Cyclic Water Trimer Research Articles

Insight into the Binding of Argon to Cyclic Water Clusters from Symmetry-Adapted Perturbation Theory.

This work systematically examines the interactions between a single argon atom and the edges and faces of cyclic H2O clusters containing three-five water molecules (Ar(H2O)n=3-5). Full geometry optimizations and subsequent harmonic vibrational frequency computations were performed using MP2 with a triple-ζ correlation consistent basis set augmented with diffuse functions on the heavy atoms (cc-pVTZ for H and aug-cc-pVTZ for O and Ar; denoted as haTZ). Optimized structures and harmonic vibrational frequencies were also obtained with the two-body-many-body (2b:Mb) and three-body-many-body (3b:Mb) techniques; here, high-level CCSD(T) computations capture up through the two-body or three-body contributions from the many-body expansion, respectively, while less demanding MP2 computations recover all higher-order contributions. Five unique stationary points have been identified in which Ar binds to the cyclic water trimer, along with four for (H2O)4 and three for (H2O)5. To the best of our knowledge, eleven of these twelve structures have been characterized here for the first time. Ar consistently binds more strongly to the faces than the edges of the cyclic (H2O)n clusters, by as much as a factor of two. The 3b:Mb electronic energies computed with the haTZ basis set indicate that Ar binds to the faces of the water clusters by at least 3 kJ mol-1 and by nearly 6 kJ mol-1 for one Ar(H2O)5 complex. An analysis of the interaction energies for the different binding motifs based on symmetry-adapted perturbation theory (SAPT) indicates that dispersion interactions are primarily responsible for the observed trends. The binding of a single Ar atom to a face of these cyclic water clusters can induce perturbations to the harmonic vibrational frequencies on the order of 5 cm-1 for some hydrogen-bonded OH stretching frequencies.

A dipole-dipole interaction tuning the photoluminescence of silicon quantum dots in a water vapor environment.

The optical properties of silicon quantum dots (Si QDs) depend on the working conditions, which are critical for their application in optoelectronic devices and fluorescent tags. However, how a humid environment, most common in daily life, influences the photoluminescence (PL) of Si QDs has not been fully understood yet. Herein, we applied time-dependent density functional calculations to show that the adsorption of water molecules would exhibit distinct effects on the PL spectra of Si QDs as a function of size. In particular, the PL of Si QDs presents dual band emission with the adsorption of the cyclic water trimer (H2O)3 under common humid conditions, completely different from the PL of Si QDs under other conditions. The transition dipole moment decomposition analysis shows that the additional emission peak originates from the single Si-Si stretched bond of Si QDs induced by the dipole-dipole interaction between the cyclic water trimer and Si QDs. Moreover, the PL characteristics are size dependent. As the size increases from Si17H24 (the diameter of 0.6 nm) to Si52H52 (1.4 nm), the dipole-dipole interaction energy between (H2O)3 and Si QDs rapidly decreases from 19.1 × 10-22 J to 6.0 × 10-26 J, resulting in a single peak of PL of (H2O)3 adsorption on Si52H52. This study not only gives a deep understanding of PL of Si QDs under humid conditions, but also provides a new perspective on the development of optical devices based on Si QDs.

Capturing the Elusive Water Trimer from the Stepwise Growth of Water on the Surface of the Polycyclic Aromatic Hydrocarbon Acenaphthene.

Polycyclic aromatic hydrocarbons (PAHs) are key players in reaction chemistry. While it is postulated that they serve as a basis for ice grains, there has been no direct detection of PAHs in astronomical environments. We aim to investigate the hydration of PAHs to set a foundation for the future exploration of potential ice formation pathways. We report results from chirped pulse Fourier transform microwave spectroscopy and quantum-chemical calculations for the PAH acenaphthene and acenaphthene complexed with up to four water molecules. The acenaphthene-(H2O)3 complex is of particular interest as the elusive cyclic water trimer was observed. It appears in a slightly distorted configuration when compared with the pure water trimer. This is explained by hydrogen-bond net cooperativity effects. Binding energies for the complexes are presented. Our results provide insight into the onset of complex aggregation that could be occurring in extraterrestrial environments as part of ice grain formation.

