Ionization Dynamics of the Small-Sized Water Clusters: A Direct Ab Initio Trajectory Study

Fragmentation of water clusters formed in helium nanodroplets by charge transfer and Penning ionization

Helium nanodroplets ("HNDs") are widely used for forming tailor-made clusters and molecular complexes in a cold, transparent, and weakly interacting matrix. The characterization of embedded species by mass spectrometry is often complicated by the fragmentation and trapping of ions in the HNDs. Here, we systematically study fragment ion mass spectra of HND-aggregated water and oxygen clusters following their ionization by charge transfer ionization ("CTI") and Penning ionization ("PEI"). While the efficiency of PEI of embedded clusters is lower than for CTI by about factor 10, both the mean sizes of detected water clusters and the relative yields of unprotonated cluster ions are significantly larger, making PEI a "soft ionization" scheme. However, the tendency of ions to remain bound to HNDs leads to a reduced detection efficiency for large HNDs containing >104 helium atoms. These results are instrumental in determining optimal conditions for mass spectrometry and photoionization spectroscopy of molecular complexes and clusters aggregated in HNDs.

Direct tracking of ultrafast proton transfer in water dimers

Upon ionization, water forms a highly acidic radical cation H₂O^+· that undergoes ultrafast proton transfer (PT)-a pivotal step in water radiation chemistry, initiating the production of reactive H₃O⁺, OH^{[Formula: see text]} radicals, and a (hydrated) electron. Until recently, the time scales, mechanisms, and state-dependent reactivity of ultrafast PT could not be directly traced. Here, we investigate PT in water dimers using time-resolved ion coincidence spectroscopy applying a free-electron laser. An extreme ultraviolet (XUV) pump photon initiates PT, and only dimers that have undergone PT at the instance of the ionizing XUV probe photon result in distinct H₃O⁺ + OH⁺ pairs. By tracking the delay-dependent yield and kinetic energy release of these ion pairs, we measure a PT time of (55 ± 20) femtoseconds and image the geometrical rearrangement of the dimer cations during and after PT. Our direct measurement shows good agreement with nonadiabatic dynamics simulations for the initial PT and allows us to benchmark nonadiabatic theory.

Mutual independence of water and n-nonane nucleation at low temperatures

The interaction of water with different substances in the earth's atmosphere lies at the heart of many processes that influence our climate. However, it is still unclear how different species interact with water on the molecular level and in which ways this interaction contributes to the water vapor phase transition. Here, we report the first measurements of water-nonane binary nucleation in the 50-110 K temperature range, along with unary nucleation data of both. The time-dependent cluster size distribution in a uniform post-nozzle flow was measured by time-of-flight mass spectrometry coupled with single-photon ionization. From these data, we extract experimental rates and rate constants for both nucleation and cluster growth. The observed mass spectra of water/nonane clusters are not or only slightly affected by the introduction of the other vapor, and the formation of mixed clusters was not observed during nucleation of the mixed vapor. Additionally, the nucleation rate of either substance is not much affected by the presence (or absence) of the other species, i.e., the nucleation of water and nonane proceeds independently, indicating that hetero-molecular clusters do not play a role during nucleation. Only at the lowest temperature of our experiment (i.e., 51 K) do the measurements suggest that interspecies interaction slows water cluster growth. The findings here are in contrast to our earlier work in which we showed that vapor components in other mixtures, e.g., CO₂ and toluene/H₂O, can interact to promote nucleation and cluster growth in a similar temperature range.

Imaging the short-lived hydroxyl-hydronium pair in ionized liquid water

[Figure: see text].

Infrared Spectroscopy of Neutral Water Dimer Based on a Tunable Vacuum Ultraviolet Free Electron Laser

Infrared (IR) spectroscopy provides detailed structural and dynamical information on clusters at the fingerprint level. Herein, we demonstrate the capability of a tunable vacuum ultraviolet free electron laser (VUV-FEL) for selective detection of a wide variety of neutral water clusters and for recording the size-dependent IR spectra. The present technique does not require the presence of an ultraviolet chromophore or a dipole moment and is generally applicable for IR spectroscopy of neutral clusters free from confinement. To show the features of our technique, we report here the IR spectra of neutral water dimer in the OH stretch region, providing benchmarks for theoretical study of the accurate description of hydrogen bonding structures involved in liquid water and ice. Quantum mechanical calculations on a 12-dimensional ab initio potential energy surface are utilized to simulate the anharmonic vibrational spectra of water dimer. These results help to resolve the controversy of the exact vibrational assignment of each band feature of the water dimer.

