Aromatic embedding wins over classical hydrogen bonding – a multi-spectroscopic approach for the diphenyl ether–methanol complex

Abstract

Dispersion interactions are omnipresent in intermolecular interactions, but their respective contributions are difficult to predict. Aromatic ethers offer competing docking sites for alcohols: the ether oxygen as a well known hydrogen bond acceptor, but also the aromatic π system. The interaction with two aromatic moieties in diphenyl ether can tip the balance towards π binding. We use a multi-spectroscopic approach to study the molecular recognition, the structure and internal dynamics of the diphenyl ether-methanol complex, employing infrared, infrared-ultraviolet and microwave spectroscopy. We find that the conformer with the hydroxy group of the alcohol binding to one aromatic π cloud and being coordinated by an aromatic C-H bond of the other phenyl group is preferred. Depending on the expansion conditions in the supersonic jet, we observe a second conformer, which exhibits a hydrogen bond to the ether oxygen and is higher in energy.

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

“…Table S3 (ESI †) summarizes the results for the three lowest-energy isomers of the DPE-t-BuOH complex (OH-O, OH-p1, and OH-p2) in comparison to the two low-energy isomers of the DPE-MeOH complex (OH-O and OH-p). 6 Generally, there is a good agreement with the more sophisticated DFT-SAPT calculations described above as similar trends are observed (cf. Tables S3 and S5, ESI †): with both approaches the stabilizing effect of the electrostatic contribution E (1) el is about 5 kJ mol À1 larger for the OH-O isomer than for OH-p1 or OH-p2, which most probably originates from the additional OH-O hydrogen bond.…”

“…In a multi-spectroscopic analysis the complex of DPE with methanol (MeOH) has been investigated in the electronic ground state (S 0 ) by our groups (Suhm, Schnell, Gerhards). 6 Within that study, the OH-p structure was found to dominate, in agreement with theoretical predictions. The combined knowledge of vibrational frequencies as well as of the rotational constants allowed a TU Kaiserslautern, Fachbereich Chemie & Research Center Optimas, us to unambiguously determine the structure of the DPE-MeOH complex and to provide a valuable dataset for benchmarking theoretical calculations.…”

“…Relative energies of each pair of dihedral angles can be obtained from the calculated potential energy surface for DPE in ref. 6, where a scan of the two dihedral angles of DPE around the two O-C Ph bonds was performed.…”

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.…”

“…10 Otherwise, they might provide information about the docking site from the shift of the electronic spectrum upon complexation. Microwave spectroscopy would be very attractive as it can provide direct structural evidence for the docking site and prove the coexistence of C-/O-docking in a supersonic jet, [11][12][13][14][15] but a quantification of the relative abundance is more challenging. Raman spectroscopy in supersonic jets could provide a higher relative sensitivity for the C-bonded complex and better spatial resolution of the concentration evolution, 16 but the computed scattering cross sections may be somewhat less reliable than IR absorption cross sections.…”