On the tunneling splitting in a cyclic water trimer

We propose an alternative explanation of the “bifurcation” splittings observed for the water trimer in the VRT experiments of Saykally’s group [Chem. Rev. 103 (2003) 2533]. In our interpretation, the splittings originate from the quantum delocalization of hydrogen bonded protons in the mean field potential between two oxygen neighbors. The pattern and the order of our calculated splittings is in the range of experimentally observed values. Consequently, quantum delocalization of protons should be considered seriously as the origin of experimentally observed fine splittings. The presented model can be extended to a water pentamer and, hopefully, advance our understanding of liquid water.

Ionization dynamics of the water trimer: A direct ab initio MD study

Ionization dynamics of the cyclic water trimer (H2O)3 have been investigated by means of direct ab initio molecular dynamics (AIMD) method. Two reaction channels, complex formation and OH dissociation, were found following the ionization of (H2O)3. In both channels, first, a proton was rapidly transferred from H2O+ to H2O (time scale is ∼15fs after the ionization). In complex channel, an ion–radical contact pair (H3O+–OH) solvated by the third water molecule was formed as a long-lived H3O+(OH)H2O complex. In OH dissociation channel, the second proton transfer further takes place from H3O+(OH) to H2O (time scale is 50–100fs) and the OH radical is separated from the H3O+. At the same time, the OH dissociation takes place when the excess energy is efficiently transferred into the kinetic energy of OH radical. The OH dissociation channel is significantly minor, and almost all product channels were the complex formation. The reaction mechanism was discussed on the basis of theoretical results.

Electron Capture Dynamics of a Water Molecule Connected to a Cyclic Water Trimer: A Direct Ab Initio MD Approach

Electron capture dynamics of the water tetramer (H(2)O)(n) (n = 4) have been investigated by means of a full-dimensional direct ab initio molecular dynamics (MD) method at the MP2/6-311++G(d,p) level. Two structural conformers (branched and cyclic forms of the water tetramer) were examined as neutral water tetramers. The structure of the branched form is that a dangling water molecule binds to the ring composed of a cyclic water trimer. In the case of electron capture of the branched form, first, an excess electron was trapped by the dangling water molecule. Next, rotation of the water molecule located in the ring occurred rapidly, while a hydrogen bond of the ring was broken. The branched structure was gradually changed to a linear one. This change was caused by the increase of the dipole moment of the neutral water tetramer oriented toward the excess electron. The time scale of hydrogen bond breaking and solvation of the excess electron were estimated to be 100 and 400 fs, respectively. In the case of the cyclic water tetramer, a planar structure was only changed to a slight bent form. The mechanism of electron capture of the water tetramer (mainly the branched form) was discussed on the basis of theoretical results.

Density functional study on chirospectra of hydrogen-bonded systems X− (H2O)3 (X = F, Cl, Br, I)

This paper calculates the molecular structures, infrared, Raman, circular dichroism spectra and optical rotatory powers of some hydrogen-bonded supramolecular systems as a cyclic water trimer, (H2O)3 and its pyramidal halide complexes, X− (H2O)3 (X = F, Cl, Br, I) with the gradient-corrected density functional theory method at the B3LYP/6–311++G(2d,2p) and B3LYP/Aug–cc–pVTZ levels. It finds that the complexation of halide anions with the water trimer can efficiently modulate the chirally optical behaviors. The calculated vibrational circular dichroism spectrum illuminates that the vibrational rotational strength of S(+)–(H2O)3 mostly originates from the O–H rocking modes, whereas chirality of S(–)–X− (H2O)3 (X = F, Cl, Br, I) has its important origin in the O–H stretching modes. The calculated optical rotatory power demonstrates that S(+)–(H2O)3 and S(+)–F−(H2O)3 are positively chiral, whereas S(–)–X−(H2O)3 (X = Cl, Br, I) are negatively chiral. With the polarizable continuum model, calculated bulk solvent effect in the solvents water and carbontetrachloride and argon shows that the positive chirality of S(+)–(H2O)3 is enhanced and the negative chirality of S(–)–X−(H2O)3 (X = Cl, Br, I) and the positive chirality of S(+)–F−(H2O)3 are reduced with an augmentation of the solvent dielectric constant.

