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Various types of interactions between halogen (X) and π moiety (X−π interaction) including halogen bonding play important roles in forming the structures of biological, supramolecular, and nanomaterial systems containing halogens and aromatic rings. Furthermore, halogen molecules such as X2 and CX4 (X = Cl/Br) can be intercalated in graphite and bilayer graphene for doping and graphene functionalization/modification. Due to the X−π interactions, though recently highly studied, their structures are still hardly predictable. Here, using the coupled-cluster with single, double, and noniterative triple excitations (CCSD(T)), the Moller–Plesset second-order perturbation theory (MP2), and various flavors of density functional theory (DFT) methods, we study complexes of benzene (Bz) with halogen-containing molecules X2 and CX4 (X = Cl/Br) and analyze various components of the interaction energy using symmetry adapted perturbation theory (SAPT). As for the lowest energy conformers (S1), X2–Bz is found to have the...

Most density functionals do not properly describe the characteristics of superoxide (O2–) (i.e., first two vertical electron detachment energies and the excitation energies of neutralized singlet state) of small even-numbered AunO2– clusters. However, the second-order Moller–Plesset theory (MP2) shows significant charge transfer from Au cluster anions to oxygen molecule and so provides proper electronic characteristics of superoxide of small even-numbered AunO2– clusters. This has allowed us to properly describe the properties of even-numbered AunO2– clusters. Even in the case of odd-numbered AunO2– clusters, we find that Au5– is a chemically O2-adsorbed singlet state at 0 K, against a commonly accepted physisorbed triplet state. This is further evidenced by our extensive coupled cluster with single, double, and perturbative triple excitations [CCSD(T)] calculations, including the relativistic effect. However, the entropy effect makes the physisorbed triplet state more stable than the chemisorbed singlet ...

Propeller-shaped triazine was used to synthesize microporous polycarbazole materials through an inexpensive FeCl3-catalyzed reaction using direct oxidative coupling (PCBZ) and extensive cross-linking (PCBZL) polymerization routes. PCBZL has a Brunauer–Emmett–Teller specific surface area of 424 m2 g–1 and shows larger CO2 uptake (64.1 mg g–1 at 273 K, 1 atm). Selective adsorption of CO2 over N2 calculated using the ideal adsorbed solution theory shows that both PCBZ (125) and PCBZL (148) exhibit selectivity at 298 K, which is significantly higher than PCBZ (110) and PCBZL (82) at 273 K. These values of selectivity are among the highest reported for any triazine-based microporous material. By introducing the electron-rich carbazole structure into the nitrogen fertile triazine-based system, the adsorption enthalpy is increased drastically, which in turn contributes to high selective adsorption values. The larger existing binding energy between CO2 and propeller specifies more stable and favorable interaction...

The CO molecule can interact with the hydroxyl radical ((•)OH) via either a weak noncovalent interaction or strong covalent bonding. Upon the ionization of neutral water clusters, the resulting water cluster cations produce protonated water clusters and hydroxyl radicals. In this regard, we investigate the interactions of a CO molecule with water dimer and trimer cations using density functional theory (DFT), Möller-Plesset second-order perturbation theory (MP2), and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. It is found that the reaction products of CO by the water dimer and trimer cations form a HOĊO radical solvated by a protonated water monomer and dimer, respectively. These radicals are useful intermediates to make oxalic acids, formic acids, metal ligands, and so on, which is important in green chemistry.

Since the aliphatic C–H···anion interaction is relatively weak, anion binding using hydrophobic aliphatic C–H (Cali–H) groups has generally been considered not possible without the presence of additional binding sites that contain stronger interactions to the anion. Herein, we report X-ray structures of organic crystals that feature a chloride anion bound exclusively by hydrophobic Cali–H groups. An X-ray structure of imidazolium-based scaffolds using Cali–H···A− interactions (A− = anion) shows that a halide anion is directly interacting with fifteen Cali–H groups (involving eleven hydrogen bonds, two bidentate hydrogen-bond-type binding interactions and two weakly hydrogen-bonding-like binding interactions). Additional supporting interactions and/or other binding sites are not observed. We note that such types of complexes may not be rare since such high numbers of binding sites for an anion are also found in analogous tetraalkylammonium complexes. The Cali–H···A− interactions are driven by the formation of a near-spherical dipole layer shell structure around the anion. The alternating layers of electrostatic charge around the anion arise because the repulsions between weakly positively charged H atoms are reduced by the presence of the weakly negatively charged C atoms connected to H atoms.

By using density functional theory (DFT) and high-level ab initio theory, we have investigated the structure, interaction energy, electronic property, and IR spectra of the water trimer cation [(H2O) 3 + ]. Two structures of the water trimer cation [the H3O+ containing linear (3Lp) structure versus the ring (3OO) structure] are compared. For the complete basis set (CBS) limit of coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)], the 3Lp structure is 11.9 kcal/mol more stable than the 3OO structure. This indicates that the ionization of water clusters produce the hydronium cation moiety (H3O+) and the hydroxyl radical. It is interesting to note that the calculation results of the water trimer cation vary seriously depending on the calculation level. At the level of Moller–Plesset second-order perturbation (MP2) theory, the stability of 3OO is underestimated due to the underestimated O…O hemibonding energy. This stability is also underestimated even for the CCSD(T) single point calculations on the MP2-optimized geometry. For the 3OO structure, the MP2 and CCSD(T) calculations give closed-ring structures with a hemi-bond between two O atoms, while the DFT calculations show open-ring structures due to the overestimated O…O hemibonding energy. Thus, in order to obtain reliable stabilities and frequencies of the water trimer cation, the CCSD(T) geometry optimizations and frequency calculations are necessary. In this regard, the DFT functionals need to be improved to take into account the proper O…O hemibonding energy.

