Showing papers on "Intermolecular force published in 1991"

Volume transition in a gel driven by hydrogen bonding

Abstract: INTERACTIONS between macromolecules fall into four categories: ionic, hydrophobic, van der Waals and hydrogen bonding. Phase transitions in polymer gels provide a means of studying these interactions. Many gels will undergo reversible, discontinuous volume changes in response to changes in, for example, temperature, gel composition or light irradiation1–5. These transitions result from the competition between repulsive intermolecular forces, usually electrostatic in nature, that act to expand the polymer network, and an attractive force that acts to shrink it. Volume transitions in gels have been observed that are driven by all of the above-mentioned forces except hydrogen bonding (ref 6–10; T.T. et al, unpublished data; H. Inomata et al., personal communication). Here we report on a phase transition in an interpenetrating polymer network of poly(acrylamide) and poly(acrylic acid) that completes this picture—it is controlled by cooperative 'zipping' interactions between the molecules which result from hydrogen bonding. Cooperativity is an essential feature of the interactions, in that independent hydrogen bonds would not provide a sufficient driving force for the transition. A further novel characteristic of this phase transition is that the swelling (in water) is induced by an increase rather than a decrease in temperature.

Extracting hydrophobic free energies from experimental data: relationship to protein folding and theoretical models.

Abstract: Solubility and vapor pressure measurements of hydrocarbons in water are general1 thought reassessment of the solubility data on the basis of new developments in solution thermodynamics suggests that the hydrophobic surface free energy for hydrocarbon solutes is 46-47 cal/(mol*A2), although the actual value depends strongly on curvature effects (Nicholls et al. (1991) Proteins (in press); Sharp et al. (1991) Science 252, 106-1091. The arguments to support such a significant increase in the estimate of the hy- drophobic effect stem partly from theoretical considerations and partly from the experimental results of De Young and Dill ((1990) J. Phys. Chem. 94,801-8091 on benzene partition between water and alkane solvents. Previous estimates of the hydrophobic effect derive from an analysis of solute partition data, which does not fully account for changes in volume entropy. We show here how the ideal gas equations, combined with experimental molar volumes, can account for such changes. Revised solubility scales for the 20 amino acids, based on cyclohexane to water and octanol to water transfer energies, are derived. The agreement between these scales, particularly the octanol scale, and mutant protein stability measurements from Kellis et al. ((1989) Biochemistry 28,4914-49221 and Shortle et al. ((1990) Biochemistry 29, 8033-80411 is good. The increased strength of the hydrophobic interaction has implications for the energetics of protein folding, substrate binding, and nucleic acid base stacking and the interpretation of computer simulations. to provide estimates of the strength of the hydrophobic effect in the range 20-30 cal/(mol- K 2). Our %e hydrophobic effect has provided a major unifying con- cept in understanding the structure and function of biological systems. Hydrophobicity can be defined phenomenologically in terms of the low solubility of nonpolar molecules in water. The underlying physical interactions that are responsible for this effect are also thought to yield the attractive forces that cause nonpolar molecules to aggregate in an aqueous medium, producing, for example, what is certainly a major driving force in protein folding (Dill, 1990). A distinction is sometimes made between hydrophobicity as measured from solubilities and from intermolecular association (Wood & Thompson, 1990), but it is likely that they represent different manifes- tations of the same physical phenomenon: the energetically costly disruption of the hydrogen-bonding network of water by solute molecules that cannot themselves form hydrogen bonds.

Generalized correlations in terms of polarizability for van der Waals interaction potential parameter calculations

Abstract: General correlations between van der Waals interaction potential parameters and polarizabilities of the interacting neutral–neutral partners of any nature are presented and discussed. To ensure the full applicability of the correlations, an evaluation of the long‐range interaction constants is performed in terms of the Slater–Kirkwood approximation whose numerical coefficients, having the meaning of effective electron numbers, are estimated interpolating the values deduced by theoretical considerations. The values of the long‐range constants so obtained are compared satisfactorily with the available experimental ones. The correlations are tested successfully over practically all systems characterized experimentally. Their use to predict the parameters of unknown systems is suggested.

