Dipole Interaction Model Research Articles

Spin dynamics in solid dilute paramagnets

The free-induction decay signals for the two-spin clusters, the “matrix,” and the entire spin system as a whole are calculated for a real dipole-dipole interaction on the basis of a method for the cluster expansion of correlation functions previously developed by the authors. The intracluster interactions, which generate a discrete spectrum, are taken into account exactly, and the interactions outside the clusters are taken into account on the basis of the Anderson-Weiss-Kubo (AWK) theory of phase relaxation. The corresponding shape functions of the resonance line are calculated. Before the clusters are separated, an AWK analysis in the limit of slow fluctuations of the local fields is in satisfactory agreement with the exact results obtained in the Anderson model of dipolar interactions, but a similar analysis for realistic fluctuation rates and Dt?1 leads to a significant qualitative difference: exp(−Dt) transforms into \(\exp ( - \sqrt {D_1 t} )\). After the clusters are separated, this difference is obliterated. The free-induction decay is almost exponential up to Dt∼10, and nonexponential behavior of the long-time asymptote appears sooner. This finding is not altered significantly when the analysis is complicated by raising the maximum rank of the clusters to 3 or more.

Positional dependence of the effects of negatively charged Glu side chains on the stability of two-stranded α-helical coiled-coils

The effects on protein stability of negatively charged Glu side chains at different positions along the length of the α-helix were investigated in the two-stranded α-helical coiled-coil. A native coiled-coil has been designed which consists of two identical 35 residue polypeptide chains with a heptad repeat QgVaGbAcLdQeKf and a Cys residue at position 2 to allow the formation of an interchain 2-2′ disulphide bridge. This coiled-coil contains no intra- or interchain electrostatic interactions and served as a control for peptides in which Glu was substituted for Gln in the e or g heptad positions. The effect of the substitutions on stability was determined by urea denaturation at 20°C with the degree of unfolding monitored by circular dichroism spectroscopy. A Glu substituted for Gln near the N-terminus in each chain of the coiled-coil stabilizes the coiled-coil at pH 7, consistent with the charge–helix dipole interaction model. This stability increase is modulated by pH change and the addition of salt (KCl or guanidine hydrochloride), confirming the electrostatic nature of the effect. In contrast, Glu substitution in the middle of the helix destabilizes the coiled-coil because of the lower helical propensity and hydrophobicity of Glu compared with Gln at pH 7. Taking the intrinsic differences into account, the apparent charge–helix dipole interaction at the N-terminus is approximately 0.35 kcal/mol per Glu substitution. A Glu substitution at the C-terminus destabilizes the coiled-coil more than in the middle owing to the combined effects of intrinsic destabilization and unfavourable charge–helix dipole interaction with the negative pole of the helix dipole. The estimated destabilizing charge–helix dipole interaction of 0.08 kcal/mol is smaller than the stabilizing interaction at the N-terminus. The presence of a 2-2′disulphide bridge appears to have little influence on the magnitude of the charge–helix dipole interactions at either end of the coiled-coil. © 1997 European Peptide Society and John Wiley & Sons, Ltd.

Thermodynamic fluctuations in two-dimensional degenerate antiferromagnetic structures

A model of a two-dimensional antiferromagnet with an arbitrary anisotropic interaction that allows for degeneracy of the ground state is proposed. The lifting of degeneracy by thermodynamic fluctuations and the accompanying effects are studied by a method of self-consistent calculations of Gaussian angular fluctuations that is asymptotically exact at low temperatures. Fluctuations are shown to lead to collinear ordering of the orientations of magnetic sublattices, an effect that initiates long-range orientational order in systems with anisotropic interaction but retains only short-range order in systems with isotropic short-range interaction. The temperature patterns of the orientational correlators are given for the particular cases of dipole and isotropic short-range interaction models. The nature of the Ising-like behavior of the system is discussed for the case of a strong anisotropy of the correlators, which corresponds to quasi-one-dimensional behavior.