Given that transition metal-hydrogen systems have been studied as a predecessor for hydrogen storage materials, we have investigated the neutral and multiply charged titanium-H2 systems (Ti-H2, Ti(+)-H2, Ti(2+)-H2, Ti(3+)-H2, and Ti(4+)-H2) using density functional theory (DFT) and high-level ab initio calculations, including coupled cluster theory with single, double, and perturbatively triple excitations [CCSD(T)]. These systems show different types of hydrogenation depending on their charged state. The neutral Ti-H2 system shows dihydride structure with covalent interaction where the Ti-H distance is 1.76 Å, while H2 is dissociated into two neigboring hydride ions by withdrawing electrons from Ti. The charged Ti(+)-H2, Ti(2+)-H2, and Ti(3+)-H2 systems show dihydrogen structures with noncovalent interaction, where the Ti(+)-H, Ti(2+)-H, and Ti(3+)-H distances are 2.00, 2.14, and 2.12 Å, respectively. The main binding energies in these systems arise from the hydrogen polarizability driven interaction by the positive charge of Ti(n+) (n = 1-3). Among Ti(n+)-H2 (n = 1-3) the Ti(+)-H2 has the shortest distance against our common expectation, while Ti(2+)-H2 has the longest distance. The Ti(+)-H2 distance is the shortest because of the d-σ* molecular orbital (MO) interaction which is not present in Ti(2+)-H2 and Ti(3+)-H2. The Ti(4+) ion does not bind H2. In this regard, we have investigated the maximal hydrogen binding capacity by Ti complexes. The coordination of titanium mono- and dications complexed with dihydrogen (H2) [Ti(+)(H2)n and Ti(2+)(H2)m] is studied along with their structures, binding energies, electronic properties, and spectra. The titanium monocations of the quartet ground state have up to the hexacoordinaton, while titanium dications of the triplet ground state have up to the octacoordination at very low temperatures. At room temperature, the monocations favor penta- to hexacoordination, while the dications favor hexacoordination. This information would be useful for the design of hydrogen storage devices of Ti complexes, such as Ti-decorated/dispersed polymer-graphene hybrid materials.

Capturing CO2 by aqueous ammonia has recently received much attention due to its advantages over other state-of-the-art CO2-capture technology. Thus, understanding this CO2-capturing mechanism, which has been causing controversy, is crucial for further development toward advanced CO2 capture. The CO2 conversion mechanism in aqueous ammonia is investigated using ab initio calculations and kinetic simulations. We show full details of all reaction pathways for the NH3-driven conversion mechanism of CO2 with the pronounced effect of microsolvation. Ammonia performs multiple roles as reactant, catalyst, base, and product controller. Both carbamic and carbonic acids are formed by the ammonia-driven trimolecular mechanism. Ammonia in microsolvation makes the formation of carbamic acid kinetically preferred over carbonic acid. As the concentration of CO2 increases, the dominant product becomes carbonic acid. The conversion from carbamic acid into carbonic acid occurs through the decomposition−recombination pathwa...

Many ligands have been experimentally designed and tested for their activities as inhibitors against bacterial enoyl-ACP reductase (FabI), ENR. Here the binding energies of the reported ligands with the E. coli ENR- were calculated, analyzed and compared, and their molecular dynamics (MD) simulation study was performed. IDN, ZAM and AYM ligands were calculated to have larger binding energies than TCL and IDN has the largest binding energy among the considered ligands (TCL, S54, E26, ZAM, AYM and IDN). The contribution of residues to the ligand binding energy is larger in E. coli ENR-NAD+-IDN than in E. coli ENR--TCL, while the contribution of is smaller for IDN than for TCL. The large-size ligands having considerable interactions with residues and have many effective functional groups such as aromatic rings, acidic hydroxyl groups, and polarizable amide carbonyl groups in common. The cation- interactions have large binding energies, positively charged residues strongly interact with polarisable amide carbonyl group, and the acidic phenoxyl group has strong H-bond interactions. The residues which have strong interactions with the ligands in common are Y146, Y156, M159 and K163. This study of the reported inhibitor candidates is expected to assist the design of feasible ENR inhibitors.

We have investigated the structure, interaction energy, electronic properties, and IR spectra of the ammonia-water cation (NH(3)H(2)O)(+) using density functional theory (DFT) and high-level ab initio theory. The ammonia-water cation has three minimum-energy structures of (a) H(2)NH(+)...OH(2), (b) H(3)N(+)...OH(2), and (c) H(3)NH(+)...OH. The lowest-energy structure is (a), followed by (c) and (b). The ammonia dimer cation has two minimum-energy structures [the lowest H(3)NH(+)...NH(2) structure and the second lowest (H(3)N...NH(3))(+) structure]. The minimum transition barrier for the interconversion between (a), (b), and (c) is approximately 6 kcal/mol. Most DFT calculations with various functionals, except a few cases, overstabilize the N...O and N...N binding, predicting different structures from Moller-Plesset second-order perturbation (MP2) theory and the most reliable complete basis set (CBS) limit of coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. Thus, the validity test of the DFT functionals for these ionized molecular systems would be of importance.

