Intermolecular Processes in Excited Electronic States in the Gas Phase (Review)

A brief review of works on the temperature dependences of the rate constants k_q of the intermolecular processes proceeding in the excited electronic states in the gas phase is given. The dependences k_q(T) for such biomolecular processes as intermolecular vibrational energy transfer in the triplet state vibrational quasi-continuum, triplet-triplet electron excitation energy transfer, and intermolecular photoinduced electron transfer have been compared. The experimental data have shown that in the gas phase for all analyzed intermolecular processes both an increase and a decrease in k_q with increasing temperature (T) is observed, which is not associated with the specific intermolecular interactions leading to the formation of long-lived components. The change in the type of temperature dependence is due to the change in the mechanisms of the radiationless transitions with increasing density of vibrational levels in the final electronic state. The applicability of the known models based on the theory of radiationless transitions for predicting the temperature dependences k_q(T) is discussed. __________ Translated from Zhurnal Prikladnoi Spektros-kopii, Vol. 72, No. 4, pp. 429–439, July–August, 2005.     

Theoretical calculation of the conformation and crystal structure of poly-L-hydroxyproline

The intra-and intermolecular forces determining the conformation of poly-L-hydroxyproline have been examined. The vacuum conformation of the isolated molecule has the left-hand trans configuration of poly-L-proline II with Ψ = 295°. A second less favorable low-energy form is the right-hand superhelix, also previously described for polyproline. No cis conformation was found to be feasible. Introduction of intermolecular interaction functions shows that the conformation in the solid state is strongly modified to accommodate such interactions. The rotation angle corresponding to the minimum energy in the solid is found to be Ψ = 343°. The crystal structure predicted from these calculations is a hexagonal lattice with a₀ = 12.5 Å and c₀ = 9.1 Å with space group P3₂. Three helices are required in the fundamental structural unit. These results are in excellent agreement with X-ray evidence.     

Thermal diffusion in binary mixtures containing molecular gases II: mixtures of N2 with He,Ne, Ar,and Kr and of CO2 with He and Ar

Expressions developed for the thermal diffusion factor, αT, for binary mixtures containing molecular species have been tested for six different atom-molecule interactions. Specifically, the interactions between N₂ and He, Ne, Ar and Kr, plus the interactions between CO₂ and He, Ar, have been examined, in each case using at least two competing modern potential energy surfaces. Detailed expressions in both the ‘total-energy-flux’ and the ‘two-flux’ representations have been developed, and the importance of the anisotropic components of the intermolecular potential energy surfaces in determining the thermal diffusion factor for such mixtures has been determined.     

Manifestation of intramolecular hydrogen bonds in the IR spectra of bioactive aminophenols

The intermolecular interactions in solutions of aminophenols in CCl₄ are studied by the methods of IR Fourier spectroscopy. If the hydroxyl groups of aminophenol molecules occupy the ortho positions with respect to the amino groups of the molecules, the hydroxyl and amino groups are involved in intramolecular interactions with the formation of hydrogen bonds O-H...N and N-H...O. The introduction of two additional tert-butyl groups into the structure of the aminophenol molecule facilitates the formation of O-H...N bonds and impedes the formation of N-H...O bonds. The occurrence of the carbonyl group in the structure of aminophenols leads to the formation of intramolecular hydrogen bonds O-H...O=C. The introduction of the methyl groups into carbonyl-containing aminophenols transforms the O-H...O=C bond into the hydrogen bond N-H...O=C.     

Ultrasonic studies of intermolecular interactions in binary mixtures of 4-methoxy benzoin with various solvents: Excess molar functions of ultrasonic parameters at different concentrations and in different solvents

Density (ρ), ultrasonic velocity (U), for the binary mixtures of 4-methoxy benzoin (4MB) with ethanol, chloroform, acetonitrile, benzene, and di-oxane were measured at 298 K. The solute–solvent interactions and the effect of the polarity of the solvent on the type of intermolecular interactions are discussed here. From the above data, adiabatic compressibility (β), intermolecular free length (L_f), acoustic impedance (Z), apparent molar volume (Ø), relative association (RA) have been calculated. Other useful parameters such as excess density, excess velocity and excess adiabatic compressibility have also been calculated. These parameters were used to study the nature and extent of intermolecular interactions between component molecules in the binary mixtures.     

