Hydrogen multicenter bonds and reversible hydrogen storage.

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 NaAlH(4) is also discussed.

Structural, energetic, and electronic properties of hydrogenated titanium clusters.

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.

Nature of hydrogen interaction and saturation on small titanium clusters.

This work explores the nature of interaction and saturation of hydrogen molecules on small titanium clusters using ab initio calculations. Molecular dynamics simulations and ensuing charge density maps were used to gain insight into the key steps involved in dissociation of the hydrogen molecule on the metal clusters. The mechanistic insights gleaned from these simulations were subsequently utilized to obtain realistic models of the hydrogen saturated titanium clusters. It was found that the most stable hydrogen saturated titanium clusters involve hydrogen multicenter bonds. The observed peaks in the experimental mass and photoelectron spectra of hydrogen saturated titanium clusters are attributed to structures possessing hydrogen multicenter bonds. Hydrogen multicenter bonds are also ascribed to the origin of the broad shoulder in the vibrational spectra of hydrogen cycled Ti-doped NaAlH4 reported in a recent study.

Supersonic jet studies of solvation effects on the spectroscopy and photophysics of 4-diethylaminopyridine.

We present the results of spectroscopic and photophysical investigations of 4-diethylaminopyridine (DEAP) and its 1 : 1 complexes with a number of protic solvents such as water and various alcohols of different acidity isolated under supersonic jet conditions. While a double resonance vibrational spectroscopic method was employed to investigate the size and geometrical structure of jet-cooled clusters, laser-induced fluorescence spectroscopy was used to examine the changes of photophysics induced by complexation of DEAP with solvent molecule(s). The results obtained from ab initio calculations enable the assignment of geometries and of the vibrational spectra of the clusters in the OH-stretch region. The comparison of the experimental and calculated vibrational spectra indicates that the solvent molecule is hydrogen-bonded to the pyridine nitrogen atom. Dual luminescence is observed only for the complexes with alcohols of relatively strong acidity.

Cation-pi-anion interaction: a theoretical investigation of the role of induction energies.

Cation-pi and the corresponding anion-pi interactions have in general been investigated as binary complexes despite their association with counterions. However, a recent study of the ammonia channel highlights the important but overlooked role of anions in cation-pi interactions. In an effort to examine the structural and energetic consequences of the presence of counterions, we have carried out detailed ab initio calculations on some model cation-pi-anion ternary complexes and evaluated the nonpair potential terms, three-body contributions, and attractive and repulsive energy components of the interaction energy. The presence of the anion in the vicinity of the pi system leads to a large redistribution of electron density and hence leads to an inductive stabilization. The resulting electronic and geometrical changes have important consequences in both chemical and biological systems. Compared to cation-pi-anion ternary complexes, the magnitude of the cation-pi interaction in pi-cation-anion ternary complexes is markedly lower because of charge transfer from the anion to the cation.

Understanding of assembly phenomena by aromatic-aromatic interactions: benzene dimer and the substituted systems.

Interactions involving aromatic rings are important in molecular/biomolecular assembly and engineering. As a consequence, there have been a number of investigations on dimers involving benzene or other substituted pi systems. In this Feature Article, we examine the relevance of the magnitudes of their attractive and repulsive interaction energy components in governing the geometries of several pi-pi systems. The geometries and the associated binding energies were evaluated at the complete basis set (CBS) limit of coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)] using a least biased scheme for the given data set. The results for the benzene dimer indicate that the floppy T-shaped structure (center-to-center distance: 4.96 A, with an axial benzene off-centered above the facial benzene) is isoenergetic in zero-point-energy (ZPE) corrected binding energy (D0) to the displaced-stacked structure (vertical interplanar distance: 3.54 A). However, the T-shaped structure is likely to be slightly more stable (D0 approximately equal to 2.4-2.5 kcal/mol) if quadruple excitations are included in the coupled cluster calculations. The presence of substituents on the aromatic ring, irrespective of their electron withdrawing or donating nature, leads to an increase in the binding energy, and the displaced-stacked conformations are more stabilized than the T-shaped conformers. This explains the wide prevalence of displaced stacked structures in organic crystals. Despite that the dispersion energy is dominating, the substituent as well as the conformational effects are correlated to the electrostatic interaction. This electrostatic origin implies that the substituent effect would be reduced in polar solution, but important in apolar media, in particular, for assembling processes.

