Interaction between aromatic amine cations and nonpolar solvents: Infrared spectra of isomeric aniline -Ar n ( ) complexes

Infrared (IR) photodissociation spectra of the aniline⁺-Ar_n cations, An ⁺ - Ar _n (n = 1, 2), are analyzed in the vicinity of the N-H stretch fundamentals. The complexes are produced in an electron impact (EI) ion source which produces predominantly the most stable cluster isomers. Two isomers of An⁺-Ar are identified by their characteristic N-H stretch frequencies: the planar proton-bound global minimum, in which the Ar ligand forms a nearly linear H-bond to the amino group, and the less stable π-bound local minimum, in which the Ar atom is attached to the π-electron system of the aromatic ring. This result is the first unambiguous detection of the most stable H-bound An⁺-Ar dimer. All previous spectroscopic studies of An⁺-Ar employed resonance enhanced multiphoton ionization (REMPI) of neutral An-Ar and identified only the less stable π-bound cation due to restrictions arising from the Franck-Condon principle. The EI-IR spectrum of An⁺-Ar₂ shows that the most stable structure of this trimer features two equivalent H-bonds (C_2v symmetry). The interpretation of the experimental data is supported by quantum chemical calculations. The ab initio potential of An⁺-Ar calculated at the UMP2/6-311G(2df, 2pd) level features H-bound global minima ( D _e = 513 cm^-1) and π-bound local minima ( D _e = 454 cm^-1), with a barrier of V _b ≈ 140 cm^-1 for isomerization from the π-bound toward the H-bound minimum. Received 4 February 2002 Published online 13 September 2002     

Infrared ion dip and ultraviolet spectroscopy of 4-phenyl imidazole, its tautomer, 5-phenyl imidazole, and its multiply hydrated clusters

Mass-resolved resonant two photon ionisation (R2PI) and infrared ion dip spectra have been recorded for 4-phenylimidazole (4PI) and its singly and multiply hydrated clusters 4PI(H₂O)_{n = 0 - 4}, under supersonic expansion conditions. In the case of 4PI(H₂O)_0,1, it has also been possible to record infrared spectra in both the ground (S₀) and excited (S₁) states. Combining the experimental data with the results of ab initio calculations has led to the structural assignment of each cluster. In each case, the water molecules bind primarily to the NH site of the imidazole ring. Clusters with n≥ 2 incorporate linear water chains, in which the proton donating terminus bridges either to the π-electron system (n = 2) or to the >N: atom site (n = 3, 4) on the imidazole ring. Despite the creation of a “water wire”, connecting the donor and acceptor sites of imidazole, there is no evidence of proton transfer in either the ground or excited state. Received 20 December 2001 Published online 13 September 2002     

Predictions of geometrical structures and ionization potentials for small barium clusters Ba

The geometrical structure of ground state Ba_n clusters (n =2-14) has been predicted from various types of calculations including two ab initio approaches used for the smaller sizes namely HF+MP2( n =2-6), DFT (LSDA)( n =2-6, 9) and one model approach HF+pairwise dispersion used for all sizes investigated here. The lowest energy configurations as well as some isomers have been investigated. The sizes n =4, 7 and 13 are predicted to be the relatively more stable ones and they correspond to the three compact structures: the tetrahedron, the pentagonal bipyramid and the icosahedron. The growth behavior from Ba₇ to Ba₁₃ appears to be characterized by the addition of atoms around a pentagonal bipyramid leading to the icosahedral structure of Ba₁₃ which is consistent with the observed size-distribution of barium clusters. Values for vertical ionization potentials calculated for n =2-5 at the CI level are seen to be in quite good agreement with recent measures. Received: 14 May 1997 / Received in final form: 2 February 1998 / Accepted: 27 February 1998     

Influence of covalence and anion symmetry on the structure of small metal hydroxide clusters: Sodium versus silver hydroxide