Reaction dynamics following electron capture of chlorofluorocarbon adsorbed on water cluster: A direct density functional theory molecular dynamics study

The electron capture dynamics of halocarbon and its water complex have been investigated by means of the full dimensional direct density functional theory molecular dynamics method in order to shed light on the mechanism of electron capture of a halocarbon adsorbed on the ice surface. The CF(2)Cl(2) molecule and a cyclic water trimer (H(2)O)(3) were used as halocarbon and water cluster, respectively. The dynamics calculation of CF(2)Cl(2) showed that both C-Cl bonds are largely elongated after the electron capture, while one of the Cl atoms is dissociated from CF(2)Cl(2) (-) as a Cl(-) ion. Almost all total available energy was transferred into the internal modes of the parent CF(2)Cl radical on the product state, while the relative translational energy of Cl(-) was significantly low due to the elongation of two C-Cl bonds. In the case of a halocarbon-water cluster system, the geometry optimization of neutral complex CF(2)Cl(2)(H(2)O)(3) showed that one of the Cl atoms interacts with n orbital of water molecules of trimer and the other Cl atom existed as a dangling Cl atom. After the electron capture, only one C-Cl bond (dangling Cl atom) was rapidly elongated, whereas the other C-Cl bond is silent during the reaction. The dangling Cl atom was directly dissociated from CF(2)Cl(2) (-)(H(2)O)(3) as Cl(-). The fast Cl(-) ion was generated from CF(2)Cl(2) (-)(H(2)O)(3) on the water cluster. The mechanism of the electron capture of halocarbon on water ice was discussed on the basis of the theoretical results.

Electron hydration dynamics in water clusters: A direct ab initio molecular dynamics approach

Electron attachment dynamics of excess electron in water cluster (H2O)n (n = 2 and 3) have been investigated by means of full-dimensional direct ab initio molecular dynamics (MD) method at the MP26-311++G(d,p) level. It was found that the hydrogen bond breaking due to the excess electron is an important process in the first stage of electron capture in water trimer. Time scale of electron localization and hydrogen bond breaking were determined by the direct ab initio MD simulation. The initial process of hydration in water cluster is clearly visualized in the present study. In n = 3, an excess electron is first trapped around the cyclic water trimer with a triangular form, where the excess electron is equivalently distributed on the three water molecules at time zero. After 50 fs, the excess electron is concentrated into two water molecules, while the potential energy of the system decreases by -1.5 kcal/mol from the vertical point. After 100 fs, the excess electron is localized in one of the water molecules and the potential energy decreases by -5.3 kcal/mol, but the triangular form still remained. After that, one of the hydrogen bonds in the triangular form is gradually broken by the excess electron, while the structure becomes linear at 100-300 fs after electron capture. The time scale of hydrogen bond breaking due to the excess electron is calculated to be about 300 fs. Finally, a dipole bound state is formed by the linear form of three water molecules. In the case of n = 2, the dipole bound anion is formed directly. The mechanism of electron hydration dynamics was discussed on the basis of theoretical results.

Infrared spectra and structure of water clusters trapped in argon and krypton matrices

The infrared spectra of water clusters trapped in argon and krypton matrices have been analyzed on the basis of recent quantum chemical calculations. Comparing the observed spectra with the band positions and intensities estimated by calculations for the water clusters, we have identified almost all bands observed in the bonded OH stretching region. In addition to the absorptions of the cyclic water trimer, tetramer, and pentamer, the present analysis reveals some infrared bands of the cage and book isomers of the water hexamer. For the cyclic pentamer, the second bonded OH stretching band is found to be absent or very weak, in contrast to the quantum chemical calculations.

