Control over the Hydrogen‐Bond Docking Site in Anisole by Ring Methylation

Abstract

The supramolecular docking of methanol to anisole may occur via an OH⋅⋅⋅O hydrogen bond or via an OH⋅⋅⋅π contact. The subtle balance between these two structures can be varied in supersonic jets by one order of magnitude through single to triple methylation of the aromatic ring and introduction of a single tert-butyl substituent, as evidenced by infrared spectroscopy. This steep variation makes it possible to assess the accuracy of relative quantum-chemical energy predictions on a kJ mol(-1) level, promising insights into inductive, mesomeric, and dispersive effects. The zero-point-corrected B3LYP-D3/aVTZ level is shown to provide an accurate relative description of the two very different hydrogen bonds, similar to a wavefunction-based protocol including CCSD(T) corrections applied to the same structures. M06-2X alone systematically overestimates the stability of π coordination.

Multi-spectroscopic and theoretical analyses on the diphenyl ether–tert-butyl alcohol complex in the electronic ground and electronically excited state

“…This assignment is based on a blue-shift of the S 1 ' S 0 transition compared to the bare 9-methoxyanthracene with small deviations occurring due to varying dispersion interactions between the alkyl chain of the alcohol and the aromatic moiety. For the related anisole case, 11 aromatic substitution allowed to tip the balance towards OH-p coordination, 7 as evidenced by FTIR spectroscopy.…”

“…As the OH-O-and OH-p-bound isomers of this complex are close in energy it seems reasonable that the balance between OH-O and OH-p can easily be modified by subtle structural changes 7 such as altering the alcohol.…”

The furan microsolvation blind challenge for quantum chemical methods: First steps

“…However, in particular, the observation of two Cdocking complexes would be inconsistent with previous findings for similar complexes. [4][5][6]14 Also, it would be difficult to explain why methylation of one or two C atoms in the ring would leave the pattern of bands so unaffected. Furthermore, Ar relaxation experiments without Ar coating carried out for Fu should have resulted in further relaxation.…”

“…In contrast, the case of substituted anisoles has shown that the narrow barriers between C-and O-docking can be sufficiently pronounced to stabilize two spectroscopically identifiable species. 6 If the limiting interconversion barriers between the two sites are shallow and narrow enough, one may approximate the ensemble of complexes in a jet expansion by a low (20-100 K) effective conformational temperature at which the interconversion freezes, together with always lower (≈10 K) rotational temperatures and uncertain as well as non-uniform (50-200 K) vibrational temperatures of individual normal modes. The lower the conformational freezing temperature is, the more sensitive the experimental switch between O-and C-docking will react to the energy difference of the competing sites.…”

“…The lower the conformational freezing temperature is, the more sensitive the experimental switch between O-and C-docking will react to the energy difference of the competing sites. Therefore, such low barrier isomerism may serve as an experimental docking energy balance, 6 which helps to decide which of the competing conformers is lower in energy, in favorable cases down to a sub-kJ/mol energy scale. 7 These experimental C/O-docking ratios (at a very low but still unknown conformational temperature) may then be compared to quantum chemical predictions of the energy difference and help to judge the performance of the employed computational protocol.…”