Search

A new strategy for reversible hydrogen storage based on the properties of hydrogen multicenter bonds is proposed. This is demonstrated by carrying out ab initio calculations of hydrogen saturation of titanium and bimetallic titanium-aluminum nanoclusters. Hydrogen saturation leads to the formation of exceptionally and energetically stable hydrogen multicenter bonds. The stabilization results from sharing of the hydrogen atom electron density with the frontier orbitals of the metal cluster. The strength of the hydrogen multicenter bonds can be modulated either by varying the degree of hydrogen loading or by suitable alloying. Mode-specific infrared excitation of the vibrational modes associated with the multicenter hydrogen bonds can release the adsorbed hydrogen, thereby enabling efficient reversible hydrogen storage. The possible formation of hydrogen multicenter bonds involving titanium atoms and its implication to hydrogen adsorption/desorption kinetics in hydrogen cycled Ti-doped NaAlH4 is also discussed. [ABSTRACT FROM AUTHOR]

Hydrogen undergoes dissociative chemisorption on small titanium clusters. How the electronic structure of the cluster changes as a function of the number of adsorbed hydrogen atoms is an important issue in nanocatalysis and hydrogen storage. In this paper, a detailed theoretical investigation of the structural, energetic, and electronic properties of the icosahedral Ti13 cluster is presented as a function of the number of adsorbed hydrogen atoms. The results show that hydrogen loaded Ti13H20 and Ti13H30 clusters are exceptionally stable and are characterized by hydrogen multicenter bonds. In Ti13H20, the dissociated hydrogen atoms are bound to each of the 20 triangular faces of Ti13, while in Ti13H30, they are bound to the 30 Ti–Ti edges of Ti13. Consequently, the chemisorption and desorption energies of the Ti13H20 (1.93 eV, 3.10 eV) are higher than that of Ti13H30 (1.13 eV, 1.95 eV). While increased hydrogen adsorption leads to an elongation of the Ti–Ti bonds, there is a concomitant increase in the electrostatic interaction between the dissociated hydrogen atoms and the Ti13 cluster. This enhanced interaction results from the participation of the subsurface titanium atom at higher hydrogen concentrations. Illustrative results of hydrogen saturation on the larger icosahedral Ti55 cluster are also discussed. The importance of these results on hydrogen saturated titanium clusters in elucidating the mechanism of hydrogen adsorption and desorption in titanium doped complex metal hydrides is discussed. [ABSTRACT FROM AUTHOR]

The nature of the interaction of paramagnetic atoms A (= [sup 4]N,[sup 4]p, ³O, ³S) with π systems and C[sub 60] in the A…C[sub 6]H[sub 6] complexes and endohedral fullerenes (A@C[sub 60]) has been investigated employing second-order Møller-Plesset perturbation level of theory and density functional theory calculations. The coupled cluster theory with single and double excitations, and with perturbed triplet excitations have also been carried out for the A … C[sub 6]H[sub 6] complexes. The calculated geometries indicate that the paramagnetic N and P atoms lie on the C[sub 6] axis of benzene in the A…C[sub 6]H[sub 6] complex and at the center of the C[sub 60] cage in A@C[sub 60]. On the other hand, the O and S atoms are slightly shifted towards the C-C bond of benzene in the A…C[sub 6]H[sub 6] complex. A comparison of the calculated binding energies (BEs) of these paramagnetic complexes and the corresponding rare gas complexes like He…C[sub 6]H[sub 6] and He@C[sub 60] indicate that the BEs of the former are much larger than those of the latter. For both the rare gas and paramagnetic atom complexes dispersive interactions dominate the BEs. The energy barriers of the penetration of the O atom through a [6,6] bond are lower than the penetration of the helium atom and higher than the penetration of a N atom. [ABSTRACT FROM AUTHOR]

Studies the nature of the pi-H interaction in ethene-H[sub 2]O, benzene-H[sub 2]O and benzene-(H[sub 2]O)[sub 2]. Minimum geometries and vibrational frequencies; Binding energy of the complexes; Results from symmetry adapted perturbation theory on the interaction of water with benzene and ethene.

Investigates the possibility of the benzene-water van der Waals bonded systems to exhibit a pi-H kind of bonding interaction. Ab initio calculations on different conformations of the fluorobenzene-water van der Waals bond; Involvement of the bond formed between benzene and water in the formation of a six-membered ring system with the F-C-C-H of the aromatic ring.

