Near-Infrared Spectroscopy and Anharmonic Theory of Protonated Water Clusters: Higher Elevations in the Hydrogen Bonding Landscape

Abstract

Near-infrared spectroscopy measurements are presented for protonated water clusters, H(HO) , in the size range of n = 1-8. Clusters are produced in a pulsed-discharge supersonic expansion, mass selected, and studied with infrared laser photodissociation spectroscopy in the regions of 3600-4550 and 4850-7350 cm. Although there is some variation with cluster size, the main features of these spectra are a broad absorption near 5300 cm, a sharp doublet near 7200 cm, as well as a structured absorption near 4100 cm for n ≥ 2. The vibrational patterns measured for the hydronium, Zundel, and Eigen ions are compared to those predicted by different forms of anharmonic theory. Second-order vibrational perturbation theory (VPT2) and a local mode treatment of the OH stretches both capture key aspects of the spectra but suffer understandable deficiencies in the quantitative description of band positions and intensities.

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

“…As shown, anharmonic theory computations predict a ν 2 + ν 3 bend-stretch combination at 5208 cm –1 in reasonable agreement with the experiment, and ν 1 + ν 3 , 2ν 1 , and 2ν 3 overtones at 7032, 7050, and 7165 cm –1 in less agreement with the experimental pattern. Results of similar mixed theory–experiment agreement were obtained recently for near-IR spectroscopy of protonated water clusters . This is another example of the limitations of the VPT2 method for such systems.…”

Bend-to-Break: Curvilinear Proton Transfer in Phenol-Ammonia Clusters

“…Figure shows 3 representative structures (2D–2F) that correspond to proton transferred structures with a magnitude of the electric field along the O–H bond that is in the range 240–252 MV cm –1 . The analysis of all of the 42 proton transferred structures indicates that the acceptor ammonia molecule has a triple donor motif with all 3 of the N–H groups hydrogen bonded to other ammonia molecules (absence of dangling NH group on the acceptor ammonia molecule), without exception, leading to the formation of a structure equivalent to that of the solvated Eigen cation of protonated water structures. − …”

How to VPT2: Accurate and Intuitive Simulations of CH Stretching Infrared Spectra Using VPT2+K with Large Effective Hamiltonian Resonance Treatments

“…More sophisticated ways of correcting for LAM have been described, 8 but such methods are not widely implemented and tend to be greatly more complicated than standard VPT. In the case of severe LAM, such as in proton-bound dimers, 9,10 errors become catastrophic, and the normal-mode harmonic oscillator starting point should be abandoned in favor of descriptions based on curvilinear internal coordinates and more expansive potential energy surfaces, such as discrete variable representation approaches 11 or extensions of VSCF. 12 In certain cases of LAM, a situation arises in which the electronic PES has multiple minima separated by a sufficiently shallow barrier, such that the ground state wave function is highly delocalized, with the largest amplitude above the barrier.…”

“…More sophisticated ways of correcting for LAM have been described, but such methods are not widely implemented and tend to be greatly more complicated than standard VPT. In the case of severe LAM, such as in proton-bound dimers, , errors become catastrophic, and the normal-mode harmonic oscillator starting point should be abandoned in favor of descriptions based on curvilinear internal coordinates and more expansive potential energy surfaces, such as discrete variable representation approaches or extensions of VSCF …”