Finding the vibrations that matter
Molecules respond to stimuli in complicated ways. The study of molecules in motion, known as chemical dynamics, often requires reduction of the problem at hand to more simple models.
Chemical dynamics often relies on how the energy of a molecule or molecules changes as they change configuration (e.g. as the molecule vibrates). Therefore, it is common to try and build an understanding of chemical dynamics mathematically by calculating the so-called “Potential Energy Surface” (PES) that defines the system being studied. However, the number of variables that define the PES of a molecule containing N atoms is 3N-5 for linear molecules and 3N-6 for non-linear molecules. This means that the number of variables increases quite rapidly as the systems become bigger. In practice, the PES is constructed through a series of calculations at points along the variables that define it and along all combinations of variables. For a non-linear system of 20 atoms, if we were to calculate only 10 points along each dimension, this would mean performing 27,540 calculations. Moreover, an extra atom will lead to an increase in the number of calculations by a factor of one thousand! Therefore, when investigating chemical dynamics problems, identifying the important variables can save a lot of effort.
In a recent paper in Nature Communications, the Meijer and Weinstein groups in collaboration with the group of Eric Bittner, of the University of Houston (TX), utilise a method to help identify the important vibrational modes for electron-transfer reactions in a family of Pt complexes, which were reported on in an earlier Nature Chemistry paper by the Meijer and Weinstein groups. These complexes are of particular note since electron-transfer in these systems can be influenced by selective vibrational excitation. The vibrations identified in the paper are consistent with the vibrations that were found to have an effect in these previous experimental and theoretical studies, highlighting the accuracy of the new technique. In the future, these methods could be used predictively to identify candidate vibrations for similar vibrational-control phenomena.
The full article by Xunmo Yang, Theo Keane, Milan Delor, Anthony J. H. M. Meijer, Julia Weinstein & Eric R. Bittner can be found here: Identifying electron transfer coordinates in donor-bridge-acceptor systems using mode projection analysis