Many‐body symmetry‐adapted perturbation theory of intermolecular interactions. H2O and HF dimers

Abstract: A many‐body version of the symmetry‐adapted perturbation theory is developed for a direct calculation of intermolecular potentials as a sum of the electrostatic, exchange, induction, and dispersion contributions. Since no multipole expansion is used, the obtained interaction energy components are properly dampened at short distance by the charge‐overlap (penetration) effects. The influence of the intramonomer correlation is accounted for by the perturbation expansion in terms of the Mo/ller–Plesset type fluctuation potentials WA and WB for the individual molecules. For the electrostatic and for the dispersion energy, the terms of the zeroth, first, and second order in WA+WB are considered. In this way, the leading three‐particle correlation contribution to the dispersion energy is taken into account. As a test of our method, we have performed calculations of the interaction energy for the water and hydrogen fluoride dimers. Both the geometry and the basis set dependence of the interaction energy component...

Effects of pressure and stress on C60 fullerite to 20 GPa

Abstract: BULK C60 (fullerite) is a solid composed of extremely hard pseudospherical molecules bonded by weak van der Waals interactions. We expect that pressurization will probe this diversity as the intermolecular distance is reduced. Here we present experimental measurements of the room-temperature pressure–volume equation of state of C60 fullerite to pressures of 20 GPa. The face-centred cubic phase remains stable under hydrostatic compression to this pressure, with an atmospheric-pressure isothermal bulk modulus of K0 = 18.1 ± 1.8 GPa and dK0/dP = 5.7 ± 0.6, characteristic of isotropic van der Waals intermolecular bonding. X-ray diffraction intensity changes are consistent with the compressibility of the C60 molecule being significantly lower than that of the bulk fullerite solid. Under non-hydrostatic compression, a transition to a crystallographic structure of lower symmetry is observed at 16±1 GPa.

Molecular simulation of methane and butane in silicalite

Abstract: Molecular simulation studies of methane in silicalite at room temperature are reported. Adsorption isotherms and diffusion coefficients have been calculated for a range of pressures. At low pressures (<20 bar) adsorption is predominantly at specific potential-energy minima in the silicalite channels while at higher pressures adsorption is determined by the total accessible channel volume. Diffusion is found to be strongly anisotropic with the fastest diffusion along the straight channels. Good agreement is obtained with experimental results for orientationally averaged diffusion coefficients. At low pressure, diffusion is well described by a pseudo-Bosanquet formula which identifies two independent contributions to diffusional resistance (from collisions with walls and from intermolecular collisions). At short times significant molecular force correlations arise due to the diffuculty of methane molecules passing each other in the channel intersections of the silicalite lattice. Some preliminary results for butane diffusion coefficients in silicaliate are also reported.

Hydrogen-bonded liquids

Abstract: 1. Introduction.- i) Basic concepts and tools of liquid state theory.- ii) Methods for calculating intermolecular energy surfaces.- iii) Quantum effects in hydrogen-bonded liquids.- iv) Transport processes, relaxation, and glass formation in hydrogen-bonded liquids.- v) Thermodynamic properties of some H-bonded liquids in their undercooled and/or overcompressed states.- i) X-ray diffraction studies of liquids..- ii) Neutron diffraction techniques.- iii) Recent structural studies of liquid D2O by neutron diffraction..- iv) H/D substitution in neutron diffraction..- v) Structural analysis of liquid formic acid using neutron diffraction..- vi) Structural studies of water near an interface..- vii) SANS and QENS studies of Vycor containing D2O/H2O mixtures..- i) Molecular dynamics simulation studies of the hydrogen-bond network in water..- ii) A molecular model for aqueous solutions..- iii) A model for hydrogen-bonding effects at aqueous interfaces..- iv) A collective approach to the dynamics of water..- i) Properties of H-bonding in the infrared spectral range.- ii) Infrared spectra of H-bonded molecules..- iii) Vibrational and collision-induced Raman scattering from water and aqueous solutions..- iv) Acoustic S- and P-wave Character of the Intermolecular 60 and 175 cm-1 Raman Bands from Liquid Water..- v) Quasi-elastic and inelastic neutron scattering and molecular dynamics of water at supercooled temperature..- vi) The properties of hydrogen-bonded liquids studied by high pressure NMR..- vii) Low frequency light scattering in hydrogen-bonded liquids..- viii)Dielectric Measurements of water, alcohols and their mixtures..- ix) Ultrasonic properties of alcoholic aqueous solutions..- x) Multi-nuclear relaxation time studies in undercooled aqueous electrolytes..- xi) Vibrational dynamics of liquids and glassy electrolytes..- 6. The Programme.- Posters.- 7. Postscript.- Author index.- Chemical index.

