Cl-O Bond Research Articles

First-principles study of the electronic, vibrational, thermodynamic properties and phase stability of orthorhombic KClO4 under pressure

The purpose of this study is to deeply understand the basic physical properties of potassium perchlorate (KClO4) as a typical ionic energetic material under pressure. KClO4 Received attention for its widespread use in explosives, fireworks, safety matches and rocket propellants. In this paper, we systematically study the basic physical properties of KClO4 under 0-20 GPa pressure, including electrons, mechanical, vibrations, and thermodynamic parameters, based on density functional theory (DFT). The accuracy of the calculations was verified by optimizing the lattice parameters of the crystal structure and comparing them with previous experimental values. Electronic structure analysis revealed that KClO4 is a broadband gap nonconductor. Electronic density of states (DOS) analysis revealed strong covalent bonding between Cl-O bonds. Based on the phonon information, this study calculates the thermodynamic properties of KClO4 and analyzes a function of the relevant thermodynamic quantities under pressure. Combined with the structural phase transition phenomena observed in high pressure experiments, we analyze the dynamic and mechanical instability. These analyses help to understand the behavior of KClO4 under high pressure conditions and are crucial for its application in energetic materials.

Tyr 118-Mediated Electron Transfer is Key to the Chlorite Decomposition in Heme-Dependent Chlorite Dismutase.

Chlorite dismutase (Cld) is a crucial enzyme that catalyzes the decomposition of chlorite ions into chloride ions (Cl-) and molecular oxygen (O2). Despite playing an important role in the detoxification of toxic chlorite ions, the mechanism of cleavage of the Cl-O bond by Cld remains highly debatable. The present study highlights the mechanism of such Cl-O bond cleavage in Cld using sophisticated computational tools such as hybrid quantum mechanical/molecular mechanical calculations and long-time scale molecular dynamics simulations. Here, we show that Cld forms a high spin ferric hexacoordinated substrate adduct in the presence of a chlorite ion, which subsequently reduces to a ferrous state. Our study shows a stepwise pathway with the homolytic cleavage of the Cl-O bond that produces a high spin Fe(III)-OH species and a diradicaloid species formed by the combination of a chlorine-based ClO• radical and a protein-based tyrosine118• radical. The findings provide significant insights into Cl-O bond cleavage and O2 formation which shows a crucial role of the tyrosine118 during the electron transfer process.

Highly selective synthesis of surface FeIV=O with nanoscale zero-valent iron and chlorite for efficient oxygen transfer reactions

High-valent iron-oxo species (FeIV=O) has been a long-sought-after oxygen transfer reagent in biological and catalytic chemistry but suffers from a giant challenge in its gentle and selective synthesis. Herein, we propose a new strategy to synthesize surface FeIV=O (≡FeIV=O) on nanoscale zero-valent iron (nZVI) using chlorite (ClO2-) as the oxidant, which possesses an impressive ≡FeIV=O selectivity of 99%. ≡FeIV=O can be energetically formed from the ferrous (FeII) sites on nZVI through heterolytic Cl-O bond dissociation of ClO2- via a synergistic effect between electron-donating surface ≡FeII and proximal electron-withdrawing H2O, where H2O serves as a hydrogen-bond donor to the terminal O atom of the adsorbed ClO2- thereby prompting the polarization and cleavage of Cl-O bond for the oxidation of ≡FeII toward the final formation of ≡FeIV=O. With methyl phenyl sulfoxide (PMS16O) as the probe molecule, the isotopic labeling experiment manifests an exclusive 18O transfer from Cl18O2- to PMS16O18O mediated by ≡FeIV=18O. We then showcase the versatility of ≡FeIV=O as the oxygen transfer reagent in activating the C-H bond of methane for methanol production and facilitating selective triphenylphosphine oxide synthesis with triphenylphosphine. We believe that this new ≡FeIV=O synthesis strategy possesses great potential to drive oxygen transfer for efficient high-value-added chemical synthesis.

The effects of the gas-liquid interface and gas phase on Cl/ClO radical interaction with water molecules.

