Bonding of benzene with excited States of Fe7

The interaction between high-spin Fe7 clusters and a benzene molecule was studied using the BPW91/6-311++G(2d,2p) method. The Fe7-C6H6 ground state has a T-shaped structure, similar to that of the benzene dimer, and a multiplicity M = 2S + 1 = 19 (S = total spin). The carbon atoms are bonded to a single equatorial iron atom, which experiences a dramatic decrease in its magnetic moment, from 3.1 to -0.8 mu(B); the magnetic moments of other Fe atoms are larger than those in the ground-state Fe7 cluster. Such unexpected magnetic behavior of the cluster is crucial for adsorption of benzene.     

Matrix isolation infrared spectroscopic and theoretical study of dinuclear chromium oxide clusters: Cr2On (n = 2, 4, 6) in solid argon

Dichromium oxide clusters, Cr2O2, Cr2O4, and Cr2O6, have been prepared and characterized by matrix isolation infrared spectroscopy and quantum chemical calculations. Laser-evaporated chromium atoms reacted with O2 in solid argon to form the previously characterized CrO2 molecules, which further reacted with chromium atoms to form Cr2O2 spontaneously on annealing. The Cr2O2 cluster is determined to have a chainlike CrOCrO structure. The rhombic ring isomer, which was predicted to be more stable than the CrOCrO structure, was not formed at the present experimental conditions. The Cr2O4 cluster was formed from the barrierless dimerization of the chromium dioxide molecules, which is characterized to have a planar D2h symmetry. TheCr2O6 cluster was produced under UV light irradiation. It is determined to have a singlet ground state with a nonplanar D2h symmetry.     

Theoretical characterization of structures and energies of benzene-(H2S)n and (H2S)n (n=1-4) clusters

An ab initio study was performed in clusters up to four H(2)S molecules and benzene using calculations at MP26-31+G(*) and MP2/aug-cc-pVDZ levels. Differences between both sets of calculations show the importance of using large basis sets to describe the intermolecular interactions in this system. The obtained binding energies reflect that benzene has not the same behavior in H(2)S as in water, pointing to a higher solubility of this molecule in H(2)S than in water. The Bz-cluster binding energy was fitted to an asymptotic representation with a maximum value of the energy of -8.00 kcal/mol that converges in a cluster with 12 H(2)S molecules. The obtained intermolecular distance in the Bz-H(2)S dimer is similar to the experimental value; however, the difference is much larger for the angles defining the orientation. The influence of benzene produces a distortion of the (H(2)S)(n) clusters, so the intermolecular distances change with regard to the (H(2)S)(n) isolated clusters. Frequency shifts are larger in clusters with benzene than without it. In the smallest clusters the shift associated to the stretching of the S-H bonded to benzene is the largest one, but for the cluster with three H(2)S molecules this stretching is combined with the other S-H stretching of the molecule so the resulting shift is not the largest one.     

Theoretical studies on hydroquinone-benzene clusters

High-level ab initio calculations were carried out to evaluate the interaction between the hydroquinone and benzene molecules. The intermolecular interaction energy was calculated using the M?ller-Plesset second-order perturbation theory at the complete basis set limit and also at the coupled cluster theory with single, double, and perturbatively triple excitations. The calculated binding energy is larger than the benzene dimer interaction energy. The T-shaped cluster (T-a) and the parallel conformation (P-a) are calculated to be nearly isoenergetic. Owing to the large energy gain in the attraction by electron correlation, the dispersion interaction is important for the attraction.     