Positional dependence of the effects of negatively charged Glu side chains on the stability of two-stranded alpha-helical coiled-coils.

The effects on protein stability of negatively charged Glu side chains at different positions along the length of the alpha-helix were investigated in the two-stranded alpha-helical coiled-coil. A native coiled-coil has been designed which consists of two identical 35 residue polypeptide chains with a heptad repeat QgVaGbAcLdQeKf and a Cys residue at position 2 to allow the formation of an interchain 2-2' disulphide bridge. This coiled-coil contains no intra- or interchain electrostatic interactions and served as a control for peptides in which Glu was substituted for Gln in the e or g heptad positions. The effect of the substitutions on stability was determined by urea denaturation at 20 degrees C with the degree of unfolding monitored by circular dichroism spectroscopy. A Glu substituted for Gln near the N-terminus in each chain of the coiled-coil stabilizes the coiled-coil at pH 7, consistent with the charge-helix dipole interaction model. This stability increase is modulated by pH change and the addition of salt (KCl or guanidine hydrochloride), confirming the electrostatic nature of the effect. In contrast, Glu substitution in the middle of the helix destabilizes the coiled-coil because of the lower helical propensity and hydrophobicity of Glu compared with Gln at pH7. Taking the intrinsic differences into account, the apparent charge-helix dipole interaction at the N-terminus is approximately 0.35 kcal/mol per Glu substitution. A Glu substitution at the C-terminus destabilizes the coiled-coil more than in the middle owing to the combined effects of intrinsic destabilization and unfavourable charge-helix dipole interaction with the negative pole of the helix dipole. The estimated destabilizing charge-helix dipole interaction of 0.08 kcal/mol is smaller than the stabilizing interaction at the N-terminus. The presence of a 2-2'disulphide bridge appears to have little influence on the magnitude of the charge-helix dipole interactions at either end of the coiled-coil.

Geometric isomerism in clusters: High resolution infrared spectroscopy of a noncyclic CO2 trimer

High resolution infrared spectra of a previously unidentified noncyclic isomer of (CO2)3 have been obtained via direct absorption of a 4.3 μm diode laser in a slit jet supersonic expansion. Two vibrational bands (labeled νI and νIII) are observed, corresponding to the two most infrared active linear combinations of the three constituent CO2 monomer asymmetric stretches: νI is redshifted −5.85 cm−1 from the monomer vibrational origin and is predominately a c-type band of an asymmetric top, while νIII is blueshifted +3.58 cm−1 and is predominately an a-type band. Transitions with Ka+Kc=odd (even) in the ground (excited) state are explicitly absent from the spectra due to the zero nuclear spin of CO2; this rigorously establishes that the noncyclic isomer has a C2 symmetry axis. The vibrational shifts and relative intensities of the bands are interpreted via a resonant dipole interaction model between the high-frequency stretches of the CO2 monomers. Rotational constants are determined by fits of transition frequencies to an asymmetric top Hamiltonian. These results are used to determine vibrationally averaged structural parameters for the complex, which is found to be stacked asymmetric but with C2 symmetry about the b inertial axis. The structural parameters are then used to test several trial CO2–CO2 interaction potentials.

Force-field dependence of Boltzmann weighting factors on predicted pi-pi* circular dichroic spectra of cyclo (Gly-Pro-Gly)2.

Semi-empirical energy calculations were performed for published conformations of cyclo (Gly-Pro-Gly)2 using different force fields (DISCOVER cvff and cff91, AMBER, and CHARMM). The resulting potential energies were then used to create Boltzmann weighting factors for an ensemble of cyclo(Gly-Pro-Gly)2 structures. The dipole interaction model was used to predict pi-pi* circular dichroic spectra (CD) for the individual structures of cyclo(Gly-Pro-Gly)2. The Boltzmann weighting factors were applied to the individual spectra so that a composite spectrum was constructed to represent a CD arising from a collection of different structures in solution. Weighting factors determined from different force fields were compared. Boltzmann-weighted spectra better resembled the experimental CD than any calculated spectrum using only a single conformation of cyclo (Gly-Pro-Gly)2. The structures most heavily weighted contained at least one type I beta-turn.

