Water at surfaces and interfaces: From molecules to ice and bulk liquid

The structure and growth of water films on surfaces is reviewed, starting from single molecules to two-dimensional wetting layers, and liquid interfaces. This progression follows the increase in temperature and vapor pressure from a few degrees Kelvin in ultra-high vacuum, where Scanning Tunneling and Atomic Force Microscopies (STM and AFM) provide crystallographic information at the molecular level, to ambient conditions where surface sensitive spectroscopic techniques provide electronic structure information. We show how single molecules bind to metal and non-metal surfaces, their diffusion and aggregation. We examine how water molecules can be manipulated by the STM tip via excitation of vibrational and electronic modes, which trigger molecular diffusion and dissociation. We review also the adsorption and structure of water on non-metal substrates including mica, alkali halides, and others under ambient humid conditions. We finally discuss recent progress in the exploration of the molecular level structure of solid-liquid interfaces, which impact our fundamental understanding of corrosion and electrochemical processes.     

Formation of stacking defects at surfaces: From atomistic simulations to density functional theory calculations

We have performed electronic structure calculations to study the evolution of the stacking fault energy at (111) surfaces of metals. We first apply an sp–d tight-binding model and then increase the accuracy on the electronic structure by using density functional theory (DFT) calculations. We show in this way the relative importance of sp–d hybridization both in the formation of defects at the surface of metals and in reconstruction phenomena as a function of band filling especially at the end of transition metal series. Comparing our results with atomistic simulations it is concluded that although atomistic calculations are powerful tools to investigate relaxation mechanisms at surfaces, a higher degree of accuracy on electronic structure is necessary to quantify the energy of some defects at surfaces like stacking faults. In particular long range interactions associated to less localized sp electrons are playing a rather important role in reconstruction phenomena for metals like platinum and gold. These results are backed up by DFT calculations applied to iridium, platinum and gold (111) surfaces.     

Photochemistry of hydrogen halides on water clusters: simulations of electronic spectra and photodynamics, and comparison with photodissociation experiments

The photochemistry of small HX·(H(2)O)(n), n = 4 and 5 and X = F, Cl, and Br, clusters has been modeled by means of ab initio-based molecular simulations. The theoretical results were utilized to support our interpretation of photodissociation experiments with hydrogen halides on ice nanoparticles HX·(H(2)O)(n), n ≈ 10(2)-10(3). We have investigated the HX·(H(2)O)(n) photochemistry for three structural types: covalently bound structures (CBS) and acidically dissociated structures in a form of contact ion pair (CIP) and solvent separated pair (SSP). For all structures, we have modeled the electronic absorption spectra using the reflection principle combined with a path integral molecular dynamics (PIMD) estimate of the ground state density. In addition, we have investigated the solvent effect of water on the absorption spectra within the nonequilibrium polarizable continuum model (PCM) scheme. The major conclusion from these calculations is that the spectra for ionic structures CIP and SSP are significantly red-shifted with respect to the spectra of CBS structures. We have also studied the photodynamics of HX·(H(2)O)(n) clusters using the Full Multiple Spawning method. In the CBS structures, the excitation led to almost immediate release of the hydrogen atom with high kinetic energy. The light absorption in ionically dissociated species generates the hydronium radical (H(3)O) and halogen radical (X) within a charge-transfer-to-solvent (CTTS) excitation process. The hydronium radical ultimately decays into a water molecule and hydrogen atom with a characteristic kinetic energy irrespective of the hydrogen halide. We have also investigated the dynamics of an isolated and water-solvated H(3)O radical that we view as a central species in water radiation chemistry. The theoretical findings support the following picture of the HX photochemistry on ice nanoparticles investigated in our molecular beam experiments: HX is acidically dissociated in the ground state on ice nanoparticles, generating the CIP structure, which is then excited by the UV laser light into the CTTS states, followed by the H(3)O radical formation.     