Molecular Dynamics Simulation of Liquid Water: Hybrid Density Functionals

The structure, dynamical, and electronic properties of liquid water utilizing different hybrid density functionals were tested within the plane wave framework of first-principles molecular dynamics simulations. The computational approach, which employs modified functionals with short-ranged Hartree-Fock exchange, was first tested in calculations of the structural and bonding properties of the water dimer and cyclic water trimer. Liquid water simulations were performed at the state point of 350 K at the experimental density. Simulations included three different hybrid functionals, a meta-functional, four gradient-corrected functionals, and the local density and Hartree-Fock approximations. It is found that hybrid functionals are superior in reproducing the experimental structure and dynamical properties as measured by the radial distribution function and self-diffusion constant when compared to the pure density functionals. The local density and Hartree-Fock approximations show strongly over- and understructured liquids, respectively. Hydrogen bond analysis shows that the hybrid functionals give slightly smaller average numbers of hydrogen bonds than pure density functionals but similar hydrogen bond populations. The average molecular dipole moments in the liquid from the three hybrid functionals are lower than those of the corresponding pure density functionals.

Natural Energy Decomposition Analysis: Extension to Density Functional Methods and Analysis of Cooperative Effects in Water Clusters

Natural energy decomposition analysis (NEDA) is a method for partitioning molecular interaction energies into physically meaningful components, including electrical interaction, charge transfer, and core repulsions. The method is a numerically stable procedure that was originally developed for analyzing Hartree-Fock (HF) wave functions based on the localized orbital description of natural bond orbital analysis. In this work, we extend NEDA to treat charge densities from density functional theory (DFT) calculations, replacing the intermolecular exchange (EX) component of the HF analysis with an exchange-correlation (XC) component. DFT/NEDA is applied to hydrogen bonding interactions and cooperative effects in water clusters. Electrical interactions and charge transfer contribute importantly to hydrogen bonding. Comparison of HF and DFT results reveals that the exchange and correlation effects of DFT slightly enhance the extent of charge transfer and core repulsions in the water clusters. Cooperative stabilization of the cyclic water trimer and tetramer is considered by performing a many-body expansion of the interaction energy. Natural energy decomposition analysis of this expansion suggests that charge transfer is the leading source of cooperative stabilization. Polarization effects have only marginal influence on cooperativity.

Dynamics of the Concerted Triple Proton Transfer in Cyclic Water Trimer Using the Multiconfiguration Molecular Mechanics Algorithm

Cyclic water clusters are important molecular species to understand the nature of hydrogen bonded networks. Theoretical studies for the dynamics of triple proton transfer in the cyclic water trimer were performed. The potential energy surface (PES) of triple proton transfer is generated by the multiconfiguration molecular mechanics (MCMM) algorithm. We have used the MP2/6-31G(d,p) level for high-level ab initio data (energies, gradients, and Hessians), which are used in the Shepard interpolation. Eight high-level reference points were added step by step, including two points for the critical configurations of the large curvature tunneling paths. The more high-level points are used, the better the potential energy surfaces become. The rate constant and kinetic isotope effect (KIE) for the triple proton transfer at 300 K, which have been calculated by the canonical variational transition-state theory with microcanonical optimized multidimensional semiclassical tunneling approximation, are 1.6 x 10(-3) s(-1) and 230, respectively. Tunneling is very important not only for the triple proton transfer but also for the triple deuterium transfer. The MCMM results show good agreement with those from the direct ab initio dynamics calculations.

Anchoring the potential energy surface of the cyclic water trimer

Six cyclic stationary points on the water trimer potential energy surface have been fully optimized at the MP2 level with the aug-cc-pVQZ basis set. In agreement with previous work, harmonic vibrational frequencies indicate that two structures are minima, three are transition states connecting minima on the surface while the remaining stationary point is a higher-order saddle point. The 1- and n-particle limits of the electronic energies of each of these six structures were estimated by systematically varying both the basis sets and theoretical methods. The former limit was approached with the cc-pVXZ and aug-cc-pVXZ families of basis sets (X=2-7) while MP2, CCSD(T), and BD(TQ) calculations helped examine the latter. Core correlation effects have also been assessed at the MP2 level with the cc-pCVXZ series of basis sets (X=2-5). These data have been combined to provide highly accurate relative energies and dissociation energies for these stationary points.