A molecular dynamics study of the far infrared spectrum of liquid water

Abstract: The far infrared spectrum of liquid water at room temperature is calculated by molecular dynamics simulation over the spectral range 0.5–1000 cm−1. It is shown that the experimental absorption intensity can be reproduced satisfactorily provided that the dipole induced dipole mechanism is conveniently implemented in the calculation and the classical profile corrected for quantum effects. The contribution due to exchange overlap dipoles between O and H atoms is also investigated but its role in the genesis of the far infrared (FIR) spectrum is negligible. Although the dipole induced dipole (DID) mechanism is found to be responsible for the peculiar band shape near 200 cm−1 by revealing the intermolecular oscillations of the hydrogen bond network, no other translational band is detected in the region 10–60 cm−1, a result in contradistinction with data put forward recently. Moreover, it is shown that the absorption spectrum is the seat of various cancellation effects between permanent and induced dipoles, eff...

The dynamics of open-shell Van der Waals complexes

Abstract: The theory of Van der Waals complexes formed from atoms and open‐shell (Σ and Π) diatomic molecules is developed, paying particular attention to the quantum numbers that are conserved in the complex and the angular momentum coupling cases that may be observed. Complexes formed from diatoms in multiplet Σ states may exhibit several different coupling schemes closely analogous to Hund’s coupling cases for diatomic molecules. Complexes formed from diatoms in Π states usually exhibit a coupling scheme in which the (signed) projection P of the diatom angular momentum j onto the intermolecular axis is nearly conserved. Correlation diagrams showing the bending energy levels as a function of potential anisotropy are given for complexes containing diatomic molecules in both Σ and Π states. The transition from free internal rotor quantum numbers to near‐rigid bender quantum numbers with increasing anisotropy is investigated. The cases of Ar–OH and Ne–OH are considered as examples.

A new flexible/polarizable water model

Abstract: Both geometrical flexibility and instantaneously responsive electrical polarization are incorporated into a newly developed 5‐site water model that includes one oxygen atom, two partially shielded protons, and two negative charges representing lone pairs. The charges are diffusively distributed. Their values are variable in accordance with the local field. The intramolecular potential function used is the one recently developed by Dang and Pettitt [J. Phys. Chem. 91, 3349 (1987)] for a free water molecule. In order to strengthen the angular dependence of the intermolecular dimer potential, a short‐range Morse‐type interaction is introduced to represent specific hydrogen bonding interactions. With this model we carry out a classical constant volume molecular dynamics simulation of liquid water at mass density 0.997 g/cm3 and room temperature 298 K. Results for the liquid structure, thermodynamic properties, transport dynamics, dielectric features, and spectroscopic characteristics are presented and compare...

Cooperative nonlinear optical response of molecular aggregates: Crossover to bulk behavior.

Abstract: Equations of motion for the nonlinear optical response of arbitrary-size molecular aggregates are derived. The relative role of intramolecular and intermolecular (two-exciton) nonlinearities and the cooperative enhancement induced by the latter are analyzed. A crossover from \ensuremath{\sim}${\mathit{N}}^{2}$ to \ensuremath{\sim}N scaling of the nonlinear polarizability is predicted as the aggregate dimension becomes comparable to the optical wavelength, N being the aggregate size. The limitations of the local-field approximation are demonstrated.