Theory of the photodissociation of ozone in the Hartley continuum: potential energy surfaces, conical intersections, and photodissociation dynamics

Ab initio potential energy and transition dipole moment surfaces are presented for the five lowest singlet even symmetry electronic states of ozone. The surfaces are calculated using the complete active space self consistent field method followed by contracted multireference configuration interaction (MRCI) calculations. A slightly reduced augmented correlation consistent valence triple-zeta orbital basis set is used. The ground and excited state energies of the molecule have been computed at 9282 separate nuclear geometries. Cuts through the potential energy surfaces, which pass through the geometry of the minimum of the ground electronic state, show several closely avoided crossings. Close examination, and higher level calculations, very strongly suggests that some of these seemingly avoided crossings are in fact associated with non-symmetry related conical intersections. Diabatic potential energy and transition dipole moment surfaces are created from the computed ab initio adiabatic MRCI energies and transition dipole moments. The transition dipole moment connecting the ground electronic state to the diabatic B state surface is by far the strongest. Vibrational-rotational wavefunctions and energies are computed using the ground electronic state. The energy level separations compare well with experimentally determined values. The ground vibrational state wavefunction is then used, together with the diabatic B<--X transition dipole moment surface, to form an initial wavepacket. The analysis of the time-dependent quantum dynamics of this wavepacket provides the total and partial photodissociation cross sections for the system. Both the total absorption cross section and the predicted product quantum state distributions compare well with experimental observations. A discussion is also given as to how the observed alternation in product diatom rotational state populations might be explained.     

Standard and direct methods in ab initio Hartree-Fock calculations: Application to polyoxometallates,organometallic clusters,and polypeptides

This article describes the present status of the program system ASTERIX and its move toward an efficient implementation on distributed memory computers with 128 processors or more. The ASTERIX program is currently used for scf and ci calculations on what we consider to be truly large systems; polyoxometallates, metallocarbohedrenes, organometallic clusters, and polypeptides. A brief review of our projects in those fields, either recently achieved or in progress, is provided. © 1996 John Wiley & Sons, Inc.     

Coherent spin control of matrix isolated molecules by IR+UV laser pulses: quantum simulations for ClF in Ar

Two coherent sequential IR+UV laser pulses may be used to generate two time-dependent nuclear wave functions in electronic excited triplet and singlet states via single (UV) and two photon (IR+UV) excitation pathways, exploiting spin-orbit coupling and vibrational pre-excitation, respectively. These wave functions evolve from different Franck-Condon domains until they overlap in a domain of bond stretching with efficient intersystem crossing. Here, the coherence of the laser pulses is turned into optimal interferences of the wave packets, yielding the total wave packet at the target place, time, and with dominant target spin. The time resolution of spin control is few femtoseconds. The mechanism is demonstrated by means of quantum model simulations for ClF in an Ar matrix.     

Effective method to compute vibrationally resolved optical spectra of large molecules at finite temperature in the gas phase and in solution

The authors present a new method for the computation of vibrationally resolved optical spectra of large molecules, including the Duschinsky rotation of the normal modes and the effect of thermal excitation. The method automatically selects the relevant vibronic contributions to the spectrum, independently of their frequency, and it is able to provide fully converged spectra with moderate computational times, both in vacuo and in solution. By describing the electronic states in the frame of the density functional theory and its time-dependent extension, they computed the room temperature absorption spectra of coumarin C153 and trans-stilbene in cyclohexane and the phosphorescence spectrum of porphyrazine in gas phase, showing that the method is fast and efficient. The comparison with experiment for trans-stilbene and coumarin C153 is very satisfactory, confirming the progress made toward a reliable method for the computation and interpretation for the optical spectra of large molecules.     

Exploring the electronic structure of elemental lithium: from small molecules to nanoclusters, bulk metal, and surfaces

Clusters of lithium atoms ranging in size from Li4 to Li40 and bulk metallic solids, including surfaces, are investigated through first principles electronic structure calculations, which are based upon density functional theory and the electron localization function (ELF). It is found that large lithium ppi-type contributions in the electronic wavefunction cause the electrons to localize in interstitial regions, which leads to multicenter bonding for both the clusters and the solids, including their surfaces. For the smaller clusters these stabilizing ppi interactions also lead to short Li-Li interatomic distances, which in conjunction with the longer bonds induces "distance alternation" in the range from 2.45 A to 3.15 A. This consequence of the additional ppi interactions is absent in simple solids due to symmetry. The electronic structure of the clusters is topologically insensitive to deformations that do not affect their general shape, but changes significantly upon isomerization. The ramifications upon dynamic properties is that the clusters are quasi-rigid at low temperatures and retain their shape though the distance alternation pattern is suppressed. The picture which emerges for bonding in the bulk solid is that the metallic state arises from the presence of a large number of partially occupied multicenter bonds. For nanoscale clusters only the surface of these clusters exhibits strong localization, whereas their interiors display localization properties similar to the bulk metallic solid. On the other hand, localized states similar to those of the clusters ("dangling bonds") are found on the (001) surface of body-centered cubic (bcc) and face-centered cubic (fcc) lithium solids.     

