Electronic and vibrational spectra of protonated benzaldehyde-water clusters, [BZ-(H2O)n≤5] H+: Evidence for ground-state proton transfer to solvent for n ≥ 3

Vibrational and electronic photodissociation spectra of mass-selected protonated benzaldehyde(water)(n) clusters, [BZ-(H2O)n] H+ with n <= 5, are analyzed by quantum chemical calculations to determine the protonation site in the ground electronic state (S-0) and pi pi* excited state (S-1) as a function of microhydration. IR spectra of [BZ-(H2O)(n)] H+ with n <= 2 are consistent with BZH(+)-(H2O)(n) type structures, in which the excess proton is localized on benzaldehyde. IR spectra of clusters with n >= 3 are assigned to structures, in which the excess proton is located on the (H2O)(n) solvent moiety, BZ-(H2O)(n)H+. Quantum chemical calculations at the B3LYP, MP2, and ri-CC2 levels support the conclusion of proton transfer from BZH(+) to the solvent moiety in the S-0 state for hydration sizes larger than the critical value n(c) = 3. The vibronic spectrum of the S-1 (<-) S-0 transition (pi pi*) of the n = 1 cluster is consistent with a cis-BZH(+) -H2O structure in both electronic states. The large blueshift of the S-1 origin by 2106 cm(-1) upon hydration with a single H2O ligand indicates that the proton affinity of BZ is substantially increased upon S-1 excitation, thus strongly destabilizing the hydrogen bond to the solvent. The adiabatic S-1 excitation energy and vibronic structure calculated at the ri-CC2/aug-cc-pVDZ level agrees well with the measured spectrum, supporting the notion of a cis-BZH(+) -H2O geometry. The doubly hydrated species, cis-BZH(+) -H2O)(2), does not absorb in the spectral range of 23 000-27 400 cm(-1), because of the additional large blueshift of the pi pi* transition upon attachment of the second H2O molecule. Calculations predict roughly linear and large incremental blueshifts for the pp* transition in [BZ-(H2O)(n)]H+ as a function of n. In the size range n >= 3, the calculations predict a proton transfer from the (H2O)(n)H+ solvent back to the BZ solute upon electronic pi pi* excitation. (c) 2014 AIP Publishing LLC.