In the marine boundary layer (MBL), chlorine (Cl) and chlorine monoxide (ClO) are powerful oxidants with high concentrations. The gas-liquid interface is also ubiquitous in the MBL as a favorable site for atmospheric reactions. Understanding the role of water in Cl/ClO radical chemistry is essential for predicting their behavior in the atmosphere and developing effective strategies for mitigating their harmful effects. However, the research studies on the system of Cl/ClO radicals on the surface of water droplets are still insufficient. In previous studies, we have found unique results related to the hydroxyl radical at the interface using ab initio molecular dynamics (AIMD). In this work, we have used AIMD to investigate interactions between Cl/ClO radicals and water molecules at the gas-liquid interface. Radical mobility, radial distribution functions, coordination, and population analyses were conducted to investigate the surface preference, bonding pattern, and track Cl/ClO radicals in the water droplets. In addition, density functional theory (DFT) analysis was conducted to compare the results at the gas-liquid interface with those in the gas phase. We found that Cl/ClO radicals tend to remain near the gas-liquid interface in water droplet systems and outside of water clusters in gas phase systems. The ClO radical can form O*-H and Cl-O bonds with water molecules; however, neither the O*-O hemibond nor the Cl-H bond was detected in all systems. Different dominant structures were obtained for ClO in the interface and gas phase. The ClO radical can be bonded to one water molecule from its oxygen side, (H2O)0-Cl-O*-(H2O)1 at the interface, or to two water molecules from the chlorine and oxygen sides, (H2O)1-Cl-O*-(H2O)1 in the gas phase. Meanwhile, the Cl radical can only form a dominant structure like Cl*-(H2O)1 at the gas-liquid interface by making a Cl*-O hemibond. Providing a thorough explanation of the Cl/ClO radical behavior at the gas-liquid interface, this study will improve our understanding of the MBL's oxidizing capacity and pollution causes.

Unveiling the activity origin of ultrathin BiOCl nanosheets for photocatalytic CO2 reduction

Ultrathin layered semiconductors have attracted particular attention for various photocatalytic applications, while their surface-structure changes under reaction conditions have been rarely concerned. Herein, the dynamic evolutions of surface atomic and electronic structures on ultrathin BiOCl photocatalysts were firstly explored by combining synchronous-illumination X-ray photoelectron spectroscopy (SI-XPS) with X-ray diffraction (SI-XRD). The related results clearly reveal that the exposed {001} facets of ultrathin BiOCl are terminated with chlorine atoms instead of generally considered oxygen atoms. Under steady states, the outward migration of chlorine atoms on BiOCl (001) surfaces resulted in the formation of Bi, Cl, and O atoms with multiple-valence states and length decrease of ClO and BiO bond within the lattice phase. Under excitation states, the surface chlorine atoms return back to the lattice phase, leading to the elongation of BiCl and BiO bonds and the valence-state normalization of Bi, Cl, and O atoms. Owing to these significant atomic- and electronic-structure changes, ultrathin BiOCl nanosheets exhibit the prominent activity enhancement (21.4 μmol g−1 h−1) for CO2 reduction to CO compare with bulk BiOCl (3.3 μmol g−1 h−1).

Density functional calculations of the adsorption and decomposition of RDX-AP on Al (1 1 1) surface

The adsorption and decomposition mechanisms of RDX-AP on Al (1 1 1) surface (ternary system) were investigated by using the first-principles calculations. After adsorption on Al (1 1 1) surface, RDXAP underwent decomposition accompanying with rupture of NNO2, NN, ClO bonds and formation of new AlN, AlO bonds. Adsorption energy increased with the number of new bonds. There was a strong charge exchange between the RDX/AP and Al surface, suggesting it belonged to chemical adsorption. In addition, reaction energy barrier of decomposition of ternary system was higher than that of binary system, and the former released more energy than the latter one. These results suggest that the ternary system was more stable than the binary system and the former could release more energy once it decomposed. The results are helpful for understanding the decomposition mechanism of composite solid propellant with multiple components at an atomistic scale.

Revisiting a Historical Concept by Using Quantum Crystallography: Are Phosphate, Sulfate and Perchlorate Anions Hypervalent?

