Ab Initio CASSCF and DFT Investigations of (H2O)2+ and (H2S)2+: Hemi-Bonded vs Proton-Transferred Structure

Radicals in aqueous solution: assessment of density-corrected SCAN functional

We study self-interaction effects in solvated and strongly-correlated cationic molecular clusters, with a focus on the solvated hydroxyl radical. To address the self-interaction issue, we apply the DC-r2SCAN method, with the auxiliary density matrix approach. Validating our method through simulations of bulk liquid water, we demonstrate that DC-r2SCAN maintains the structural accuracy of r2SCAN while effectively addressing spin density localization issues. Extending our analysis to solvated cationic molecular clusters, we find that the hemibonded motif in the [CH3S∴CH3SH]+ cluster is disrupted in the DC-r2SCAN simulation, in contrast to r2SCAN that preserves the (three-electron-two-center)-bonded motif. Similarly, for the [SH∴SH2]+ cluster, r2SCAN restores the hemibonded motif through spin leakage, while DC-r2SCAN predicts a weaker hemibond formation influenced by solvent-solute interactions. Our findings demonstrate the potential of DC-r2SCAN combined with the auxiliary density matrix method to improve electronic structure calculations, providing insights into the properties of solvated cationic molecular clusters. This work contributes to the advancement of self-interaction corrected electronic structure theory and offers a computational framework for modeling condensed phase systems with intricate correlation effects.

Spectroscopic evidence of S∴N and S∴O hemibonds in heterodimer cations

Computational and condensed phase experimental evidence for the existence of S∴N and S∴O hemibonded structures has been reported previously, but no gas phase experimental evidence has been reported. To experimentally explore the existence of the S∴N and S∴O hemibonds in the gas phase, we recorded the infrared photodissociation action spectra of four cationic clusters: [CH₃SH-NH₃]⁺, [CH₃SCH₃-NH₃]⁺, [CH₃SCH₃-H₂O]⁺, and [CH₃OCH₃-H₂O]⁺. Combined with the calculation results, it is found that the S∴N hemibonded structure is competitive with the S⋯HN H-bonded structure, though only the latter structure is actually observed in [CH₃SH-NH₃]⁺. The spectral and theoretical results show that hemibonds can form between the second- (oxygen or nitrogen) and the third-period elements (sulfur) in the heterodimer clusters of [CH₃SCH₃-NH₃]⁺ and [CH₃SCH₃-H₂O]⁺. However, the S∴N and S∴O hemibonded structures are found competitive with the C⋯HN and CH⋯O H-bonded structures, respectively, and both the structures coexist. On the other hand, the O∴O hemibonded structure is much less stable than other hydrogen bonded (H-bonded) structures in [CH₃OCH₃-H₂O]⁺, and it shows no clear contribution to the observed spectrum. This study provides direct spectroscopic evidence for the existence of S∴N and S∴O hemibonds in the gas phase and their competition with the H-bonds, which may be also fundamentally important in biological processes.

Probing solvation and reactivity in ionized polycyclic aromatic hydrocarbon–water clusters with photoionization mass spectrometry and electronic structure calculations

Synchrotron based mass spectrometry coupled with theoretical calculations provides insight into polycyclic aromatic hydrocarbon water interactions.

Shrinking Self-Interaction Errors with the Fermi-Löwdin Orbital Self-Interaction-Corrected Density Functional Approximation

The self-interaction error (SIE) is one of the major drawbacks of practical exchange-correlation functionals for Kohn-Sham density functional theory. Despite this, the use of methods that explicitly remove SIE from approximate density functionals is scarce in the literature due to their relatively high computational cost and lack of consistent improvement over standard modern functionals. In this article we assess the performance of a novel approach recently proposed by Pederson, Ruzsinszky, and Perdew [ J. Chem. Phys. 2014, 140, 121103] for performing self-interaction free calculations in density functional theory based on Fermi orbitals. To this end, we employ test sets consisting of reaction energies that are considered particularly sensitive to SIE. We found that the parameter-free Fermi-Löwdin orbital self-interaction correction method combined with the standard local spin density approximation (LSDA) and Perdew-Burke-Ernzerhof (PBE) functionals gives a much better estimate of reaction energies compared to their parent LSDA and PBE functionals for most of the reactions in these two sets. They also perform on par with the global PBE0 and range-separated LC-ωPBE hybrids, which partially eliminate the SIE by including Hartree-Fock exchange. This shows the potential of the Fermi-Löwdin orbital self-interaction correction (FLOSIC) method for practical density functional calculations without SIE.

