Self-Association and Microhydration of Phenol: Identification of Large-Amplitude Hydrogen Bond Librational Modes

Abstract

The self-association mechanisms of phenol have represented long-standing challenges to quantum chemical methodologies owing to the competition between strongly directional intermolecular hydrogen bonding, weaker non-directional London dispersion forces and C-H⋯π interactions between the aromatic rings. The present work explores these subtle self-association mechanisms of relevance for biological molecular recognition processes via spectroscopic observations of large-amplitude hydrogen bond librational modes of phenol cluster molecules embedded in inert neon "quantum" matrices complemented by domain-based local pair natural orbital-coupled cluster DLPNO-CCSD(T) theory. The spectral signatures confirm a primarily intermolecular O-H⋯H hydrogen-bonded structure of the phenol dimer strengthened further by cooperative contributions from inter-ring London dispersion forces as supported by DLPNO-based local energy decomposition (LED) predictions. In the same way, the hydrogen bond librational bands observed for the trimeric cluster molecule confirm a pseudo-C₃ symmetric cyclic cooperative hydrogen-bonded barrel-like potential energy minimum structure. This structure is vastly different from the sterically favored "chair" conformations observed for aliphatic alcohol cluster molecules of the same size owing to the additional stabilizing London dispersion forces and C-H⋯π interactions between the aromatic rings. The hydrogen bond librational transition observed for the phenol monohydrate finally confirms that phenol acts as a hydrogen bond donor to water in contrast to the hydrogen bond acceptor role observed for aliphatic alcohols.

Keywords: London dispersion forces; hydrogen bonding; large-amplitude librational motion; local energy decomposition; neon matrices; phenol cluster molecules; vibrational spectroscopy.

Figure 1

The infrared absorption spectra of phenol embedded in solid neon at 4 K. In the two different experiments shown, the phenol source sublimation vessel was thermostated at 0 °C (bottom) and at −5 °C (top and bottom, respectively) during deposition to achieve different mixing ratios with neon. The spectra of the 0 °C experiment were recorded before (blue trace) and after annealing of the matrix at 9.5 K (red trace). The spectra have been normalized to the phenol monomer transitions in the 650–575 cm⁻¹ range. The experimentally observed transitions associated with the intramolecular OH-stretching and the large-amplitude hydrogen bond librational modes of (PhOH)₂ and (PhOH)₃ are indicated with the respective band origins.

Figure 2

(a) The spectral dependence of the phenol/neon mixing ratio by spatial spot probing. The three shown spectra collected for spots with increasing phenol/ratio (the blue and red traces collected for the lowest and highest phenol/neon mixing ratios, respectively) have been normalized to the monomer transitions. The proposed dimer and trimer absorption features are differentiated based on their different growth rates relative to the monomer bands. (b) The evolution of the neon matrix spectra over time during annealing at 9.5 K. A series of difference spectra (annealing spectrum subtracted the pre-annealing spectrum) collected at 3 min intervals during the annealing procedure is shown. The colored difference spectra (the blue trace collected after 3 min and the red trace collected after 15 min, respectively) show excess absorption relative to the pre-annealing spectrum (black trace) demonstrating the progressive formation of phenol cluster molecules.

Figure 3

The optimized potential energy minima structures of the pure phenol cluster molecules (a) (PhOH)₂ (MP2/AVQZ level), (b) (PhOH)₂ (PW6B95-D4/ma-def2-QZVP level) and (c–f) the four different conformations of (PhOH)₃. The relative zero-point energy corrected dissociation energies D₀ (the change of zero-point energy ($\Delta$ZPE) calculated at PW6B95-D4/ma-def2-QZVP//SCS-MP2/AVQZ levels with electronic energies obtained at the DLPNO-CCSD(T)/AVQZ level, both values in kJ·mol⁻¹) are given in brackets for each conformation.

Figure 4

The simulated vibrational spectra of PhOH, (PhOH)₂ and the predicted conformations of (PhOH)₃ using the DFT (PW6B95-D4) and SCS-MP2 methodologies. Harmonic vibrational mode frequencies have been scaled separately using a scaling factor of 0.95 for the large-amplitude hydrogen bond librational modes and 0.97 for less perturbed intramolecular transitions. The predicted large-amplitude hydrogen bond librational transitions are marked with filled areas. The striped area indicates spectral overlap between a librational transition and a less perturbed intramolecular transition. The band positions for the experimentally assigned librational transitions of (PhOH)₂ and (PhOH)₃ are indicated on the trace for the identified conformation. The experimental pre- and post-annealing spectra are provided above the simulations.

Figure 5

Animations of the large-amplitude hydrogen bond librational modes of (a) (PhOH)₂, (b,c) (PhOH)₃ and (d) PhOH·H₂O associated with the experimentally assigned absorption bands.

Figure 6

The infrared absorption spectra of phenol (black trace, small traces of H₂O), H₂O (purple trace) and D₂O (brown trace) together with spectra of phenol/H₂O (blue trace) and phenol/D₂O (green trace) mixtures. For the two mixtures the spectra after annealing of the matrix to 9.5 K (red and orange traces, respectively) are shown below 650 cm⁻¹. The new assigned transitions associated with the large-amplitude hydrogen bond librational modes of the PhOH·H₂O and PhOH·D₂O monohydrates are indicated in the spectra with their respective band positions.

Figure 7

The optimized global (a) and local (b) potential energy minima structures of the phenol monohydrate (PhOH·H₂O) employing the SCS-MP2/AVQZ level of theory.