Gas-phase ion mobilities and structures of benzene cluster cations (C6H6)n+, n = 2-6

Benzene clusters are generated by pulsed supersonic beam expansion, ionized by electron impact, mass-selected and then injected into a drift cell for ion mobility measurements in a helium buffer gas. The measured collision cross sections and theoretical calculations are used to determine the structures of the cluster cations (C(6)H(6))(n)(+) with n = 2-6. Density functional theory calculation, at an all-electron level and without any symmetry constraint, predicts that the dimer cation has two nearly degenerate ground state structures with the sandwich configuration more stable than the T-configuration by only 0.07 eV. The ion mobility experiment indicates that only one structure is observed for the mass-selected dimer cation at room temperature. The calculated cross section for the sandwich structure agrees very well (within 2.4%) with the experimental value. For the n = 3-6 clusters, the experiments suggest the presence of at least two structural isomers for each cluster. A Monte Carlo minimum-energy search technique using the 12-site OPLS potential for benzene is used to determine the structures of the lowest-energy isomers. The calculated cross sections for the two lowest-energy isomers of the n = 3-6 clusters agree well with the experimental results. The clusters' structures reveal two different growth patterns involving a sandwich dimer core or a pancake trimer stack core. The lowest-energy isomers of the n = 3-6 clusters incorporate the pancake trimer stack as the cluster's core. The trimer stack allows the charge to hop between two dimers, thus maximizing charge resonance interaction in the clusters. For larger clusters, the appearance of magic numbers at n = 14, 20, 24, 27, and 30 is consistent with the incorporation of a sandwich dimer cation within icosahedral, double icosahedral, and interpenetrating icosahedral structures. On the basis of the ion mobility results and the structural calculations, the parallel-stacked motif among charged aromatic-aromatic interactions is expected to play a major role in determining the structures of multi aromatic components. This conclusion may provide new insights for experimental and theoretical studies of molecular design and recognition involving aromatic systems.     

Path integral ground state study of finite-size systems: application to small (parahydrogen)N (N=2-20) clusters

We use the path integral ground state method to study the energetic and structural properties of small para-H2 clusters of sizes ranging from 2 to 20 molecules. A fourth order formula is used to approximate the short imaginary-time propagator and two interaction potentials are considered. Our results are compared to those of exact basis set calculations and other quantum Monte Carlo methods when available. We find that for all cluster sizes considered, our results show a lower ground state energy than literature values obtained by diffusion Monte Carlo and variational Monte Carlo. For the dimer and trimer, ground state energies are in good agreement with exact results obtained using the discrete variable representation. Structural properties are found to be insensitive to the choice of interaction potential. We explore the use of Pekeris coordinates to analyze the importance of linear arrangement in trimers and for trimers within clusters of larger size.     

Surface S(N)2 reaction by H2O on chlorinated Si(100)-2 x 1 surface

The potential energy surfaces of one, two, and three water molecule sequential adsorptions on the symmetrically chlorinated Si(100)-2 x 1 surface were theoretically explored with SIMOMM:MP2/6-31G(d). The first water molecule adsorption to the surface dimer requires a higher reaction barrier than the subsequent second water molecule adsorption. The lone pair electrons of the incoming water molecule nucleophilically attack the surface Si atom to which the leaving Cl group is bonded, yielding an S(N)2 type transition state. At the same time, the Cl abstracts the H atom of the incoming water molecule, forming a unique four-membered ring conformation. The second water molecule adsorption to the same surface dimer requires a much lower reaction barrier, which is attributed to the surface cooperative effect by the surface hydroxyl group that can form a hydrogen bond with the incoming second water molecule. The third water molecule adsorption exhibits a higher reaction barrier than the first and the second water molecule adsorption channels but yields a thermodynamically more stable product. In general, it is expected that the surface Si-Cl bonds can be subjected to the substitution reactions by water molecules, yielding surface Si-OH bonds, which can be a good initial template for subsequent surface chemical modifications. However, oversaturations can be a competing side reaction under severe conditions, suggesting that the precise control of surface kinetic environments is necessary to tailor the final surface characteristics.     

