Local bond flips

Quasar not only considers one conformer per molecule but also represents each molecule by an ensemble of conformers in different orientations and protonation states (called fourth dimension [21]), thereby reducing the bias associated with the choice of the bioactive conformation. The fifth dimension refers to the possibility to consider an ensemble of different induced-fit models [22] and the sixth dimension allows for the simulation of local induced H-bond flip-flop and various solvation effects [23]. 

The dipole associated with the maleimide unit, which is located in the plane of the maleimide cycle and perpendicular to the C - C bond taking part to the chain backbone. The motion of this resultant dipole implies a rotation of the rigid maleimide unit around the local chain axis. This structure is, in some way, analogous to that encountered with iso-phthalic units in aryl-aliphatic polyamides (Sect. 6) where the zso-phthalic rings only undergo small amplitude oscillations but no flips at temperatures below the glass transition temperature. Thus, it is unlikely that the maleimide dipole could be involved in the transition motions of CMIMx copolymers. 

At first, the bulkiness of the CMI group creates sites with a lower local density, in which ester group 7r-flips with a smaller activation energy occur. This effect gradually disappears for MMA units located farther away from the CMI group along the chain, in such a way that the pure PMMA behaviour is recovered for MMA units separated from CMI cycle by five or more C - C bonds. 

The same local update algorithm can be applied to systems with longer-range interactions and with coupling constants that vary from bond to bond. For more complex classical models, such as Heisenberg models, local updates will no longer consist of simple spin flips, but of arbitrary rotations of the local spin vectors. 

In this section, the flip motion of the phenylene around the bonding axis is used as an example for the naturally abundant C CSA analysis. When the local field of the H nuclei is averaged out to zero by the dipolar decoupling, the Hamiltonian H of the system is given by... 

Because of their two-fold symmetry, the phenolic side-chains of Tyr can execute a 7r-flip motion about the Cp-Cy bond between two orientations of locally equal energy. Generally, the effect of molecular motion reduces the quadrupole coupling to a time-averaged value which is smaller than the rigid lattice constant. Thus, the small inner doublet with a splitting of 30 kHz, which is observed both for B. mori and S.c. ricini silk fibroin, is attributed to a fast... 

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