Structures and IR/UV spectra of neutral and ionic phenol–Arncluster isomers (n ≤ 4): competition between hydrogen bonding and stacking

Abstract

The structures, binding energies, and vibrational and electronic spectra of various isomers of neutral and ionic phenol–Ar_nclusters with n ≤ 4, PhOH⁽⁺⁾–Ar_n, are characterized by quantum chemical calculations. The properties in the neutral and ionic ground electronic states (S₀, D₀) are determined at the M06-2X/aug-cc-pVTZ level, whereas the S₁ excited state of the neutral species is investigated at the CC2/aug-cc-pVDZ level. The Ar complexation shifts calculated for the S₁ origin and the adiabatic ionisation potential, ΔS₁ and ΔIP, sensitively depend on the Ar positions and thus the sequence of filling the first Ar solvation shell. The calculated shifts confirm empirical additivity rules for ΔS₁ established recently from experimental spectra and enable thus a firm assignment of various S₁ origins to their respective isomers. A similar additivity model is newly developed for ΔIP using the M06-2X data. The isomer assignment is further confirmed by Franck–Condon simulations of the intermolecular vibrational structure of the S₁ ← S₀ transitions. In neutral PhOH–Ar_n, dispersion dominates the attraction and π-bonding is more stable than H-bonding. The solvation sequence of the most stable isomers is derived as (10), (11), (30), and (31) for n ≤ 4, where (km) denotes isomers with k and m Ar ligands binding above and below the aromatic plane, respectively. The π interaction is somewhat stronger in the S₁ state due to enhanced dispersion forces. Similarly, the H-bond strength increases in S₁ due to the enhanced acidity of the OH proton. In the PhOH⁺–Ar_n cations, H-bonds are significantly stronger than π-bonds due to additional induction forces. Consequently, one favourable solvation sequence is derived as (H00), (H10), (H20), and (H30) for n ≤ 4, where (Hkm) denotes isomers with one H-bound ligand and k and m π-bonded Ar ligands above and below the aromatic plane, respectively. Another low-energy solvation motif for n = 2 is denoted (11)^H and involves nonlinear bifurcated H-bonding to both equivalent Ar atoms in a C_2v structure in which the OH group points toward the midpoint of an Ar₂ dimer in a T-shaped fashion. This dimer core can also be further solvated by π-bonded ligands leading to the solvation sequence (H00), (11)^H, (21)^H, and (22) for n ≤ 4. The implications of the ionisation-induced π → H switch in the preferred interaction motif on the isomerisation and fragmentation processes of PhOH⁽⁺⁾–Ar_n are discussed in the light of the new structural and energetic cluster parameters.