There are many examples of atoms in molecules that violate Lewis' octet rule, because they have more than four electron pairs assigned to their valence. These atoms are referred to as hypervalent. However, hypervalency may be regarded as an artifact arising from Lewis' description of molecules, which is based on the assumption that electrons are localized in two-center two-electron bonds and lone pairs. In the present paper, the isoelectronic phosphate (PO4 3- ), sulfate (SO4 2- ) and perchlorate (ClO4 - ) anions were examined with respect to the concept of hypervalency. Lewis formulas containing a hypervalent central atom exist for all three anions. Based on X-ray wavefunction refinements of high-resolution X-ray diffraction data of representative crystal structures (MgNH4 PO4 ⋅6 H2 O, Li2 SO4 ⋅H2 O, and KClO4 ), complementary bonding analyses were performed. In this way, experimental information from the new field of quantum crystallography validate long-known facts, or refute long-standing misunderstandings. It is shown that the P-O and S-O bonds are highly polarized covalent bonds and, thus, the increase in the valence population following three-center four-electron bonding is not sufficient to yield hypervalent phosphorus or sulfur atoms, respectively. However, for the highly covalent Cl-O bond, most bonding indicators imply a hypervalent chlorine atom.

Basis Set Effects in the Description of the Cl-O Bond in ClO and XClO/ClOX Isomers (X = H, O, and Cl) Using DFT and CCSD(T) Methods

The performance of a group of density functional methods of progressive complexity for the description of the ClO bond in a series of chlorine oxides was investigated. The simplest ClO radical species and the two isomeric structures XClO/ClOX for each X = H, Cl, and O were studied using the PW91, TPSS, B3LYP, PBE0, M06, M06-2X, BMK, and B2PLYP functionals. Geometry optimizations and reaction enthalpies and enthalpies of formation for each species were calculated using Pople basis sets and the (aug)-cc-pVnZ Dunning sets, with n = D, T, Q, 5, and 6. For the calculation of enthalpies of formation, atomization and isodesmic reactions were employed. Both the precision of the methods with respect to the increase of the basis sets, as well as their accuracy, were gauged by comparing the results with the more accurate CCSD(T) calculations, performed using the same basis sets as for the DFT methods. The results obtained employing composite chemical methods (G4, CBS-QB3, and W1BD) were also used for the comparisons, as well as the experimental results when they are available. The results obtained show that error compensation is the key for successful description of molecular properties (geometries and energies) by carefully selecting the method and basis sets. In general, expansion of the one-electron basis set to the limit of completeness does not improve results at the DFT level, but just the opposite. The enthalpies of formation calculated at the CCSD(T)/aug-cc-pV6Z for the species considered are generally in agreement with experimental determinations and the most accurate theoretical values. Different sources of error in the calculations are discussed in detail.

The Structure and Cl-O Dissociation Energy of the ClOO Radical: Finally, the Right Answers for the Right Reason.

The chlorine peroxy radical (ClOO) has historically been a highly problematic system for theoretical studies. In particular, the erratic ab initio predictions of the Cl-O bond length reported in the literature thus far exhibit unacceptable errors with respect to the experimental structure. In light of the widespread disagreement observed, we present a careful and systematic investigation of the ClOO geometry toward the basis set and correlation limits of single reference ab initio theory, employing the cc-pVXZ (X = D, T, Q, 5, 6) basis sets extrapolated to the complete basis set limit and coupled cluster theory through single, double, triple, and perturbative quadruple excitations [CCSDT(Q)]. We demonstrate a considerable sensitivity of the Cl-O bond length to both electron correlation and basis set size. The CCSDT(Q)/CBS structure is found to be re(ClO) = 2.082, re(OO) = 1.208, and θe(ClOO) = 115.4°, in remarkable agreement with Endo's semi-experimentally determined values re(ClO) = 2.084(1), re(OO) = 1.206(2), and θe(ClOO) = 115.4(1)°. Moreover, we compute a Cl-O bond dissociation energy of 4.77 kcal mol-1, which is likewise in excellent agreement with the most recent experimental value of 4.69 ± 0.10 kcal mol-1.

Light-Driven C-H Oxygenation of Methane into Methanol and Formic Acid by Molecular Oxygen Using a Perfluorinated Solvent.

The chlorine dioxide radical (ClO2. ) was found to act as an efficient oxidizing agent in the aerobic oxygenation of methane to methanol and formic acid under photoirradiation. Photochemical oxygenation of methane occurred in a two-phase system comprising perfluorohexane and water under ambient conditions (298 K, 1 atm). The yields of methanol and formic acid were 14 and 85 %, respectively, with a methane conversion of 99 % without formation of the further oxygenated products such as CO2 and CO. Ethane was also photochemically converted into ethanol (19 %) and acetic acid (80 %). The methane oxygenation is initiated by the photochemical Cl-O bond cleavage of ClO2. to generate Cl. and O2 . The produced Cl. reacts with CH4 to form a methyl radical (CH3. ). Finally, the oxygenated products such as methanol and formic acid were given by the radical chain reaction. A fluorous solvent plays an important role of inhibiting the deactivation of reactive radical species such as Cl. and CH3. .

