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Through Ring Umbrella Inversion of a Molecular Rattle

Bidisa Das and K. L. Sebastian*

Inorganic and Physical Chemistry, Indian Institute of Science,
Bangalore, Karnataka 560012 INDIA

This is an abstract for a presentation given at the
10th Foresight Conference on Molecular Nanotechnology


Molecules, which can perform any kind of interesting motion, have drawn a lot of attention recently, in connection with nanotechnology. Here we suggest the design of a molecular rattle, which can perform an 'umbrella inversion' kind of motion (see figure).

We report theoretical investigations on some [Ring]-Li+ compounds, which can exhibit a through ring umbrella inversion. Our obvious choices were the molecules, which become aromatic in anionic form. Since we are looking for a through ring motion, trials with small sized rings were unsuccessful due the high activation barrier associated with the process. With cis-cyclononatetraenyllithium, the energy barrier for the Li+ to pass through the ring is small, so that it should be possible to observe this experimentally. The stable equilibrium geometry is the one where Li+ is sitting symmetrically above the ring (point group C9v). For through ring motion, the transition state is planar with Li+ occupying the center of the nine membered ring (D9h).

Our studies predict cyclononatetraenylithium to be molecular rattle, in which umbrella inversions can occur. The potential energy for the umbrella inversion is a double well, with an activation barrier of 11.89 kcal/mol. The inversion rate was estimated to be 556 sec-1 at 298°K. Assuming that the trough ring motion can be taken to be one dimensional, we solved the Schrodinger equation numerically to obtain the energy levels. We find that the Li+ should go through the ring easily by an excitation to v = 12 vibrational level.

The calculations were done using GAUSSIAN 94 software using B3LYP hybrid-correlation-functional with 6-311G** basis-sets.

larger image for print

The top three figures are schematic representation of the 'umbrella inversion' and the lower three figures show the same for cis-cyclononatetraenyl lithium molecule.

  1. ground state with Li+ above the planer ring,
  2. transition state with Li+ at the center of the ring,
  3. other ground state when Li+ has passed through the ring.

Abstract in Microsoft Word® format 99,142 bytes

*Corresponding Address:
K. L. Sebastian
Inorganic and Physical Chemistry, Indian Institute of Science
Bangalore, Karnataka 560012 INDIA
Phone: 091-080-309-2385 Fax: 091-080-360-1552


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