Structures, energetics, and spectra of aqua-cesium (I) complexes: an ab initio and experimental study.

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.

Hydration profiles of aromatic amino acids: conformations and vibrations of L-phenylalanine-(H2O)n clusters.

IR-UV double resonance spectroscopy and ab initio calculations were employed to investigate the structures and vibrations of the aromatic amino acid, L-phenylalanine-(H(2)O)(n) clusters formed in a supersonic free jet. Our results indicate that up to three water molecules are preferentially bound to both the carbonyl oxygen and the carboxyl hydrogen of L-phenylalanine (L-Phe) in a bridged hydrogen-bonded conformation. As the number of water molecules is increased, the bridge becomes longer. Two isomers are found for L-Phe-(H(2)O)(1), and both of them form a cyclic hydrogen-bond between the carboxyl group and the water molecule. In L-Phe-(H(2)O)(2), only one isomer was identified, in which two water molecules form extended cyclic hydrogen bonds with the carboxyl group. In the calculated structure of L-Phe-(H(2)O)(3) the bridge of water molecules becomes larger and exhibits an extended hydrogen-bond to the pi-system. Finally, in isolated L-Phe, the D conformer was found to be the most stable conformer by the experiment and by the ab initio calculation.

Hydration and dissociation of hydrogen fluoric acid (HF).

The hydration and dissociation phenomena of HF(H(2)O)(n)() (n < or = 10) clusters have been studied by using both the density functional theory with the 6-311++G[sp] basis set and the Møller-Plesset second-order perturbation theory with the aug-cc-pVDZ+(2s2p/2s) basis set. The structures for n > or = 8 are first reported here. The dissociated form of the hydrogen-fluoric acid in HF(H(2)O)(n) clusters is found to be less stable at 0 K than the undissociated form until n = 10. HF may not be dissociated at 0 K solely by water molecules because the HF H bond is stronger than the OH H bond, against the expectation that the dissociated HF(H(2)O)(n) would be more stable than the undissociated one in the presence of a number of water molecules. The dissociation would be possible for only a fraction of a number of hydrated HF clusters by the Boltzmann distribution at finite temperatures. This is in sharp contrast to other hydrogen halide acids (HCl, HBr, HI) showing the dissociation phenomena at 0 K for n > or = 4. The IR spectra of dissociated and undissociated structures of HF(H(2)O)(n) are compared. The structures and binding energies of HF(H(2)O)(n) are found to be similar to those of (H(2)O)(n+1). It is interesting that HF(H(2)O)(n=5,6,10) are slightly less stable compared with other sizes of clusters, just like the fact that (H(2)O)(n=6,7,11) are slightly less stable. The present study would be useful for the experimental/spectroscopic investigation of not only the dissociation phenomena of HF but also the similarity of the HF-water clusters to the water clusters.

Modulation of molecular conductance induced by electrode atomic species and interface geometry.

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 Angstrom 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.

Why the hydration energy of Au+ is larger for the second water molecule than the first one: skewed orbitals overlap.

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.

Role of molecular orbitals of the benzene in electronic nanodevices.

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.

Substituent effects on the edge-to-face aromatic interactions.