Structures and excitation energies of ozone–water clusters O 3(H 2O) n ( n = 1–4)

Hybrid density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been carried out for ozone–water clusters O 3(H 2O) n ( n = 1–4) in order to obtain hydration effects on the absorption spectrum of ozone. The first water molecule in n = 1 is bound to the ozone molecule by an oxygen orientation form in which the oxygen atom of H 2O orients the central oxygen atom of O 3. In n = 2, the water dimer is bound to O 3 and then the cyclic structure is formed as the most stable structure. For n = 3 (or n = 4), the cyclic water trimer (or tetramer) is bound by a hydrogen bond to the ozone molecule. The TD-DFT calculations of O 3(H 2O) n ( n = 0–4) show that the first and second excitation energies of O 3 are blue-shifted by the interaction with the water clusters. The magnitude of the spectral shift is largest in n = 2, and the shifts of the excitation energies are +0.07 eV for S 1 and +0.13 eV for S 2 states. In addition to the spectral shifts (S 1 and S 2 states), it is suggested that a charge-transfer band is appeared as a low-lying excited state above the S 1 and S 2 states. The origin of the spectrum shifts was discussed on the basis of theoretical results.

Anisole-(H2O)n (n=1–3) complexes: An experimental and theoretical investigation of the modulation of optimal structures, binding energies, and vibrational spectra in both the ground and first excited states

We present the results obtained from spectroscopic investigations and quantum chemical calculations of the interaction of anisole (methoxybenzene) with small water clusters. The experiments have been carried out using resonant two-photon ionization (R2PI) and IR-UV double-resonance vibrational spectroscopy (IR/R2PI) in the region of the OH stretches. Apart from the vibrational spectra of the water moiety in the clusters, their intermolecular vibrations in the electronically excited S1 state are identified by IR/R2PI hole burning spectroscopy and assigned according to the vibrations calculated for the S1 state and compared with the vibrations calculated for the S0 state. The calculations for the S0 state were carried out at the second order Møller-Plesset level of theory using both the 6-31+G* and aug-cc-pVDZ basis sets and for the S1 state at the configuration interaction singles (CIS) level with the 6-31+G* basis set. In the electronic ground state (S0), the interaction of a water monomer to anisole is mediated through its oxygen atom, and that of a water dimer both through the oxygen atom (σ type of interaction) and the arene ring (π type of interaction). Thus in contrast to the interaction of fluorinated benzenes with water clusters, wherein a conformational transition from an in-plane σ to a on-top π bonding emerges starting with a water trimer, this conformational transition appears in case of anisole already with a water dimer. In the excited state (S1) of the investigated systems, there is a pronounced weakening of the interaction of the water cluster with the aromatic chromophore, which is also responsible for the blue shift of the electronic transitions. Consequently, the structures of the complexes of anisole with a water monomer or dimer are very different in both states. The weakening results from a diminished electron density of the oxygen atom and of the π system of anisole in the excited state. The calculated binding energies of the ground-state conformers indicate that these small water clusters are bound more strongly to anisole than to other π systems like benzene, toluene, fluorobenzene, and p-difluorobenzene. The many-body decomposition of the binding energy (S0) reveals that a progressive increase in the size of the water clusters results in a weakening of the σ O⋯H interaction and a concurrent strengthening of the π-H interaction. The complex containing a cyclic water trimer exhibits in the excited state also a π-type H-bonding interaction, but its stability emerges from a delocalization of the electron density from the water trimer to the anisole oxygen. Excepting the water dimer complexes, there is a good agreement between the calculated OH vibrational frequencies and the experimental IR spectra recorded for the ground state. The surprisingly good agreement of the calculated and the experimentally observed intermolecular modes in the excited state of these complexes provides convincing evidence that the experimental spectra emerge from structures similar to those predicted at the CIS/6-31+G* level.