Activation of CO2 in Photoirradiated CO2-H2O Clusters: Direct Ab Initio Molecular Dynamics (MD) Study

Carbon dioxide (CO₂) is one of the stable and inactive molecules that contribute to greenhouse gases. The development of new reactions of CO₂ activation, chemical fixation, and conversion is a very important issue. In this report, the reactions of CO₂-H₂O binary clusters were investigated using a direct ab initio molecular dynamics (AIMD) method to find a new reaction of CO₂ activation. Clusters composed of carbon dioxide and water molecules, CO₂(H₂O) _n ( n = 2-5), were utilized as a model of the binary cluster. The reaction dynamics of [CO₂(H₂O) _n]⁺ following the ionization of parent neutral clusters were also investigated. Two electronic states of [CO₂(H₂O) _n]⁺ were examined for direct AIMD surfaces: CO₂[(H₂O) _n]⁺ (ground state) and (CO₂)⁺(H₂O) _n (excited charge transfer (CT) state). After the ionization of the clusters, a proton-transfer (PT) reaction occurred within the (H₂O) _n⁺ moiety at the ground state, whereas the reactive HCO₃ radical was formed at the CT state for OH addition to CO₂⁺: CO₂⁺(H₂O) _n → HCO₃ + H⁺(H₂O) _n-1. The mechanisms of the PT process and the HCO₃ radical formation were discussed based on the theoretical results.

Study of the transient "free" OH radical generated in H2O-H2O2 mixtures by stimulated Raman scattering

Forward and backward stimulated Raman scattering (SRS) were studied in the H₂O₂-H₂O mixtures by a strong excitation laser with 532nm. Only the backward SRS (BSRS) of the H₂O₂-H₂O system shows an unexpected SRS shoulder peak at around 3600cm^-1, which is similar to the characteristic peak of "free" OH radical. The generation of the "free" OH radical is mainly attributed to the dissociation of hydrogen peroxide (HP) molecules. Simultaneously, the ionization of HP-water clusters generates a part of "free" OH radical under the Laser-induced breakdown (LIB). The interaction of water and HP is also discussed.

Ab initio investigation of structure, stability, thermal behavior, bonding, and infrared spectra of ionized water cluster (H2O)6. J Chem Phys 2016;145:154307. [PMID: 27782468 DOI: 10.1063/1.4964860]

The low-lying isomers of cationic water cluster (H₂O)₆⁺ have been globally explored by using particle swarm optimization algorithm in conjunction with quantum chemical calculations. Compared with previous results, our searching method covers a wide range of structural isomers of (H₂O)₆⁺ and therefore turns out to be more effective. With these local minima, geometry optimization and vibrational analysis are performed for the most interesting clusters at second-order Møller-Plesset (MP2)/aug-cc-pVDZ level, and their energies are further refined at MP2/aug-cc-pVTZ and coupled-cluster theory with single, double, and perturbative triple excitations/aug-cc-pVDZ level. The interaction energies using the complete basis set limits at MP2 level are also reported. The relationships between their structure arrangement and their energies are discussed. Based on the results of thermal simulation, structural change from a four-numbered ring to a tree-like structure occurs at T ≈ 45 K, and the relative population of six lowest-free-energy isomers is found to exceed 4% at some point within the studied temperature range. Studies reveal that, among these six isomers, two new-found isomers constitute 10% of isomer population at 180 K, and the experimental spectra can be better explained with inclusions of the two isomers. The molecular orbitals for six representative cationic water clusters are also studied. Through topological and reduced density gradient analysis, we investigated the structural characteristics and the bonding strengths of these water cluster radical cations.

Proton transfer rates in ionized water clusters (H2O)n (n = 2–4)

A proton transfer process is usually dominant in several biological phenomena such as the energy relaxation of photo-excited DNA base pairs and a charge relay process in Ser-His-Glu.

