Theoretical Chemistry

Professor K. L. Sebastian works on applications of quantum mechanics and statistical mechanics in chemistry. The topics studied include molecular devices, nanotechnology and surfaces, molecular ratchets, equilibrium and non-equilibrium statistical mechanics of polymers, biophysical chemistry and chemical dynamics. Some of the recently studied problems are briefly described here.

Molecular and Nanomechanical Devices:

Can one design molecules that prefers to diffuse on a surface by rolling?  The question is of interest and importance in connection with nano-technology, where the dream is to design nano-motors that make use of chemical energy or nano-robots that can go inside a cell and perform useful work.  We have explored the possibility of having a molecule chemically bonded to the surface, and capable of moving on the surface by rolling, because of its fluxional nature. Thus, our theoretical studies predict hypostrophene, adsorbed on Al(100) would diffuse on the surface by rolling, rather than sliding – it is a molecular roller. We have studied cyclopentadienyl adsorbed on Ge and shown that it acts as a molecular wheel.

Molecular Rattle.

We have also investigated the dynamics of a mechanical two-level system (mechanical equivalent of the ‘bit’). The compression of a nano-rod or a rod like molecule would cause it to buckle. There are now two possible states, and the system is interesting, as a potential nano-sized device. We have developed a theoretical approach for the calculation of the rate of transitions between the two states, due to thermal fluctuations and tunnelling. Another interesting suggestion from our group is the umbrella inversion in a molecular rattle.  The molecule cyclononatetraenyl lithium (see figure) has a pyramidal geometry.  Our calculations show that it can undergo umbrella like inversions in which the lithium goes through the ring to the other side.  The barrier height was found to be about 12 kcal/mol. Appropriate vibrational excitation would allow the Li to go from one side of the ring to the other (see figure).

Statistical mechanics of polymers and Biophysical Chemistry:

How do long chain molecules cross a free energy barrier in space? The question is of interest in problems involving the passage of a long chain molecule through a pore in a membrane. We found that they can do this, rather efficiently, through a kink mechanism. Other related problems that we have studied are 
Molecular Rattle.
  1. Pulling out of a polymer from a potential well, which is of interest in connection with the mechanical unzipping of DNA
  2. Breaking of a polymer molecule under tension
  3. Forcing a long chain molecule out of a potential well, by the application of an electric field
  4. Packaging of a viral DNA in to its capsid, under the influence of an external force.

Other Problems

In addition to the above, we have been interested in other theoretical problems and these will only be mentioned here. They are
  1. Theory of electron transfer at interfaces
  2. Density functional theory and hardness
  3. Coupled cluster theory
  4. Path integrals

Representative Publications