Spectral Characterization of Three-Electron Two-Center (3e-2c) Bonds of Gaseous CH3S∴S(H)CH3 and (CH3SH)2+ and Enhancement of the 3e-2c Bond upon Protonation

The three-electron two-center (3e-2c) bond plays an important role in structures and electron communication in biological systems involving cationic sulfur compounds. Although the nature of 3e-2c bonds and their theoretical formalism have attracted great interest, direct spectral identifications of 3e-2c-bound molecules are scarce. We observed the infrared spectra of the weakly 3e-2c-bound CH₃S∴S(H)CH₃ and the strongly 3e-2c-bound (CH₃SH)₂⁺ in a supersonic jet using infrared (IR) dissociation with vacuum-ultraviolet photoionization and time-of-flight detection. Protonation of CH₃S∴S(H)CH₃ to form [CH₃(H)S∴S(H)CH₃]⁺ significantly enhances the 3e-2c bond, characterized by a large red shift of the SH-stretching band with enhanced IR intensity, shortening of the calculated S-S distance from 3.00 to 2.86 Å, and a dissociation energy increased from ∼23 to 162 kJ mol^-1.

Spectroscopic observation of two-center three-electron bonded (hemi-bonded) structures of (H2S) n+ clusters in the gas phase

A two-center three-electron 2c-3e bond (hemi-bond) is a non-classical chemical bond, and its existence has been supposed in radical cation clusters with lone pairs. Though the nature of the hemi-bond and its role in the reactivity of radical cations have attracted great interest, spectroscopic observations of hemi-bonded structures have been very scarce. In the present study, the presence of a stable hemi-bonded core (H2S∴SH2)+ in (H2S) n+ (n = 3-6) in the gas phase is demonstrated by infrared spectroscopy combined with quantum chemical calculations. The spectral features of the free SH stretch of the ion core show that the hemi-bond motif of the ion core is maintained up to the completion of the first H-bonded solvation shell. All of the observed spectra are well reproduced by the minimum energy hemi-bonded isomers, and no sign of the proton-transferred ion core type H3S+-SH, which is estimated to have a much higher energy, is found. Spin density calculations show that the excess charge is almost equally delocalized over the two H2S molecules in the cluster for n = 3 to 6. This also indicates the hemi-bond nature of the (H2S∴SH2)+ ion core and the small impact of the formation of a solvation shell on the ion core.

Ab Initio Study of Ionized Water Radical Cation (H2O)8+ in Combination with the Particle Swarm Optimization Method

The structures of cationic water clusters (H₂O)₈⁺ have been globally explored by the particle swarm optimization method in combination with quantum chemical calculations. Geometry optimization and vibrational analysis for the 15 most interesting clusters were computed at the MP2/aug-cc-pVDZ level and infrared spectrum calculation at MPW1K/6-311++G** level. Special attention was paid to the relationships between their configurations and energies. Both MP2 and B3LYP-D3 calculations revealed that the cage-like structure is the most stable, which is different from a five-membered ring lowest energy structure but agrees well with a cage-like structure in the literature. Furthermore, our obtained cage-like structure is more stable by 0.87 and 1.23 kcal/mol than the previously reported structures at MP2 and B3LYP-D3 levels, respectively. Interestingly, on the basis of their relative Gibbs free energies and the temperature dependence of populations, the cage-like structure predominates only at very low temperatures, and the most dominating species transforms into a newfound four-membered ring structure from 100 to 400 K, which can contribute greatly to the experimental infrared spectrum. By topological analysis and reduced density gradient analysis, we also investigated the structural characteristics and bonding strengths of these water cluster radical cations.