Analyzes binding energies, vibrational frequencies and equilibrium structures of benzene-hydrogen halide interactions. Methods used in ab initio calculations; Analysis of geometries and binding energies; Results of vibrational frequency shifts; Description of benzene complex quantum mechanical structure.

We present a systematic theoretical investigation of the interaction of an organic molecule with gold and palladium electrodes. We show that the chemical nature of the electrode elicits significant geometrical changes in the molecule. These changes, which are characteristic of the electrode atomic species and the interface geometry, are shown to occur at distances as great as 10 Å from the interface, leading to a significant modification of the inherent electronic properties of the molecule. In certain interface geometries, the highest occupied molecular orbital (HOMO) of the palladium-contacted molecule exhibits enhanced charge delocalization at the center of the molecule, compared to gold. Also, the energy gap between the conductance peak of the lowest unoccupied molecular orbital (LUMO) and the Fermi level is smaller for the case of the palladium electrode, thereby giving rise to a higher current level at a given bias than the gold-contacted molecule. These results indicate that an optimal choice of the atomic species and contact geometry could lead to significantly enhanced conductance of molecular devices and could serve as a viable alternative to molecular derivatization. [ABSTRACT FROM AUTHOR]

The interactions of the first-row hydrides (NH3, H2O, HF) with ethene have been investigated by carrying out calculations, at the second order Møller–Plesset (MP2) level of theory using both the 6-31+G* and aug-cc-pVDZ basis sets. Unlike previous investigations of these systems, the geometries and vibrational frequencies in the present study were obtained by carrying out explicit counterpoise corrected optimizations. In an effort to understand the nature of the H⋯π interactions prevalent in these complexes, the interaction energies were decomposed into individual energy components using the symmetry adapted perturbation theory. Given the goals of the present investigation, the geometries, vibrational frequencies and interaction energy components of the water dimer have also been evaluated. While the interaction energy of the conventional H-bond is dominated by electrostatic interactions, electrostatic, dispersive and inductive interactions are important in the description of the π H-bond. An important distinction between conventional H-bonded complexes and the π H-bonded complexes is that the inductive interaction gets magnified at the MP2 level. Thus, the inclusion of electron correlation is an important prerequisite both for the magnification of the inductive interaction and to obtain an accurate estimate of the dispersion energies. It is observed that changes in various geometrical and vibrational parameters of these π H-bonded complexes can be correlated to the magnitude of either the individual or a combination of various interaction energy components. [Copyright &y& Elsevier]

Investigates the nature and origin of the pi-hydrogen interaction in both the ethene (olefinic) and benzene (aromatic) complexes of the first-row hydrides. Enhancement of the pi-hydrogen interaction; Contributions of each of the attractive and repulsive exchange components of the total binding energy; Investigation of weak interactions.

The van der Waals complexes benzene-argon (BAr), fluorobenzene-argon (FAr), p-difluorobenzene-argon (DAr) are investigated at the second-order Møller–Plesset (MP2) level of theory using the 6-31+G(d), cc-pVDZ, aug-cc-pVTZ, and [7s4p2d1f/4s3p1d/3s1p] basis sets. Geometries, binding energies, harmonic vibrational frequencies, and density distribution are calculated where basis set superposition errors are corrected with the counterpoise method. Binding energies turn out to be almost identical (MP2/[7s4p2d1f/4s3p1d/3s1p]: 408, 409, 408 cm-1) for BAr, FAr, and DAr. Vibrationally corrected binding energies (357, 351, 364 cm-1) agree well with experimental values (340, 344, and 339 cm-1). Symmetry adapted perturbation theory (SAPT) is used to decompose binding energies and to examine the influence of attractive and repulsive components. Fluorine substituents lead to a contraction of the π density of the benzene ring, thus reducing the destabilizing exchange-repulsion and exchange-induction effects. At the same time, both the polarizing power and the polarizability of the π-density of the benzene derivative decreases thus reducing stabilizing induction and dispersion interactions. Stabilizing and destabilizing interactions largely cancel each other out to give comparable binding energies. The equilibrium geometry of the Ar complex is also a result of the decisive influence of exchange-repulsion and dispersive interactions. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The optimal structures, binding energies, and harmonic vibrational frequencies of clusters containing a substituted benzene molecule microsolvated by four water molecules, termed as π-(water tetramer) clusters (π: p-difluorobenzene, fluorobenzene, benzene, toluene) have been evaluated at the second order perturbation level of theory (MP2) using both the 6-31+G* and aug-cc-pVDZ basis sets. In sharp contrast to the complexes of smaller water clusters with these π systems, wherein the water subcluster is most strongly bound to toluene, the water tetramer is most strongly bound to fluorobenzene. This exceptionally high binding energy results from both a π···OH H-bond and a competing σ F···OH bond between the water tetramer moiety and the aromatic molecule. The magnitudes of the many-body energy terms and their contribution to the binding energies of these π-(water tetramer) systems indicates that the contributions of three- and higher-order terms are much smaller when compared to the neutral water clusters. The two-body terms associated with the π- and σ-type of interaction indicates that in both the fluorobenzene and p-difluorobenzene complexes, the increase in the size of the water cluster enhances the π-H-bonding interaction and weakens the σ F···H interaction. This observation is in consonance with the calculated and experimentally observed redshifts of the OH vibrational frequencies. Thus, with an increase in the size of a water cluster bound to the fluorinated π system, there is a lowering of the redshift induced by the σ F···H interaction and an increase in the redshift due to the π-H interaction. The calculated redshift of the π H-bonded OH mode is very much dependent on the basis set, with larger basis sets yielding shifts which are in better agreement with the experimentally determined shifts. © 2001 American ... [ABSTRACT FROM AUTHOR]

