Structure and dynamics of phthalocyanine-argonn (n = 1-4) complexes studied in helium nanodroplets

Van der Waals clusters of phthalocyanine with 1-4 argon atoms formed inside superfluid helium nanodroplets have been investigated by recording fluorescence excitation spectra as well as emission spectra. The excitation spectra feature a multitude of sharp lines when recorded in superfluid helium droplets in contrast to the respective spectra measured in a seeded supersonic beam (Cho et al. Chem. Phys. Lett. 2000, 326, 65). The pickup technique used for doping of the phthalocyanine and the argon into the droplets allows for nondestructive analysis of the cluster sizes. Alternation of the pickup sequence gives information on the binding site of the argon atoms. The investigation of dispersed emission spectra in helium droplets can be used as a special tool for the identification of 0(0)0 transitions within the variety of sharp lines seen in the excitation spectra. Thus, different isomers of the clusters can be distinguished. Moreover, the emission spectra reveal information on dynamic processes such as vibrational predissociation of the van der Waals complexes and interconversion among isomeric species. The binding energy of the phthalocyanine-argon1 complex in helium droplets was estimated to be at most 113 cm-1.     

Structures and bonding nature of small monoligated copper clusters (HCN-Cun, n = 1-3) through high-resolution infrared spectroscopy and theory

The structures, C-H stretching frequencies, and dipole moments of HCN-Cun (n = 1-3) clusters are determined through high-resolution infrared spectroscopy. The complexes are formed and probed within superfluid helium droplets, whereby the helium droplet beam is passed over a resistively heated crucible containing copper shot and then through a gas HCN pickup cell. All complexes are found to be bound to the nitrogen end of the HCN molecule and on the "atop site" of the copper cluster. Through the experimental C-H vibrational shifts of HCN-Cun and ab initio calculations, it was found that the HCN-metal interaction changes from a strong van der Waals bond in n = 1 to a partially covalent bond in HCN-Cu3. Comparisons with existing infrared data on copper surfaces show that the HCN-Cun bond must begin to weaken at very large copper cluster sizes, eventually returning to a van der Waals bond in the bulk copper surface case.     

Halogen‐bonded adduct of 1,2‐dibromo‐1,1,2,2‐tetrafluoroethane and 1,4‐diazabicyclo[2.2.2]octane

Halogen bonding is an intermolecular interaction capable of being used to direct extended structures. Typical halogen‐bonding systems involve a noncovalent interaction between a Lewis base, such as an amine, as an acceptor and a halogen atom of a halofluorocarbon as a donor. Vapour‐phase diffusion of 1,4‐diazabicyclo[2.2.2]octane (DABCO) with 1,2‐dibromotetrafluoroethane results in crystals of the 1:1 adduct, C₂Br₂F₄·C₆H₁₂N₂, which crystallizes as an infinite one‐dimensional polymeric structure linked by intermolecular N...Br halogen bonds [2.829 (3) Å], which are 0.57 Å shorter than the sum of the van der Waals radii.     

Synthesis of a heptacoordinate trichlorosilane with a tetradentate ligand and unusual stability for nucleophilic substitution

A trichlorosilane bearing a tetradentate ligand with three coordinating arms was synthesized by the reaction of triarylmethyllithium with tetrachlorosilane. Its heptacoordinate structure around the central silicon atom has been revealed by X-ray crystallographic analysis. Three methoxy oxygen atoms are located at the opposite side to the chlorine atoms,and the average Si ··· O interatomic distance is 2.807Å, shorter than the sum of van der Waals radii (Si··· O: 3.40 Å). Chlorine atoms on the central silicon atom were not replaced by any nucleophiles such as alkyllithium, hydride anion, and alcohols. Effective suppression of nucleophilic attack is attributed to the intramolecular interactions of lone pairs of oxygen atoms with σ^* _Si?Cl antibonding orbital aswell as steric hindrance.     

A Crystallographic Charge Density Study of the Partial Covalent Nature of Strong N⋅⋅⋅Br Halogen Bonds

The covalent nature of strong N?Br???N halogen bonds in a cocrystal ( 2 ) of N‐bromosuccinimide ( NBS ) with 3,5‐dimethylpyridine ( lut ) was determined from X‐ray charge density studies and compared to a weak N?Br???O halogen bond in pure crystalline NBS ( 1 ) and a covalent bond in bis(3‐methylpyridine)bromonium cation (in its perchlorate salt ( 3 ). In 2 , the donor N?Br bond is elongated by 0.0954 Å, while the Br???acceptor distance of 2.3194(4) is 1.08 Å shorter than the sum of the van der Waals radii. A maximum electron density of 0.38 e Å^?3 along the Br???N halogen bond indicates a considerable covalent contribution to the total interaction. This value is intermediate to 0.067 e Å^?3 for the Br???O contact in 1 , and approximately 0.7 e Å^?3 in both N?Br bonds of the bromonium cation in 3 . A calculation of the natural bond order charges of the contact atoms, and the σ*(N1?Br) population of NBS as a function of distance between NBS and lut , have shown that charge transfer becomes significant at a Br???N distance below about 3 Å.     

