Degenerate symmetry-adapted perturbation theory of weak interactions between closed- and open-shell monomers: application to Rydberg states of helium hydride

Symmetry-adapted perturbation theory is extended to the (quasi) degenerate, open-shell case. The new formalism is tested in calculations of the interaction energies for a helium atom in the ground state interacting with an excited hydrogen atom. It is shown that the method gives satisfactory results if the coupling with higher Rydberg states of the dimer is small, as is the case for the A²Σ⁺,B²Π,E²Π,3²Π, and 1²Δ states of HeH. For the C²Σ⁺ state convergence of the method is very slow, but it can be improved by including the n=3 states in the model space. Received: 3 June 1998 / Accepted: 9 September 1998 / Published online: 7 December 1998     

Energy transfer in chemical dimers of porphyrins and chlorins by the methods of selective laser spectroscopy at 1.8–10 K

The methods of selective laser spectroscopy applied at 1.8–10 K to a glass-forming tetrahydrofuran-toluene (3∶1) mixture have revealed that in the dimers of tetrapyrrole compounds (ethane-bis-octaethylporphyrins, ethane-bis-Zn-octaethylchlorins, heterodimers of Zn-cyclopentane porphyrins) the exciton interactions (V₁₂≈118 cm⁻¹) and S-S-transfer of energy (of probability F≈11 sec⁻¹) are implemented under conditions of substantial inhomogeneous broadening (δF_1/2≈50 cm⁻¹). Upon excitation of a donor component in the dimers, one observes a strong quenching of its emission, the disappearance of the fine structure in the fluorescence spectra, a decrease in the efficiency of spectral hole burning by, approximately, an order of magnitude, and a 12-fold increase in their halfwidth. To whom correspondence should be addressed. Reported at the VIIIth International Conference on Spectroscopy of Porphyrins and Their Analogs, Minsk, September 22–26, 1998. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 500–504, July–August, 1999.     

The mechanism of double proton transfer in dimers of uracil and 2-thiouracil--the reaction force perspective

The intermolecular double proton transfer in dimers of uracil and 2-thiouracil is studied through density functional theory calculations. The reaction force framework provides the basis for characterizing the mechanism that in all cases has been associated to a dynamic balance between polarization and charge transfer effects. It has been found that the barriers for proton transfer depend upon the nature of the acceptor atoms and its position within the seminal monomer. Actually, the change in the nature of the hydrogen bonds connecting the two monomers along the reaction coordinate may favor or disfavor the double-proton transfer.     

A combining rule calculation of the ground-state van der Waals potentials of the magnesium rare-gas complexes

The potential energy curves and spectroscopic constants of the ground-state of the Mg–Rg (Rg = He, Ne, Ar, Kr, and Xe) van der Waals complexes are generated by the Tang–Toennies potential model and a set of derived combining rules. The parameters of the model are calculated from the potentials of the homonuclear magnesium and rare-gas dimers. The predicted spectroscopic constants are comparable to other available theoretical and experimental results, except in the case of Mg–He, we note that there are large differences between various determinations. Moreover, in order to reveal relative differences between species more obviously we calculated the reduced potential of these five systems. The curves are clumped closely together, but at intermediate range the Mg–He reduced potential is clearly very different from the others.     

Deep-ultraviolet surface plasmon resonance of Al and Al_(core)/Al_2O_(3shell) nanosphere dimers for surface-enhanced spectroscopy

The localized surface plasmon resonance properties of Al and Alcore/Al2O3 shell nanosphere dimers with Al and Al core nanosphere radii of 20 nm and Al2O3 shell of 2 nm in the deep-ultraviolet region have been studied using the finite difference time domain method. The extinction spectra and the electric field distribution profiles of the two dimers for various gap distances between two individual nanospheres are compared with those of the corresponding monomers to reveal the extent of plasmon coupling. It is found that with the interparticle distance decreasing, a strong plasmon coupling between two Al or Alcore/Al2O3 shell nanospheres is observed accompanied by a significant red shift in the extinction spectra at the parallel polarization direction of the incident light related to the dimer axis, while for the case of the perpendicular polarization direction, a weak plasmon coupling arises characterized by a slight blue shift in the extinction spectra. The electric field distribution profiles show that benefiting from the dielectric Al2O3 shell, the gap distance of Alcore/Al2O3 shell nanosphere dimers can be tailored to < 1 nm scale and results in a very high electric field enhancement. The estimated surface-enhanced Raman scattering enhancement factors suggests that the Alcore/Al2O3 shell nanosphere dimers with the gap of < 1 nm gave rise to an enhancement as high as 8.1 × 107 for interparticle gap = 0.5 nm. Our studies reveal that the Alcore/Al2O3 shell nanosphere dimers may be promising substrates for surface-enhanced spectroscopy in the deep-ultraviolet region.     

