Calculation of normal frequencies of adsorbed molecules by the LCGTO-MCP-LSD method

Density functional calculations have been performed to determine the equilibrium geometry and normal frequencies of the Pd₃(CCH₃) cluster (ethylidyne on Pd₃). We find promising agreement between the theoretically obtained frequencies and those of experimental studies for CCH₃ on the Pd(111) surface. The effect of the metal cluster relaxation on the calculated equilibrium geometry and normal frequencies is examined.     

Symmetries of electrostatic interaction between DNA molecules

A model for pair interaction U of DNA molecules generated by the discrete dipole moments of base-pairs and the charges of phosphate groups is studied. A noncommutative group of eighth order ℒ of symmetries that leave U invariant is found. The minima are classified with the use of group ℒ and numerical methods are employed for finding them. The minima may correspond to several cholesteric phases, as well as to phases formed by crosslike conformations of molecules at an angle close to 90°—the “snowflake phase.” The results depend on the effective charge Q of the phosphate group, which can be modified by the polycations or the ions of metals. The snowflake phase could exist for Q above the threshold Q _C. Below Q _C, there could be several cholesteric phases. Close to Q _C, the snowflake phase could change into the cholesteric one at constant distance between adjacent molecules. The text was submitted by the authors in English.     

Chemical effects on vibrational properties of adsorbed molecules on metal surfaces

Vibrational properties of chemisorbed molecules on metal surfaces are studied with a focus on the coverage (θ) dependent chemical shift of the frequencies. The electronic properties of an incomplete monolayer of adsorbates are calculated by means of the coherent potential approximation, in which the electron hopping between the adsorbates (band formation effect) and the depolarization effect due to the proximity of ionized adsorbed molecules are taken into account consideration. It is found that in a weakly chemisorbed system, such as CO/Cu, the negative shift in chemical origin amounts to the positive dipole shift at low coverage. It is also shown that the variation of the back-donated charge with θ gives rise to the coverage dependent polarizability, which in turn influences the frequency shift estimated by the previous dipole coupling theory. The coverage dependent back-donation also plays a significant role in the work function change Δϕ of the substrate. The polarity of a weakly chemisorbed CO remains unchanged compared to a free CO (⁻CO⁺) so that Δϕ exhibits the initial lowering in the presence of the positive dipoles. The increase in the back-donated charge with θ causes the decrease in the effective dipole moment towards a compensation of the positive hole due to 5σ donation. A simple explanation is offered to clarify the characteristic difference of the work function change between the strongly and weakly chemisorbed CO molecules on metal surfaces. In particular, a possible origin of the work function minimum observed for CO/Cu systems is discussed in terms of the coverage dependent back-donation.     

Symmetry of electrostatic interaction between pyrophosphate DNA molecules

We study chiral electrostatic interaction between artificial ideal homopolymer DNA-like molecules in which a number of phosphate groups of the sugar-phosphate backbone are exchanged for the pyrophosphate ones. We employ a model in which the DNA is considered as a one-dimensional lattice of dipoles and charges corresponding to base pairs and (pyro)phosphate groups, respectively. The interaction between molecules of the DNA is described by a pair potential U of electrostatic forces between the two sets of dipoles and charges belonging to respective lattices describing the molecules. Minima of the potential U indicate orientational ordering of the molecules and thus liquid crystalline phases of the DNA. We use numerical methods for finding the set of minima in conjunction with symmetries verified by the potential U . The symmetries form a non-commutative group of 8th order, S . Using the group S we suggest a classification of liquid crystalline phases of the DNA, which allows several cholesteric phases, that is polymorphism. Pyrophosphate forms of the DNA could clarify the role played by charges in their liquid crystalline phases, and open experimental research, important for nano-technological and bio-medical applications.     

The interaction polarizability of two hydrogen molecules

The incremental polarizability which occurs when two hydrogen molecules interact has been calculated by the ab initio SCF method using a [3s, 2p] gaussian basis set. Results are reported for various separations and orientations. Where symmetry allows, the polarizabilities are compared with those of the helium diatom. The general shapes of the curves are similar, though they differ in detail.     

The change of the vibrational frequencies of a chemisorbed diatomic caused by the electrostatic interactions with the metal surface

Results of density functional calculations are used to discuss the change of the vibrational frequency of an adsorbed molecule caused by the electrostatic interaction between a chemisorbed molecule — simulated by two effective charges — and a metallic surface.     

