Torsional profiles

In order to achieve a force field that will provide sufficiently accurate results, the addition of a dihedral potential that adequately describes the complex torsional profile is required. This should allow vdW parameters, especially well-depths, to remain at reasonable magnitudes and at values that reproduce the intermolecular distances found in the solid state. To achieve this, a six-term cosine potential of the form... 

A force field for solid state modeling of fluoropolymers predicted a suitable helical conformation but required further improvement in describing intermole-cular effects. Though victory cannot yet be declared, the derived force fields improve substantially on those previously available. Preliminary molecular dynamics simulations with the interim force field indicate that modeling of PTFE chain behavior can now be done in an all-inclusive manner instead of the piecemeal focus on isolated motions and defects required previously. Further refinement of the force field with a backbone dihedral term capable of reproducing the complex torsional profile of perfluorocarbons has provided a parameterization that promises both qualitative and quantitative modeling of fluoropolymer behavior in the near future. 

Figure 4.4 Torsional profiles for selected molecules in the gas phase and different solutions.

Fig. 14. (a) Potential energy profile of 67 in the excited state with respect to the torsional angle about the sp-sp2 single bond. Note that the excited state has a much steeper torsional profile than the ground state, (b) Expected time-dependence of the emission in a system for which quadratic coupling is significant. 

Figure 28-12 A plot of Equation 28-20 with 1/i = 16 = 0. and Vi is a positive number. Note that this curve has threefold symmetry. The third term in Equation 28-20 is used to reproduce ethane-like torsion profiles about single bonds.

However, energies are just as crucia 1, and these are sometimes harder to come by. For hydrocarbons and simpler organics, there is a large database of heats of formation, and, hence, strain energies, and these are valuable in parameterization. Other energies include rotation barriers and conformational differences. A competent force field should reproduce the butane torsional profile of Figure 2.6, and should obtain the A values for many cyclohexane substituents. Due to the similarity to real molecular vibrational modes, IR vibrations should be a valuable source for a force field, but in practice few modern force fields use them in their parameterization. 

From molecular mechanistic point of view, the computed torsional profiles are sufficiently invariant with respect to the molecular environment and could be utilized for generation of torsional parameters for the rotation about the C-O bond in ethers. 

---