By using density functional theory (DFT) and high-level ab initio theory, the structure, interaction energy, electronic property, and IR spectra of the water dimer cation [(H2O)2(+)] are investigated. Two previously reported structures of the water dimer cation [disproportionated ionic (Ion) structure and hydrazine-like (OO) structure] are compared. For the complete basis set (CBS) limit of coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)], the Ion structure is much more stable (by 11.7 kcal/mol). This indicates that the ionization of water clusters produce the hydronium cation moiety (H3O(+)) and the hydroxy radical. The transition barrier for the interconversion from the Ion/OO structure is ∼15/∼9 kcal/mol. It is interesting to note that the calculation results of the water dimer cation vary seriously depending on calculation methods. Moller-Pleset second-order perturbation (MP2) theory gives reasonable relative energies in favor of the Ion structure but reports unreasonable frequencies for the OO structure. On the other hand, most DFT calculations with various functionals overstabilize the OO structure. However, the DFT results with MPW1K and BH&HLYP functionals are very close to the CCSD(T)/CBS results. Thus, as for the validity test of the DFT functionals for ionized molecular systems, the energy comparison of two water dimer cation structures would be a very important criterion.

We have reinvestigated the structures of hydrated hydride anion clusters, using density functional theory and high-level ab initio theory. We find new low-lying energy structures for H(H2O)n3,4,6 which are compatible with previously reported structures. The binding energies, electronic properties, and IR spectra of these competing low-energy hydrated hydride anion clusters are reported to facilitate experiments. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem 109: 1820 -1826, 2009

Using high-level ab initio calculations and excited state ab initio molecular dynamics simulations, we show that hydrated iodic acids release hydrogen radicals and/or hydrogen molecules as well as iodine radicals upon excitation. Its photoreaction process involving charge transfer to the solvent takes place in four steps: 1) hydration of the acid, 2) charge transfer to water upon excitation of hydrated acid, 3) detachment of the neutral iodine atom, and 4) detachment of the hydrogen radical. The iodine detachment process from excited hydrated hydro-iodic acids is exothermic and the detachment of hydrogen radicals from hydrated hydronium radicals is spontaneous if the initial kinetic energy of the cluster is high enough to get over the activation barrier of the detachment. The complete release of the radicals can be understood in terms of kinetics. This study shows how the hydrogen and halogen radicals are dissociated and released from their hydrated acids. Simple experiments corroborate our predicted mechanism for the release of hydrogen molecules from iodic acid in water by ultraviolet light.

In contrast to the extensive theoretical investigation of the solvation phenomena, the dissolution phenomena have hardly been investigated theoretically. Upon the excitation of hydrated halides, which are important substances in atmospheric chemistry, an excess electron transfers from the anionic precursor (halide anion) to the solvent and is stabilized by the water cluster. This results in the dissociation of hydrated halides into halide radicals and electron-water clusters. Here we demonstrate the charge-transfer-to-solvent (CTTS)-driven femtosecond-scale dissolution dynamics for I-(H2O)n=2-5 clusters using excited state (ES) ab initio molecular dynamics (AIMD) simulations employing the complete-active-space self-consistent-field (CASSCF) method. This study shows that after the iodine radical is released from I-(H2O)n=2-5, a simple population decay is observed for small clusters (2

A hydroiodic acid molecule is dissociated by more than four water molecules. Here, the effect of an excess electron on the hexahydated hydroiodic acid where the dissociated structure [H3O+(H2O)5I−] is much more stable than the undissociated one [(H2O)6HI], is investigated. Upon binding an excess electron (e−), the cluster releases a hydrogen radical and forms the stable hexahydrated iodide [(H2O)6I−] when the initial kinetic energy is above ∼200 K, due to the small transition barrier (∼0.5 eV). The system with the hydrogen radical released, [(H2O)6I− + H·], is much more stable than the systems with an excess electron, [(H3O)+{e−(H2O)5}I−] or [e−(H2O)2(H3O)+(H2O)3I−].

In this account, we highlight the theoretical investigations of various cluster systems comprising of water clusters, π-containing clusters, and metallic clusters. We illustrate how these investiga...

In this mini-review we briefly describe intermolecular interactions ranging from hydrogen bonding to ionic interactions to aromatic interactions. Manifestation of these interaction forces is in the design and realization of various ionophores with chemo-sensing capability for biologically important cations and anions. We also explain how the understanding of hydrogen bonding and π-interactions has led to the design of self-assembled organic nanotubes. We further discuss the conformational changes between stacked and edge-to-face conformers in benzoquinone-benzene complexes, which are controlled by alternating electrochemical potential. The resulting flapping motion illustrates a promising pathway toward the design of nanomechanical devices.

Electron(e)-binding hydrated hydrogen fluoride clusters [e–HF(H2O)n=1–10] have been studied with density functional and ab intio calculations. The hydrofluoric acid in e–HF(H2O)n clusters is found to be undissociated at 0 K till n = 10. The e–HF(H2O)3 cluster is particularly unstable compared with the corresponding neutral structures, which reflects the particularly unstable antimagic number of e–water tetramer. The characteristic of “magic” numbers of electron–water clusters appears in these e–(HF)(H2O)n clusters. The vertical detachment energies of e–HF(H2O)n are enhanced by the HF acid as compared with those of the e-binding water clusters [e–(H2O)n + 1], and the excess electron is surface bound near the terminal water molecule with two dangling hydrogen atoms. The coordination number of HF is one for n = 1–4 as a linear structure in contrast to two for n = 5–6, and three for n = 7–10. The phase transition from 2- to 3-dimensional structures appears at penta-hydrated system in contrast to hepta-hydrated system for neutral HF–water clusters. The structures for e–HF(H2O)n=2,3 are quite different from those of the corresponding e–(H2O)n + 1 = 3,4, and the structures for e–HF(H2O)n = 2–6 are quite different from those of the corresponding HF(H2O)n = 2–6.