Coordination of methanol clusters to benzene: a computational study

Benzene-methanol cluster structures were investigated with theoretical chemistry methods to describe the microsolvation of benzene and the benzene-methanol azeotrope. Benzene-methanol (MeOH) clusters containing up to six methanol molecules have been calculated by ab initio [MP2/6-311++G(d,p)//MP2/6-31+G(d,p) + BSSE correction] method. The BSSE was found quite large with this basis set, hence, different extrapolation schemes in combination with the aug-cc-pVxZ basis sets have been used to estimate the complete basis set limit of the MP2 interaction energy [ΔE(MP2/CBS)]. For smaller clusters, n ≤ 3, DFT procedures (DFTB+, MPWB1K, M06-2X) have also been applied. Geometries obtained for these clusters by M06-2X and MP2 calculations are quite similar. Based on the MP2/CBS results, the most stable C(6)H(6)(MeOH)(3) cluster is characterized by a hydrogen bonded MeOH trimer chain interacting with benzene via π···H-O and O···H-C(benzene) hydrogen bonds. Larger benzene-MeOH clusters with n ≥ 4 consist of cyclic (MeOH)(n) subclusters interacting with benzene by dispersive forces, to be denoted by C(6)H(6) + (MeOH)(n). Interaction energies and cooperativity effects are discussed in comparison with methanol clusters. Besides MP2/CBS calculations, for selected larger clusters the M06-2X/6-311++G(d,p)//M06-2X/6-31+G(d,p) procedure including the BSSE correction was also used. Interaction energies obtained thereby are usually close to the MP2/CBS limit. To model the benzene-MeOH azeotrope, several structures for (C(6)H(6))(2)(MeOH)(3) clusters have been calculated. The most stable structures contain a tilted T-shaped benzene dimer interacting by π···H-O and O···H-C (benzene) hydrogen bonds with a (MeOH)(3) chain. A slightly less negative interaction energy results for a parallel displaced benzene sandwich dimer with a (MeOH)(3) chain atop of one of the benzene molecules.     

Study of adsorption and decomposition of H2O on Ge(100)

The adsorption and decomposition of water on Ge(100) have been investigated using real-time scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. The STM results revealed two distinct adsorption features of H2O on Ge(100) corresponding to molecular adsorption and H-OH dissociative adsorption. In the molecular adsorption geometry, H2O molecules are bound to the surface via Ge-O dative bonds between the O atom of H2O and the electrophilic down atom of the Ge dimer. In the dissociative adsorption geometry, the H2O molecule dissociates into H and OH, which bind covalently to a Ge-Ge dimer on Ge(100) in an H-Ge-Ge-OH configuration. The DFT calculations showed that the dissociative adsorption geometry is more stable than the molecular adsorption geometry. This finding is consistent with the STM results, which showed that the dissociative product becomes dominant as the H2O coverage is increased. The simulated STM images agreed very well with the experimental images. In the real-time STM experiments, we also observed a structural transformation of the H2O molecule from the molecular adsorption to the dissociative adsorption geometry.     

Conformational control of benzyl-o-carboranylbenzene derivatives and molecular encapsulation of acetone in the dynamically formed space of 1,3,5-tris(2-benzyl-o-carboran-1-yl)benzene

A 1,3,5-substituted benzene platform has been widely used in the fields of supramolecular chemistry and molecular recognition. Here, we show that 1,3,5-tris(2-benzyl-o-carboran-1-yl)benzene 6 exhibits solvent-dependent conformation in the crystalline state. Recrystallization from dichloromethane-n-pentane gave the anti conformation 6-anti, while recrystallization from methanol-acetone gave the syn conformation 6-syn, in which the three benzyl-o-carboranyl moieties are located to one side of the central benzene ring. Interestingly, one acetone molecule is captured in the π-rich space of 6-syn and two complexes facing each other encapsulate two acetone molecules in a π-rich container formed by the eight benzene rings. The inclusion involves several weak interactions, that is, T-shaped C-H···π interactions, and C-H···O and C-H···π interactions. Two C-H···O interactions involving benzylic C-H hydrogens activated by the electron-withdrawing character of the o-carborane cage and the oxygen atom of the acetone seem to be the most important. DFT calculations indicate that the binding energy for entrapment of acetone is 6.6 kcal/mol. Inclusion of acetone is achieved through not only multiple C-H···O interactions but also a number of C-H···π interactions. The third benzyl-o-carborane moiety is fixed in the syn conformation by intramolecular and intermolecular C-H···π interactions.     