Halogen bonding in a series of Br(CF2)nBr–DABCO adducts (n = 4, 6, 8)

Halogen bonding (XB) is a highly‐directional class of intermolecular interactions that has been used as a powerful tool to drive the design of crystals in the solid phase. To date, the majority of XB donors have been iodine‐containing compounds, with many fewer involving brominated analogues. We report the formation of adducts in the vapour phase from a series of dibromoperfluoroalkyl compounds, BrCF₂(CF₂)_n CF₂Br (n = 2, 4, 6), and 1,4‐diazabicyclo[2.2.2]octane (DABCO). Single‐crystal X‐ray diffraction studies of the colourless crystals identified 1,4‐diazabicyclo[2.2.2]octane–1,4‐dibromoperfluorobutane (1/1), C₄Br₂F₈·C₆H₁₂N₂, (I), 1,4‐diazabicyclo[2.2.2]octane–1,6‐dibromoperfluorohexane (1/1), C₆Br₂F₁₂·C₆H₁₂N₂, (II), and 1,4‐diazabicyclo[2.2.2]octane–1,8‐dibromoperfluorooctane (1/1), C₈Br₂F₁₆·C₆H₁₂N₂, (III), each of which displays a one‐dimensional halogen‐bonded network. All three adducts exhibit N…Br distances less than the sum of the van der Waals radii, with butane analogue (I) showing the shortest N…Br halogen‐bond distances yet reported between a bromoperfluorocarbon and a nitrogen base [2.809 (3) and 2.818 (3) Å], which are 0.58 and 0.59 Å shorter than the sum of the van der Waals radii.     

Über arsen-haltige heterocyclen : III. Kristalistruktur von 2,6-dimethyl-4,4-diphenyl-1,4-oxarsenaniumbromid-monohydrat

The crystal structure of the title compound has been determined from single crystal X-ray data and refined to a conventional R factor of 0.046. The coordination of the As atom is tetrahedral with a mean As—C distance of 1.92 Å. The six-membered heterocycle has chair conformation with two equatorial methyl and one equatorial and one axial phenyl substituent. The transannular 1,4-As?O distance is 3.11 Å, interactions are discussed. The connection of the structure is more van der Waals than ionic type. Some unspecific gaps are statistically occupied by one molecule of crystal water.     

Metastable impurity-helium solid phase. Experimental and theoretical evidence

A novel solid metastable phase of “sticking-together” helium van der Waals coats surrounding polarizable heavy impurities - Ne, Ar, Kr, Xe, N and N₂ - has been obtained and investigated using the experimental technique based on gas beam injection into superfluid helium. The element composition for this phase was measured and for all inert gases the solid samples were found to consist of more than 90% helium atoms and to keep stability up to T ≈ 8 K. The cluster approach to the theoretical analysis of the model NeHe₁₂ structures developed makes it possible to suggest the main physical reason for its formation to be the breakdown of quantum-mechanical helium atoms behavior, namely, strong suppression of amplitudes of radial zeroth vibrations for helium atoms belonging to the van der Waals shell in the field of dispersion forces of the central impurity particle. The phase diagram for the impurity-helium substance has been simulated and the direction of future investigations have also been outlined.     

Electronic spectroscopy of nonalternant hydrocarbons inside helium nanodroplets

We have recorded the electronic spectra of three polycyclic aromatic hydrocarbons (acenaphtylene, fluoranthene, and benzo(k)fluoranthene) containing a five-member ring and their van der Waals complexes with argon and oxygen with a molecular beam superfluid helium nanodroplet spectrometer. Although the molecules, which differ by addition of one or two fused benzene rings to acenaphtylene, have the same point group symmetry, the spectral lineshapes show distinct differences in the number of zero phonon lines and shapes of the phonon wings. Whereas the smallest molecule (acenaphtylene) has the most complicated line shape, the largest molecule (benzo(k)fluoranthene) shows different lineshapes for different vibronic transitions. The van der Waals complexes of fluoranthene exhibit more peaks than the theoretically allowed number of isomeric complexes with argon/oxygen. The current models of molecular solvation in liquid helium do not adequately explain these discrepancies.     