Structural evolution and solvation of the OH radical in ionized water radical cations (H2O)n+, n = 5–8

Structural evolution of ionized water radical cations (H₂O)_n⁺, n = 5–8, is studied by ab intio methods.

Characterization of a solvent-separated ion-radical pair in cationized water networks: infrared photodissociation and Ar-attachment experiments for water cluster radical cations (H2O)n+(n = 3-8)

We present infrared spectra of nominal water cluster radical cations (H(2)O)(n)(+) (n = 3-8), or to be precise, ion-radical complexes H(+)(H(2)O)(n-1)(OH), with and without an Ar tag. These clusters are closely related to the ionizing radiation-induced processes in water and are a good model to characterize solvation structures of the ion-radical pair. The spectra of Ar-tagged species show narrower bandwidths relative to those of the bare clusters due to the reduced internal energy via an Ar-attachment. The observed spectra are analyzed by comparing with those of the similar system, H(+)(H(2)O)(n), and calculated ones. We find that the observed spectra are attributable to ion-radical-separated motifs in n ≥ 5, as reported in the previous study (Mizuse et al. Chem. Sci.2011, 2, 868-876). Beyond the structural trends found in the previous study, we characterize isomeric structures and determine the number of water molecules between the charged site and the OH radical in each cluster size. In all of the characterized cluster structures (n = 5-8), the most favorable position of OH radical is the next neighbor of the charged site (H(3)O(+) or H(5)O(2)(+)). The positions and cluster structures are governed by the balance among the hydrogen-bonding abilities of the charged site, H(2)O, and OH radical. These findings on the ionized water networks lead to understanding of the detailed processes of ionizing radiation-initiated reactions in liquid water and aqueous solutions.

Chasing charge localization and chemical reactivity following photoionization in liquid water

The ultrafast dynamics of the cationic hole formed in bulk liquid water following ionization is investigated by ab initio molecular dynamics simulations and an experimentally accessible signature is suggested that might be tracked by femtosecond pump-probe spectroscopy. This is one of the fastest fundamental processes occurring in radiation-induced chemistry in aqueous systems and biological tissue. However, unlike the excess electron formed in the same process, the nature and time evolution of the cationic hole has been hitherto little studied. Simulations show that an initially partially delocalized cationic hole localizes within ~30 fs after which proton transfer to a neighboring water molecule proceeds practically immediately, leading to the formation of the OH radical and the hydronium cation in a reaction which can be formally written as H(2)O(+) + H(2)O → OH + H(3)O(+). The exact amount of initial spin delocalization is, however, somewhat method dependent, being realistically described by approximate density functional theory methods corrected for the self-interaction error. Localization, and then the evolving separation of spin and charge, changes the electronic structure of the radical center. This is manifested in the spectrum of electronic excitations which is calculated for the ensemble of ab initio molecular dynamics trajectories using a quantum mechanics/molecular mechanics (QM∕MM) formalism applying the equation of motion coupled-clusters method to the radical core. A clear spectroscopic signature is predicted by the theoretical model: as the hole transforms into a hydroxyl radical, a transient electronic absorption in the visible shifts to the blue, growing toward the near ultraviolet. Experimental evidence for this primary radiation-induced process is sought using femtosecond photoionization of liquid water excited with two photons at 11 eV. Transient absorption measurements carried out with ~40 fs time resolution and broadband spectral probing across the near-UV and visible are presented and direct comparisons with the theoretical simulations are made. Within the sensitivity and time resolution of the current measurement, a matching spectral signature is not detected. This result is used to place an upper limit on the absorption strength and/or lifetime of the localized H(2)O(+) ((aq)) species.

Ionization of Water Clusters Mediated by Exciton Energy Transfer from Argon Clusters

The exciton energy deposited in an argon cluster (Arn, ⟨n = 20⟩) using VUV radiation is transferred to softly ionize doped water clusters ((H2O)n, n = 1-9), leading to the formation of nonfragmented clusters. Following the initial excitation, electronic energy is channeled to ionize the doped water cluster while evaporating the Ar shell, allowing identification of fragmented and complete water cluster ions. Examination of the photoionization efficiency curve shows that cluster evaporation from excitons located above 12.6 eV is not enough to cool the energized water cluster ion and leads to their dissociation to (H2O)n-2H(+) (protonated) clusters.