C 1s -->pi* excitation in variable size benzene clusters

The C 1s -->pi* transition in molecular benzene and benzene clusters is investigated by photoion yields at high energy resolution. The vibrationally resolved band shows the same shape in clusters as in the bare molecule, but it is redshifted by 50 meV in small clusters, i.e. near the threshold of cluster formation. This redshift increases to 70 meV with increasing cluster size. The results are assigned in comparison with ab initio calculations on model structures of dimers, trimers, and tetramers. These indicate that different carbon sites in the molecular moieties give rise to distinct spectral shifts, where carbon sites that are pointing to the pi-system of another molecule show a larger redshift than the other ones. Such structural properties are found in solid benzene, so that the gas-to-solid shift of C 1s -->pi* excited benzene is derived to be a redshift which is of the order of 100-180 meV.     

The electronic structure of free water clusters probed by Auger electron spectroscopy

(H2O)(N) clusters generated in a supersonic expansion source with N approximately 1000 were core ionized by synchrotron radiation, giving rise to core-level photoelectron and Auger electron spectra (AES), free from charging effects. The AES is interpreted as being intermediate between the molecular and solid water spectra showing broadened bands as well as a significant shoulder at high kinetic energy. Qualitative considerations as well as ab initio calculations explain this shoulder to be due to delocalized final states in which the two valence holes are mostly located at different water molecules. The ab initio calculations show that valence hole configurations with both valence holes at the core-ionized water molecule are admixed to these final states and give rise to their intensity in the AES. Density-functional investigations of model systems for the doubly ionized final states--the water dimer and a 20-molecule water cluster--were performed to analyze the localization of the two valence holes in the electronic ground states. Whereas these holes are preferentially located at the same water molecule in the dimer, they are delocalized in the cluster showing a preference of the holes for surface molecules. The calculated double-ionization potential of the cluster (22.1 eV) is in reasonable agreement with the low-energy limit of the delocalized hole shoulder in the AES.     

Intermolecular pi-pi interactions in solids

A parameter-free DFT/CCSD(T) correction scheme is proposed for precise calculations (close to CCSD(T) accuracy) of weakly bound molecular solids. The correction scheme has been tested for solid benzene and graphite. The CCSD(T)/CBS correction to planewave DFT calculations reproduces the experimentally determined lattice constants for solid benzene. The calculated cohesive energy of benzene (457 meV per molecule) compares well with the experimental values of the heat of sublimation (460-560 meV per molecule). For graphite, the correction yields structural parameters (a = 2.46 A, c = 6.60 A) in good agreement with experiment (a = 2.46 A, c = 6.67 A). The calculated exfoliation energy of 54 meV per atom agrees fairly well with the most recent experimental value of 52 +/- 5 meV per atom.     

Addition of water to Al5O4- determined by anion photoelectron spectroscopy and quantum chemical calculations

The anion photoelectron spectra of Al5O4- and Al5O5H2- are presented and interpreted within the context of quantum chemical calculations on these species. Experimentally, the electron affinities of these two molecules are determined to be 3.50(5) eV and 3.10(10) eV for the bare and hydrated cluster, respectively. The spectra show at least three electronic transitions crowded into a 1 eV energy window. Calculations on Al5O4- predict a highly symmetric near-planar structure with a singlet ground state. The neutral structure calculated to be most structurally similar to the ground state structure of the anion is predicted to lie 0.15 eV above the ground state structure of the neutral. The lowest energy neutral isomer does not have significant Franck-Condon overlap with the ground state of the anion. Dissociative addition of water to Al5O4- is energetically favored over physisorption. The ground state structure for the Al5O4- +H(2)O product forms when water adds to the central Al atom in Al5O4- with -H migration to one of the neighboring O atoms. Again, the ground state structures for the anion and neutral are very different, and the PE spectrum represents transitions to a higher-lying neutral structure from the ground state anion structure.     

