Yu. V. Novakovskaya

Stationary states and dissociation of H3O radical in water clusters

The ground and low-lying excited states of H3O(H2O)k radicals are studied. The character of the unpaired electron localization in the systems is analyzed, and the relative probability of the radical dissociation onto a water cluster and atomic hydrogen is estimated. Reaction coordinates of the dissociation are constructed and conditions of metastable existence of an H3O radical are determined. Structures, in which H3O can spontaneously dissociate, are found. Lifetimes of H3O(H2O)k clusters before the hydrogen atom detachment at the initial conditions of two kinds, namely, upon the vertical attachment of an electron to H3O+(H2O)k cations and upon the vibrational excitation of metastable neutral H3O(H2O)k systems, are estimated.

Structure reorganization dynamics of water clusters upon vertical ionization: Quantum chemical investigation

Quantum dynamics simulations of (H2O)n+ water cluster cations comprising up to six molecules with the initial configurations of stable neutral isomers are combined with stationary calculations of the same cations in order to get nonempirical information about water cluster relaxation after the absorption of energy in a range of from 11.0 to 12.0 eV. The electronic problem was solved and the potential energy of the system was estimated in terms of the restricted or unrestricted Hartree-Fock approximation with the 6–31++G** atomic basis set taking into account second-order Möller-Plesset perturbation theory corrections. Calculations of dynamic trajectories were based on the Born-Oppenheimer approximation. The formation of stable water cluster cations is shown to be a multistage process, the principal stages of which are qualitatively correctly reflected by the sequence of steps in cation geometry optimization runs.

Theoretical estimation of the ionization potential of water in condensed phase. II. Superficial water layers

Quantum chemical calculations of neutral and charged (H2O)n water clusters modeling fragments of a real hydrogen-bond network of water and dynamic simulations of the clusters either upon the electron removal from a stable neutral cluster or upon the excitation of various cluster vibrations enabled us to distinguish separate stages of the structure reorganization. Thermal motion of molecules in a liquid modeled by the low-frequency rotational vibrations and swinging of molecules prevents the formation of cations with the optimum mutual arrangement of the OH radical and H3O+ ion. Judging from the typical periods of reactive vibrations and those motions, which impede the desired structure reorganization, the most probable should be the formation of OH…H2O…H3O+ fragments in liquid water. The energy necessary for the ionization of superficial water layers (at the irradiation) can be estimated from the intermediate ionization potentials of water clusters. Extrapolating the dependence of these potentials on the number of water molecules constituting the cluster provided the first ever theoretical estimate of the threshold ionization energy of water: 9.5 eV.

Adiabatic ionization of water clusters: Nonempirical dynamic model

To determine the probability and mechanism of water cluster ionization initiated by the absorption of energy equal to the adiabatic ionization potential, the evolution of the vibrationally excited ring-like water tetramer was studied. The simulations were carried out in terms of the classical dynamics approach in the Born-Oppenheimer approximation. The adiabatic potential of the system and the forces acting on the nuclei were calculated in the second order of the Möller-Plesset perturbation theory with the use of the extended double-zeta 6-31++G** basis set. The initial states of the cluster system differed in the energy distribution over the intra-and intermolecular vibrational degrees of freedom. The initial conditions that promote the formation of an H3O ... H2O ... OH sequence of fragments, when the vertical electron detachment requires the energy equal to the adiabatic ionization potential of the system, are found.

Possible transformations of the ozone molecule in the presence of water associates

Possible channels of ozone transformations in the atmosphere in the presence of isolated water molecules or water associates were studied by quantum-chemical calculations of model systems comprising ozone and water molecules. The calculations were performed using the multiconfigurational self-consistent field approach in the complete active space, including perturbation theory corrections. The electronic excitation of the ozone molecule coordinated to a water associate should result in the formation of a strongly excited hydrogen peroxide molecule, which easily decomposes to two OH radicals. An alternative, less probable, transformation channel involves the formation of the HO2 radical and atomic hydrogen. The interaction of the ozone molecule with the OH radical in turn results in the formation of the HO2 radical and oxygen molecule. The MP2 variant of the one-configuration Möller-Plesset perturbation theory was shown to be inapplicable to describing the HO4 system.