We investigate the interaction of the water trimer with the aromatic π systems (benzene, toluene, fluorobenzene, and p-difluorobenzene) at the second-order Møller-Plesset level of theory using both the 6-31+G* and aug-cc-pVDZ basis sets. The minimum-energy structures, binding energies, and the harmonic vibrational frequencies were calculated for all these systems. The minimum-energy structures reveal that the water trimer has only a π type of interaction with benzene and toluene, but has both a π and σ type (H-bond with the fluorine atom) of interaction with fluorobenzene and p-difluorobenzene. The binding energies reveal that, contrary to what has been observed in previous theoretical studies, the strength of the interaction of these π systems with the water trimer is greater than with the water dimer. A good agreement is obtained between the calculated and experimentally measured vibrational frequencies of these complexes. An analysis of the calculated OH stretching modes reveals some unusual facets of the π and σ type of interaction observed in the fluorobenzene and p-difluorobenzene complexes. Thus one observes that the OH red-shift associated with the π type of interaction is larger than with the σ type of interaction. This is particularly interesting because in both the water monomer and water dimer complexes, the OH red-shift associated with the interaction of the water cluster with fluorobenzene and p-difluorobenzene is due to the formation of the H-bond with the fluorine atom. Thus the increase in the size of the water cluster seems to favor the π type of interaction over the σ type of interaction. This inference is also supported by the magnitude of the two-body terms associated with the π and σ type of interaction in both the fluorobenzene and p-difluorobenzene complexes. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The interaction of the water dimer with both the olefinic and aromatic π systems (ethene, benzene, toluene, fluorobenzene, and p-difluorobenzene) has been investigated using both the supermolecular [second-order Møller-Plesset (MP2)] and perturbational (symmetry adapted perturbation theory) approaches. The geometry optimizations, harmonic vibrational frequencies, and the components of the binding energy were evaluated using fairly large basis sets (6-31+G[sup *] and aug-cc-pVDZ). The minimum energy structures obtained at the MP2/6-31+G[sup *] and MP2/aug-cc-pVDZ levels of theory indicate that the water dimer exhibits a π-type of interaction with ethene, benzene, and toluene and a σ-type of interaction with both fluorobenzene and p-difluorobenzene. This is demonstrated from the vibrational frequencies which are in good agreement with the experimentally determined numbers. Our calculations indicate that the nature and strength of the interaction of the donor water molecule (water dimer) with the π system has a significant bearing on the total binding energy of the complex. Apart from the interaction of the water dimer with the π system, we also show how this interaction influences the hydrogen bond characteristics of the water dimer by evaluating the hydrogen bond strengths. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electron-bound water clusters [e-(H2O)n] show very strong peaks in mass spectra for n=2, 6, 7, and (11), which are called magic numbers. The origin of the magic numbers has been an enigma for the last two decades. Although the magic numbers have often been conjectured to arise from the intrinsic properties of electron-bound water clusters, we attributed them not to their intrinsic properties but to the particularly weak stability of the corresponding neutral water clusters (H2O)n=2,6,7, and (11). As the cluster size increases; this nonsmooth characteristic feature in stability of neutral water clusters is contrasted to the smooth increase in stability of e--water clusters. As the magic number clusters have significant positive adiabatic electron affinities, their abundant distributions in atmosphere could play a significant role in atmospheric thermodynamics. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Owing to the utility of redox phenomena of silver in many chemical systems, it is important to understand the coordination chemistry of Ag[sup +] ion and hence the hydration structure. The lowest-energy conformations of Ag[sup +](H[sub 2]O)[sub 1–6] are sensitive to the calculation method employed. The coordination number (N[sub c]) of Ag[sup +](H[sub 2]O)[sub n] is predicted to be 2 for n=2–6 at the density functional theory level, while the N[sub c] for n=3–5 is 3, and that for n=6 is 4 at the second-order Møller–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 N[sub c] of 4 is also as competitive as N[sub c] of 3 for n=5. To identify the correct N[sub c], 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[sup +] ion is somewhat different from that of alkali metal ions. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Since the energy conversion reactions in most biological systems involve the interactions of various negatively charged quinones with water molecules, we have investigated the equilibrium structures, binding energies, and vibrational frequencies of the water monomer/dimer complexes of p-benzoquinones (Q) and its corresponding negatively charged anionic species (Q[sup -] and Q[sup 2-]). The calculations, which include the evaluation of charges and aromaticity, indicate the existence of double hydrogen bonds in the water monomer and water dimer complexes of neutral p-benzoquinone. When two water molecules are complexing with p-benzoquinone, a conformer exhibiting an H-bond between two water molecules (W[sub 2]Q) is energetically more favored than the conformer WQW in which there is no direct interaction between the water molecules. However, an increase in the negative charge of p-benzoquinone leads to the energetic stabilization of the WQ[sup 2-]W conformer over the Q[sup 2-]W[sub 2] conformer. The charge transfer Q to water molecules is found to be significant in the case of Q[sup 2-], up to -0.23 a.u. in the presence of two water molecules. An increase in the aromaticity of the p-benzoquinone molecule upon its interaction with the water molecules is noted from an analysis of the Nucleus Independent Chemical Shift (NICS) values. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The highly stereospecific and regiospecific recognition of α-amino acids exhibited by a novel Co(III) metal complex embodied in the experimental work (Nature 1999, 401, 254) is rationalized from the energetics and structural characteristics with the use of density functional calculations. The steric repulsion between the chiral center of the receptor [Co(III) complex] and alanine has been a cause for the discrimination of complex stabilities. The energies evaluated for all possible alanine binding modes clearly reveal regiospecificity. Our main emphasis is laid on the basecatalyzed epimerization reaction that drives the stereospecific recognition to near completion. The conducted tour mechanism is found to be the most likely candidate. A similar role by the equivalent Zn(II) complex is found. [ABSTRACT FROM AUTHOR]