Intermolecular coulomb decay at weakly coupled heterogeneous interfaces

Surface ejection of H(+)(H(2)O)(n=1-8) from low energy electron irradiated water clusters adsorbed on graphite and graphite with overlayers of Ar, Kr or Xe results from intermolecular Coulomb decay (ICD) at the mixed interface. Inner valence holes in water (2a(1)(-1)), Ar (3s(-1)), Kr (4s(-1)), and Xe (5s(-1)) correlate with the cluster appearance thresholds and initiate ICD. Proton transfer occurs during or immediately after ICD and the resultant Coulomb explosion leads to H(+)(H(2)O)(n=1-8) desorption with kinetic energies that vary with initiating state, final state, and interatomic or molecular distances.

Fragmentation pathways of H+(H2O)2 after extreme ultraviolet photoionization

Photofragmentation of the protonated water dimer H+(H2O)_{2}, a fundamental system both in aqueous solutions and gas-phase water clusters, has been studied at 13.8 nm using the Free Electron Laser FLASH in Hamburg. In a crossed-beam experiment using time-resolved, single-molecule fragment imaging, the two-body breakup into H2O++H3O+ was found as a prominent fragmentation channel with a kinetic energy release of up to 10 eV. This channel was observed with at least a similar yield as events with stronger fragmentation, producing protons together with neutral fragments and showing an absolute cross section of (0.5 ± 0.2) × 10(-18) cm2.

Ion-molecule reactions of ammonia clusters with C60 aggregates embedded in helium droplets

Helium nanodroplets are co-doped with C(60) and ammonia. Mass spectra obtained by electron ionization reveal cations containing ammonia clusters complexed with up to four C(60) units. The high mass resolution of Δm/m≈ 1/6000 makes it possible to separate the contributions of protonated, unprotonated and dehydrogenated ammonia. C(60) aggregates suppress the proton-transfer reaction which usually favors the appearance of protonated ammonia cluster ions. Unprotonated C(x)(NH(3))(n)(+) ions (x = 60, 120, 180) exceed the abundance of the corresponding protonated ions if n < 5; for larger values of n the abundances of C(60)(NH(3))(n)(+) and C(60)(NH)(n-1)NH(4)(+) become about equal. Dehydrogenated C(60)NH(2)(+) ions are relatively abundant; their formation is attributed to a transient doubly charged C(60)-ammonia complex which forms either by an Auger process or by Penning ionization following charge transfer between the primary He(+) ion and C(60). The abundance of C(x)NH(3)(+) and C(x)NH(4)(+) ions (x = 120 or 180) is one to two orders of magnitude weaker than the abundance of ions containing one or two additional ammonia molecules. However, a model involving evaporation of NH(3) or NH(4) from the presumably weakly bound C(x)NH(3)(+) and C(x)NH(4)(+) ions is at odds with the lack of enhancement in the abundance of C(120)(+) and C(180)(+). Mass spectra of C(60) dimers complexed with water complement a previous study of C(60)(H(2)O)(n)(+) recorded at much lower mass resolution.