Comparative analysis of the state of lithium and sodium atoms in water clusters

Structures and energetic characteristics of Li(H2O)n and Li+(H2O)n clusters with n = 1–6, 19, and 27 determined in the second order of the Møller-Plesset perturbation theory with 6–31++G(d,p) basis set are analyzed. The electron density redistribution, which takes place upon the electron addition to a Li+(H2O)n cluster, is found to be provided by hydrogen-bonded water molecules: initially almost neutral molecules, which are most distant from lithium, become negatively charged. The calculated energies of the electron capture by Li+(H2O)n clusters are approximated with the appropriate electrostatic model, and estimates of the lithium ionization energy in water clusters of various sizes are found. Similar estimates obtained earlier for sodium are made more accurate.

Ionization of sodium in water clusters

Structures of Na(H2O)n and Na+(H2O)n clusters with n = 1−6, 19, and 28 are determined in the second order of the Møller-Plesset perturbation theory with the use of extended atomic basis set 6–31++G**. It is found that when the number of molecules is sufficient for the formation of two solvation shells around sodium, a continuous hydrogen-bond network is formed in both neutral and charged clusters, and the orientation of each molecule is determined by the balance between interactions with the neighboring water molecules and with the field of the central particle. In the cations, this field is stronger, and up to the third solvation shell, molecules have a predominant orientation with respect to sodium. In the neutral clusters, with an increase in the number of water molecules, the maximum of the electron density distribution of the highest occupied molecular orbital becomes more distant from the sodium nucleus, being shifted toward the cluster surface. The energy of this orbital accordingly decreases in absolute value approaching 22 kcal/mol inmicroparticles. In the charged clusters, the distribution of the positive charge generally correlates with the character of the highest occupied orbital in the neutral systems, so that with an increase in the number of molecules, the atomic charge of sodium decreases and tends to zero as n → ∞. The ionization potential of sodium changes in inverse proportion to the linear size of the cluster, and should not exceed 1.1 eV in watermicroparticles.

The nature of hydrogen bonds and conjugation in H-bonded systems

Analysis of the electron density distributions in the spatial regions of hydrogen bonds reveals the existence of both σ and π binding. The σ-type electron density distribution is shown to be determined by occupied orbitals of both bonding and antibonding character. The π system can be conjugated and provides the predominant binding of molecules in nonplanar structures. The mutual orientation of the molecules involved in a hydrogen bond is found to be optimal when the X-H bond in the proton donor and the orbital of the acceptor, the main contributor to σ binding, have a common axis of symmetry. The conjugated π electron system in molecular clusters determines the so-called cooperativity that is manifested in the deviation of the properties of hydrogen-bonded systems from pairwise additivity.

Nonempirical Description of the Atmospherically Important Anionic Species. III. Hydrated Nitrogen Dioxide Anions

Neutral and negatively charged NO2(H2O)n clusters are simulated at the second order of the Møller–Plesset perturbation theory with 6-31G basis set extended with diffuse and polarization functions on all nuclei. For better reliability, configuration interaction and multiconfiguration self-consistent field calculations with the active spaces, formed by all single and double excitations to the basic determinant, are carried out. The weak binding of a neutral NO2 molecule to water clusters is provided by its coordination to two water molecules, either directly H bonded to each other or joined in an H-bond network via the third molecule. The presence of an excess electron strongly decreases the summary energy of the NO2(H2O)n system, so that its adiabatic affinity exceeds the summary affinity of NO2 and water system, although the excess electron is localized predominantly by NO2 fragment.

Large amplitude oscillations of protons in water clusters

Synchronous oscillations of bridge protons in molecular rings of (H2O)n water clusters with n = 4–6, 8, and 12 are analyzed as large amplitude motions that result in the inversion of the hydrogen bond sequence in a single ring. The corresponding one-dimensional cross sections of the potential energy surfaces (symmetric double-well potentials) are constructed at the MP2/6–31++G(d, p) level. The eigenstates of the systems in the constructed potentials in the energy range up to the top of the barrier are determined. The synchronous oscillations of protons are shown to be coupled to vibrations of the oxygen skeleton of the molecular ring, and the typical frequencies of the complex vibrations lie in the range of 230–330 cm−1.