Van der Waals complexes of perylene with rare-gas atoms

Fluorescence excitation spectra of weakly bound complexes between perylene (P) and rare-gas atoms (R) have been observed. Model calculations of potential surfaces of vdW complexes consisting of perylene and rare-gas atoms show that geometrical re-organisation is important and that thermal population of intermolecular P-R vibrational states may account in part for unresolved broadening at moderate stagnation pressures.     

Van der Waals complexes of polar aromatic molecules: unexpected structures for dimers of azulene

Full geometry optimizations at the dispersion corrected DFT-BLYP/TZV2P level of theory have been performed for dimers of azulene that may serve as a model system for the van der Waals complexes of polar pi systems. The structures and binding energies for 11 dimers are investigated in detail. The DFT-D interaction energies have been successfully checked against results from the accurate SCS-MP2/aug-cc-pVTZ approach. Out of the nine investigated stacked complexes, eight have binding energies larger than 7.4 kcal/mol (SCS-MP2) that exceed the value of 7.1 kcal/mol for the best naphthalene dimer. T-shaped arrangements (CH...pi) are significantly less stable. Two out of the three best structures have an antiparallel alignment of the monomer dipole moments in the complex, although the best ones with a parallel orientation are only about 0.5 kcal/mol less strongly bound which points to a minor importance of dipole-dipole interactions to binding. Quite surprisingly, the energetically lowest structure (DeltaE = -9.2 kcal/mol) corresponds to a situation where the two seven-membered rings are located almost on top of each other (7-7) and the long molecular axes are rotated against each other by 130 degrees. The 7-7 structural motif is found also in other energetically low-lying structures, and the expected 5-7 (two-side) arrangement is less strongly bound by about 2 kcal/mol. This can be explained by the electrostatic potential of azulene that only partially reflects the charge separation according to the common 4n + 2 pi electron rule. General rules for predicting stable van der Waals complexes of polar pi systems are discussed.     

Van der Waals Onteractions in molecular complexes of tetracyanoethylene

The interaction energy of complexes formed between methylbenzenes and tetracyanoethylene is calculated by two procedures. The first one is the monopoles-bond polarizabilities procedure previously described, while the second is derived from the semi-empirical treatment proposed by Kitaygorodsky. A satisfactory agreement is obtained between the calculated energies and the observed energies of formation in the gas phase. The dipole moment induced by mutual electronic polarization of the components is calculated for the Durene-Tetracyanoethylene complex, and is found to account for the major part of the observed dipole.

---

Zusammenfassung Die Wechselwirkungsenergie von Komplexen, die aus Methylbenzenen und Tetracyanäthylen gebildet werden, wird mit Hilfe von zwei Verfahren berechnet. Das erste ist das Verfahren der Monopol-Bindungs Polarisierbarkeiten, welches bereits beschrieben wurde. Das zweite Verfahren wird aus der halbempirischen Methode, die durch Kitaygorodsky vorgeschlagen wurde, hergeleitet. Eine befriedigende Übereinstimmung zwischen den berechneten und beobachteten Formationsenergien in der Gasphase wird erzielt. Das Dipolmoment, welches durch die gegenseitige elektrische Polarisation der Komponenten induziert wird, wird für den Durol-Tetracynäthylen-Komplex ausgerechnet. Es stimmt im wesentlichen Teil mit dem beobachteten Dipolmoment überein.

---

Résumé L'énergie d'interaction des complexes formés entre les methylbenzenes et le tetracyanoéthylene est évaluée au moyen de deux procédés: le premier est le procédé dit «monopoles-polarisabilités de liaison» tandis que le second est dérivé du traitement semi-empirique proposé par Kitaygorodsky. Un accord satisfaisant est obtenu entre les valeurs calculées et les énergies expérimentales de formation en phase gazeuse. Le dipole induit par polarisation électronique mutuelle des constituants est calculé dans le cas du complexe durene-tetracyano éthylène et rend compte de la majeure partie du dipole observé.