Triaziridine and tetrazetidine vs. cyclic water trimer and tetramer: A computational approach to the relationship between molecular and supramolecular conformational analysis

The potential energy surfaces of cyclic water trimer and tetramer, as typical examples of supramolecular systems, have been compared with those of triaziridine and tetrazetidine, which can be considered as their suitable molecular analogues. The survey was performed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) level of theory and includes also the monomethyl and monofluoro derivatives of both the trimer and the tetramer. This comparison leads to the conclusion that supramolecular systems, where the molecular components are held together by weak interactions, exhibit the same conformational features as the analogous molecular compounds, where the constituent atoms are held together by strong covalent bonds. More importantly, even though in the former the energy required to induce conformational rearrangements is much smaller than that needed for typical molecular systems, there is a fairly good linear relationship between both magnitudes. We have also found that this quantitative relationship can be perturbed when the through-space interactions, either repulsive or attractive, between the substituents are sufficiently strong.

IR−UV Ion-Dip Spectroscopy of N-Benzylformamide Clusters: Stepwise Hydration of a Model Peptide

Fluorescence excitation, resonant two-photon ionization (R2PI) and IR−UV ion dip spectroscopy have been used to study conformers of N-benzylformamide (NBFA) and associated clusters including hydrates with up to n = 3 water molecules. The most stable conformer has a trans arrangement of the HNCO atoms. It is distinguished from the cis conformer by a higher frequency for the NH stretch (3478 cm-1, compared to 3443 cm-1) and lower frequency for the amide I overtone (3435 cm-1, compared to 3465 cm-1). The cis conformer forms cyclic H-bonded structures with one or two water molecules, binding via strong H-bonds to the neighboring CO and NH groups. With the addition of a third water molecule, the cyclic water trimer binds to both these groups in preference to a linear chain of three waters. For trans-NBFA, a single water binds to the carbonyl group and is further stabilized by dispersive CH···Owater interactions. Two water molecules bind to the NH group instead and form a bridge to the π-system of the aromatic ...

Spectroscopic determination of the water pair potential.

A polarizable water pair potential was determined by fitting a potential form to microwave, terahertz, and mid-infrared (D2O)2 spectra through a rigorous calculation of the water dimer eigenstates. It accurately reproduces most ground state vibration-rotation-tunneling spectra and yields excellent second viral coefficients. The calculated dimer structure and dipole moment are very close to those determined from microwave spectroscopy and high-level ab initio calculations. The dimer binding energy and acceptor switching and donor-acceptor interchange tunneling barriers are in excellent agreement with recent ab initio theory, as are cyclic water trimer and tetramer structures and binding energies.

Four-dimensional model calculation of torsional levels of cyclic water tetramer

Quantum four-dimensional model calculations of the coupled intermolecular torsional vibrations of the cyclic homodromic water tetramers (H2O)4 and (D2O)4 are presented, based on the analytical modEPEN4B potential energy surface [S. Graf and S. Leutwyler, J. Chem. Phys. 109, 5393 (1998), preceding paper] and a four-dimensional discrete variable representation approach. The lowest 50 torsional levels were calculated up to 420 and 500 cm−1 for (D2O)4 and (H2O)4, respectively. For both clusters, the torsional ground state is split by a synchronous O–H torsional inversion process, similar to inversion tunneling in ammonia, with calculated tunnel splittings of 21.8 and 0.000 12 MHz for (H2O)4 and (D2O)4, respectively. As for the cyclic water trimer and pentamer, the four torsional fundamentals of the tetramer lie above the torsional interconversion barriers, between 185–200 cm−1 for (D2O)4 and 229–242 cm−1 for (H2O)4, but also lie below the one-dimensional torsionally adiabatic barriers. The anharmonic fundamental frequencies lie both above and below the normal-mode frequencies, by up to 33%. Slightly above the fundamental torsional excitations, at 257–260 and 280–281 cm−1 for (H2O)4 and (D2O)4, respectively, lie four states corresponding to four versions of the {uudd} isomer, which form a pseudorotational manifold; the torsional interconversion occurs by a sequence of double O–H flips. Higher excited pseudorotational states are calculated up to a vibrational angular momentum of k=3. At ≈295 and ≈300 cm−1, a further group of eight states is found, corresponding to the eight permutationally equivalent versions of yet another isomer, the {uuud} structure. The four {uudd} and eight {uuud} states of (H2O)4 exhibit inverse isotope effects, and lie at lower energy than their (D2O)4 counterparts.