From molecules to materials: molecular paddle-wheel synthons of macromolecules, cage compounds and metal-organic frameworks

Metal-organic frameworks (MOF) are becoming a more and more important class of functional materials. Yet, very often, the synthesis of MOFs is not easy to control and requires a profound knowledge and experience in solid state chemistry. One of the most frequently used metal connectors is the so-called 'paddle-wheel' (PW) unit, which is a well-known molecular compound type in inorganic coordination chemistry. Depending on the ligands, the geometry of PWs strictly directs the assembly of ordered networks. This review focuses on the question, to what extent ordered network structures can be accessed by typical molecular syntheses in solution, starting from molecular PW complexes to ordered macromolecules, finite cage compounds and finally, three-dimensional superstructures.     

Bond-selective photodissociation of partially deuterated ammonia molecules: photodissociations of vibrationally excited NHD(2) in the 5nu(NH) state and NH(2)D in the 5nu(ND) state

Ultraviolet photodissociation of NHD(2) excited to the fourth overtone state of the NH stretching mode (5nu(NH)) and NH(2)D excited to that of the ND stretching mode (5nu(ND)) has been investigated by using a crossed laser and molecular beams method. Branching ratio between the NH and ND bond dissociations has been determined by utilizing a (2+1) resonance enhanced multiphoton ionization scheme of H and D atoms. For the photolysis of NHD(2) in the 5nu(NH) state, the NH dissociation cross section is 5.1+/-1.4 times as large as the ND dissociation cross section per bond. On the other hand, for the photolysis of NH(2)D in the 5nu(ND) state, the ratio of the NH dissociation cross section per bond to the ND dissociation cross section decreases to 0.68+/-0.16. In comparison with the branching ratios for the photolysis of vibrationally unexcited NH(2)D and NHD(2), the present results indicate that the excitation of the NH stretching mode enhances the NH dissociation with ca. two times larger NH/ND branching ratio, whereas the excitation of the ND stretching mode results in the preferential ND dissociation with ca. 3-4 times larger ND/NH branching ratio than that for the vibrational ground states. The mechanism of the bond-selective enhancement has been discussed in terms of the energetics and dynamics of wave packet.     

Model studies in heterogeneous catalysis at the microscopic level: from the structure and composition of surfaces to reaction kinetics

Heterogeneous catalysis is one of the fields of modern technology, in which a characterization of structural and chemical properties of solid surfaces at the microscopic level is of enormous importance. For a long time, such insights have been precluded by the complexity of most catalytically active materials. Recently, substantial progress has been made, however, toward a microscopic-level understanding of complex catalyst surfaces. We discuss the driving factors for these advancements, which are based on the development of new well-defined model systems as well as on advances in experimental technology and theory. Scrutinizing the example of planar model catalysts, we identify the process of linking structural and chemical information to microscopic reaction kinetics as a particular challenging aspect of today’s work. We review the kinetic effects which may have an influence on the reaction kinetics on complex surfaces. As an example how structural and kinetic information can be correlated at the microscopic level we discuss the case of surface oxidation and oxygen induced restructuring of Pd nanoparticles as studied by molecular beam methods.     

Conformational changes in proteins at interfaces: From solution to the interface,and back

After adsorption, polymer molecules, including proteins, undergo conformational relaxation. It involves a certain degree of spreading of the polymer molecule over the sorbent surface. Adsorption kinetics experiments reveal that the characteristic time of spreading is much longer for globular proteins than for highly flexible polymers. Based on differential scanning calorimetry it is concluded that the degree of spreading of protein molecules decreases with increasing rate of covering the surface with the protein. It is furthermore inferred that the relaxation does not proceed gradually but rather in steps. According to circular dichroism spectra the relaxed protein molecules still contain a large fraction of ordered structure. As a rule, protein molecules that, by homomolecular exchange, are released from the surface re-adopt their original structure; however, an exception to this rule is reported.     