For the design of functional molecules and nanodevices, it is very useful to utilize nanorecognition (which is governed mainly by interaction forces such as hydrogen bonding, ionic interaction, π-H/π-π interactions, and metallic interactions) and nanodynamics (involving capture, transport, and release of electrons, photons, or protons). The manifestation of these interaction forces has led us to the design and realization of diverse ionophores/receptors, organic nanotubes, nanowires, molecular mechanical devices, molecular switches, enzyme mimetics, protein folding/unfolding, etc. In this review, we begin with a brief discussion of the interaction forces, followed by some of our representative applications. We discuss ionophores with chemo-sensing capability for biologically important cations and anions and explain how the understanding of hydrogen bonding and π-interactions has led to the design of self-assembled nanotubes from calix[4]hydroquinone (CHQ). The binding study of neutral and cationic transition metals with the redox system of hydroquinone (HQ) and quinone (Q) predicts what kind of nanostructures would form. Finally, we look into the conformational changes between stacked and edge-to-face conformers in π-benzoquinone-benzene complexes controlled by alternating electrochemical potential. The resulting flapping motion illustrates a promising pathway toward the design of mobile nanomechanical devices.

There are no clear conclusions over the structures of the acetylene clusters. In this regard, we have carried out high-level calculations for acetylene clusters (C2H2)2-5 using dispersion-corrected density functional theory (DFT-D), Møller-Plesset second-order perturbation theory (MP2); and coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] at the complete basis set limit. The lowest energy structure of the acetylene dimer has a T-shaped structure of C2v symmetry, but it is nearly isoenergetic to the displaced stacked structure of C2h symmetry. We find that the structure shows the quantum statistical distribution for configurations between the T-shaped and displaced stacked structures for which the average angle (|θ̃|) between two acetylene molecules would be 53-78°, close to the T-shaped structure. The trimer has a triangular structure of C3h symmetry. The tetramer has two lowest energy isomers of S4 and C2h symmetry in zero-point energy (ZPE)-uncorrected energy (ΔEe), but one lowest energy isomer of C2v symmetry in ZPE-corrected energy (ΔE0). For the pentamer, the global minimum structure is C1 symmetry with eight sets of T-type π-H interactions and a set of π-π interactions. Our high-level ab initio calculations are consistent with available experimental data.

The cation-π interactions have been intensively studied. Nevertheless, the interactions of π systems with heavy transition metals and their accurate conformations are not well understood. Here, we theoretically investigate the structures and binding characteristics of transition metal (TM) cations including novel metal cations (TM(n+) = Cu(+), Ag(+), Au(+), Pd(2+), Pt(2+), and Hg(2+)) interacting with benzene (Bz). For comparison, the alkali metal complex of Na(+)-Bz is also included. We employ density functional theory (DFT) and high levels of ab initio theory including Moller-Plesset second-order perturbation (MP2) theory, quadratic CI method with single and double substitutions (QCISD), and the coupled cluster theory with single, double, and perturbative triple excitations (CCSD(T)). Each of the transition metal complexes of benzene exhibits intriguing binding characteristics, different from the typical cation-π interactions between alkali metal cations and aromatic rings. The complexes of Na(+), Cu(+), and Ag(+) favor the conformation of C6v symmetry with the cation above the benzene centroid (πcen). The formation of these complexes is attributed to the electrostatic interaction, while the magnitude of charge transfer has little correlation with the total interaction energy. Because of the TM(n+)←π donation, cations Au(+), Pd(2+), Pt(2+), and Hg(2+) prefer the off-center π conformation (πoff) or the π coordination to a C atom of the benzene. Although the electrostatic interaction is still important, the TM←π donation effect is responsible for the binding site. The TM(n+)-Bz complexes give some characteristic IR peaks. The complexes of Na(+), Cu(+), and Ag(+) give two IR active modes between 800 and 1000 cm(-1),which are inactive in the pure benzene. The complexes of Au(+), Pd(2+), Pt(2+), and Hg(2+) give characteristic peaks for the ring distortion, C-C stretching, and C-H stretching modes as well as significant red-shifts in the CH out-of-plane bending.

Extended electron-deficient arenes are investigated as potential neutral receptors for polyanions. Anion binds via σ interaction with extended arenes, which are composed solely of C and N ring atoms and CN substituents. As a result, the positive charge on the aromatic C is enhanced, consequently maximizing binding strength. Selectivity is achieved because different charge distributions can be obtained for target anions of a particular geometry. The halides F(-) and Cl(-) form the most stable complex with 6, while the linear N3(-) interacts most favorably with 7. The trigonal NO3(-) and tetrahedral ClO4(-) fit the 3-fold rotational axis of 6 but do not form stable complexes with 5 and 7. The Y-shaped HCOO(-) forms complexes with 4, 5, and 7, with the latter being the most stable. Thus, the anion complexes exhibit strong binding and the best geometrical fit between guest and host, reminiscent of Lego blocks.