Suppressed charge trapping characteristics of (NH4)2Sx passivated GaN MOS device with atomic layer deposited HfAlOx gate dielectric

The charge trapping behaviors of ammonium poly-sulfide, (NH4)2Sx, passivated GaN MOS device with atomic layer deposited HfAlOx gate dielectric and DC-sputtered TiN gate electrode are investigated and compared with those of GaN MOS device with hydrochloric acid, HCl, passivation. Compared to non-S passivated device, higher peak capacitance (>20% @1MHz), reduced frequency dispersion (~30% ↓), lower hysteresis (~34% ↓), higher breakdown (1.3×), lower stress induced flat-band voltage (VFB) shift (~30% ↓), and lower interface state density (Dit), stronger immunity Dit generation (ΔDit) against gate bias voltage are attained with S-passivated device. Further improved characteristics are observed with post deposition annealing at 400°C for 10min. These behaviors are mainly attributed to higher bond strength energy of SO and SN than those of ClO and ClN bonds.

Residual chlorine in TiO2 films grown at low temperatures by plasma enhanced atomic layer deposition

We have studied the presence of residual chlorine in thin TiO2 films grown by plasma enhanced atomic layer deposition (PEALD) at temperatures within 40–250°C range, using x-ray photoemission spectroscopy, secondary ion mass spectrometry, grazing incidence x-ray diffraction and scanning electron microscopy. The source of the residual chlorine is TiCl4, which was used as a titanium precursor in ALD. The PEALD results are compared with the results on ALD films grown thermally in the same reactor. Films deposited by PEALD show a lower amount of residual chlorine, while chlorine concentration decreases with the processing temperature in both ALD techniques. In addition to the standard signal from residual chlorine bonded to Ti, a strong signal from ClO bonds was observed in PEALD samples grown at low temperatures. Our study also shows the development of an anatase phase in TiO2 films grown above 200°C, which correlates well with the reduction of both types of residual chlorine in PEALD samples.

First principles investigations of structural, electronic and elastic properties of ammonium perchlorate under high pressures

Ammonium perchlorate (NH4ClO4) is a highly energetic oxidizer widely used in solid propellants and explosives. Under extreme pressure conditions, significant changes are observed in the structures and properties of NH4ClO4. However, many studies of structural transformations of NH4ClO4 under high pressures have not formed a more consistent conclusion. In this study, the structural, electronic, and elastic properties of NH4ClO4 are investigated by first-principles calculations based on the density functional theory with dispersion correction (DFT-D) method in a range of 0-15 GPa. The unit cell volume and lattice parameters are optimized by GGA/PBE-TS, which leads to good agreement with the experimental structure parameters at 0 GPa, suggesting the reliability of the present calculation method. The calculated P-V data are fitted to the third-order Birch-Murnaghan equation of state, and the result provides better agreement with experimental result than other calculations for the unit cell with a volume V0 and bulk moduli B0 and B'. The comprehensive analyses of the lattice parameters, bond lengths, and hydrogen bonds under high pressure indicate that three structural transformations occur in NH4ClO4 at 1 GPa, 4 GPa, and 9 GPa. With increasing pressure, hydrogen bonding interaction gradually increases, and intra- and intermolecular hydrogen bonds are present in crystals. Results obtained from the band structures and state densities under high pressure indicate that NH4ClO4 exhibits good insulating properties. Valence band shifts towards low energy, conduction band shifts towards high energy, and electronic localization is enhanced. The charge density differences and Mulliken charge populations at different pressures reveal that the covalent interaction between the N-H and Cl-O bonds increases, and the ionicity of crystal decreases. The band gaps in different structural transition regions exhibit different linear increase trends with increasing pressure. The calculated elastic constants of NH4ClO4 satisfy elastic stability criteria of orthorhombic systems at pressures ranging from 0 GPa to 15 GPa, indicating that NH4ClO4 is mechanically stable. The bulk modulus, shear modulus, and Young's modulus are estimated by the Voigt-Reuss-Hill approach. The Cauchy pressures and B/G values indicate that NH4ClO4 exhibits ductility, attributed to the fact that NH4ClO4 is an ionic crystal, and ionic bonds are non-directional bonds; hence, NH4ClO4 is ductile and can be easily bended or reshaped. The results indicate that the ductility properties of NH4ClO4 increase with increasing pressure. All calculated properties are in excellent agreement with the available experimental results. These results will not only help to understand the structural transformations of NH4ClO4 under high pressures but also provide an important theoretical reference for the safe application of NH4ClO4 in solid propellants and explosives.

