [148-152]. The amphi-philes are then forced into sheets by tethering the head groups to two-dimensional surfaces, either via a harmonic potential or via a rigid constraint. 

The nature of the covalent bonds in the polypeptide backbone places constraints on structure. The peptide bond has a partial doublebond character that keeps the entire six-atom peptide group in a rigid planar configuration. The N—C and Ca—C bonds can rotate to assume bond angles of (p and ip, respectively. 

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