     

Average and extreme multi-atom Van der Waals interactions: Strong coupling of multi-atom Van der Waals interactions with covalent bonding

Background  

The prediction of ligand binding or protein structure requires very accurate force field potentials – even small errors in force field potentials can make a 'wrong' structure (from the billions possible) more stable than the single, 'correct' one. However, despite huge efforts to optimize them, currently-used all-atom force fields are still not able, in a vast majority of cases, even to keep a protein molecule in its native conformation in the course of molecular dynamics simulations or to bring an approximate, homology-based model of protein structure closer to its native conformation.     

Spin rovibronic symmetry in the formation of Van der Waals complexes

To include weak electron spin rovibronic and nuclear interaction other than electrostatic interaction we invoke the coupling of the spin rovibronic states of the molecular fragments as they form Van der Waals complexes. It is shown how concerted and cooperative rotation/vibration, etc. will preserve the relative geometry and potential for the complex. Stateto-state symmetry correlations, including the Fermion symmetry of protons are performed for the formation of a hypothetical dimer of C_2h symmetry from a monomer ABH₃ of C_3v symmetry, and for the formation of a hypothetical complex of C_3v symmetry from ABH₃ coupled with C₆H₆ Cor C₆F₆) of D_6h symmetry.This paper is dedicated to Professor J. Peter Toennies for his 60th birthday. It was due to Peter's encouragement and hospitality and under the intellectual and serene atmosphere of Max-Planck Institut für Strömungsforschung, that the author spent the last few summers of the most enjoyable and productive research life at Göttingen during which many ideas were formed, one of which pertains to this paper.Supported by private funds.Academician, Academia Sinica.Tamkang Chair Lecturer, March 1990.     

Intermolecular vibronic spectroscopy of small Van Der Waals clusters: phenol- and aniline-(argon)2 complexes

We report the clear observation and assignment of the symmetric stretching and bending van der Waals modes in two three-bodyC _2v complexes, phenol- and aniline-(Ar)₂, using resonant two-photon ionization.     

On the angular shape of van der Waals dimers of small polar molecules

Ab initio calculations on twenty van der Waals dimers of small polar molecules at the experimentally observed intermolecular separation R, using Hartree-Fock molecular moments, show that the minima in the electrostatic interaction expanded up to R⁻⁶ converges to angular structures which are close to those observed experimentally for such complexes.     

Van der Waals complexes of Cu, Ag, and Au with hydrogen sulfide. The bonding character

The electronic and structural features of the Cu...SH2, Ag...SH2, and Au...SH2 complexes are investigated by using the spin-adapted restricted open-shell HF coupled cluster CCSD(T) method combined with the second-order spin-free Douglas-Kroll-Hess (DKH) relativistic approach. M...SH2 complexes are nonplanar with bonding energies -5.99, -1.99, and -9.08 mHartree, respectively. Comparison with analogous M...OH2 and M...NH3 complexes allows us to establish general features of the bonding between coinage metal atoms and ligand molecules with the participation of their lone electron pairs. Consistent interpretation of the interaction effects can be obtained by using the molecular orbital picture of the M...L region. The bonding character is explained by stressing the importance of the charge transfer from the lone pair of the ligand to the metal atom. Relativistic changes of the metal element electron affinity and polarizability facilitate the understanding of major trends in the pattern of interactions between the coinage metal atoms and different lone pair donating ligands.     

Two-color resonance enhanced multiphoton ionization of Van der Waals molecules: studies of spectroscopy shifts and ionization thresholds of paraxylene clustered with argon

Van der Waals molecules comprised of paraxylene (PX) clustered with Ar atoms were formed in pulsed supersonic jet, subjected to two-color resonance enhanced two-photon ionization (2C R2PI), and analyzed in a time-of-flight mass spectrometer. Perturbed S₁ states of the complexes display spectral shifts relative to the nascent molecule (PX). The appearance potentials (AP) of PX·^Ar_n clusters were measured for n = 1 to 6. On average, the AP is red-shifted ≈ 120 cm⁻¹ per Ar atom addition, but there are deviations from a smooth trend at the dimer and pentamer.     

