Bond dissociation energies (BDEs) can give quick insight into which bonds are the most easily cleaved by UV or degradants. This is important for the design of everything from drugs to OLEDs, where the long-term stability can be important for the effectiveness.
Modeling BDEs can be done easily with Rowan's suite of computational tools. There are two types of BDE calculations: static and dynamic. Static BDEs provide a simple comparison of the molecule before and after breaking a bond, without any relaxation of the broken molecule, and are good for a quick screening of energies to narrow down which bonds should be targeted for further investigation. Dynamic BDEs require an optimization of the molecule after the bond is broken, showing how the relaxation effects can lower the reaction barrier. However, for molecules with large amounts of conformational flexibility, this can lead to anomalous results due to rearrangement of unrelated parts of the molecule.
To run a BDE computation, first optimize the molecule, optionally running the conformational search workflow first if there is any conformational ambiguity. Then remove the target atom/fragment with the molecule editor. If a static BDE is desired, run a single point energy computation on the individual fragments. If a dynamic BDE is desired, optimize the individual fragments. Then subtract the initial energy from the sum of the fragment energies to obtain the BDE. Compare this BDE to the BDEs of other bonds in the molecule or other molecules.
The bonds which are the weakest are the most likely sites for degradation reactions, and a variety of steps may be taken to increase the stability. One common technique is to swap hydrogens for deuteriums, improving the metabolic properties of pharmaceuticals and increasing the working lifetimes of OLEDs due to the kinetic isotope effect. Other substitutions may include the addition of electron-withdrawing/-donating group or substitution of a hydrogen with a methyl group if space allows.
