Microsolvation in V⁺(H₂O)_n Clusters Studied with Selected-Ion Infrared Spectroscopy

Abstract

Gas-phase ion–molecule clusters of the form V+(H2O) n (n = 1–30) are produced by laser vaporization in a supersonic expansion. These ions are analyzed and mass-selected with a time-of-flight mass spectrometer and investigated with infrared laser photodissociation spectroscopy. The small clusters (n ≤ 7) are studied with argon tagging, while the larger clusters are studied via the elimination of water molecules. The vibrational spectra for the small clusters include only free O–H stretching vibrations, while larger clusters exhibit redshifted hydrogen bonding vibrations. The spectral patterns reveal that the coordination around V+ ions is completed with four water molecules. A symmetric square-planar structure forms for the n = 4 ion, and this becomes the core ion in larger structures. Clusters up to n = 8 have mostly two-dimensional structures, but hydrogen bonding networks evolve to three-dimensional structures in larger clusters. The free O–H vibration of acceptor–acceptor–donor (AAD)-coordinated surface molecules converges to a frequency near that of bulk water by the cluster size of n = 30. However, the splitting of this vibration for AAD- versus AD-coordinated molecules is still different compared to other singly charged or doubly charged cation–water clusters. This indicates that cation identity and charge-site location in the cluster can produce discernable spectral differences for clusters in this size range.

Photochemistry and UV/vis spectroscopy of hydrated vanadium cations, V+(H2O)n, n = 1–41, a model system for photochemical hydrogen evolution

“…The measured photodissociation cross sections for n = 1-4 are shown in Figure 1. As shown in previous studies, 33,35,41 35,41 For n = 3, the T-shaped isomer reported by Ohashi and coworkers 35 is within error limits isoenergetic with the Y-shaped structure from Figure 1, and exhibits a small imaginary frequency in our calculations. This indicates that the position of the oxygen atoms is not very well defined for this cluster size and that the water molecules undergo large-amplitude motions even at low temperatures in the coordination plane of the metal center for n = 3.…”

“…This observation is in line with earlier infrared spectroscopy that the first solvation shell is fully occupied with four water molecules. 35,41 Further hydration in the second solvation shell does not significantly affect the electronic structure of the V + ion in the core and mostly broadens the bands.…”

“…40 Duncan and co-workers also studied V + (H 2 O) n , n ≤ 30, by infrared photodissociation spectroscopy and found a transition from twoto three-dimensional structures for n > 8. 41 We showed that large clusters V + (H 2 O) n , n > 10, are unreactive against a series of small neutral molecules, including O 2 , CO 2 , N 2 O, NO and C 3 H 7 I. [42][43][44] Important and in part conflicting results were obtained in guided ion beam studies of the V + + H 2 O/D 2 O reaction.…”

Microsolvation of Zn cations: infrared multiple photon dissociation spectroscopy of Zn+(H2O)n (n = 2–35)

“…72 The most red-shifted band is also present for clusters n 4 4, centred at 3220 cm À1 for n = 7, and even in the largest cluster with n = 35, a slightly blue-shifted maximum at 3230 cm À1 can be identified in an overall broad absorption band that is typical for water clusters. The shift to the red, as outlined in many previous publications, 3,[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][73][74][75] is due to the metal-induced polarisation on the water molecules, which removes electron density from bonding orbitals on the O-H bond, causing an observed red-shift. As discussed previously for Zn + (H 2 O) n complexes, 43 the polarizing effect of Zn + , when compared to other singly-charged metal ions, 3,[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][73][74]…”

Asymmetric Solvation of the Zinc Dimer Cation Revealed by Infrared Multiple Photon Dissociation Spectroscopy of Zn2+(H2O)n (n = 1–20)

“…Our group has an extended history in studying these systems [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51]. Infrared action spectroscopy, coupled with quantum chemical calculations, is a powerful technique used to investigate the structures of gas-phase molecular complexes, [52][53][54][55][56][57][58][59][60] in particular hydrated metal ions [61][62][63][64][65][66][67][68][69]. We recently published a study investigating hydrated zinc monomer ions Zn + (H 2 O) n with up to 35 water molecules, [70] where we found evidence for surface solvation of Zn + , which was previously established by Duncan and co-workers for n ≤ 4 [71].…”