Ab initio SCF-CI studies of the intermolecular interaction between two hydrogen molecules near the Van der Waals minimum

SCF-CI calculations have been used to study the intermolecular energy between two hydrogen molecules in four different geometrical configurations. The CI matrix was diagonalized using perturbation techniques. The importance of the perturbation expansion order upon the intermolecular energy could therefore be studied. The wave function includes all singly and doubly excited configurations. The natural orbitals have been determined and their relative importance on the intermolecular energy is considered.     

Adsorption of small aromatic molecules on gold: a DFT localized basis set study including van der Waals effects

Schrödinger equation for harmonium and related models may be transformed to the biconfluent Heun equation. The solubility of this equation and its applications in quantum chemistry are briefly discussed.     

Reactions of O with CO van der Waals molecules and clusters

The reaction of O(³ P) with COR _m clusters to produce electronically excited CO₂ was studied under molecular beam conditions. It was found that the spectrum of the chemiluminescence produced extended from the blue all the way to the near infrared. The dependence of the total emission intensity on stagnation pressure was investigated for (CO) _m as well as for COR _m , R=He, Ne, Ar, N₂, CO₂ and SO₂. The low pressure data indicate that small (CO) _m polymers are more efficient than clusters of CO with other species in inducing the chemiluminescent reaction. The larger CO-rare gas clusters, however, exhibited larger reaction cross-sections than those of the CO polymers. Rare gas clusters ofm≧5, on the other hand have successively smaller cross sections for reaction. The reactivity of the CO₂ and SO₂ clusters seems to peak at aboutm=1 and then decreases for larger species. An equilibrium model for cluster formation was proposed and it was found capable of explaining and simulating the experimental observations. Contrary to what was reported from afterglow experiments, no barrier for the reaction was detected.     

Van der Waals interactions: evaluations by use of a statistical mechanical method

In this work the induced van der Waals interaction between a pair of neutral atoms or molecules is considered by use of a statistical mechanical method. With use of the Schro?dinger equation this interaction can be obtained by standard quantum mechanical perturbation theory to second order. However, the latter is restricted to electrostatic interactions between dipole moments. So with radiating dipole-dipole interaction where retardation effects are important for large separations of the particles, other methods are needed, and the resulting induced interaction is the Casimir-Polder interaction usually obtained by field theory. It can also be evaluated, however, by a statistical mechanical method that utilizes the path integral representation. We here show explicitly by use of this method the equivalence of the Casimir-Polder interaction and the van der Waals interaction based upon the Schro?dinger equation. The equivalence is to leading order for short separations where retardation effects can be neglected. In recent works [J. S. H?ye, Physica A 389, 1380 (2010); Phys. Rev. E 81, 061114 (2010)], the Casimir-Polder or Casimir energy was added as a correction to calculations of systems like the electron clouds of molecules. The equivalence to van der Waals interactions indicates that the added Casimir energy will improve the accuracy of calculated molecular energies. Thus, we give numerical estimates of this energy including analysis and estimates for the uniform electron gas.     

Absorption spectra of small niobium and gold clusters measured by photodepletion of their rare gas van der Waals complexes: some preliminary experiments

The optical absorption spectra of small niobium clusters have been determined over the wavelength range 260 – 740 nm by photodetaching Krypton atoms from the corresponding neutral van der Waals, vdW, complexes, Nb_nKr_m, n=5–15, m=1–3. Cross sections for small gold clusters were determined by photodetachment experiments oncharged vdW complexes [Au_nXe_m]⁺, m=1, 2. The absorption cross sections are observed to increase monotonically with decreasing wavelength. At the long wavelength end of the range, the cross section is practically independent of the cluster nuclearity, n; whereas, at the short wavelength end of the range, the cross section increases monotonically with n.