Ionization of doped helium nanodroplets: complexes of C60 with water clusters

Water clusters are known to undergo an autoprotonation reaction upon ionization by photons or electron impact, resulting in the formation of (H(2)O)(n)H(3)O(+). Ejection of OH cannot be quenched by near-threshold ionization; it is only partly quenched when clusters are complexed with inert gas atoms. Mass spectra recorded by electron ionization of water-doped helium droplets show that the helium matrix also fails to quench OH loss. The situation changes drastically when helium droplets are codoped with C(60). Charged C(60)-water complexes are predominantly unprotonated; C(60)(H(2)O)(4)(+) and (C(60))(2)(H(2)O)(4)(+) appear with enhanced abundance. Another intense ion series is due to C(60)(H(2)O)(n)OH(+); dehydrogenation is proposed to be initiated by charge transfer between the primary He(+) ion and C(60). The resulting electronically excited C(60)(+*) leads to the formation of a doubly charged C(60)-water complex either via emission of an Auger electron from C(60)(+*), or internal Penning ionization of the attached water complex, followed by charge separation within {C(60)(H(2)O)(n)}(2+). This mechanism would also explain previous observations of dehydrogenation reactions in doped helium droplets. Mass-analyzed ion kinetic energy scans reveal spontaneous (unimolecular) dissociation of C(60)(H(2)O)(n)(+). In addition to the loss of single water molecules, a prominent reaction channel yields bare C(60)(+) for sizes n=3, 4, or 6. Ab initio Hartree-Fock calculations for C(60)-water complexes reveal negligible charge transfer within neutral complexes. Cationic complexes are well described as water clusters weakly bound to C(60)(+). For n=3, 4, or 6, fissionlike desorption of the entire water complex from C(60)(H(2)O)(n)(+) energetically competes with the evaporation of a single water molecule.

Water-chloride and water-bromide hydrogen-bonded networks: influence of the nature of the halide ions on the stability of the supramolecular assemblies

Two compounds, namely, [TTPH(2)](Cl)(2) x 4 H(2)O (1) and [TTPH(2)](Br)(2) x 4 H(2)O (2), (TTP = 4'-p-tolyl-2,2':6',2''-terpyridine) were synthesized from purely aqueous media and characterized by physical techniques. In the solid-state structures of these compounds, interesting supramolecular assemblies are observed. In 1, an unusual staircase-like architecture of the tape of edge-shared planar water hexamer is of importance, where the chloride ions are at the two edges of the tape. In 2, the polymeric nature of the water-bromide assembly is of interest, where discrete open-cube water octamers are doubly bridged by bromide ions. Semiempirical and DFT calculations confirm that the nature of the anion indeed affects the topology of the water-halide assemblies. We conclude that the protonated [TTPH(2)](2+) species can act as appropriate receptors for halide ions, which in turn act as a matrix for the formation of polymeric 1D water-halide assemblies.

Degree of initial hole localization/delocalization in ionized water clusters

The electronic structure of ionized bulk liquid water presents a number of theoretical challenges. Not the least of these is the realization that the detailed geometry of the hydrogen bonding network is expected to have a strong effect on the electronic couplings between water molecules and thus the degree of delocalization of the initially ionized system. This problem is approached from a cluster perspective where a high-level coupled cluster description of the electronic structure is still possible. Building on the work and methodology developed for the water dimer cation [J. Phys. Chem. A 2008, 112, 6159], the character and spectrum of electronic states of the water hole and their evolution from the dimer into higher clusters is presented. As the time evolution of the initially formed hole can in principle be followed by the system's transient absorption spectrum, the state spacings and transition strengths are computed. An analysis involving Dyson orbitals is applied and shows a partially delocalized nature of states. The issue of conformation disorder in the hydrogen bonding geometry is addressed for the water dimer cation.

Interaction between thymine dimer and flavin-adenine dinucleotide: a DFT and direct ab initio molecular dynamics study

The interaction between the fully reduced flavin-adenine dinucleotide (FADH (-)) and thymine dimer (T) 2 has been investigated by means of density functional theory (DFT) calculations. The charges of FADH (-) and (T) 2 were calculated to be -0.9 and -0.1, respectively, at the ground state. By photoirradiation, an electron transfer occurred from FADH (-) to (T) 2 at the first excited state. Next, the reaction dynamics of electron capture of (T) 2 have been investigated by means of the direct ab initio molecular dynamics (MD) method (HF/3-21G(d) and B3LYP/6-31G(d) levels) in order to elucidate the mechanism of the repair process of thymine dimer caused by the photoenzyme. The thymine dimer has two C-C single bonds between thymine rings (C 5-C 5' and C 6-C 6' bonds) at the neutral state, which is expressed by (T) 2. After the electron capture of (T) 2, the C 5-C 5' bond was gradually elongated and then it was preferentially broken. The time scale of the C-C bond breaking and formation of the intermediate with a single bond (T) 2 (-) was estimated to be 100-150 fs. The present calculations confirmed that the repair reaction of thymine dimer takes place efficiently via an electron-transfer process from the FADH (-) enzyme.