Laser-induced gas-surface interactions

Chemical reactions in homogeneous systems activated by laser radiation have been extensively investigated for more than a decade. The applications of lasers to promote gas-surface interactions have just been realized in recent years. The purpose of this paper is to examine the fundamental processes involved in laser-induced gas-surface chemical interactions. Specifically, the photon-enhanced adsorption, adsorbate-adsorbate and adsorbate-solid reactions, product formation and desorption processes are discussed in detail. The dynamic processes involved in photoexcitation of the electronic and vibrational states, the energy transfer and relaxation in competition with chemical interactions are considered. These include both single and multiple photon adsorption, and fundamental and overtone transitions in the excitation process, and inter- and intra-molecular energy transfer, and coupling with phonons, electron-hole pairs and surface plasmons in the energy relaxation process. Many current experimental and theoretical studies on the subject are reviewed and discussed with the goal of clarifying the relative importance of the surface interaction steps and relating the resulting concepts to the experimentally observed phenomena. Among the many gas-solid systems that have been investigated, there has been more extensive use of CO adsorbed on metals, and SF₆ and XeF₂ interactions with silicon as examples to illustrate the many facets of the electronically and vibrationally activated surface processes. Results on IR laser stimulated desorption of C₅H₅N and C₅D₅N molecules from various solid surfaces are also presented. It is clearly shown that rapid intermolecular energy exchange and molecule to surface energy transfer can have important effects on photodesorption cross sections and isotope selectivities. It is concluded that utilization of lasers in gas-surface studies not only can provide fundamental insight into the mechanism and dynamics involved in heterogeneous interactions, but also offer the possibility for technical innovation for practical applications.     

Rovibrational matrix elements of the multipole moments and of the polarizability of the H2 molecule in the solid phase: Effect of intermolecular potential

Rovibrational matrix elements of the multiple moments Q _l up to rank 10 and of the linear polarizability of the H₂ molecule in the condensed phase have been computed taking into account the effect of the intermolecular potential. Comparison with gas phase matrix elements shows that the effect of solid state interactions is marginal.     

The revision of intermolecular interactions in 1,3‐dinitrobenzene crystal—the role of nitro groups in optical nonlinearity

Polarized Fourier transform‐infrared (FT‐IR) reflectance spectra and powder Raman spectra have been measured for 1,3‐dinitrobenzene crystal in order to revise the assignments of bands by means of the oriented gas model reinforced with quantum chemical [density functional theory (DFT)] calculations. Longitudinal optical/transverse optical (LO‐TO) splitting of some bands is observed indicating medium strong, long‐range, dipole–dipole interactions. The analysis of overtones in the polarized FT‐NIR spectra has allowed us to estimate the anharmonicity of vibrations in the crystal. The molecular motions of the nitro groups are analyzed on the basis of temperature‐dependent polycrystalline IR spectra. Based on the values of the energy difference (Δν_el) between the forbidden A_1g→B_2u transition in the benzene molecule in the gas phase and the first electronic transition in 1,3‐dinitrobenzene, it has been concluded that the intermolecular interactions are medium strong. The nitro group interactions are proposed to play the main role in the optical nonlinearity. Copyright © 2010 John Wiley & Sons, Ltd.     

Nanoencapsulation of Fullerenes in Organic Structures with Nonpolar Cavities

The formation of supramolecular structures, assemblies, and arrays held together by weak intermolecular interactions and non-covalent binding mimicking natural processes has been used in applications being anticipated in nanotechnology, biotechnology and the emerging field of nanomedicine. Encapsulation of C₆₀ fullerene by cyclic molecules like cyclodextrins and calixarenes has potential for a number of applications. Similarly, biomolecules like lysozyme also have been shown to encapsulate C₆₀ fullerene. This poster article reports the recent trends and the results obtained in the nanoencapsulation of fullerenes by biomolecules containing nonpolar cavities. Lysozyme was chosen as the model biomolecule and it was observed that there is no covalent bond formed between the bimolecule and the C₆₀ fullerene. This was confirmed from fluorescence energy transfer studies. UV–Vis studies further supported this observation that it is possible to selectively remove the C₆₀ fullerene from the nonpolar cavity. This behavior has potential in biomedical applications     

The influence of the nature of a substituent on the parameters of the intra- and intermolecular interactions in molecules of cross-conjugated ketones

We have quantitatively analyzed the vibronic parameters of two cross-conjugated δ-dimethylaminoketones. The presence of the –N(CH₃)₂, C=O, and –NO₂ groups in the benzene ring has been shown to affect the manifestation of the vibronic parameters of characteristic bands that describe the state (vibrations, types of deformation upon excitation) of polyene systems with aromatic rings. Data on the influence of the nature of the substituent on the parameters of intra- and intermolecular interactions in the examined compounds have been presented.     

