Spectroscopic Signatures of Mode-Dependent Tunnel Splitting in the Iodide–Water Binary Complex

Abstract

The gas-phase vibrational spectrum of the isolated iodide−water cluster ion (I − •H 2 O), first reported in 1996, presents one of the most difficult, long-standing spectroscopic puzzles involving ion microhydration. Although the spectra of the smaller halides are well described in the context of an asymmetrical ground-state structure in which only one OH group is hydrogen-bonded to the ion, the I − •H 2 O spectrum displays multiplet structures with partially resolved rotational patterns that are additionally influenced by quantum nuclear spin statistics. In this study, this complex behavior is unraveled with a combination of experimental methods, including ion preparation in a temperature-controlled ion trap and spectral simplification through applications of tag-free, two-color IR−IR double-resonance spectroscopy. Analysis of the double-resonance spectra reveals a vibrational ground-state tunneling splitting of about 20 cm −1 , which is on the same order as the spacing between the peaks that comprise the multiplet structure. These findings are further supported by the results obtained from a fully coupled, six-dimensional calculation of the vibrational spectrum. The underlying level structure can then be understood as a consequence of experimentally measurable, vibrational mode-dependent tunneling splittings (which, in the case of the ground vibrational state, is comparable to the rotational energy spacing between levels with K a = 0 and 1), as well as Fermi resonance interactions. The latter include the hydrogen-bonded OH stretches and combination bands that involve the HOH bend overtones and soft-mode excitations of frustrated translation and rotation displacements of the water molecule relative to the ion. These anharmonic couplings yield closely spaced bands that are activated in the IR by borrowing intensity from the OH stretch fundamentals.

Electrostatics, Charge Transfer, and the Nature of the Halide–Water Hydrogen Bond

“…Although some systematic errors in CCSD(T)-F12b basis-set extrapolations have been noted, 79−81 the binding energies computed in ref 77 are more consistent with the experimental dissociation energy of I − (H 2 O), estimated at 3200−3500 cm −1 (9.1−10.0 kcal/ mol). 37,82 This minor discrepancy poses no serious problem, as the present work is focused on understanding the physical nature of the halide−water interaction, for which we rely on SAPT calculations.…”

“…There have been numerous calculations of the anharmonic vibrational frequencies of X − (H 2 O) complexes, 4,10,[29][30][31][32][33][34][35][36][37] including calculations of the HB L ↔ HB R tunneling splitting. [29][30][31][32][33][34][35][36][37] This splitting has been measured experimentally in I − (H 2 O), 2,37 but is much smaller (and not observed) in F − (H 2 O) due to the much stronger hydrogen bond and concomitantly larger barrier height. The very strong hydrogen bond in F − (H 2 O) manifests as extreme anharmonicity along the hydrogen-bonded O-H stretching coordinate (not shown in Fig.…”

Neat, Simple, and Wrong: Debunking Electrostatic Fallacies Regarding Noncovalent Interactions

“…For Cl – and larger halides, the double-well potential is nearly flat, as shown for Cl – (H 2 O) in Figure b, although even in I – (H 2 O) it is possible to measure tunneling splittings, meaning that a left ↔ right barrier must still exist. The simple charge–dipole picture suggests that E int (θ XOH ), or at least E elst (θ XOH ), should have a minimum at θ XOH = 0, but no such minimum is observed even for Br – .…”

Infrared Signatures of Isomer Selectivity and Symmetry Breaking in the Cs+(H2O)3 Complex Using Many-Body Potential Energy Functions

“…Gauss-Hermite quadrature was used for evaluation of the potential energy and dipole matrix elements. [124][125][126] The theoretical methods used in these calculations included harmonic normal-mode analysis; localmode vibrational self-consistent field theory (VSCF); 127,128 second-order, local-mode vibrational degenerate perturbation theory (VDPT2) 129 on VSCF states; the local-monomer approximation (LMon); [130][131][132][133][134][135][136][137] and a perturbatively corrected local-monomer model (cLMon) that included twomode, cross-monomer couplings. 138 Across all models, the small system sizes allowed ten quanta of excitation to be included along each mode.…”