X-ray, vibrational spectra and quantum chemical studies on a new semiorganic crystal: 4-Chloroanilinium perchlorate

A new semi-organic material 4-chloroanilinium perchlorate was synthesized and grown as a single crystal by slow evaporation solution growth technique. A good X-ray quality single crystal was selected from the grown crops and used for the single crystal diffraction studies. The asymmetric part of the unit cell contains a 4-chloroanilinium cation and a perchlorate anion. The protonation on the N site of the chloroaniline is confirmed from the CN bond distance and the deprotonation on perchloric acid is confirmed from ClO bond geometry. The molecular aggregations are stabilized through intricate three dimensional hydrogen bonding network formed by the classical NH⋯O hydrogen bonds. It form two infinite chains running along the b-axis of the unit cell which are cross-linked through another NH⋯O bond leading to alternate ring R44(12) motifs. These ring and chain motifs lead to alternate hydrophilic and hydrophobic layers along c-axis of the unit cell. The presence of different functional groups and the nature of their vibrations were identified in experimental vibrational studies through Infra-Red and Raman measurements in the range of 4000–400cm−1. The optimized molecular structure, vibrational mode, computed spectra, molecular properties and NBO analysis of the 4-chloroanilinium perchlorate were found out by quantum chemical calculations with HF and DFT/B3LYP methods invoking 6-311++G(d,p) basis sets. Computed geometrical parameters and harmonic frequencies of fundamental, combination and overtone transitions were found in satisfactory agreement with the experimental data. The electronic properties such as HOMO and LUMO energies were carried out.

Peroxidase-Type Reactions Suggest a Heterolytic/Nucleophilic O–O Joining Mechanism in the Heme-Dependent Chlorite Dismutase

Heme-containing chlorite dismutases (Clds) catalyze a highly unusual O-O bond-forming reaction. The O-O cleaving reactions of hydrogen peroxide and peracetic acid (PAA) with the Cld from Dechloromonas aromatica (DaCld) were studied to better understand the Cl-O cleavage of the natural substrate and subsequent O-O bond formation. While reactions with H2O2 result in slow destruction of the heme, at acidic pH heterolytic cleavage of the O-O bond of PAA cleanly yields the ferryl porphyrin cation radical (compound I). At alkaline pH, the reaction proceeds more rapidly, and the first observed intermediate is a ferryl heme. Freeze-quench EPR confirmed that the latter has an uncoupled protein-based radical, indicating that compound I is the first intermediate formed at all pH values and that radical migration is faster at alkaline pH. These results suggest by analogy that two-electron Cl-O bond cleavage to yield a ferryl-porphyrin cation radical is the most likely initial step in O-O bond formation from chlorite.

3-(Ammoniomethyl)pyridinium bis(perchlorate)

In the title molecular salt, C6H10N2 2+·2ClO4 −, the Cl—O bond lengths [anion 1: 1.369 (3)–1.415 (3); anion 2: 1.420 (2)–1.441 (2) Å] and the O—Cl—O angles [anion 1: 105.4 (2)–111.8 (4); anion 2: 107.8 (1)–110.3 (1)°] indicate a slight distortion of the perchlorate anions from regular tetra­hedral symmetry. In the crystal, the components are linked into columns along the a-axis direction via N—H⋯O and C—H⋯O hydrogen bonds, with stacks of the organic mol­ecules being surrounded by stacks of perchlorate anions.

2-Trifluoromethyl-1H-benzimidazol-3-ium perchlorate

In the title salt, C8H6F3N2 +·ClO4 −, the atoms of the benzimidazole ring (including H atoms) are nearly coplanar (r.m.s. deviation of the fitted atoms = 0.0122 Å) and the triflouromethyl group lies out of this plane. The perchlorate anion adopts a distorted tetra­hedral conformation with the Cl—O bond distances ranging from 1.412 (3) to 1.439 (2) Å. The benzimidazolium cations are linked to adjacent anions by inter­molecular N—H⋯O hydrogen bonds, forming chains.