Topological analysis of charge density in heptasulfur imide (S7NH) from isolated molecule to solid

Intra and intermolecular interactions of heptasulfur imide (S₇NH) are investigated in terms of topological properties analyses, such analyses are applied to both experimental (multipole model) and theoretically calculated (DFT and PDFT calculations) charge densities of the isolated molecule and of the crystal. The same analyses are also applied to a multipole model density obtained from theoretically (PDFT) derived structural amplitudes. The covalent bond character of S-N, N-H and S-S bonds are well described in terms of density, ρ_b, and total energy density, H_b, at the bond critical point r_c, though it is clear that the S-S bonds are weaker shared interactions than those of N-H and S-N bonds. Lone pair electron regions of sulfur and nitrogen atoms are revealed as the local charge concentration site from the Laplacian of charge density. The even weaker intermolecular interactions are well characterized; these include the N-H?S hydrogen bonding, N?S binding interactions and S?S binding interactions. All these intermolecular binding interactions are closed-shell interactions. The Laplacian of charge density demonstrates a directional intermolecular binding interaction. The corresponding intermolecular binding energies are derived by MP2/6-311+G(d,p) calculations. Atomic graph of each atom of the molecule is described in detail by the vertices, edges and faces of the polyhedron around the nucleus to illustrate such directional interactions.     

Recent Advance in Terahertz Wave and Material Basis

The unexplored terahertz (THz) region involves important phenomena of both fundamental and applied natures. Examples include phonon interactions, rotational transitions, and intermolecular dynamics. Frequency tunable high-power THz wave generation has been successfully achieved utilizing lattice resonance of LiNbO₃ and GaP crystals, respectively. Semiconductor devices utilizing the electron tunneling effect have also been developed.     

Structural studies of liquid alcohols by neutron diffraction

Neutron diffraction measurements have been made on methyl alcohol at room temperature for an incident wavelength of 0·94 Å. Cross sections have been obtained for CD₃OD, CD₃OH, and mixtures of these compounds. These data are subtracted to obtain the separated structure factors for intermolecular H₀H₀, the hydroxyl components, and the non-hydroxyl components. The Fourier transformations of the structure factors show components of both intra- and intermolecular distribution functions. Width parameters obtained from model fits are too large for thermal vibrations and are interpreted as geometrical broadening due to the stretching of bonds, variations in bond angles, and rotation of the methyl group. Differences in the real space distribution function between hydrogen and deuterium are noted.     

Isotopic effects on non-linearity, molecular radius and intermolecular free length

Computation of non-linearity parameter (B/A), molecular radius (r _m) and intermolecular free length (L _f) for H₂O, C₆H₆, C₆H₁₂, CH₃OH, C₂H₅OH and their deuterium-substituted compounds have been carried out at four different temperatures, viz., 293.15, 303.15, 313.15 and 323.15 K. The aim of the investigation is an attempt to study the isotopic effects on the non-linearity parameter and the physicochemical properties of the liquids, which in turn has been used to study their effect on the intermolecular interactions produced thereof.     

Ultrasonic and IR study of intermolecular association through hydrogen bonding in ternary liquid mixtures

Complex formation in ternary liquid mixtures of dimethylsulfoxide (DMSO) with phenol and o-cresol in carbontetrachloride has been studied by measuring ultrasonic velocity at 2 MHz, in the concentration range of 0.019-0.162 (in mole fraction of DMSO) at varying temperatures of 20, 30 and 40 degrees C. Using measured values of ultrasonic velocity, other parameters such as adiabatic compressibility, intermolecular free length, molar sound velocity, molar compressibility, specific acoustic impedance and molar volume have been evaluated. These parameters have been utilized to study the solute-solute interactions in these systems. The ultrasonic velocity shows a maxima and adiabatic compressibility a corresponding minima as a function of concentration for these mixtures. The results indicate the occurrence of complex formation between unlike molecules through intermolecular hydrogen bonding between oxygen atom of DMSO molecule and hydrogen atom of phenol and o-cresol molecules. The excess values of adiabatic compressibility and intermolecular free length have also been evaluated. The variation of both these parameters with concentration also indicates the possibility of the complex formation in these systems. Further, to investigate the presence of O-HO bond complexes and the strength of molecular association with concentrations, the infrared spectra of both the systems, DMSO-phenol and DMSO-o-cresol, have been recorded for various concentrations at room temperature (20 degrees C). The results obtained using infrared spectroscopy for both the systems also support the occurrence of complex formation through intermolecular hydrogen bonding in these ternary liquid mixtures.     