Molecular dynamics simulations of D2O ice photodesorption

Molecular dynamics (MD) calculations have been performed to study the ultraviolet (UV) photodissociation of D(2)O in an amorphous D(2)O ice surface at 10, 20, 60, and 90 K, in order to investigate the influence of isotope effects on the photodesorption processes. As for H(2)O, the main processes after UV photodissociation are trapping and desorption of either fragments or D(2)O molecules. Trapping mainly takes place in the deeper monolayers of the ice, whereas desorption occurs in the uppermost layers. There are three desorption processes: D atom, OD radical, and D(2)O molecule photodesorption. D(2)O desorption takes places either by direct desorption of a recombined D(2)O molecule, or when an energetic D atom produced by photodissociation kicks a surrounding D(2)O molecule out of the surface by transferring part of its momentum. Desorption probabilities are calculated for photoexcitation of D(2)O in the top four monolayers and are compared quantitatively with those for H(2)O obtained from previous MD simulations of UV photodissociation of amorphous water ice at different ice temperatures [Arasa et al., J. Chem. Phys. 132, 184510 (2010)]. The main conclusions are the same, but the average D atom photodesorption probability is smaller than that of the H atom (by about a factor of 0.9) because D has lower kinetic energy than H, whereas the average OD radical photodesorption probability is larger than that of OH (by about a factor of 2.5-2.9 depending on ice temperature) because OD has higher translational energy than OH for every ice temperature studied. The average D(2)O photodesorption probability is larger than that of H(2)O (by about a factor of 1.4-2.3 depending on ice temperature), and this is entirely due to a larger contribution of the D(2)O kick-out mechanism. This is an isotope effect: the kick-out mechanism is more efficient for D(2)O ice, because the D atom formed after D(2)O photodissociation has a larger momentum than photogenerated H atoms from H(2)O, and D transfers momentum more easily to D(2)O than H to H(2)O. The total (OD + D(2)O) yield has been compared with experiments and the total (OH + H(2)O) yield from previous simulations. We find better agreement when we compare experimental yields with calculated yields for D(2)O ice than when we compare with calculated yields for H(2)O ice.     

Understanding adsorption and interactions of alkane isomer mixtures in isoreticular metal-organic frameworks

Novel metal-organic frameworks (MOFs) may lead to advances in adsorption and catalysis owing to their superior properties compared to traditional nanoporous materials. A combination of the grand canonical Monte Carlo method and configurational-bias Monte Carlo simulation was used to evaluate the adsorption isotherms of C4-C6 alkane isomer mixtures in IRMOF-1 and IRMOF-6. The amounts of adsorbed linear and branched alkanes increase with increasing pressure, and the amount of branched alkanes is larger than that of the linear ones. The locations of the alkane isomer reveal that the Zn4O clusters of the IRMOFs are the preferential adsorption sites for the adsorbate molecules. The interaction energy between the Zn4O cluster and the adsorbate is larger than that between the organic linker and the adsorbate. It was further confirmed that the Zn4O cluster plays a much more important role in adsorption by pushing a probe molecule into the pore at positions closer to the Zn4O cluster. It is difficult for branched alkane molecules to approach the Zn4O cluster of IRMOF-6 closely owing to strong spatial hindrance. In addition, the adsorption selectivity is discussed from the viewpoints of thermodynamics and kinetics, and the diffusion behavior of n-butane and 2-methylpropane were investigated to illustrate the relationship between diffusion and adsorption.     

