Conformational Flexibility of Manxane Revealed by Adiabatic Mapping, Normal-Mode Analysis, and Molecular-dynamics

We have investigated the relative stability of the conformational minima and the barriers and modes for interconversion of manxane (bicyclo[3.3.3]undecane) using a molecular mechanics approach. The study involved adiabatic mapping of the energy surface as a function of the torsion angles, normal mode analysis, and molecular dynamics simulations followed by digital signal analysis. Energy mapping revealed two conformational minima, C-3h and C-s. The energy surface has a single C-3h --> C-s transition state, in which the plane of symmetry is maintained. There are two types of C-s --> C-s transitions. One of these (II) involves a symmetric transition state, and the other (II') has two nonsymmetric transition states. The calculated overall energy barrier for C-3h --> G(3h) interconversion agreed with the observed value. Partitioning of the energy of the minimum energy conformations and the various transition states revealed the underlying causes for relative stability. The low-frequency modes of the two conformations, as obtained by normal mode (NM) analysis and by extracting characteristic modes of motion from the molecular dynamics (MD) trajectory, were very similar, with some differences due to the inclusion of anharmonicity in the MD treatment. The evolution of modes with time was monitored using sliding frequency distributions (SFD). The results highlighted the Importance of energy transfer between modes. Channeling of energy into one of the low-frequency, torsionally active, modes is a prerequisite for the occurrence of a conformational transition. The characteristic features of the active mode in the period preceding a conformational transition can determine the nature of the transition.