Water Dimer Cation Research Articles

Capture of Hydroxyl Radicals by Hydronium Cations in Water Microdroplets

AbstractDespite the high stability of bulk water, water microdroplets possess strikingly different properties, such as the presence of hydroxyl radicals (OH⋅) at the air–water interface. Previous studies exhibited the recombination of OH⋅ into H2O2 molecules and the capture of OH⋅ by oxidizing other molecules. By spraying pure water microdroplets into a mass spectrometer, we detected OH⋅ in the form of (H4O2)+ that is essentially OH⋅−H3O+, a hydroxyl radical combined with a hydronium cation through hydrogen bonding. We also successfully captured it with two OH⋅ scavengers, caffeine and melatonin, and key oxidation radical intermediates that bear important mechanistic information were seen. It is suggested that some previous reactions involving (H4O2)+ should be attributed to reactions with OH⋅−H3O+ rather than with the water dimer cation (H2O−OH2)+.

Autocorrelation of electronic wave-functions: a new approach for describing the evolution of electronic structure in the course of dynamics

ABSTRACTWe introduce a new approach for analysing changes in electronic structure in the course of ab initio molecular dynamics simulations. The analysis is based on the time autocorrelation function of the many-body electronic wave-function. The approach facilitates the interpretation of dynamical events that may not be easily revealed by consideration of nuclear configurations alone. We apply the method to several illustrative examples: the shared proton vibration in the F−(H2O) complex, representing changes in strength of non-covalent interactions; proton transfer in the water dimer cation, as an example for chemical reactions in weakly bound systems; and the intramolecular proton transfer in malonaldehyde. In all cases, we observe distinct features in the time autocorrelation function when chemical changes occur. The autocorrelation function serves as an effective reaction coordinate, incorporating all degrees of freedom, including electronic ones. The method is also sensitive to changes in the electronic wave-function not accompanied by significant nuclear motions.

Hemibonding between Water Cation and Water.

The hemibonding interaction in the water dimer cation is studied using coupled cluster electronic structure methods. The hemibonded dimer cation geometry is a local minimum structure characterized by the two participating monomers having both a very short separation and a near parallel relative orientation. It is shown that the vertically ionized dimer at its optimum neutral geometry can convert to the hemibonded dimer cation structure with essentially no energetic hindrance. Direct conversion to the hemibonded structure is therefore an energetically facile alternative to the minimum energy path that connects the vertically ionized neutral water dimer to the global minimum proton-transferred structure. A substantial barrier must be surmounted to convert the hemibonded dimer cation to the proton-transferred structure. The optical absorption spectrum of the hemibonded dimer cation is characterized by three excited near-UV states, two of which have very large oscillator strengths. Relative resonance Raman intensities are estimated for the hemibonded dimer cation vibrational modes, finding the intermolecular stretching mode to be the most strongly enhanced when in near resonance with each of the near-UV excited states, and the anharmonicity and overtones of this mode are estimated. These results provide guidance for the possible observation of hemibonded cations in irradiated liquid water.

Vibrational Signatures of Electronic Properties in Oxidized Water: Unraveling the Anomalous Spectrum of the Water Dimer Cation.

The water dimer cation, (H2O)2+, has long served as a prototypical reference system for water oxidation chemistry. In spite of this status, a definitive explanation for the anomalous—and dominant—features in the experimental vibrational spectrum [Gardenier, G. H.; Johnson, M. A.; McCoy, A. B. J. Phys. Chem. A, 2009, 113, 4772–4779] has not been determined, and harmonic analyses qualitatively fail to reproduce these features. In this computational study, accurate quantum chemistry methods are combined with a fully coupled, six-dimensional anharmonic model to show that the unassigned bands are the result of resonant mode interactions and strong anharmonic coupling. Such coupling is fundamentally due to the unique electronic structure of this open-shell ion and the manner in which auxiliary modes affect the natural charge-transfer properties of the shared-proton stretch. These unique vibrational signatures provide a key reference point for modern spectroscopic and mechanistic analyses of water-oxidation catalysts.

Modeling Charge Resonance in Cationic Molecular Clusters: Combining DFT-Tight Binding with Configuration Interaction.

In order to investigate charge resonance situations in molecular complexes, Wu et al. (J. Chem. Phys. 2007, 127, 164119) recently proposed a configuration interaction method with a valence bond-like multiconfigurational basis obtained from constrained DFT calculations. We adapt this method to the Self-Consistent Charge Density-Functional-based Tight Binding (SCC-DFTB) approach and provide expressions for the gradients of the energy with respect to the nuclear coordinates. It is shown that the method corrects the wrong SCC-DFTB behavior of the potential energy surface in the dissociation regions. This scheme is applied to determine the structural and stability properties of positively charged molecular dimers with full structural optimization, namely, the benzene dimer cation and the water dimer cation. The method yields binding energies in good agreement with experimental data and high-level reference calculations.

Spectroscopic signatures of proton transfer dynamics in the water dimer cation

Using full-dimensional EOM-IP-CCSD/aug-cc-pVTZ potential energy surfaces, the photoelectron spectrum, vibrational structure, and ionization dynamics of the water dimer radical cation, (H(2)O)(2) (+), were computed. We also report an experimental photoelectron spectrum which is derived from photoionization efficiency measurements and compares favorably with the theoretical spectrum. The vibrational structure is also compared to the recent experimental work of Gardenier et al. [J. Phys. Chem. A 113, 4772 (2009)] and the recent theoretical calculations by Cheng et al. [J. Phys. Chem. A 113, 13779 (2009)]. A reduced-dimensionality nuclear Hamiltonian was used to compute the ionization dynamics for both the ground state and first excited state of the cation. The dynamics show markedly different behavior and spectroscopic signatures depending on which state of the cation is accessed by the ionization. Ionization to the ground state cation surface induces a hydrogen transfer which is complete within 50 fs, whereas ionization to the first excited state results in a much slower process.

Electronic and molecular structure of the water dimer cation. A theoretical study

Electronic and molecular structure of the water dimer cation. A theoretical study