An ab initio study of the Na_n(OH)_n, Na_n(OH)_n-1 ⁺, Ag_n(OH)_n, and Ag_n(OH)_n-1 ⁺ clusters with n up to four is presented. The results of this study show that, in accordance with experimental observations, the sodium hydroxide clusters are almost purely ionic, while the Ag-O bond exhibits a significant covalent character. The perturbation caused by the non-spherical OH^- group relatively to an atomic anion, as well as the influence on structures and energies of the covalent character of the metal-oxygen bond are determined. The appearance of metal-metal bonds in the silver hydroxide clusters is also discussed. Finally, the theoretical results obtained on the Na-OH clusters are compared to experimental results available on the dissociation of the Na_n(OH)_n-1 ⁺ clusters. Received 9 August 1999 and Received in final form 1st December 1999     

Negative-ion photoelectron spectroscopy of acrylonitrile clusters containing a sodium atom

Cluster anions of a sodium atom with acrylonitrile molecules, (n = 0–6), have been studied by negative-ion photoelectron spectroscopy. In addition, theoretical calculations by using density functional theory have been performed to obtain optimized structures and vertical detachment energies. For Na(AN)^–, the spectrum can be explained by excitation of two different isomers of the anion. For , a broad band is found in the photoelectron spectrum, whose profile is almost identical with those of previously reported photoelectron spectra of and a negative ion of chemically synthesized 1,3,5-cyclohexanetricarbonitrile (CHTCN) molecule. From this resemblance of band profiles, we conclude that oligomerization of (AN)₃ takes place in and the CHTCN is formed as the intracluster reaction product.     

Can we rationalize the structure of small silicon-carbon clusters?

A theoretical study of clusters with using density functional theory is presented. Tests of various functionals demonstrate that local spin density approximation (LSDA) is the most adequate functional for the study of these systems. Structures, vibrational frequencies, and IR intensities of the lowest energy isomer of the studied clusters obtained using LSDA are described, and the unusual properties of the Si-C clusters are discussed. A quantitative analysis of the obtained structures was carried out, and relations between the coordinations, interatomic distances, and angles observed in the Si-C clusters were obtained through introduction of the notion of coordination. This analysis also shows that the carbon atoms mainly exhibit sp and sp² hybridizations, and that a majority of silicon atoms do not hybridize. This study is the fi rst step of the implementation of a semi-empirical potential, which would describe the moderately small Si-C clusters. Received: 20 October 1997 / Received in final form: 16 December 1997 / Accepted: 17 December 1997     

Cohesive properties of mercury clusters in the ground and excited states

Optimized structures and cohesive energies of small mercury clusters (Hg_N; N = 3–7, 13, 19) are calculated with the spin-orbit diatomics-in-molecules method. The theory takes into account the effect of s-p mixing which tends to enhance the binding energies in the ground state. It is shown that excimer clusters have significantly short optimum bond lengths and their atomic geometries differ considerably from those in the ground state. Excitation energy gap depends sensitively on both cluster size and nearest-neighbor separation. Numerical results are compared with other theories and experiments.     

Chirality, defects, and disorder in gold clusters

Theoretical and experimental information on the shape and morphology of bare and passivated gold clusters is fundamental to predict and understand their electronic, optical, and other physical and chemical properties. An effective theoretical approach to determine the lowest-energy configuration (global minimum) and the structures of low energy isomers (local minima) of clusters is to combine genetic algorithms and many-body potentials (to perform global structural optimizations), and first-principles density functional theory (to confirm the stability and energy ordering of the local minima). The main trend emerging from structural optimizations of bare Au clusters in the size range of 12-212 atoms indicates that many topologically interesting low-symmetry, disordered structures exist with energy near or below the lowest-energy ordered isomer. For example, chiral structures have been obtained as the lowest-energy isomers of bare Au₂₈ and Au₅₅ clusters, whereas in the size-range of 75-212 atoms, defective Marks decahedral structures are nearly degenerate in energy with the ordered symmetrical isomers. For methylthiol-passivated gold nanoclusters [Au₂₈(SCH₃)₁₆ and Au₃₈(SCH₃)₂₄], density functional structural relaxations have shown that the ligands are not only playing the role of passivating molecules, but their effect is strong enough to distort the metal cluster structure. In this work, a theoretical approach to characterize and quantify chirality in clusters, based on the Hausdorff chirality measure, is described. After calculating the index of chirality in bare and passivated gold clusters, it is found that the thiol monolayer induces or increases the degree of chirality of the metallic core. We also report simulated high-resolution transmission electron microscopy (HRTEM) images which show that defects in decahedral gold nanoclusters, with size between 1-2 nm, can be detected using currently available experimental HRTEM techniques.     