Rovibrational dynamics of the RbCs molecule in static electric fields. Classical study

We study the classical dynamics of the RbCs molecule in the presence of a static electric field. Under the Born–Oppenheimer approximation, we perform a rovibrational investigation which includes the interaction of the field with the molecular polarizability. The stability of the equilibrium points and the phase space structure of the system are explored in detail. We find that, for strong electric fields or for energies close to the dissociation threshold, the molecular polarizability causes relevant effects on the system dynamics.     

Dimer statistics of honeycomb lattices on Klein bottle,Möbius strip and cylinder

Dimer statistics is a central problem in statistical physics. In this paper the enumerations of close-packed dimers of honeycomb lattices on Klein bottle, Möbius strip and cylinder are considered. By establishing a Pfaffian orientation or a crossing orientation, and then computing the determinants of the skew-symmetric matrices of the resulting orientation graphs, we obtain explicit expressions of the number of close-packed dimers of the Klein-bottle polyhex, the Möbius polyhex and the cylindrical polyhex.     

Stability of functionalized C60 paramagnetic dimers and monomers

Density functional theory is used to calculate the bond dissociation energy to cleave the C₆₀C₆₀ bond of the paramagnetic X-C₆₀C₆₀-X and X-C₆₀C₆₀ dimers where X is F, OH, O and H. The results show that these dimers would not be stable much above room temperature and therefore cannot constitute the paramagnetic phase needed to form the observed ferromagnetism which has been shown to be stable up to 800 K. The calculated bond dissociation energies to remove an F, OH or H from a single C₆₀ are large suggesting that they could be the source of the unpaired spin needed for the high temperature ferromagnetism.     

Elucidation of the Active Conformation of Vancomycin Dimers with Antibacterial Activity against Vancomycin‐Resistant Bacteria

Covalently linked vancomycin dimers have attracted a great deal of attention among researchers because of their enhanced antibacterial activity against vancomycin‐resistant strains. However, the lack of a clear insight into the mechanisms of action of these dimers hampers rational optimization of their antibacterial potency. Here, we describe the synthesis and antibacterial activity of novel vancomycin dimers with a constrained molecular conformation achieved by two tethers between vancomycin units. Conformational restriction is a useful strategy for studying the relationship between the molecular topology and biological activity of compounds. In this study, two vancomycin units were linked at three distinct positions of the glycopeptide (vancosamine residue (V), C terminus (C), and N terminus (N)) to form two types of novel vancomycin cyclic dimers. Active NC‐VV‐linked dimers with a stable conformation as indicated by molecular mechanics calculations selectively suppressed the peptidoglycan polymerization reaction of vancomycin‐resistant Staphylococcus aureus in vitro. In addition, double‐disk diffusion tests indicated that the antibacterial activity of these dimers against vancomycin‐resistant enterococci might arise from the inhibition of enzymes responsible for peptidoglycan polymerization. These findings provide a new insight into the biological targets of vancomycin dimers and the conformational requirements for efficient antibacterial activity against vancomycin‐resistant strains.     

A comparative study of the dimers of selected hydroxybenzenes

Hydroxybenzenes are the parent compounds of large classes of derivatives, many of which exhibit biological activities. This work presents the results of a comparative study of the dimers of selected hydroxybenzenes, considering all the possible mutual geometrical arrangements of the two monomers and comparing their relative stabilities and interaction energies. The OH···OH hydrogen bond between the two monomers is the dominant stabilizing factor, with frequent preference for mutual perpendicularity of the two aromatic rings. C? H···O unconventional H‐bonds, OH···π unconventional H‐bonds, H···π interactions and π··π interactions also may play significant roles. The factors stabilizing individual hydroxybenzenes (presence of intramolecular hydrogen bonds; number, positioning and orientation of the OH groups; symmetry features) have greater influence on the dimers' relative energy than on the interaction energy between monomers. While results from different calculations methods (HF, MP2, and DFT/B3LYP) show consistency for all the features just‐mentioned, they show some relevant differences in the way they take into account different types of interactions between monomers, resulting in some differences in the geometry arrangements of the monomers in the lowest energy dimers and in differences in the relative preferences among higher‐energy dimer geometries. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012