We have studied geometries, energies and vibrational spectra of disulfuric acid (H2S2O7) and its anion (HS2O7(-)) hydrated by a few water molecules, using density functional theory (M062X) and ab initio theory (SCS-MP2 and CCSD(T)). The most noteworthy result is found in H2S2O7(H2O)2 in which the lowest energy conformer shows deprotonated H2S2O7. Thus, H2S2O7 requires only two water molecules, the fewest number of water molecules for deprotonation among various hydrated monomeric acids reported so far. Even the second deprotonation of the first deprotonated species HS2O7(-) needs only four water molecules. The deprotonation is supported by vibration spectra, in which acid O-H stretching peaks disappear and specific three O-H stretching peaks for H3O(+) (eigen structure) appear. We have also kept track of variations in several geometrical parameters, atomic charges, and hybrid orbital characters upon addition of water. As the number of water molecules added increases, the S-O bond weakens in the case of H2S2O7, but strengthens in the case of HS2O7(-). It implies that the decomposition leading to H2SO4 and SO3 hardly occurs prior to the 2nd deprotonation at low temperatures.

The CO2 capturing and sequestration are of importance in environmental science. Understanding of the CO2-interactions with various functional molecules including multi-N-containing superbases and heteroaromatic ring systems is essential for designing novel materials to effectively capture the CO2 gas. These interactions are investigated using density functional theory (DFT) with dispersion correction and high level wave function theory (resolution-of-identity (RI) spin-component-scaling (scs) Moller-Plesset second-order perturbation theory (MP2) and coupled cluster with single, double and perturbative triple excitations (CCSD(T))). We found intriguing molecular systems of melamine, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 7-azaindole and guanidine, which show much stronger CO2 interactions than the well-known functional systems such as amines. In particular, melamine could be exploited to design novel materials to capture the CO2 gas, since one CO2 molecule can be coordinated by four melamine molecules, which gives a binding energy (BE) of ∼85 kJ mol(-1), much larger than in other cases.

To understand the self-assembly process of the transition metal (TM) nanoclusters and nanowires self-synthesized by hydroquinone (HQ) and calix[4]hydroquinone (CHQ) by electrochemical redox processes, we have investigated the binding sites of HQ for the transition-metal cations TM(n+)=Ag(+), Au(+), Pd(2+), Pt(2+), and Hg(2+) and those of quinone (Q) for the reduced neutral metals TM(0), using ab initio calculations. For comparison, TM(0)-HQ and TM(n+)-Q interactions, as well as the cases for Na(+) and Cu(+) (which do not take part in self-synthesis by CHQ) are also included. In general, TM-ligand coordination is controlled by symmetry constraints imposed on the respective orbital interactions. Calculations predict that, due to synergetic interactions, silver and gold are very efficient metals for one-dimensional (1D) nanowire formation in the self-assembly process, platinum and mercury favor both nanowire/nanorod and thin film formation, while palladium favors two-dimensional (2D) thin film formation.

In earlier studies, the interactions of isolated ionic species with various solvents were investigated using ab initio calculations. The ionic species investigated included cations (proton, hydronium, ammonium, and metal cations) and anions (single electron, hydroxide, and halide anions). However in the present study, we investigate the interactions of these ionic species with the solvent in the presence of other competing ionic species. We also elaborate on how the information obtained from these extensive studies have been employed in designing and synthesizing various kinds of novel ionophores and receptors.

We review our theoretical work done on a variety of different chemical systems, which show different H-bonding characteristics. The systems include water clusters, its interactions with polar molecules and π-systems, organic nanotubes, enzymes, and ionophores/receptors. Special features of normal, short, short strong, and π-type H-bonding interactions in these systems are discussed in terms of structures, interaction energies, and spectra.

We have investigated the solvent rearrangement process for the excited electron in the iodide–water pentamer using density functional and ab initio calculations. Upon excitation of the iodide–water pentamer, an electron transfers from the iodide anion to the water cluster, resulting in release of the iodine atom and formation of the anionic water pentamer undergoing rearrangement process toward the most stable conformation. Thus, the transformation pathway is elucidated.

e − W2( 2 S1/2)] are studied by using ab initio methods. This study is interesting and important in order to understand the formation mechanism and stabilization of electron-water dimer, the relationships between anion-water and electronwater dimer, and the electronic properties and phenomena of solvated or hydrated anions and electrons. The structures of di-hydrated iodide 17 and electron 6 previously studied at B3LYP/6-311++G**, B3LYP/6-311++G** [sp] and MP2/aug-cc-pVDZ+(2s2p/2s) levels were adapted in this study. The extra diffuse functions in brackets and parentheses have the exponent values scaled by 1/8 from those of the outermost basis functions in conventional basis sets. The CTTS energies of halide-water clusters were predicted at the RPA-B3LYP/6-311++G**, CIS/6-311++ G**(sp) and CIS/aug-cc-pVDZ+(2s2p/2s) levels of theory. The IR spectra were calculated at B3LYP/6-311++G**(sp), MP2/6-311++G**(sp), CIS/6-311++G**(sp), MP2/aug-ccpVDZ+(2s2p/2s), and CIS/aug-cc-pVDZ+(2s2p/2s) levels of theory. All calculations were performed by using a Gaussian 03 suite of programs. 29 Some important figures