Dielectric relaxation dynamics of water and methanol solutions associated with the ionization of N,N-dimethylaniline : theoretical analyses

Abstract: The solvation dynamics associated with the ionization of N,N‐dimethylaniline (DMA) in water and methanol solutions has been studied theoretically. Potential energy surfaces of DMA and DMA+ were computed by ab initio molecular orbital (MO) methods. Intermolecular pair potential functions between DMA and H2O were developed with the aid of the electron distributions of DMA and H2O and the results of MO calculations for the DMA–H2O system. Potential functions between DMA and MeOH were also determined empirically using the parameters for DMA–H2O interaction. Equilibrium and nonequilibrium molecular dynamics calculations were carried out for the DMA–water and DMA–methanol solutions. The simulation results were analyzed comparing two solvents in order to obtain a realistic molecular model for the solvation dynamics of DMA in polar solvents. The solvation coordinate was defined by the potential energy difference between neutral and cation states and free energy curves along it were constructed using the umbrella ...

Intermolecular forces : an introduction to modern methods and results

Abstract: General Aspects.- I Intermolecular Forces.- II Quantum Chemistry of the Hydrogen Bond.- III How to Understand Liquids?.- IV Dynamic Aspects of Intermolecular Interactions.- Spectroscopic Methods.- V Vibration Aspects of the Hydrogen Bond.- VI Experimental Vibrational Characteristics of the Hydrogen Bond.- VII IR-Overtone Vibration Spectroscopy.- VIII Intermolecular Interactions at Low Temperature. Matrix Isolation Spectroscopy Applied to Hydrogen-Bonded Complexes and Charge Transfer Complexes.- IX Water - The Most Anomalous Liquid.- X Cooperative Effects Involved in H-Bond Formation.- XI NMR Studies of Elementary Steps of Multiple Proton and Deuteron Transfer in Liquids, Crystals, and Organic Glasses.- XII Cluster Research with Spectroscopic Molecular Beam Techniques.- Other Methods.- XIII Molecular Beam Scattering: Method and Results on Intermolecular Potentials.- XIV Molecular Dynamics (MD) Computer Simulations of Hydrogen-Bonded Liquids.- XV The Energy of Intermolecular Interactions in Solution.- XVI The Mobile Order Created by Hydrogen Bonds in Liquids.- XVII Hydrogen Bonding and Entropy.- XVIII Specific Intermolecular Forces and the Permittivity and Conducivity of Solutions.- XIX The Role of Hydrogen Bonds in Biochemistry.- XX Hydrogen Bonds in Crystals.- XXI Role of Intermolecular Interactions in Chromatographic Separations.

Electronic spectral shifts of aromatic molecule–rare‐gas heteroclusters

Abstract: In this paper, a semiempirical theory for the spectral shifts of the electronic origin of the S0→S1 transition of (aromatic molecule)⋅(rare‐gas)n heteroclusters is advanced and applied. Neglecting the modifications of intermolecular overlap and exchange interactions upon electronic excitation, the dispersive contributions to the spectral shift are evaluated to second order, accounting for finite‐size structural features of the large molecule by the utilization of the multicenter monopole representation of the intermolecular interactions. The spectral shifts for nonpolar aromatic hydrocarbons in or on rare‐gas heteroclusters are represented in terms of differences between electrostatic interactions involving an electrostatic field (due to the molecular transition monopoles charge distribution) and an induced dipole (originating from the rare‐gas polarizability). The transition monopoles incorporated all the one‐ and two‐electron ππ* excitations of the aromatic molecule, which were represented by Huckel or ...

Computer Simulations of Mesogenic Molecules Using Realistic Atom-Atom Potentials

Abstract: Molecular dynamics simulations are reported for an atom-based model of the mesogen truns-4-(trans-4-n-pentylcyclohexyl)cyclohexylcarbonitrile (CCHS). Atom-atom potential functions take account of intermolecular contributions to the potential energy and internal molecular structure is modelled by a standard empirical force field. The calculations are carried out in the isothermal-isobaric ensemble at a range of temperatures, and the results provide predictions for phase behaviour and orientational ordering which are in good agreement with experiment. Time-averaged molecular properties are calculated from the simulation data. We discuss the possible uses of this technique in the study of liquid crystalline systems and in the prediction of the phase behaviour of real molecules.