Is NaI soluble in water clusters?

clusters (solvents being , or ) have been studied by resonance enhanced two photons ionization, leading to the detection of clusters. When water is the solvent, large clusters up to n>50 can be observed, whereas for and no clusters larger than 10 could be evidenced. Because the first step in the ionization process is the excitation from the ground solvated () ion pair state to a covalent excited state, the differences in the cluster size distribution for different solvent may be interpreted as a difference in cluster structures leading to a difference in the charge separation in the ground state. Received: 30 September 1997 / Revised in final form: 30 October 1997 / Accepted: 30 October 1997     

Temperature-dependent ionization potential of sodium clusters

The ionization potential of sodium clusters () at a finite temperature is studied using density functional theory and ab initio molecular dynamics. The threshold regions of the photoionization efficiency curves are deduced from the integrated IP distributions, which are obtained from the energy eigenvalues of the highest occupied Kohn-Sham states during molecular dynamics by applying a theoretically well-defined shift. The calculated ionization potentials are directly compared to the experimental values. The energetically best geometry of Na₅₅ is found to be a slightly distorted icosahedron. Received 16 April 1999 and Received in final form 6 July 1999     

Geometric magic numbers of sodium clusters: Interpretation of the melting behaviour

Putative global minima of sodium clusters with up to 380 atoms have been located for two model interatomic potentials in order to identify the structures responsible for the size-dependence of the thermodynamic properties in experiments. Structures based upon the Mackay icosahedra predominate for both potentials, and the magic numbers for the Murrell-Mottram model show excellent agreement with the sizes at which maxima in the latent heat and entropy change at melting have been found in experiment. In particular, the magic numbers at sizes intermediate between the complete Mackay icosahedra are due to unusual twisted icosahedral structures.     

Spectroscopic manifestations of phase changes in CaAr n clusters: finite size effects

The finite temperature optical spectroscopy of CaAr_n clusters in the range 6 n 146 is investigated using a Diatomics-In-Molecule (DIM) Hamiltonian and classical parallel tempering Monte Carlo simulations. The absorption spectrum is calculated in the vertical approximation at various temperatures between 2 K and 50 K. Several typical situations are reported. CaAr₆ shows a strong thermal broadening and shift of the spectral lines, possibly associated with isomerization. CaAr₁₃ only shows some broadening. CaAr₃₇ exhibits features corresponding to coexisting isomers at low temperature. Finally, the abrupt changes in the absorption spectrum in CaAr₁₄₆ at about 20 K are indicative of surface diffusion.     

Interactions of lanthanide atoms: Comparative ab initio study of YbHe,Yb<Subscript> 2</Subscript> and TmHe,TmYb potentials

The results of high-level ab initio calculations are reported for the interatomic potentials describing YbHe, Yb₂, TmHe and TmYb van der Waals interactions. It is found that the interaction properties of Tm and Yb are very similar and the interaction anisotropy in the TmHe and TmYb complexes is very small. We analyze the long-range behavior of the isotropic and anisotropic interaction potentials and discuss some implications for cold and ultracold atomic collisions of the lanthanide atoms.     