This study details the interactions prevailing in aqueous clusters of monovalent alkali metal, ammonium, and hydronium cations. The calculations involve a detailed evaluation of the structures, thermodynamic energies, andIR spectra of several plausible conformers of M + .(H 2 O) 1 - 6 (M = Li, Na, K, Rb, Cs, NH 4 , H 3 O) clusters at the second-order Miller-Plesset (MP2) and density functional levels of theory. A detailed decomposition of the interaction energies has been carried out for complexes involving one or two water molecules using symmetry adapted perturbation theory. Some of the salient insights on the structures include the emergence of the second solvent shell even before the realization of the maximal coordination number of the cation. This effect was more pronounced in clusters involving the larger cations. The quantitative estimates of various components of the interaction energy indicate the predominance of electrostatic energies in the binding of these cations to water molecules. Interestingly, for all the hydrated alkali metal cation complexes, the contribution of electrostatic energy is almost the same as the total attractive interaction energy, whereas the sum of the induction and dispersion energies are almost canceled out by exchange-repulsion energy. However, the contribution of dispersion energies slowly starts increasing as the size of the cation increases and is quite substantial in case of the Cs + complexes. In the organic cations, the dispersion energies become significant, though not comparable to the electrostatic energies. In addition to the evaluation of the harmonic frequencies of -OH stretching mode of all the structures, the anharmonic frequencies were evaluated for the smaller clusters. As the size of the cation and the size of the water cluster increases, the red shifts associated with the -OH stretching mode progressively become larger for the alkali metal cation containing complexes. For the organic cation (NH 4 +, H 3 O + ) containing complexes, an opposite trend is observed. Compared to the isolated water monomer, the ratio of the infrared intensities of the asymmetric and symmetric -OH stretching modes is very small. However, this ratio progressively becomes larger as the size of the cation increases.

Using ab initio and density functional calculations, we studied photoexcitation of a charge-balanced electron donor–acceptor (DA) complex comprised of tetracyanoethylene (TCE) and tetramethylethylene (TME). We considered both the TCE-TME stacked conformer and a possible conformer with a solvent molecule (dichloromethane) inserted between TCE and TME. The photoexcitation of the DA complex can directly form a charge transfer (CT) state. Our theoretical investigations show that the CT state can also be produced from the decay of higher excited states. Using the continuum model, we investigated the solvent effects on CT absorption, local excitation, and CT emission in the polar solvent. The equilibrium solvation energies of the ground and excited states of the DA complex were calculated using the self-consistent reaction field method, and then the correction of nonequilibrium solvation energies for the vertical transitions was made. The transition energies (i.e., CT absorption for the DA complexes and CT emis...

Owing to the utility of redox phenomena of silver in many chemical systems, it is important to understand the coordination chemistry of Ag+ ion and hence the hydration structure. The lowest-energy conformations of Ag+(H2O)1–6 are sensitive to the calculation method employed. The coordination number (Nc) of Ag+(H2O)n is predicted to be 2 for n=2–6 at the density functional theory level, while the Nc for n=3–5 is 3, and that for n=6 is 4 at the second-order Moller–Plesset perturbation level. Further accurate analysis based on coupled-cluster singles and doubles theory with perturbative corrections for triple excitations agrees with the MP2 results except that Nc of 4 is also as competitive as Nc of 3 for n=5. To identify the correct Nc, it would be useful to facilitate the IR experimental characterization. We thus provide the OH spectra for various possible structures. It is interesting to note that the hydration chemistry of Ag+ ion is somewhat different from that of alkali metal ions.

The structures of pure gold and silver clusters (Auk, Agk, k = 1−13) and neutral and anionic gold−silver binary clusters (AumAgn, 2 ≤ k = m + n ≤ 7) have been investigated by using density functional theory (DFT) with generalized gradient approximation (GGA) and high level ab initio calculations including coupled cluster theory with relativistic ab initio pseudopotentials. Pure Auk clusters favor 2-D planar configurations, while pure Agk clusters favor 3-D structures. In the case of Au, the valence orbital energies of 5d are close to that of 6s. This allows the hybridization of 6s and 5d orbitals in favor of planar structures of Auk clusters. Even 1-D linear structures show reasonable stability as local minima (or as global minima in a few small anionic clusters). This explains the ductility of gold. On the other hand, the Ag-4d orbital has a much lower energy than the 5s. This prevents hybridization, and so the coordination number (Nc) of Ag in Agk tends to be large in s-like spherical 3-D coordination i...

Although various low-lying energy structures of electron–water clusters, e−–(H2O)2–6, have been reported, some of the global minimum energy structures (in particular, for the tetramer and pentamer) are still not clearly characterized yet. Therefore, using high-level ab initio calculations, we have investigated several new low-lying energy conformers in addition to previously reported ones. The lowest energy conformer for the pentamer is found to have a wedge-like structure which has never been studied before. Based on the experimental vertical electron-detachment energies and OH vibrational spectra of the electron–water clusters, we report the most probable structures and their nearly isoenergetic structures. The OH vibrational frequencies of e−(H2O)2–6 and e−HOD(D2O)1–5 are investigated, and are found to be in excellent agreement with the available experimental data. Their O–H stretch frequency shifts are classified in terms of the types of water molecules.

The electron–water heptamer is one of the “magic” numbers in the mass spectra of electron–water clusters, but up to now the structure of the electron–water heptamer is not known. Thus we have investigated a number of low-energy structures, and report the lowest-energy structure using ab initio calculations. The relative energies, vertical electron-detachment energies, and OH vibrational frequencies of several lowest energy conformers are discussed. As in the case of e−(H2O)6, the electron affinity for e−(H2O)7 is predicted to be positive, which would explain the intense peak observed in the time-of-flight mass spectra.