Describes the type of ionophore family utilizing the cation-olefinic &pie; interaction. Development of novel ionophores; Comparison of the binding efficacies of collarenes and beltenes; Use of free energy perturbation methods to obtain binding affinities.

The design of cesium-selective ionophores must include the nature of cesium-water interactions. The authors have carried out extensive ab initio and density functional theory calculations of hydrated cesium cations to obtain reasonably accurate energetics, thermodynamic quantities, and IR spectra. An extensive search was made to find the most stable structures. Since water...water interactions are important in the aqua-Cs+ clusters, the authors investigated the vibrational frequency shifts as a function of the number of water molecules and the frequency characteristics with and without the presence of outer-shell water molecules. The predicted vibrational frequencies were then compared with the infrared photodissociation spectra of argon-tagged hydrated cesium cluster ions. This comparison allowed the identification of specific hydrogen-bonding structures present in the experimental spectra. [ABSTRACT FROM AUTHOR]

Using molecular-orbital analysis, we have elucidated the quantum-chemical origin of the intriguing phenomena in sequential hydration energies of the gold cation, which is known to be the most conspicuous among all transition metals. The hydration energy of Au+ with the second water molecule is found to be much larger than that with the first water molecule. Owing to the large relativistic effect of gold (i.e., significant lowering of the 6s orbital energy and significant raising of the 5d orbital energy), the highest occupied molecular orbital of the hydrated gold cation has a large portion of the 6s orbital. As the electron density of the 6s orbital populates in a large outer spherical shell far off the gold nucleus, the p orbitals (or sp hybridized lone-pair orbitals) of the water molecules are able to overlap with the outer part of the 6s orbital in the dihydrated gold cation, resulting in the unusual skewed overlap of p-6s-p orbitals (not the atom-to-atom bond overlap). No previous molecular-orbital analysis has reported this peculiar skewed orbitals overlap. Since this skewed orbitals overlap is saturated with two water molecules, this property is responsible for the low coordination number of the gold ion. [ABSTRACT FROM AUTHOR]

