Structures and IR/UV spectra of neutral and ionic phenol-Ar-n cluster isomers (n <= 4): competition between hydrogen bonding and stacking

The structures, binding energies, and vibrational and electronic spectra of various isomers of neutral and ionic phenol-Ar-n clusters with n <= 4, PhOH(+)-Ar-n, are characterized by quantum chemical calculations. The properties in the neutral and ionic ground electronic states (S-0, D-0) are determined at the M06-2X/aug-cc-pVTZ level, whereas the S-1 excited state of the neutral species is investigated at the CC2/aug-cc-pVDZ level. The Ar complexation shifts calculated for the S-1 origin and the adiabatic ionisation potential, Delta S-1 and Delta IP, 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 Delta S-1 established recently from experimental spectra and enable thus a firm assignment of various S-1 origins to their respective isomers. A similar additivity model is newly developed for Delta IP using the M06-2X data. The isomer assignment is further confirmed by Franck-Condon simulations of the intermolecular vibrational structure of the S-1 <- S-0 transitions. In neutral PhOH-Ar-n, dispersion dominates the attraction and pi-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 pi interaction is somewhat stronger in the S-1 state due to enhanced dispersion forces. Similarly, the H-bond strength increases in S-1 due to the enhanced acidity of the OH proton. In the PhOH+-Ar-n cations, H-bonds are significantly stronger than pi-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 pi-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 Ar-2 dimer in a T-shaped fashion. This dimer core can also be further solvated by pi-bonded ligands leading to the solvation sequence (H00), (11)(H), (21)(H), and (22) for n <= 4. The implications of the ionisation-induced pi -> 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.