The structures, vibrational frequencies, electronic properties, and cis-trans photoisomerization process of retinal chromophore and its derivatives (i.e., Schiff base and protonated Schiff base) are studied using the density-functional theory with Becke’s three-parameter exchange functional together with the correlation functionals of Lee–Yang–Parr (B3LYP) and the second order Moller–Plesset perturbation theory (MP2). The optical transition energies for photoisomerization are reported at the configuration interaction (CI) level with single excitations (CIS) as well as at the B3LYP/6-31G* level using the random-phase-approximation (RPA). For slightly simplified model systems of retinal chromophore and its derivatives, the RPA values are very close to those of the complete active-space self-consistent-field (CASSCF) method and the multireference CI (MRCI) method, and are also in reasonable agreement with the experiments. We have also tried to investigate the solvent effect of the vertical transition energies in the presence of one or two water molecules. The present study deals with the mechanism of the cis-trans (or trans-cis) photoisomerization based on the molecular orbital (MO) analysis.

Various structures of halide-water pentamers and hexamers have been investigated using extensive ab initio calculations. Then, we compare the structures, spectra, and electronic properties of the hydrated fluoride, chloride, bromide, and iodide systems. Although some of the fluoride and iodide systems have been investigated earlier, we have carried out more accurate calculations on an enlarged conformational ensemble. The chloride-, bromide-, and iodide-water pentamers and hexamers behave somewhat similarly, but differently from the fluoride-water pentamer and hexamer. Fluoride-water clusters show semisurface (or semi-internal) structures, while chloride-, bromide-, and iodide-water clusters show surface structures. We substantiate our findings by evaluating various electronic properties such as ionization potentials, natural bond orbital charges, dipole moments, and charge-transfer-to-solvent energies, as well as vibrational frequencies of the low-energy halide-water pentamers and hexamers.

The structures, energies, Spectra, and charge transfer to solvent (CTTS) energies of the hydrated superoxide anion clusters have been investigated using both density functional calculations (DFT) with Becke-3-parameters employing Lee-Yang-Parr functionals (B3LYP) and second-order M⊘ller Plesset perturbation (MP2) calculations employing highly diffuse basis sets. Given the good agreement of our predicted results with the experimental spectra and enthalpy, it would be interesting if the CTTS energies could be experimentally determined.

Ab initio calculations have been carried out to study the water clusters of undecamer [(H2O)11] and dodecamer [(H2O)12]. At the level of Moller–Plesset second-order perturbation theory using TZ2P++ basis set, the lowest-energy conformer of the undecamer has the skeletal structure of Prism56 that a cyclic pentamer and a cyclic hexamer are fused into a prism shape with 16 hydrogen bonds (HBs). In this case, there are quite a number of nearly isoenergetic conformers with different hydrogen orientations. Among these, more stable conformers tend to have dangling H atoms separated (i.e., less clustered). The lowest energy conformer of the undecamer is different in hydrogen orientation from any previously suggested structure (including the ones obtained from various minimization algorithms). A second lowest energy skeletal structure is of Prism56B that a cyclic pentamer and an open-book hexamer are fused into a prism shape with 17 HBs. The most stable dodecamer is a fused cubic or tetragonal prism skeletal struc...

The water hexamer is known to have a number of isoenergetic structures. The first experimental identification of the O–H stretching vibrational spectra of the water hexamer was done in the presence of benzene. It was followed by the identification of the pure water hexamer structure by vibration-rotational tunneling (VRT) spectroscopy. Although both experiments seem to have located only the Cage structure, the structure of the benzene–water hexamer complex is not clearly known, and the effect of benzene in the water hexamer is unclear. In particular, it is not obvious how the energy difference between nearly isoenergetic water hexamer conformers changes in the presence of benzene. Thus, we have compared the benzene complexes with four low-lying isoenergetic water hexamers, Ring, Book, Cage, and Prism structures, using ab initio calculations. We also investigated the effects of the presence of benzene on the structures, harmonic vibrational frequencies, and infrared (IR) intensities for the four low-lying energy conformers. There is little change in the structure of the water hexamer upon its interaction with the benzene molecule. Hence the deformation energies are very small. The dominant contribution to the benzene–water cluster interaction mainly comes from the π–H interactions between benzene and a single water molecule. As a result of this π–H interaction, O–Hπ bond length increases and the corresponding stretching vibrational frequencies are redshifted. The IR spectral features of both (H2O)6 and benzene–(H2O)6 are quite similar. From both the energetics and the comparison of calculated and experimental spectra of the benzene–(H2O)6, the water structure in these complexes is found to have the Cage form. In particular, among the four different Cage structures, only one conformer matches the experimental O–H vibrational frequencies.

The correlation of various properties of water clusters (H2O)n=1–10 to the cluster size has been investigated using extensive ab initio calculations. Since the transition from two dimensional (2-D) (from the dimer to pentamer) to 3-D structures (for clusters larger than the hexamer) is reflected in the hexamer region, the hexamer can exist in a number of isoenergetic conformers. The wide-ranging zero-point vibrational effects of the water clusters having dangling H atoms on the conformational stability by the O–H flapping or proton tunneling through a small barrier (∼0.5 kcal/mol) between two different orientations of each dangling H atom are not large (∼0.1) kcal/mol). Large dipole moments (>2.5 D) are found in the dimer and decamer, and significant dipole moments (∼2 D) are observed in the monomer, hexamer, and nonamer. The polarization per unit monomer rapidly increases with an increasing size of the cluster. However, this increase tapers down beyond the tetramer. The O–H vibrational frequencies serve as sensitive indicators of the status of proton donation (“d”) and acceptance (“a”) (i.e., the structural signature of H-bond type) for each water monomer in the cluster. In general, the magnitudes of the O–H frequencies (ν) for each cluster can be arranged in the following order: ν3da (single donor–single acceptor) ≅ν3daa (single donor–double acceptor) >ν3dda (double donor–single acceptor) >ν1dda>ν1da> (or ≅) ν1daa. The increase in the cluster size has a pronounced effect on the decrease of the lower frequencies. However, there are small changes in the higher frequencies (ν3da and ν3daa). The intensities of ν1daa and ν1da are very high, since the increased atomic charges can be correlated to the enhanced H-bond relay effect. On the other hand, the intensities of the ν1dda modes are diminished by more than half. Most of the above data have been compared to the available experimental data. Keeping in view the recent experimental reports of the HOH bending modes, we have also analyzed these modes, which show the following trend: ν2dda>ν2daa≅ν2da. The present study therefore would be useful in the assignments of the experimental O–H stretching and HOH bending modes.

