LOW-TEMPERATURE INTERMOLECULAR BALANCES
IN THE GAS PHASE
Martin A. Suhm
InstitutfürPhysikalischeChemie, Georg-August-UniversitätGöttingen, Tammannstr. 6, 37077 Göttingen, Germany
The accurate experimental determination of intermolecular binding energies between organic molecules remains a major challenge. This is an important challenge, because theoretical models for biochemical docking preferences need to be tested for small prototype systems, if they are to be trusted in their predictions without relying on fortuitous error compensation. Cold molecular complexes allow for a focus on enthalpic rather than entropic aspects. If the molecules offer various binding sites with sufficiently different spectroscopic signature, the relative abundance of isomeric complexes at low temperature can provide an energy ranking of docking preferences. If the interconversion barriers between binding sites are low, supersonic jet cooling can achieve conformational temperatures well below 100 K. If the molecules are tuned by chemical substitution to invert their docking preference, one can thus hope for a rather sharp switch in docking conformation, with an energy resolution on the order of 1 kJ/mol.
We recently found that anisole fulfills all these requirements with respect to the docking of a protic solvent molecule like methanol to its ether oxygen or its polarizable π system [1]. By aromatic ring substitution, we were able to tune the docking preference from almost exclusively oxygen (in the parent compound) to mostly π (in heavily alkylated anisoles). The transition is so sharp that different quantum chemical methods and protocols can be rigorously tested with respect to their ability to reproduce the conformational switch [2]. This new kind of intermolecular balance experiment using FTIR spectroscopy will be described and extended to other O/π competition situations in the gas phase. The concept is reminiscent of room temperature solution phase intramolecular balance experiments based on NMR spectroscopy [3], which are closer to equilibrium but less accessible to accurate modeling.
[1] Heger M, Altnöder J, Poblotzki A,Suhm MA, Phys. Chem. Chem. Phys. 17 (2015) 13045-13052
[2] Gottschalk HC, Altnöder J, Heger M, Suhm MA, Angew. Chem. Int. Ed. 55 (2016) 1921-1924
[3] Yang L, Adam C, Nichol GS, Cockroft SL, Nat. Chem. 5 (2013) 1006 – 1010.