Electronic absorption spectra of molecules and aggregates with interatomic charge transfer using a normal mode treatment of the atomic monopole–dipole interaction model

The theory of the atom monopole–dipole interaction model for electronic polarizability is extended to the case of complex frequency-dependent atom polarizabilities, allowing the prediction of absorption spectra in terms of a set of electronic normal modes. The charge constraints on the system are taken into account using Lagrangian and penalty function methods. Expressions are obtained for the complex polarizability of an aggregate of molecules in which intermolecular charge transfer is precluded. The results are illustrated by calculations for naphthalene using atom polarizability parameters which give an approximate fit to the experimentally observed absorption spectrum. We also treat helical stacked aggregates of naphthalene molecules and predict intensity shifts analogous to those observed in the stacked aromatic bases of helical nucleic acids.

Atom Monopole−Dipole Interaction Model with Limited Delocalization Length for Polarizabilities of Polyenes

The atom monopole−dipole interaction (AMDI) model allows charge transfer along a conjugated chain and has been used to predict the polarizabilities of aromatic hydrocarbons. But as this model does not predict the polarizabilities of very long polyenes, we introduce the concept of “delocalization length” that restricts charge transfer such that the maximum length that the electron traverses is a certain number of atoms and not the entire chain. We test this approach for a series of polyenes for two delocalization lengths of 11 atoms and 13 atoms. We find that the polarizabilities computed using this method agree well with the existing quantum mechanical calculations.

A New Optimization of Atom Polarizabilities in Halomethanes, Aldehydes, Ketones, and Amides by Way of the Atom Dipole Interaction Model

A new optimization of atom polarizabilities is carried out by fitting observed mean molecular polarizabilities, polarizability anisotropies, and Kerr constants with values calculated by way of the atom dipole interaction (ADI) model which was used for similar purposes by this laboratory in 1972. New values are obtained for the polarizabilities of F, Cl, and Br in halomethanes, C‘ and O in aldehydes, ketones, and amides, H in the aldehyde C‘HO and the amide NH group, and N in amides. Most of the new values differ slightly from atom polarizabilities that were optimized to fit only observed mean polarizabilities in 1972, though differences up to 50% occur. The new values result in a considerably improved fit of calculated and observed anisotropies and Kerr constants, with a somewhat poorer fit to mean polarizabilities. The results are compared with those from studies by Birge, Thole, and Miller, who used modified versions of the ADI model. It is concluded that the unmodified model reproduces observed anistotropies about as well as any other model with the revised atom polarizabilities.

Improved Theoretical π−π* Absorption and Circular Dichroic Spectra of Helical Polypeptides Using New Polarizabilities of Atoms and NC‘O Chromophores

Polarizability and π−π* transition parameters for the amide NC‘O chromophore are reoptimized from optical properties of amides and peptides using the dipole interaction model and updated atom polarizabilities. The parameters differ somewhat from those of a similar optimization in 1979 due to the inclusion of the new atom values and additional experimental target data. The effects of shifting the NC‘O center within a small “optimal” neighborhood of the NC‘ bond center are examined. The new parameters are used to calculate π−π* absorption and circular dichroic spectra of several helical polypeptide structures, including the α-helix, the parallel and antiparallel β-sheets, the 310-helix, and the poly(proline) I and II helices. The results are in a form that permits estimation of properties for any choice of NC‘O center within the optimal neighborhood. New findings include the following: (i) the predicted CD intensities of the undistorted parallel and antiparallel β-sheets are sensitive to the choice of NC‘O...

Polarizabilities of Nitrogen Heterocyclic Molecules from Atom Monopole−Dipole Interaction Theory

The atom monopole−dipole interaction (AMDI) model has been previously used to treat aromatic hydrocarbons with success. This model is used here to obtain the atom parameters of nitrogen and neighbo...