The 1,2-bis-(3-thienyl)-ethene derivatives are known to be good photoswitches. A large number of experiments have been carried out on different classes of these molecules to find out the most effective photoswitch. We have selected several highly efficient representative model photoswitch molecules of this class and studied their structures, photophysics, and different molecular properties at the ground and vertically excited states using density functional technique together with its time-dependent analog. These analyses are motivated toward the understanding of the effective molecular criteria, which are to be satisfied by a molecule in order to be good photoswitch. The theoretical investigations indicate that the capped ethene derivatives of this class of molecules are more effective photoswitches than the uncapped ones. Our contention has been verified by carrying out similar calculations on a well-known thermally irreversible photoswitch molecule of this class. Since the transition of the open to the closed form in photoswitching devices is the key factor for the molecules to exhibit such properties, and, moreover, since the molecules are thermally stable, they could be used for designing nonlinear optical (NLO) switches. One such possibility has been explored theoretically using a model molecular system.

A number of conformers of aqua-K+ complexes, K+(H2O)n (n=1–10) have been investigated using high level ab initio calculations, to elucidate the structures and thermodynamic energies of the hydrated potassium ions. Since the coordination number of K+ is around six in the bulk water, the focus of the present study has been the n=5 and 6 clusters. In contrast to previous studies which have used only the enthalpies to compare against the experimental numbers, the present study also employs free energies. As a result, the predictions of a number of hitherto unknown conformers are in excellent agreement with the experimental results. The maximum coordination number for K+ in ligands containing O atoms is evaluated to be around eight from the energetics of structures possessing only the first hydration shell of water molecules around the K+ ion. It is of interest to note that the hydration of the K+ ion is less structured than that of the Na+ ion, since the water–water interaction becomes more important in the aqua–K+ clusters. The predicted vibrational frequencies of the aqua–K+ clusters reflect the H-bonding signature, and hence, could be utilized in the identification of the hydration structures of K+ in experiments.

In spite of a spate of studies of various water clusters, a few theoretical studies on the water heptamer are available. State-of-the-art ab initio calculations are thus carried out on twelve possible water heptamer structures to explore the conformation as well as spectroscopic properties of this water cluster. Two three-dimensional cagelike structures comprised of seven-membered cyclic rings with three additional hydrogen bondings were found to be the lowest-lying energy heptamer conformers. The global minimum energy structure was found to be 0.5 kcal/mol lower than the other. The zero-point energy uncorrected and corrected binding energies of the global minimum energy structure are 55.2 and 37.9 kcal/mol, respectively. An almost two-dimensional ring conformer, which is only 1 kcal/mol above the global minimum at 0 K, could be more stable above 150 K. The vibrational spectra of different heptamer conformers are discussed and compared with the spectra of the hexamer and octamer water clusters.

Despite utmost importance in understanding water ionization process, reliable theoretical results of structural changes and molecular dynamics (MD) of water clusters on ionization have hardly been reported yet. Here, we investigate the water cations [(H2O)n = 2–6+] with density functional theory (DFT), Moller–Plesset second-order perturbation theory (MP2), and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. The complete basis set limits of interaction energies at the CCSD(T) level are reported, and the geometrical structures, electronic properties, and infrared spectra are investigated. The characteristics of structures and spectra of the water cluster cations reflect the formation of the hydronium cation moiety (H3O+) and the hydroxyl radical. Although most density functionals fail to predict reasonable energetics of the water cations, some functionals are found to be reliable, in reasonable agreement with high-level ab initio results. To understand the ionization process of water clusters, DFT- and MP2-based Born-Oppenheimer MD (BOMD) simulations are performed on ionization. On ionization, the water clusters tend to have an Eigen-like form with the hydronium cation instead of a Zundel-like form, based on reliable BOMD simulations. For the vertically ionized water hexamer, the relatively stable (H2O)5+ (5sL4A) cluster tends to form with a detached water molecule (H2O). © 2013 Wiley Periodicals, Inc.

Among five structures suggested by four different groups, the lowest energystructure is found to have the skeletal structure of Prism56 (Pr56-24) that a cyclic pentamer and a cyclichexamer are fused into a prism-shape with 16 hydrogen-bonds (HBs).Key Words : Water clusters, Undecamer, Hydrogen bondIntroductionSmall water clusters (H

Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, KoreaReceived April 24, 2003The relative stabilities of weak binding clusters are sensitive to temperature due to the entropy effect. Thus,here we report significant changes in relative stabilities between two low-energy electron-water trimerstructures and those between three low-energy electron-water pentamer structures, as the temperatureincreases. The trimer and pentamer show structural changes around 200 K.Key Words : Solvated electron, Electron-bound water cluster, Electron-water clusters, Thermodynamics,Vertical-electron detachment energyIntroductionAs water is the most important solvent, the electron-waterclusters have been an interesting subject in physics,chemistry and biochemistry.