Literature

Excited-state aromaticity in naphthalene

Felix

A recent JPCA article by Karadakov and Al-Yassiri highlights the differences in singlet and triplet aromaticity in naphthalene. To me this paper contains several striking observations:

  • The singlet HOMO/LUMO transition (S2, 1La) is shown to be strongly aromatic whereas the triplet HOMO/LUMO transition (T1, 3La) is antiaromatic. Does this mean states reached by the same kind of orbital transition behave differently depending on their spin-multiplicity?
  • The aromatic S2 lies above the antiaromatic S1 even though S2 is the HOMO/LUMO transition. Does this mean that singlet antiaromaticity is actually a stabilising effect?

We have discussed the excited states of naphthalene from an entirely different viewpoint in a recent J. Chem. Theory Comput. article. It would be fascinating to combine the two viewpoints.

Best-Practice DFT Protocols

Felix

I enjoyed this paper from the Grimme group:

Best-Practice DFT Protocols for Basic Molecular Computational Chemistry, Angew. Chem. Int. Ed., 2022, 61, e202205735.

The authors discuss the best modern methods for running a DFT computations – a guide through the jungle. For me the main conclusions regarding functionals are:

  • The composite method r2SCAN-3c offers great cost-benefit a ratio and we are starting to adopt it as a default method for ground-state optimisations.
    (This should be taken with a grain of salt, since it is the authors’ own method, but the arguments are sound.)
  • If you want to go beyond r2SCAN-3c, you have to try really really hard, as in range-separated hybrid meta-GGA with a triple/quadruple-zeta basis set. There is not really any reason to ever use double-zeta basis sets – better use a compound method.
  • B3LYP/6-31G* is outdated.