Insights into the ultraviolet spectrum of liquid water from model calculations

With a view toward a better molecular level understanding of the effects of hydrogen bonding on the ultraviolet absorption spectrum of liquid water, benchmark electronic structure calculations using high level wave function based methods and systematically enlarged basis sets are reported for excitation energies and oscillator strengths of valence excited states in the equilibrium water monomer and dimer and in a selection of liquid-like dimer structures. Analysis of the electron density redistribution associated with the two lowest valence excitations of the water dimer shows that these are usually localized on one or the other monomer, although valence hole delocalization can occur for certain relative orientations of the water molecules. The lowest excited state is mostly associated with the hydrogen bond donor and the significantly higher energy second excited state mostly with the acceptor. The magnitude of the lowest excitation energies is strongly dependent on where the valence hole is created, and only to a lesser degree on the perturbation of the excited electron density distribution by the neighboring water molecule. These results suggest that the lowest excitation energies in clusters and liquid water can be associated with broken acceptor hydrogen bonds, which provide energetically favorable locations for the formation of a valence hole. Higher valence excited states of the dimer typically involve delocalization of the valence hole and/or delocalization of the excited electron and/or charge transfer. Two of the higher valence excited states that involve delocalized valence holes always have particularly large oscillator strengths. Due to the pervasive delocalization and charge transfer, it is suggested that most condensed phase water valence excitations intimately involve more than one water molecule and, as a consequence, will not be adequately described by models based on perturbation of free water monomer states. The benchmark calculations are further used to evaluate a series of representative semilocal, global hybrid, and range separated hybrid functionals used in efficient time-dependent density functional methods. It is shown that such an evaluation is only meaningful when comparison is made at or near the complete basis set limit of the wave function based reference method. A functional is found that quantitatively describes the two lowest excitations of water dimer and also provides a semiquantitative description of the higher energy valence excited states. This functional is recommended for use in further studies on the absorption spectrum of large water clusters and of condensed phase water.

Electron density and energy decomposition analysis in hydrogen-bonded complexes of azabenzenes with water, acetamide, and thioacetamide

Ab initio and density functional theoretical studies on hydrogen-bonded complexes of azabenzenes with water, acetamide, and thioacetamide have been carried out to explore the controversy involved in the relative order of their stability in a systematic way. The interaction energies of these complexes have been analyzed using the Morokuma energy decomposition method, and the nature of the various hydrogen bonds formed has been investigated through topological aspects using Bader's atom in a molecule (AIM) theory. Morokuma energy decomposition analysis reveals that the major contributions to the energetics are from the polarization (PL) and charge transfer (CT) energies. From the calculated topological results, excellent linear correlation is shown to exist between the hydrogen-bond length, electron density [rho(r)], and its Laplacian [nabla(2)rho(r)] at the bond critical points for all the complexes considered.

Structural Investigation of Asymmetrical Dimer Radical Cation System (H2O−H2S)+: Proton-Transferred or Hemi-Bonded?

Ab initio molecular orbital and hybrid density functional methods have been employed to characterize the structure and bonding of (H2O-H2S)+, an asymmetrical dimer radical cation system. A comparison has been made between the two-center three-electron (2c-3e) hemi-bonded system and the proton-transferred hydrogen-bonded systems of (H2O-H2S)+. Geometry optimization of these systems was carried out using unrestricted Hartree Fock (HF), density functional theory with different functionals, and second-order Møller-Plesset perturbation (MP2) methods with 6-311++G(d,p) basis set. Hessian calculations have been done at the same level to check the nature of the equilibrium geometry. Energy data were further improved by calculating basis set superposition error for the structures optimized through MP2/6-311++G(d,p) calculations. The calculated results show that the dimer radical cation structure with H2O as proton acceptor is more stable than those structures in which H2O acts as a proton donor or the 2c-3e hemi-bonded (H2O thereforeSH2)+ system. This stability trend has been further confirmed by more accurate G3, G3B3, and CCSD(T) methods. On the basis of the present calculated results, the structure of H4OS+ can best be described as a hydrogen-bonded complex of H3O+ and SH with H2O as a proton acceptor. It is in contrast to the structure of neutral (H2O...H2S) dimer where H2O acts as a proton donor. The present work has been able to resolve the ambiguity in the nature of bonding between H2O and H2S in (H2O-H2S)+ asymmetrical dimer radical cation.