Structure of hydrated clusters of dibenzo-18-crown-6-ether in a supersonic jet--encapsulation of water molecules in the crown cavity

The structure of dibenzo-18-crown-6-ether (DB18C6) and its hydrated clusters has been investigated in a supersonic jet. Two conformers of bare DB18C6 and six hydrated clusters (DB18C6-(H(2)O)(n)) were identified by laser-induced fluorescence, fluorescence-detected UV-UV hole-burning and IR-UV double-resonance spectroscopy. The IR-UV double resonance spectra were compared with the IR spectra obtained by quantum chemical calculations at the B3LYP/6-31+G* level. The two conformers of bare DB18C6 are assigned to "boat" and "chair I" forms, respectively, among which the boat form is dominant. All the six DB18C6-(H(2)O)(n) clusters with n = 1-4 have a boat conformation in the DB18C6 part. The water molecules form a variety of hydration networks in the boat-DB18C6 cavity. In DB18C6-(H(2)O)(1), a water molecule forms the bidentate hydrogen bond with the O atoms adjacent to the benzene rings. In this cluster, the water molecule is preferentially hydrogen bonded from the bottom of boat-DB18C6. In the larger clusters, the hydration networks are developed on the basis of the DB18C6-(H(2)O)(1) cluster.     

Gas phase infrared spectroscopy of mono- and divanadium oxide cluster cations

The vibrational spectroscopy of the mono- and divanadium oxide cluster cations VO(1-3)+ and V2O(2-6)+ is studied in the region from 600 to 1600 wave numbers by infrared photodissociation of the corresponding cluster cation-helium atom complexes. The comparison of the experimental depletion spectra with the results of density functional calculations on bare vanadium oxide cluster cations allows for an unambiguous identification of the cluster geometry in most cases and, for VO(1-3)+ and V2O(5,6)+, also of the electronic ground state. A common structural motif of all the studied divanadium cluster cations is a four-membered V-O-V-O ring, with three characteristic absorption bands in the 550-900 wave number region. For the V-O-V and V=O stretch modes the relationship between vibrational frequencies and V-O bond distances follows the Badger rule.     

A theoretical study of carbazole dimers: Does carbazole form an excimer that undermines the performance of organic light emitting diodes?

Carbazole (Cz) dimers in various cofacial conformations, including staggered (Stg), anti, and syn, were explored by means of ab initio calculations at scaled opposite-spin (SOS)-MP2, SOS-CIS(D₀), and additional coupled cluster calculation levels. Similar to other π-conjugated molecules, strong Cz excimers form in the syn conformation in both S₁ and T₁ states, leading to significantly reduced optical excitation energies. Upon excitation, the dimers in the Stg and anti-conformations remain simple excited dimers, exhibiting similar optical energy gaps to those of the monomer. Being far more stable in the ground state, however, the Stg dimer is nearly isoenergetic to the syn dimer in the S₁ state and even more stable in the T₁ state. Given that the intermolecular interactions in the ground state are expected to govern the dimer conformations of Cz-based materials in the solid-state films of organic electronics, our results strongly demonstrate that the electronic excitation of Cz dimers do not necessarily lead to the strong excimer formation unless Cz molecules are forced to be arranged in the syn conformation.     

Matrix infrared spectra and theoretical studies of thorium oxide species: ThOx and Th2Oy

Infrared spectra of three new thorium oxide species have been obtained in argon and neon matrixes. All of the products are experimentally characterized using isotopic oxygen samples with the aid of electronic structure calculations. Ground state thorium atoms react with O(2) to form the ThO(2) molecules, which can dimerize to give Th(2)O(4) products. Th(2)O(4) is predicted to have nonplanar C(2h) symmetry for its closed shell singlet ground state. The rhombus-shaped Th(2)O(2) molecule in the (1)A(g) (D(2h)) ground state is also observed and its formation is proposed via the reaction of Th(2) with O(2). In addition, electron capture of neutral thorium dioxide results in the formation of the ThO(2)(-) anion. It is predicted to have a doublet ground state with a geometry similar to that of the neutral ThO(2) molecule. Electronic structure calculations on the unobserved Th(2)O and Th(2)O(3) molecules are also provided.