Oxidation of Chloride and Subsequent Chlorination of Organic Compounds by Oxoiron(IV) Porphyrin π‐Cation Radicals

Ironing it out: oxoiron(IV) porphyrin π-cation radical complexes serve as models for the oxidation of Cl(-) into an active chlorinating reagent that chlorinates various organic compounds. Evidence suggests that Cl(-) is oxidized to Cl(2) via Cl·. The mechanism involving either direct electron transfer or iron(III) hypochlorite formation, and then homolysis of the Cl-O bond is discussed.

Photodissociation dynamics of ClOOCl at 248.4 and 308.4 nm

The dynamics of ClOOCl photodissociation at 248.4 and 308.4 nm was studied with photofragment translational spectroscopy. At 248.4 nm photoexcitation, the observed products are Cl, O(2), ClO and O. Product translational energy distributions P(E) and anisotropy parameters β were deduced from the measured time-of-flight spectra of the Cl, O(2), and ClO photoproducts. The photodissociation mechanisms have been discussed and compared with available theoretical results. Synchronous and fast sequential breaking of the two Cl-O bonds may both contribute to the dissociation. The relative product yields for [ClO]: [Cl] was measured to be 0.15 ± 0.04:1. The relative amounts of [O]:[O(2)] products were estimated to be 0.12:1. The branching ratios among the Cl + O(2) + Cl:ClO + ClO:ClO + Cl + O product channels were estimated to be 0.82:0.08:0.10. At 308.4 nm excitation, time-of-flight spectra of the O(2) and ClO photoproducts were recorded while there was interference from Cl(2) impurity in detecting the Cl product. Nonetheless, the observed ClO yield relative to the O(2) yield at 308.4 nm is 1.5 times that at 248.4 nm. The branching ratio between the Cl + O(2) + Cl:ClO + ClO product channels was estimated to be 0.81:0.19 at 308.4 nm. This result suggests that the ClO product may contribute a noticeable yield in the photolysis of ClOOCl at the atmospherically important wavelengths above 300 nm.

Structure of Hydronium (H3O+)/Chloride (Cl−) Contact Ion Pairs in Aqueous Hydrochloric Acid Solution: A Zundel-like Local Configuration

A comprehensive analysis of the H(3)O(+) and H(2)O structure in the first solvation shell about Cl(-) in aqueous HCl solutions is reported from X-ray absorption fine structure (XAFS) measurements. Results show increasing degree of contact ion pairing between Cl(-) and H(3)O(+) as the HCl concentration increases from 6.0 m, 10.0 m, and finally 16.1 m HCl (acid concentrations are expressed as molality or mole HCl/1000 g water). At the highest acid concentration there are on average, approximately 1.6 H(3)O(+) ions and 4.2 H(2)O's in the first shell about Cl(-). The structure of the Cl(-)/H(3)O(+) contact ion pair is distinctly different from that of the H(2)O structure about Cl(-). The Cl-O bond length (2.98 A) for Cl(-)/H(3)O(+) is approximately 0.16 A shorter than the Cl(-)/H(2)O bond. The bridging proton resides at an intermediate position between Cl and O at 1.60 A from the Cl(-) and approximately 1.37 A from the O of the H(3)O(+). The bridging-proton structure of this contact ion pair, (Cl-H-OH(2)), is similar to the structure of the water Zundel ion, (H(2)O-H-OH(2)(+)). In both cases there is a shortened Cl-O or O-O bond, and the intervening proton bond distances are substantially longer than for the covalent bonds of either HCl or H(2)O. A detailed structural analysis of the aqueous chloride species, Cl(-)/(H(2)O)(n), was also completed as part of this study in order to understand the relative importance of various XAFS photoelectron scattering paths. For aqueous Cl(-) the measured Cl-O and Cl-H distances of 3.14 A and 2.23 A, respectively, are in excellent agreement with earlier neutron and X-ray diffraction results. Overall, these results significantly improve our understanding of the interaction of H(3)O(+) with Cl(-). The results are of interest to fundamental physical chemistry and they have important consequences in biochemical, geochemical, and atmospheric processes.