A density functional theory study of van der Waals interaction in carbon nanotubes

In this article, we investigate the effect of van der Waals force in zigzag carbon nanotubes (CNTs) including single-wall CNT (SWCNT) and double-walled CNT (DWCNT) structures with several interaction configurations. The solid-state density functional theory is employed to calculate the geometric optimization, normal mode frequencies, and IR and Raman spectra with the periodic boundary condition. For SWCNTs, we find that the Raman intensity is not affected by the tube diameter or the electronic structure. The IR absorption, however, increases with the tube diameter. We find that the close metallicity of the electronic structure has a significant impact on the IR simulations. When the van der Waals force is applied outside the CNTs at a distance longer than 3.0, the effect on Raman spectra is minimal but some effects can still be confirmed by IR absorption. When the van der Waals force acts inside the CNTs, the effect on the spectrum can be observed, especially at a distance of 2.8 Å, both IR and Raman can be significantly enhanced in many modes.     

Microsolvation of phthalocyanines in superfluid helium droplets.

Experimental and theoretical investigations of the spectroscopy of molecules in superfluid helium droplets provide evidence for the key role of the first helium layer surrounding the dopant molecule in determining the molecule's spectroscopic features. Recent investigations of emission spectra of phthalocyanine in helium droplets revealed a doubling of all transitions. Herein, we present the emission spectra of Mg-phthalocyanine and of phthalocyanine-argon clusters in helium droplets, which confirm the splitting as a general effect of the helium environment. A scheme of levels is deduced from the emission spectra and attributed to quantized states of the first helium layer surrounding the dopant molecule.     

Atomic bonding of neutral and cationic Mn clusters

By using the spin-polarized DV-X-LCAO method, electronic states of neutral and cationic Mn _N clusters (N=25) are calculated to study atomic bonding of Mn clusters. For the neutral Mn₂ cluster, the equilibrium interatomic distance is much larger than that of the bulk crystal. The 3d-derived states are nearly half-filled, and the 4s-derived states are almost fully occupied,i.e. the electronic configuration is close to that of the isolated atom. These indicate that the Mn₂ cluster is bound by the van der Waals force. The same situation is true for the larger neutral clusters while the equilibrium interatomic distance becomes smaller and thes-d mixing becomes larger. For the cationic clusters, the behaviors tend to become metallic. The equilibrium interatomic distances are smaller and thes-d mixings are larger than those of the corresponding neutral clusters. However, the Mn ₂ ⁺ and Mn ₄ ⁺ clusters still remain the van der Waals characters. Contrary to these clusters, the Mn ₅ ⁺ cluster, whose interatomic distance is smaller than that of the bulk crystal, shows strong metallic bonding. These results seem to correspond to the magic number observed on the mass spectroscopy of cationic Mn clusters.     

Regio- and stereoselectivity in halomethoxylation of 7-phenyl-1-phenylsulfonyltricyclo[4.1.0.02,7]heptane

The reaction of 7-phenyl-1-phenylsulfonyltricyclo[4.1.0.0^2,7]heptane with N-halo(chloro, bromo, iodo)succinimide in methanol led to the formation of a single product of conjugate syn-addition to the central C¹–C⁷ bond of the norpinane structure with a halogen atom in the geminal position to the sulfonyl substituent. The stereochemical result of the addition originates from the features of the reaction intermediate structure, a norpinanyl cation of a benzyl type where the reaction site is shielded from the nucleophile attack from the anti-direction by the sulfonyl group whose oxygen atom is approached to the reaction site to a distance of ～1.45 Å according to the nonempirical calculations by the 6-31G method; this distance is nearly by half less than the sum of van der Waals radii of C?O.     

On the solid state structure of 4-iodobenzoic acid

The solid-state structure of 4-iodobenzoic acid has been confirmed by variable temperature X-ray diffraction, variable temperature solid-state NMR and differential scanning calorimetry. 4-iodobenzoic acid crystallizes in the space group P2(1)/n, and dimerizes in the solid state about a center of inversion. Using extensive X-ray crystallographic data collections, the placement of the carboxylate H atoms from the residual electron density in difference Fourier maps was determined. The position of the electron density associated with the proton is found to vary with temperature in that the population of the disordered sites changes with varying temperature. Determination of the crystal structure between the temperatures of 248 and 198 K was not possible due to a phase transition, an endothermic event occurring at 230.77 K. The phase transition is also indicated by a change in the relaxation time of the ring carbon atoms in the solid-state NMR data. Though the dominating force in the dimeric unit in the solid state is the presence of strong hydrogen bonds, there are also van der Waals forces present between the iodine atoms. In the layered structure, the iodine-iodine distance is within the van der Waals contact radii, an interaction which causes a deformation in the electron density of the iodine atoms.     

