A collaboration between the University of Warwick, Technische Universität and ISIS has used inelastic neutron spectroscopy (INS) and density functional theory (DFT) to study the effect of vibrations on a molecule that acts like a molecular machine.
In areas such as catalysis and molecular machines, people often study the energy barriers associated with the reactions or movements of the molecules involved, but there are next-to-no examples where people look at the effect of vibrational energy.
In general, C-H bonds are very stable, and hard to activate to encourage them to react. This activation is possible in some metal organic complexes, where the metal causes polarisation in the C‑H bond.
In this study, published in Journal of Physical Chemistry Letters, the group studied a metal organic complex which acts like a molecular machine. The molecule has a Cp* ligand that rotates like a little helicopter blade, bringing the –CH3 groups on the Cp* close to the Rh-OH/D centre one at a time, leading to bond activation and proton abstraction.
This movement causes the Cp* ligand to progressively self-deuterate when in the presence of a deuterated solvent. Although electronic effects explain some of the features of the reaction pathway for these deuteriation reactions, the group were able to identify the vibrations relevant for this carousel movement and proximal positioning.
They were able to identify these vibrational modes thanks to the unique properties of inelastic neutron spectroscopy when compared to infra-red or Raman: the lack of selection rules means that all modes are visible.
In addition to this, the ability of neutrons to distinguish between hydrogen and deuterium allowed them to carry out contrast measurements to distinguish vibrations from different molecular groups. After using INS to obtain a complete map of the vibrational modes, they used DFT to assign and validate them.
They then investigated how these vibrations influence the energy pathway of deprotonation, and found that the effect of the vibrations on the associated energy barriers is responsible for over a 400-fold increase in the rate of operation/reaction.
This finding could be particularly useful when designing molecular machines as, if vibrational considerations could be used to lower the energy barriers associated with their movements, then their rate of operation could be greatly increased.
As well as applications as molecular machines, this class of compounds have also attracted interest for their anticancer activity and ability to act as transfer hydrogenation catalysts.
Jeff Armstrong, ISIS beamline scientist adds; “The ideas presented in the paper, I believe, have applications in a huge amount of other systems and can be used to provide design principles for new types of molecular machines."
The full paper can be found onine at DOI: 10.1021/acs.jpclett.0c03292