Isolating the Contributions of Specific Network Sites to the Diffuse Vibrational Spectrum of Interfacial Water with Isotopomer-Selective Spectroscopy of Cold Clusters

Decoding the structural information
contained in the interfacial
vibrational spectrum of water requires understanding how the spectral
signatures of individual water molecules respond to their local hydrogen
bonding environments. In this study, we isolated the contributions
for the five classes of sites that differ according to the number
of donor (D) and acceptor (A) hydrogen bonds that characterize each
site. These patterns were measured by exploiting the unique properties
of the water cluster cage structures formed in the gas phase upon
hydration of a series of cations M+·(H2O)n (M = Li, Na, Cs, NH4,
CH3NH3, H3O, and n = 5, 20–22). This selection of ions was chosen to systematically
express the A, AD, AAD, ADD, and AADD hydrogen bonding motifs. The
spectral signatures of each site were measured using two-color, IR–IR
isotopomer-selective photofragmentation vibrational spectroscopy of
the cryogenically cooled, mass selected cluster ions in which a single
intact H2O is introduced without isotopic scrambling, an
important advantage afforded by the cluster regime. The resulting
patterns provide an unprecedented picture of the intrinsic line shapes
and spectral complexities associated with excitation of the individual
OH groups, as well as the correlation between the frequencies of the
two OH groups on the same water molecule, as a function of network
site. The properties of the surrounding water network that govern
this frequency map are evaluated by dissecting electronic structure
calculations that explore how changes in the nearby network structures,
both within and beyond the first hydration shell, affect the local
frequency of an OH oscillator. The qualitative trends are recovered
with a simple model that correlates the OH frequency with the network-modulated
local electron density in the center of the OH bond.