二静电体系相互作用力与相互作用能一般关系的正确表达式

指出二静电体系的相互作用力的普遍表达式F=-↓△w1=-↓△w并不恰当。正确的表达式应当是Fj=δW1/δqj=-δW1/δqj，其中Fj是沿广义坐标qj方向的广义力，W1是静电场总能量，W1是二静电体系的相互作用能，qj为表征二静电体系相对位移的某一广义坐标。     

Inelastic scattering of electrons by vibrational motion of molecules adsorbed at metal surfaces

A model for computing the inelastic scattering power of infrared active molecular vibrations for low energy electrons reflected from gas-covered metal surfaces is constructed and the results are compared to recent observations. The metallic substrate has two major effects on the inelastic intensity: (i) at or near normal incidence, reflected electrons can strongly excite vibrations perpendicular to the surface, a process which is forbidden for forward acattering (and also for excitation by normally incident infrared light); (ii) the inelastic cross section for low energy electrons is enhanced as a result of image effects. The theory also predicts that the observed inelastic intensity should be highly sensitive to the spectrometer aperture.     

The role of electrodynamic interactions on energy loss intensities of adsorbed molecules

The effect on energy loss intensities of the electrodynamic interactions with the substrate and neighboring adsorbed molecules is discussed. The experimentally observed large variations in the loss intensity from molecules adsorbed on various sites are considered. A simple dipole model of the molecule interacting with its image in the substrate and its neighbors on equivalent and inequivalent sites is used. Good agreement with experiment for a well characterized surface coverage of bridge and on-top bonded CO molecules on Ni(111) is found.     

Appearance of intermolecular interaction in the vibrational spectra of polyatomic XY3 molecules

We present the Raman spectra of liquid and solid PBr₃, AsCl₃, AsBr₃, and BiCl₃ between 77 and 500°K. We conjecture that ethane-like dimers of these phosphorus and arsenic trihalides exist between some temperature and the melting point. On the basis of our experimental results we analyze the change in the intramolecular force field of these molecules induced by the phase transitions and temperature, whence we calculate the frequency of vibration of the molecules relative to each other in the dimers: 2PBr₃, 2AsCl₃, 2AsBr₃. The same approach also gives the frequencies of the librational and translational modes of bismuth trichloride at 77°K.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 42–49, April, 1977.     

Field enhancement at crystal steps: An intensity effect in vibrational spectra of adsorbed molecules

An enhancement of the electric field near a step in a single-crystal metal surface could give rise to an increase in the molar extinction coefficient for molecules adsorbed on the step. For CO adsorbed on a stepped surface. Pt(S)-[4(111)×(100)]. we calculate the extinction coefficient to be about 2.2 times greater for molecules adsorbed on the step than for molecules adsorbed on the middle of the terrace.     

Linear dependence of the interaction energy on intramolecular distance for adsorbed or clustered diatomic molecules

The dependence of several molecular properties (polarizabilities, multipole moments) of adsorbed diatomic molecules on the intramolecular distance are discussed within time-independent perturbation theory. The nearly linear dependence of the intermolecular interaction energy and molecular properties rationalizes the small variation of the property functions compared with those for the gas state. This relative independence of the property functions was demonstrated numerically in a previous paper using an iterative procedure for molecular hydrogen adsorbed in a zeolite-A-type framework.     

On the force of electrostatic interaction between two conducting spheres

A compact solution is obtained to the problem on the force of interaction between two conducting spheres with preset charges on their surfaces in zero external field. The derivation is based on exact solution of the problem of the potential distribution in the bispherical coordinate system. The expression for the force was derived by differentiating the potential energy of interaction between the spheres with respect to the distance between their centers. It is shown using numerical calculations that with decreasing distance between the spheres, the ratio of their charges for which the forces of interaction between the charges are zero tends to the ratio of the charges of contacting spheres. It follows hence that for any ratio of charges of the same polarity, which differs from the ratio of charges of the contacting spheres, there always exists a small distance between the spheres, at which they attract each other.     

New equations for derivatives of vibrational frequencies

A new equation is presented that expresses first partial derivatives of eigenvalues of the secular equation in terms of first derivatives of coefficients of the secular equation and the eigenvalues themselves. The utility of this equation is demonstrated using the example of isotopic shifts in the vibrational frequencies of molecules with second-order submatrices. Simple expressions for second, third, and forth derivatives of the eigenvalues are derived for this case.