Aromaticity is a ubiquitous yet elusive concept in chemistry and chemists have spent a great deal of effort on developing methods to quantify and visualise aromaticity. One particularly popular method is the nucleus independent shift (NICS), which can be seen as a virtual NMR experiment carried out within a conjugated ring to evaluate the enhanced chemical shielding induced by aromatic ring-currents. Strikingly NICS also allows to quantify antiaromaticity, as this induces a net deshielding effect within the ring. NICS provides a powerful quantitative aromaticity criterion but the main challenge for its graphical representation is that the chemical shielding is a 3×3 tensor, which is difficult to visualise with the existing methods.
Therefore, we have developed a new method for the visualisation of chemical shielding tensors (VIST), which provides a local representation of the shielding tensor along with the molecular structure. The method, thus, allows to probe local aromaticity along with the underlying anisotropy of the shielding. The method is described in the preprint “3D Visualisation of chemical shielding tensors to elucidate aromaticity and antiaromaticity” available on ChemRxiv.
Within the preprent we exemplify the main concepts in the benzene and phenanthrene molecules and continue by studying
the interplay of ground state antiaromaticity and Baird triplet state aromaticity in the potential singlet fission chromophore cyclobuta[l]phenanthrene,
local aromaticity in the neutral formally antiaromatic ground state along with global aromaticity in the doubly reduced state of paaracyclophanetetraene,
Aromaticity, despite its ubiquity in the discussions, is still surprisingly hard to visualise and quantify. We will endeavour to compare the different available techniques – nucleus-independent chemical shifts, current density plots, and the GIMIC method – with the goal of identifying the most promising ones and streamlining the workflows. In particular, we are interested in 1D, 2D, or even 3D scans of NICS values as inspired by a recent paper on excimers.
A new release of Columbus (version 7.0.1) is available. The main improvement compared to previous releases is that the binary distribution now contains a pre-compiled parallel CI executable as well as an interface to Molcas.
The parallel CI executable is linked against the Intel MPI libraries, which should be available on most computing centres and can otherwise be downloaded freely from Intel.
The Molcas interface proceeds via the free Molcas@UU distribution. Note: At the moment, there is no interface to OpenMolcas available in the distribution.
Source and binaries as well as more detailed instructions are available via the usual download page (register here). Let me know about any successes or problems regarding these features.
A new study led by C. Heshmatpour and J. Hauer from TU München studies exciton-exciton annihilation in a squaraine trimer. The experiment exploits 5th-order optical spectroscopy to study the evolution of the trimer after two-photon excitation into its bi-exciton state. Quantum chemistry computations performed by M. Menger, now located at Groningen, provide the required parameters to model the experimental signals within a Frenkel exciton model. The associated article Annihilation Dynamics of Molecular Excitons Measured at a Single Perturbative Excitation Energy just appeared in J. Phys. Chem. Lett.
Quantum chemical computations were used to aid in the assignment of the structures produced and characterised via infrared multiple photon dissociation spectroscopy. An interactive model showing the relevant molecular vibrations can be found here.
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