In an effort to examine the intricacies of electronic nanodevices, we present an atomistic description of the electronic transport properties of an isolated benzene molecule. We have carried out ab initio calculations to understand the modulation of the molecular orbitals (MOs) and their energy spectra under the external electric field, and conducting behavior of the benzene molecule. Our study shows that with an increase in the applied electric field, the energy of the third lowest unoccupied molecular orbital (LUMO) of benzene decreases, while the first and second LUMO energies are not affected. Above a certain threshold of the external electric field, the third LUMO is lowered below the original LUMO and becomes the real LUMO. Since the transport through a molecule is to a large extent mediated by the molecular orbitals, the change in MOs can lead to a dramatic increase in the current passing through the benzene molecule. Thus, in the course of this study, we show that the modulation of the molecular orbitals in the presence of a tuning parameter(s) such as the external electric field can play important roles in the operation of molecular devices. We believe that this understanding would be helpful in the design of electronic nanodevices. [ABSTRACT FROM AUTHOR]

The equilibrium structures and binding energies of the benzene complexes of p-benzoquinones (PBQ) and its negatively charged anionic species (PBQ- and PBQ2-) have been investigated theoretically using second-order Møller-Plesset calculations. While neutral p-benzoquinone-benzene clusters (PBQ-Bz) prefer to have a parallel displaced geometry (P-c), CH...π interactions (T-shaped geometries) prevail in the di-anionic PBQ-benzene (PBQ2--Bz) complexes (T-e2-). Studies on dianionic p-benzoquinone-benzene clusters showed that two nonbonded intermolecular interactions compete in the most stable conformation. One is H-bonding interaction (C–H...O type) between carbonyl oxygen of p-benzoquinone and one of the hydrogen atoms of benzene, and the other is a π–H interaction between π-electron cloud of PBQ2- and another hydrogen atom of benzene. Blueshifted H-bonds were observed in T-shaped clusters. The changes in the geometrical preference of PBQ-Bz complex upon addition of electrons would be useful in designing optimized molecular mechanical devices based on the edge-to-face and face-to-face aromatic interactions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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 S[SUB1] state are identified by IR/R2PI hole burning spectroscopy and assigned according to the vibrations calculated for the S[SUB1] state and compared with the vibrations calculated for the S[SUB0] state. The calculations for the So state were carried out at the second order Moller-Plesset level of theory using both the 6-31+G* and aug-cc-pVDZ basis sets and for the S[SUB1] state at the configuration interaction singles (CIS) level with the 6-31 + G* basis set. In the electronic ground state (S[SUB0]), 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 (ρ-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 (S[SUB1]) 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.... [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, we report the use of femtosecond time-resolved degenerate four-wave mixing rotationally resolved spectroscopy to obtain very accurate structural information on the symmetric top cyclohexane. Apart from highlighting the versatility of this method in determining accurate structures of large and complex molecules without dipole moment, the present study also details the comparison of the experimentally determined rotational constant B[subo] with that obtained from high-level ab initio calculations. The theoretical calculations, which were carried out at both the second-order Møller-Plesset (MP2) and coupled-cluster with single, double, and perturbative triple substitutions [CCSD(T)] levels of theory, also take into account vibrational averaging effects. A detailed investigation of the vibrational averaging effects reveals that the corrections emerge from only the six highly symmetric A19 modes, a justification of which is provided by an analysis of these modes. [ABSTRACT FROM AUTHOR]

Using the computer-aided molecular design approach, we recently reported the synthesis of calix[4]hydroquinone (CHQ) nanotube arrays self-assembled with infinitely long one-dimensional (1-D) short hydrogen bonds (H-bonds) and aromatic-aromatic interactions. Here, we assess various calculation methods employed for both the design of the CHQ nanotubes and the study of their assembly process. Our calculations include ab initio and density functional theories and first principles calculations using ultrasoft pseudopotential plane wave methods. The assembly phenomena predicted prior to the synthesis of the nanotubes and details of the refined structure and electronic properties obtained after the experimental characterization of the nanotube crystal are reported. For better characterization of intriguing 1-D short H-bonds and exemplary displaced π-π stacks, the X-ray structures have been further refined with samples grown in different solvent conditions. Since X-ray structures do not contain the positions of H atoms, it is necessary to analyze the system using quantum theoretical calculations. The competition between H-bonding and displaced π-π stacking in the assembling process has been clarified. The IR spectroscopic features and NMR chemical shifts of 1-D short H-bonds have been investigated both experimentally and theoretically. The dissection of the two most important interaction components leading to self-assembly processes would help design new functional materials and nanomaterials. [ABSTRACT FROM AUTHOR]

Analyzes the catalytic mechanism of enzymes. Importance of hydrogen bonds in enzymatic reactions; Stabilization of intermediates; Dissipation of negative charge from the substrates during the process.

