Microsolvation of H 2 O + , H 3 O + , and CH 3 OH 2 + by He in a cryogenic ion trap: structure of solvation shells.

Due to the weak interactions of He atoms with neutral molecules and ions, the preparation of size-selected clusters for the spectroscopic characterization of their structures, energies, and large amplitude motions is a challenging task. Herein, we generate H 2 O + He n ( n ≤ 9) and H 3 O + He n ( n ≤ 5) clusters by stepwise addition of He atoms to mass-selected ions stored in a cryogenic 22-pole ion trap held at 5 K. The population of the clusters as a function of n provides insight into the structure of the first He solvation shell around these ions given by the anisotropy of the cation-He interaction potential. To rationalize the observed cluster size distributions, the structural, energetic, and vibrational properties of the clusters are characterized by ab initio calculations up to the CCSD(T)/aug-cc-pVTZ level. The cluster growth around both the open-shell H 2 O + and closed-shell H 3 O + ions begins by forming nearly linear and equivalent OH⋯He hydrogen bonds (H-bonds) leading to symmetric structures. The strength of these H-bonds decreases slightly with n due to noncooperative three-body induction forces and is weaker for H 3 O + than for H 2 O + due to both enhanced charge delocalization and reduced acidity of the OH protons. After filling all available H-bonded sites, addition of further He ligands around H 2 O + ( n = 3-4) occurs at the electrophilic singly occupied 2p z orbital of O leading to O⋯He p-bonds stabilized by induction and small charge transfer from H 2 O + to He. As this orbital is filled for H 3 O + , He atoms occupy in the n = 4-6 clusters positions between the H-bonded He atoms, leading to a slightly distorted regular hexagon ring for n = 6. Comparison between H 3 O + He n and CH 3 OH 2 + He n illustrates that CH 3 substitution substantially reduces the acidity of the OH protons, so that only clusters up to n = 2 can be observed. The structure of the solvation sub-shells is visible in both the binding energies and the predicted vibrational OH stretch and bend frequencies.