Reliable Predictions on the Dynamics and Energetics of Internal Rotation Motions by ab initio Theory

The phenomenon of molecular internal motions (or torsional motions) has been a subject of considerable interest to both chemists and physicists for many years. Studies of torsional potential energy surface, barrier to internal rotations and stability of rotation isomers all provide basic information for understanding the origin of barriers and the forces responsible for conformational preference and structural properties. In this project, the researchers propose to carry out theoretical studies on the dynamics and energetics of single and coupled internal rotations for a series of molecules: haloethanes, hydrocarbon radicals and their cations as well as dipeptides. They will obtain the torsional potential energy surfaces for different halogen-substituted ethanes and solve for the torsional energy levels and the corresponding wavefunctions. The researchers intend to study the effects of the substituents on the internal rotation motions of the haloethanes. The second part is to predict how differences between the torsional potential in a hydrocarbon neutral/radical and its cation can influence the photoionization spectrum of a molecule or radical. Together with the information obtained from torsional analyses, the researchers examine:).how the intramolecular interactions and molecular conformations affect the energetic profiles of the torsional motions; andthe dynamical changes in torsional energies and rotation barriers as well as the conformational changes associated with the torsional motions during the ionization process of the hydrocarbon radicals.The last part of this project aims at performing torsional analyses on the coupled internal rotations of the amine and carboxylic moieties in the dipeptide molecules. The researchers determine the potential energy functions for the coupled motions at the advanced CCSD(T) level and compare the internal rotations barriers of different dipeptides. The torsional potential energy functions determined here are invaluable information to developing the molecular mechanics force field parameterization database for peptides and proteins. Finally, in order to carry out the above proposed work on torsional analyses, the researchers develop and implement a set of MathCad programs specifically for theoretical predictions of the torsional energy levels, wavefunctions and thus the frequencies. The programs are applicable to the torsional analyses for molecules with a single or coupled torsion mode(s).