A model for intermolecular cooperativity in conformational relaxations near the glass transition

Abstract: A model is proposed for segmental relaxation that requires intermolecular cooperativity. A domain of cooperativity is defined as a group of segments that must undergo relaxation simultaneously. A relaxation function is formulated based on the domain size distribution. The resulting equation gives a better fit to the dielectric and viscoelastic data than the Kohlrausch-Williams-Watts equation. The theory is extended into the nonequilibrium glassy state by fixing the entropy at the fictive temperature

Ab initio study of the intermolecular potential of Ar–H2O

Abstract: The combination of supermolecular Mo/ller–Plesset treatment with the perturbation theory of intermolecular forces is applied in the analysis of the potential‐energy surface of Ar–H2O. The surface is very isotropic with the lowest barrier for rotation of ∼35 cm−1 above the absolute minimum. The lower bound for De is found to be 108 cm−1 and the complex reveals a very floppy structure, with Ar moving freely from the H‐bridged structure to the coplanar and almost perpendicular arrangement of the C2 –water axis and the Ar–O axis, ‘‘T‐shaped’’ structure. This motion is almost isoenergetic (energy change of less than 2 cm−1 ). The H‐bridged structure is favored by the attractive induction and dispersion anisotropies; the T‐shaped structure is favored by repulsive exchange anisotropy. The nonadditive effect in the Ar2–H2O cluster was also calculated. Implications of our results on the present models of hydrophobic interactions are also discussed.

An intermolecular potential function model applied to acetylene dimer, carbon dioxide dimer, and carbon dioxide acetylene

Abstract: A general model to describe intermolecular potential functions for weakly bound molecular complexes is described. The model is designed to be complex enough to reproduce many observed details of van der Waals molecules, yet be simple enough that the required input data are readily available. The distributed multipole picture of monomer charge distributions is used to calculate electrostatic energies and atom–atom Lennard‐Jones terms describe repulsion and dispersion. The individual atom C6 coefficients are derived from long‐range molecular C6 values and the C12 coefficients are based on conventional van der Waals atomic radii. This model is applied to (HCCH)2, (CO2)2, and HCCH–CO2. The resulting potential functions are compared in detail, with particular emphasis placed on understanding why (HCCH)2 and (CO2)2 have such different structures.

Ab initio calculations on the structure, stabilization, and dipole moment of benzene⋅⋅⋅Ar complex

Abstract: The potential energy surface (PES) of the benzene⋅⋅⋅Ar complex was investigated ab initio using the second‐order Moller–Plesset theory demonstrating the practical use of such calculations for these complexes Among five structures studied, the highest symmetry C6v structure for the Ar appeared to be most stable (stabilization energy: 429 cm−1; distance of molecular centers: 3526 A) The PES is much more isotropic than was found in previous papers using an empirical potential The calculated intermolecular distance is in excellent agreement with recent high resolution measurements—also the dipole moment is in excellent agreement with known data

Multidimensional intermolecular dynamics from tunable far-infrared laser spectroscopy : Angular-radial coupling in the intermolecular potential of argon-H2O

Abstract: Five new vibration–rotation tunneling states of Ar–H2O [the Σ and Π(111) and the Σ and Π(212) internal rotor states and the n=1, Π(101) stretching‐internal rotor combination level] have been accessed by tunable far‐infrared laser spectroscopy. The measured vibrational band origins of transitions to these states are within 2% of predictions made from an anisotropic three‐dimensional intermolecular potential surface (denoted AW1) derived from a nonlinear least‐squares fit to previous far‐infrared spectral data [J. Phys. Chem. 94, 7991(1990)]. This provides strong evidence that the AW1 intermolecular potential surface incorporates much of the essential physics of the intermolecular forces which bind the cluster. However, larger deviations from the predictions are found in the observed rotational term values. A detailed analysis of these deviations clearly demonstrates the need for even stronger angular‐radial coupling in the Ar–H2O intermolecular potential than the already substantial coupling present in the...