The edge-to-face interactions for either axially or facially substituted benzenes are investigated by using ab initio calculations. The predicted maximum energy difference between substituted and unsubstituted systems is approximately 0.7 kcal/mol (approximately 1.2 kcal/mol if substituents are on both axially and facially substituted positions). In the case of axially substituted aromatic systems, the electron density at the para position is an important stabilizing factor, and thus the stabilization/destabilization by substitution is highly correlated to the electrostatic energy. This results in its subsequent correlation with the polarization and charge transfer. Thus, the stabilization/destabilization by substitution is represented by the sum of electrostatic energy and induction energy. On the other hand, the facially substituted aromatic system depends on not only the electron-donating ability responsible for the electrostatic energy but also the dispersion interaction and exchange repulsion. Although the dispersion energy is the most dominating interaction in both axial and facial substitutions, it is almost canceled by the exchange repulsion in the axial substitution, whereas in the facial substitution, together with the exchange repulsion it augments the electrostatic energy. The systems with electron-accepting substituents (NO2, CN, Br, Cl, F) favor the axial substituent conformation, while those with electron-donating substituents (NH2, CH3, OH) favor the facial substituent conformation. The interactions for the T-shape complex systems of an aromatic ring with other counterpart such as H2, H2O, HCl, and HF are also studied.

Origin of the magic numbers of water clusters with an excess electron.

Electron-bound water clusters [e(-)(H(2)O)(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 (H(2)O)(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.

Modulation of the electronic structure of semiconducting nanotubes resulting from different metal contacts.

Examined in this paper is the role of the metal electrode influencing the structure and electronic properties of semiconducting carbon nanotubes near the interface at low bias. Specifically, we present quantum-chemical calculations of finite sections of a (8,0) semiconducting single wall nanotube contacted with gold and palladium clusters. The calculations at the density functional level of theory, which included full geometry optimizations, indicate the formation of bonds between the metal atoms of the electrode and the carbon atoms of the nanotube. The local work function of the metal electrode can be expected to exhibit significant variations as a result of this bond formation. Compared to the gold-contacted nanotubes, the palladium-contacted nanotubes have a small but interesting increase in both length and diameter. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the gold-contacted nanotube are shown localized at the edges. In contrast, the HOMO and LUMO of the palladium-contacted nanotube are extended over the entire nanotube and the metal cluster contacted to it, providing thereby a better conduction path in the contact region of the electrode and the nanotube. The involvement of the highly directional d orbitals in the interactions involving the palladium cluster leads to an enhanced pi electron density in the nanotube. This enhanced pi electron density is synonymous with an improved electron transmission.

p-benzoquinone-benzene clusters as potential nanomechanical devices: a theoretical study.

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...pi 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 pi-H interaction between pi-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.

Insights into the structure of cyclohexane from femtosecond degenerate four-wave mixing spectroscopy and ab initio calculations.

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(0) 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 A(1g) modes, a justification of which is provided by an analysis of these modes.

Highly stereospecific epimerization of alpha-amino acids: conducted tour mechanism.

The highly stereospecific and regiospecific recognition of alpha-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 base-catalyzed 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.

Assembling phenomena of calix[4]hydroquinone nanotube bundles by one-dimensional short hydrogen bonding and displaced pi-pi stacking.

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 pi-pi 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 pi-pi 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.

Catalytic mechanism of enzymes: preorganization, short strong hydrogen bond, and charge buffering.

During the past decade, there has been much debate about the enormous catalytic rate enhancement observed in enzymatic reactions involving carbanion intermediates. Our recent theoretical study has demonstrated the importance of the short strong hydrogen bond (SSHB) in the enzymatic reactions. Nevertheless, other recent theoretical studies espouse the role of preorganization over that of the SSHB. To achieve a consensus on this issue and to find the truth, a more clarified explanation must be given. To this end, we have carried out an elaborate analysis of these enzymatic reactions. We here clarify that the catalytic mechanism needs to be explained with three important factors, viz., SSHB, preorganization, and charge buffering/dissipation. Since the charge buffering role is different from the commonly used concepts of the SSHB and preorganization (unless these definitions are expanded), we stress that the charge buffering role of the catalytic residues is an important ingredient of the enzymatic reaction in reducing the level of accumulation of the negative charge on the substrate during the reaction process. This charge reduction is critical to the lowering of activation barriers and the stabilization of intermediates.