The crystal and molecular structure of dodecafluorotriphenylene, C18F12

The dodecafluorotriphenylene molecule is considerably distorted from planarity by steric interactions between the “ortho” fluorine atoms which approach to within 2.41 Å of each other (or about 0.3 Å closer than the sum of the van der Waals radii for two fluorine atoms, 2.70 Å).     

CEPA calculations of potential energy surfaces for open-shell systems.: III. Van der Waals interaction between O(3P) and He(1S)

Quantum-chemical ab initio calculations have been performed for the van der Waals interaction between helium and oxygen atoms in their respective ground states: He(¹S)+ O(³P). As long as fine-structure effects are neglected, there are two low-lying electronic states, ³Σ^? and ³Π resulting from the degeneracy of the O(³P) ground state. Both states are purely repulsive at the SCF level, after inclusion of electronic correlation by the CEPA method they exhibit shallow van der Waals (dispersion) minima at large interatomic separation: R_? = 3.61 Å, ? = 1.0 meV (³Σ^?) and R_? = 3.05 Å, ? = 2.3 meV (³Π). The analysis of the results shows the very slow convergence of the dispersion interaction with increasing basis size, while SCF repulsion and the repulsion due to the change of the intra-atomic correlation are obtained reasonably accurately with moderate basis stes. Van der Waals coefficients C₆, C₈, C₁₀, potential curves of the type HFD (i.e. Hartree-Fock plus damped dispersion) and the influence of fine-structure effects (mainly spin-orbit coupling) on the shape of the adiabatic potential curves are discussed as well.     

Van der Waals radii of metals from spectroscopic data

The lack of information about the van der Waals radii of metals can be compensated for by using the results of spectroscopic investigations of van der Waals molecules. It has been shown that the interatomic distances in these molecules obey an additive scheme if one allows for the polarization effects. The van der Waals radii of the alkali metals, Ag, Mg, Zn, Cd, Hg, B, Al, In, and Si, have been determined from the interatomic distances in their heteroatomic molecules with atoms of noble gases. Use of the obtained radii for crystal chemistry is discussed.Translated fromIzyestiya Akademil Nauk. Seriya Khimicheskaya, No. 8, pp. 1374–1378, August, 1994.     

Intermolecular interactions in cyanodimethylarsine and cyanodimethyl­stibine

Cyano­di­methyl­arsine, [As(CH₃)₂(CN)], and cyano­di­methyl­stibine, [Sb(CH₃)₂(CN)], have closely related, but not isomorphous, crystal structures containing XCN?XCN? chains. The N?As distance of 3.185 (3) Å is slightly shorter than the expected van der Waals distance of 3.5 Å, while the N?Sb distance of 2.862 (9) Å, compared with the expected value of 3.7 Å, is much shorter. This is consistent with Sb being a stronger Lewis acid than As.     

A New Class of Two-Deck Dicationic Heteropentalenes Existing by the Balance between Interdeck Aromaticity and Covalent P−P Bonding

A number of two-deck dicationic 3a,6a-diaza-1,4-diphosphapentalenes (DDP)₂X₂ (X=halogen or complex ion) have been characterized. Interdeck distances P(1)⋅⋅⋅P(3) and P(2)⋅⋅⋅P(4) in crystal structures, as a rule, significantly exceed the sum of covalent radii, however they are still within the sum of van der Waals radii. The ³¹P NMR data indicate that phosphorus atoms are equivalent in solution even at low temperatures (233 K). Based on DFT calculations, the formation of two equivalent P−P covalent bonds (<2.4 Å) is energetically unfavorable, despite the absence of steric barriers. The total energy of the model dication as a function of the P⋅⋅⋅P separation has a minimum at the P−P distance of 2.85 Å. This distance corresponds to the highest aromaticity index in the interdeck space (NICS(1.0)=−20.72). The energy of interdeck interactions is estimated at 10–15 kcal/mol.     

Revealing the Preferred Interlayer Orientations and Stackings of Two‐Dimensional Bilayer Gallium Selenide Crystals

Characterizing and controlling the interlayer orientations and stacking orders of two‐dimensional (2D) bilayer crystals and van der Waals (vdW) heterostructures is crucial to optimize their electrical and optoelectronic properties. The four polymorphs of layered gallium selenide (GaSe) crystals that result from different layer stackings provide an ideal platform to study the stacking configurations in 2D bilayer crystals. Through a controllable vapor‐phase deposition method, bilayer GaSe crystals were selectively grown and their two preferred 0° or 60° interlayer rotations were investigated. The commensurate stacking configurations (AA′ and AB stacking) in as‐grown bilayer GaSe crystals are clearly observed at the atomic scale, and the Ga‐terminated edge structure was identified using scanning transmission electron microscopy. Theoretical analysis reveals that the energies of the interlayer coupling are responsible for the preferred orientations among the bilayer GaSe crystals.