Reactions associated with ionization in water: A directab initiodynamics study of ionization in (H2O)17

Quasiclassical ab initio simulations of the ionization dynamics in a (H(2)O)(17) cluster, the first water cluster that includes a fourfold coordinated (internally solvated) water molecule, have been carried out to obtain a detailed picture of the elementary processes and energy redistribution induced by ionization in a model of aqueous water. General features observable from the simulations are the following: (i) well within 100 fs following the ionization, one or more proton transfers are seen to take place from the "ionized molecule" to neighboring molecules and beyond, forming a hydronium ion and a hydroxyl radical; (ii) two water molecules close to the ionized water molecule play an important role in the reaction, in what we term a "reactive trimer." The reaction time is gated by the encounter of the ionized water molecule with these two neighboring molecules, and this occurs anytime between 10 and 50 fs after the ionization. The distances of approach between the ionized molecule and the neighboring molecules indeed display best the time characteristics of the transfer of a proton, and thus of the formation of a hydronium ion and a OH radical. These findings are consistent with those for smaller cyclic clusters, albeit the dynamics of the proton transfer displays more varieties in the larger cluster than in the small cyclic clusters. We used a partitioning scheme for the kinetic energy in the (H(2)O)(17) system that distinguishes between the reactive trimer and the surrounding "medium." The analysis of the simulations indicates that the kinetic energy of the surrounding medium increases markedly right after the event of ionization, a manifestation of the local heating of the medium. The increase in kinetic energy is consistent with a reorganization of the surrounding medium, electrostatically forced in a very short time by the water cation and in a longer time by the formation of the hydronium ion.

Static dipole polarizability and binding energy of sodium clusters Nan (n=1–10): A critical assessment of all-electron based post Hartree–Fock and density functional methods

A systematic all electron post Hartree-Fock as well as density functional theory (DFT) based calculations for the polarizability and binding energy of sodium metal clusters have been performed and an in-depth analysis of the discrepancy between the experimental and theoretical results is presented. A systematic investigation for the assessment of different DFT exchange-correlation functionals in predicting the polarizability values has also been reported. All the pure DFT functionals have been found to considerably underestimate the calculated polarizability values as compared to the MP2 results. DFT calculations using the full Hartree-Fock exchange along with one-parameter progressive correlation functional have, however, been shown to yield results in good agreement with the MP2 and experimental results. The possible sources of error present in the experimental measurements as well as in the different theoretical methods have also been analyzed. One of the most important conclusions of the present study is that the effect of electron correlation plays a significant role in determining the polarizability of the clusters and the MP2 method can be considered to be one of the most reliable methods for their prediction. It has also been noted that the polarizability value of the lower member clusters (Na2 and Na4) calculated by highly sophisticated methods such as, CCSD and CCSD(T) are found to be very close to the corresponding MP2 values. The polarizability and the binding energy of the clusters are found to be inversely related to each other and their correlation is rationalized by invoking the minimum polarizability principle. A good linear correlation between the polarizability and volume of the cluster has also been found to exist.

Experimental and theoretical characterization of H(2)OOO(+)

This report presents the preparation and characterization of H2OOO+, an important intermediate in water-oxygen chemistry. The H2OOO+ cation was produced by co-deposition of H2O/Ar with radio frequency discharged O2/Ar at 4 K and was identified by four fundamental infrared absorptions. Quantum chemical calculations indicate a doublet ground state with a H2O-O2 hemi-bonded Cs structure.