Combined experimental and theoretical study of small aluminum oxygen clusters

We report a combined experimental and computational investigation of small Al_nO_m species (n ≤20, m ≤ 12), produced in a laser vaporization cluster source. The oxygen content in the clusters was tuned by varying the oxygen concentration in the carrier gas. Ionization energies are bracketed using different ionizing photon energies in the energy range between 5.37 and 7.89 eV. Among the singly doped Al_nO species, Al₃O and Al₁₅O are found to have relatively low ionization energies, which can be related to the magic character of the corresponding cations. Peculiarly low ionization energies also are observed for specific oxygen rich species (m > 1), suggesting the formation of ionically bound subunits. The structures and ionization energies of singly doped Al_nO^0,+ (n = 1 - 7) clusters were determined using density functional theory (B3LYP/6-311+G(d)). Electronic supplementary material Supplementary Online Material     

Photoinduced dissociation reactions of silver fluoride cluster ions

Photoinduced dissociation in the ultraviolet region has been investigated for Ag _nF _n-1 ⁺ cluster ions. Photodissociation spectrum of Ag₂F⁺ in the energy of 3.8–5.6 eV exhibits several sharp bands corresponding to the transition to electronically excited states. In this dissociation, only the Ag₂ ⁺ ion was observed as a fragment ion. Theoretical calculation indicates that the parent Ag₂F⁺ ion has a linear Ag-F-Ag equilibrium geometries in the ground and excited states. Since conformational changes by excitation of bending vibration are necessary for the fragmentation of an F atom, this indicates that production of Ag₂ ⁺ from Ag₂F⁺ is a result of internal conversion and following conformational changes.     

Membrane-mediated interactions of rod-like inclusions

Inclusions embedded in lipid membranes undergo a mediated force, due to the tendency of the membrane to relax its excess of elastic energy. In this paper we determine the exact shape of a two-dimensional vesicle hosting two different inclusions, and we analyse how the inclusion conformation influences the mediated interaction. We find non-trivial equilibrium configurations for the inclusions along the hosting membrane, and we derive the complete phase diagram of the mediated interaction. In particular, we find a non-vanishing mediated force even when the distance between the inclusions is much greater than their size. Our model can be applied to describe the mediated interactions of parallel, elongated inclusions embedded in three-dimensional membranes. Received 22 October 2001 and Received in final form 8 March 2002     

Metallic evolution of small magnesium clusters

Structural and electronic properties of small magnesium clusters (N≤13) are studied using a first-principles simulation method in conjunction with the density functional theory and generalized gradient correction approximation for the exchange-correlation energy functional. It is observed that the onset of metallization of magnesium clusters is hard to assign since both the s-p hybridization and the energy gap between the valence and conduction bands do not evolve rapidly towards the known bulk properties. Instead these quantities show a slow and nonmonotonic evolution. Received 15 November 2000     

Molecular interactions in fullerenes and equilibrium of higher fullerites C76 and C84 with their vapors

From simple topological considerations on the molecular shapes, a new method for calculating the coefficients of the Girifalco intermolecular potential for various fullerenes is proposed. This eliminates the necessity for fitting the coefficients to data of measurements for each specific fullerene. We calculate them for C₇₆ and C₈₄ and apply this potential to perform research on the equilibrium of these fullerites with their vapors. The temperature dependence of the lattice parameters, the saturated vapor pressures and the enthalpies of sublimation is studied. Results are in good agreement with available experimental data. Received 13 January 2000 and Received in final form 18 June 2000     

DFT studies of ionic vibrations in Na clusters

We use time-dependent density functional theory coupled to molecular dynamics for ionic motion to compute the spectra of ionic vibrations in small Na clusters. Comparison with results from the distance dependent tight-binding approach shows good agreement between these two very different methods. We discuss the evolution of the spectra with cluster size and charge and the impact of ionic vibrations on the optical response. Received 23 July 2001 / Received in final form 5 July 2002 Published online 8 October 2002 RID="a" ID="a"e-mail: suraud@irsamc.ups-tlse.fr