Study of intermolecular interactions in binary mixtures of 2′-chloro-4-methoxy-3-nitro benzil in various solvents and at different concentrations by the measurement of acoustic properties

Density (ρ), ultrasonic velocity (u), adiabatic compressibility (β), apparent molar volume (Ø), acoustic impedance (Z), intermolecular free length (L_f), relative association (RA) of binary mixtures of 2′-chloro-4-methoxy-3-nitro benzil (abbreviated as 2CBe) in ethanol, acetonitrile, chloroform, dioxane and benzene were measured at different concentrations at 298 K. Several useful parameters such as excess density, excess ultrasonic velocity, excess adiabatic compressibility, excess apparent molar volume, excess acoustic impedance and excess intermolecular free length have been calculated. These parameters are used to explain the nature of intermolecular interactions taking place in the binary mixture. The above study is useful in understanding the solute – solvent interactions occurring in different concentrations at room temperature.     

Adsorption geometry of PTCDA on 2H-NbSe2

Low-temperature scanning tunnelling microscopy of submonolayers of PTCDA adsorbed on 2H-NbSe₂ reveals an adsorbate layer that is neither commensurate with the substrate lattice nor with the charge density wave periodicity. The observed smallness of deviations from the PTCDA bulk-state unit-cell dimensions indicates that PTCDA film formation is dominated by intermolecular interactions rather than by interaction between PTCDA and 2H-NbSe₂. PACS 61.66.Hq; 68.35.Bs; 68.37.Ef; 68.43.Fg     

Structural origin of the gradual spin transition in a mononuclear iron(II) complex

Variable temperature single crystal X-ray diffraction and SQUID magnetometry experiments have revealed a gradual spin transition in [Fe^II(L)](ClO₄)₂ (where L=1,4,7-tris(2-aminophenyl)-1,4,7-triazacyclononane), centred around room temperature. The gradual nature of the spin transition has been attributed to the lack of significant intermolecular interactions between iron centres and the propensity of the counter ions to accommodate the internal strain in the crystal caused by spin crossover.     

Orientational behaviour of O2 molecules: interplay of van der Waals,quadrupole and magnetodipole interactions

Orientational states for two molecules have been described, which are caused by van der Waals, quadrupole-quadrupole, and magnetodipole (or electric dipoledipole) interactions, their competition giving rise to three phases, with parallel-perpendicular (T-like phase I), inclined (phase II), and parallel (phase III) molecular configurations relative to the line connecting molecular centres. For weak dipoldipole interactions, the three phases change with increasing intermolecular distance (from the intermolecular repulsion radius on) in the order: III-II-I-II-III. As revealed for the O₂ molecules, the phase I vanishes and the phase II results in the range of 6.78 to 43.3 A°.     

Importance of hydrogen bonding for second harmonic generation effect: X-ray diffraction and DFT study on S-benzyl isothiouronium chloride

X-ray diffraction study of the nonlinear optical (NLO) material S-benzyl isothiouronium chloride (C₈H₁₁N₂SCl) (SBTC) is reported for the first time. The single crystal of SBTC is orthorhombic with space group Pbca. SBTC exhibits second-order NLO susceptibility, and this study shows that hydrogen bonding is, in part, responsible for this. The present work shows that C-H?Cl and N-H?Cl hydrogen bonds direct the nature of the three-dimensional lattice. Such intermolecular interactions help to extend the molecular charge transfer into the supramolecular realm, the charge transfer originating as a consequence of the high level of molecular planarity and strong donor-to-acceptor interactions. Density functional theory (DFT) calculation and atom-in-molecule (AIM) analysis has been carried out to study the nature of hydrogen involved in the SBTC complex.