Absence of a metallic phase at high pressures in C60

Abstract: THE bonding between molecules in bulk solid C60 is extremely weak, making it a narrow-band semiconductor with an energy gap of 1.5 eV (ref. 1) for the face-centred cubic phase. Doping with alkali metals produces a metallic state which can be superconducting at temperatures as high as 33 K (ref. 2). The intermolecular coupling should have an important influence on the conductivity of the pure, semiconducting state: stronger coupling might induce a transition to a metallic or possibly even superconducting state, as is the case for silicon3, or it may result in a covalent solid such as diamond4. We have explored these possibilities by measuring the electrical resistivity of solid granular C60 up to pressures of 25 GPa. Our results show that the magnitude of the gap and the resistivity decrease with increasing pressures as the sample volume5 decreases. But eventual gap closure to give a metallic state is not observed; instead, there is a sudden transition at 15–20 GPa to a more insulating phase, possibly with covalent intermolecular bonding.

Perturbation theory calculations of intermolecular interaction energies

Abstract: Intermolecular interactions play an essential role in determining the structure and conformation of biomolecules, in particular, in aqueous solutions. With the recent development of computer capabilities, it is now possible to calculate the interactions of biologically relevant molecules using the standard self-consistent field approximation. For most systems, this approximation is not sufficient and the correlation component of the interaction energy must be included. Unfortunately, the supermolecular method, which is mostly used to calculate the intermolecular interactions at the correlated level, is plagued by the basis-set superposition error and does not provide any physical interpretation of the interaction energy. An alterative approach is to use the symmetry-adapted (exchange) perturbation theory developed by us. This theory is free from the basis-set superposition error, provides a clear physical picture of the interaction energy, and involves less computational effort than does a standard many-body perturbation theory calculation of equivalent order. We have developed a system of ab initio computer codes performing calculations for arbitrary molecules. For small systems—where the accuracy could be tested—our results are in excellent agreement with experiment. Large-scale calculations performed for systems such as (H2O)2, (HF)2, and uracil…water demonstrate the high efficiency and accuracy of our method.

Properties of urea–water solvation calculated from a new ab initio polarizable intermolecular potential

Abstract: In this work, we present a quantum chemical‐based urea–water potential which includes many‐body effects by using explicit polarizabilities, and mimics the dipole and quadrupole moments of the individual molecules. We compare this potential in detail with explicit ab initio calculations and with other potentials used in molecular simulations of urea–water systems. Several deficiencies in earlier work are pointed out, such as the quality of the basis set, the accuracy of the electrostatics, and polarization effects. In the minimum energy configuration of the urea–water dimer, the water molecule is engaged in two hydrogen bonds with urea, forming a cyclic structure, with an energy of −10.9 kcal/mol. In order to test the reliability of using polarizable molecular models in simulations of dipolar solutes in liquid water, a molecular dynamics simulation with one urea molecule and 210 water molecules was carried out. The hydration of urea is characterized by the ability of urea to fit into the water structure as a ‘‘waterlike’’ molecule. The dynamics of the system was investigated by studying diffusion and relaxation processes. The relative values of the diffusion coefficients for urea and bulk water are preserved, but as in a previous polarizable water simulation, their magnitudes are too small. The shell waters are more tightly bound to urea than bulk waters are to each other. Supported by previous simulations, we conclude that urea does not act as a water structure breaker and that this effect is unimportant in biological systems.

Ab initio studies of internal rotation barriers and vibrational frequencies of (C2H2)2, (CO2)2 and C2H2-CO2

Abstract: We have performed large-scaleab initio calculations using second order Moller-Plesset perturbation theory (MP2) on the three van der Waals dimers formed from acetylene and carbon dioxide. Intermolecular geometrical parameters are reliably computed at this level of theory. Calculations of vibrational frequencies of the van der Waals modes, currently unobtainable by experimental means, give important information about the intermolecular potential and predict significant large-amplitude motion. Zero point energy contributions are shown to be vital in assessing the relative stability of conformations which are close in energy. Our studies suggest that the barrier to interconversion tunnelling in (CO2)2 is significantly smaller than previously inferred and is approximately the same as in (C2H2)2. The reason for the rigidity of (CO2)2 is the difference in monomer centre-of-mass separation between ground state and transition state. We also show that, in addition to the previously observedC 2v form, the collinear form of C2H2-CO2 is a local minimum on its potential energy surface.