Presents corrections to notations of water monomer conformers described in the article 'Structures, energies, vibrational spectra and electronic properties of water monomer to decamer.'

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-H[sub π] bond length increases and the corresponding stretching vibrational frequencies are redshifted. The IR spectral features of both (H[sub 2]O)[sub 6] and benzene-(H[sub 2]O)[sub 6] are quite similar. From both the energetics and the comparison of calculated and experimental spectra of the benzene-(H[sub 2]O)[sub 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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The correlation of various properties of water clusters (H[sub 2]O)[sub 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: ν[sub 3][sup da] (single donor-single acceptor) ≅ν[sub 3][sup daa] (single donor-double acceptor) >ν[sub 3][sup dda] (double donor-single acceptor) >ν[sub 1][sup dda]>ν[sub 1][sup da]> (or ≅) ν[sub 1][sup daa]. 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 (ν[sub 3][sup da] and ν[sub 3][sup daa]). The intensities of ν[sub 1][sup daa] and ν[sub 1][sup da] 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 ν[... [ABSTRACT FROM AUTHOR]

Laser spectroscopy, i.e., resonant two-photon ionization (R2PI), IR/R2PI ion depletion and hole burning spectroscopy have been applied in an experimental study of heterogenous clusters consisting of fluorobenzene (FB) or p-difluorobenzene (pDFB), respectively, microsolvated by up to three methanol molecules. Their infrared ion depletion spectra were taken in the region of the OH and CH stretches of methanol. In these complexes the methanol molecules form subclusters, which are weakly hydrogen bonded to either the aromatic π-system (π[sub OH]) or to the fluorine substituent (σ[sub F]) and a CH group (σ[sub CH]). In FB·(MeOH)[sub 1], pDFB·(MeOH)[sub 1], pDFB·(MeOH)[sub 2] and one isomer of FB·(MeOH)[sub 2] the methanol subunits exclusively exhibit σ[sub F] and σ[sub CH] H-bonds. A further isomer of FB·(MeOH)[sub 2] exhibits a π[sub OH] type interaction. For FB·(MeOH)[sub 3] and pDFB·(MeOH)[sub 3] (1:3) complexes the methanol subcluster may take on either a chainlike or a ringlike conformation. In the chainlike isomer of FB·(MeOH)[sub 3] the methanol trimer interacts with the chromophore via a π[sub OH] H-bond while with the corresponding pDFB cluster a σ[sub F] H-bond is formed. High level ab initio calculations, still feasible for molecular clusters of this size, have also been carried out to supplement the experimental investigations. These calculations include complete geometry optimizations of FB·(MeOH)[sub n] (n=1-2) and pDFB·(MeOH)[sub 1] at the MP2/6-31+G[sup *] and B3LYP/6-31+G* levels of theory. The minimum energy structures of the microsolvates together with their harmonic vibrational spectra allow a reliable assignment of the experimentally observed spectra and the determination of structure of the complexes. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This combined experimental and computational study is aimed at elucidating the structure and reactivity of heterogeneous molecular clusters. We report results for the system para-chlorofluorobenzene/methanol (pClFB/MeOH). Particularly, three different van der Waals (vdW) isomers of the neutral (1:1) aggregate (π,σ[sub F],σ[sub Cl]) have been assigned by comparison of experimental infrared frequencies in the O-H and C-H stretch regions measured by IR/R2PI depletion spectroscopy with calculated frequencies at MP2/6-31+G(d) and B3LYP/6-31+G(d) level. The isomers are weakly hydrogen-bonded complexes with methanol's OH group coordinated toward the aromatic π-electron cloud, the fluorine or the chlorine substituent, respectively. This assignment is corroborated by the UV and IR spectra of the corresponding monosubstituted benzene/methanol complexes. After resonant photoionization of pClFB·(MeOH)[sub n], for n=1 besides vdW fragmentation, no chemical reactivity was observed for any isomer. The investigation of aggregates with n>=5 indicated a substitution reaction taking place to produce chloroanisole[sup +], emphasizing the importance of neutral precursor cluster size over cluster structure for intracluster ion-molecule reactions. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]