Studies in hydrogen bonding: Association within small clusters of water,methanol, and ammonia molecules using Jorgensen's intermolecular pair potentials

The geometries of the dimer, trimer, and tetramer hydrogen-bonded clusters of water, methanol, and ammonia molecules have been derived using previously published intermolecular pair potentials containing constants optimized from ab initio calculations. The lowest energy forms for the dimers of all three types of molecules have an open structure, while the trimers and tetramers have cyclic structures. The results are compared with those previously described using another empirical potential, EPEN .     

Production of metal atoms,clusters and complexes in pulsed discharge-excited jets : Studies via mass-resolved laser multiphoton ionization

A pulsed, high-voltage, discharge-excited nozzle source has been developed and exploited to study the possibility of sputtering and entraining various metal atoms into the gas expansion from the discharge electrodes. By appropriate choice of electrode materials, atomic beams of copper, silver, tin and lead have been generated and spectroscopically characterized by 2 + 1 laser multiphoton ionization with mass analysis. Sufficiently high atomic densities are achieved with this nozzle system that metal clustering also takes place, producing dimers such as Ag₂, species of mixed composition such as AgSn and trimers, Ag₃. In addition, chemical reactions have been observed which can be initiated in the discharge and which lead to the formation of metal–ligand complexes when suitable molecules are seeded into the carrier gas. Mass spectral evidence for two such silver complexes is presented.     

Density functional theory study of the properties of N-H...N, non-cooperativities, and intermolecular interactions in linear trans-diazene clusters up to ten molecules

We investigate aspects of N-H...N hydrogen bonding in the linear trans-diazene clusters (n=2-10) such as the N...H and N-H lengths, n(N) --> sigma(N-H) interactions, N...H strengths, and frequencies of the N-H stretching vibrations utilizing the DFT/B3LYP theory, the natural bond orbital (NBO) method, and the theory of atoms in molecules (AIM). Our calculations indicate that the structure and energetics are qualitatively different from the conventional H-bonded systems, which usually exhibit distinct cooperative effects, as cluster size increases. First, a shortening rather than lengthening of the N-H bond is found and thus a blue rather than red shift is predicted. Second, for the title clusters, any sizable cooperative changes in the N-H and N...H lengths, n(N) --> sigma(N-H) charge transfers, N...H strengths, and frequencies of the N-H stretching vibrations for the linear H-bonded trans-diazene clusters do not exist. Because the n(N) --> sigma(N-H) interaction hardly exhibits cooperative effects, the capability of the linear trans-diazene cluster to localize electrons at the N...H bond critical point is almost independent of cluster size and thereby leads to the noncooperative changes in the N...H lengths and strengths and the N-H stretching frequencies. Third, the dispersion energy is sizable and important; more than 30% of short-range dispersion energy not being reproduced by the DFT leads to the underestimation of the interaction energies by DFT/B3LYP. The calculated nonadditive interaction energies show that, unlike the conventional H-boned systems, the trans-diazene clusters indeed exhibit very weak nonadditive interactions.     

From determination of enantiopurity to the construction of complex molecules: The Horeau principle and its application in synthesis

In 1973, Horeau and co-workers reported a technique capable of upgrading the enantiopurity of a scalemic mixture (for example, [(R)-enantiomer]?>?[(S)-enantiomer]). This method involved coupling the compound of interest to a simple, bifunctional reagent in order to form a statistical mixture of (R,R)-, (S,S)-, and (R,S)-stereoisomers. Separation of the two diastereomers and cleavage of the incorporated coupling agent effectively removes the minor, (S)-enantiomer from the major, (R)-enantiomer; thereby increasing the enantiopurity of the starting compound. Since that time, this concept of removing a minor enantiomer through the statistical formation of diastereomeric compounds (the Horeau principle) has also been applied when sequential enantioselective reactions are performed on a substrate, when multiple enantioenriched fragments are coupled together, and for determining the enantiopurity of the initial scalemic mixture. Under the right circumstances, the Horeau principle can be deployed in an iterative fashion in order to access material with extremely high enantiopurity (>99.999% ee). This review provides comprehensive coverage of the underlying theory behind the Horeau principle and its application in organic synthesis.