Free jet IR spectroscopy of (32SF6)2 in the 10 μm region

The rotation-vibration spectra of (32SF6)2 have been studied near the ν3 band of the 32SF6 monomer. The parallel band 14 cm−1 below the monomer band origin shows a well resolved J-structure, while the perpendicular band 8 cm−1 above the origin exhibits several Q-branch peaks as the only resolved strong lines. The structure of (32SF6)2 is consistent with a D2d symmetry from the intensity alternation and the existence of a first-order Coriolis interaction observed in the perpendicular band. The energy difference between the two bands is very close to the value calculated by a dipole–dipole and dipole-induced dipole interaction model, while the location of the two bands is blueshifted from the calculated values by about 2 cm−1. The possible influence of internal rotation is discussed.

High-resolution infrared diode laser spectroscopy of (CO2)3: Vibrationally averaged structures, resonant dipole vibrational shifts, and tests of CO2–CO2 pair potentials

High-resolution infrared spectra of (CO2)3 formed in a slit jet supersonic expansion are obtained via direct absorption of a tunable diode laser in the ν3 asymmetric stretch region of CO2. Over 100 distinct transitions are recorded in the trimer spectrum, which can be modeled as a perpendicular band of a planar symmetric top with C3h symmetry and no observable tunneling splittings. Results from the spectroscopic fit indicate that the complex is vibrationally averaged planar, with a carbon–carbon atom separation of RCC=4.0376(2) Å. An analysis of the vibrational blue shift for (CO2)3 of 2.5755(2) cm−1 via a resonant dipole–dipole interaction model yields an angular orientation for each CO2 axis of β=33.8(5)° away from a line tangent to the vertex and parallel to the opposite side of the equilateral triangle connecting the centers of mass of each CO2 monomer. Several model CO2–CO2 interaction potentials are tested against the vibrationally averaged structural parameters for (CO2)3. In particular, the potential of Murthy et al. [Mol. Phys. 50, 531 (1983)] reproduces RCC for the complex, but similar to all potentials tested, does not accurately predict the angular orientation β of the monomers within the trimer. Lastly, spectral evidence and model predictions suggest that there is an asymmetric top isomer of the trimer that is energetically comparable to the observed cyclic isomer.

Corrected electrostatic model for dipoles adsorbed on a metal surface

We present a dipole–dipole interaction model for polar molecules vertically adsorbed on a idealized metal surface in an approximate analytic form suitable for estimating the coverage dependence of the work function, binding energies, and thermal desorption activation energies. In contrast to previous treatments, we have included all contributions to the interaction energy within the dipole model, such as the internal polarization energy and the coverage dependence of the self-image interaction with the metal. We show that these can contribute significantly to the total interaction energy. We present formulae for both point and extended dipole cases.

Pressure Dependence of the CN(B2Σ+–X2Σ+) Emission Spectra in an Ar Flowing Afterglow

Abstract The CN(B2Σ+–X2Σ+) emission spectrum produced in the reaction of Ar(3P2) with BrCN was observed by the use of a flowing afterglow method at argon pressures ranging from 9 mTorr to 2 Torr (1 Torr = 133.322 Pa). The pressure dependences of the intensity anomalies in the rotational lines due to the B2Σ+∼A2Πi and the B2Σ+∼4Σ+ perturbations were measured by introducing collision-partner gases in the reaction region. These pressure dependences can be explained by the “doorway” model. On the other hand, the measured pressure dependence of the effective rotational temperature in the CN(B2Σ+) state can also be reproduced by the collision-induced rotational relaxation model. The cross section for the rotational relaxation in the A2Πi state was found to be smaller than those in the B2Σ+ and 4Σ+ states. This finding is interpreted by the use of the dipole–induced dipole interaction model for the collision–induced rotational relaxation.

Structure and optical anisotropies of critical polymer solutions in electric fields

We present simultaneous measurements of both the small angle light scattering (SALS) structure factor and the dichroism for near-critical polymer solutions in electric fields. The theory of flow birefringence and dichroism proposed by Onuki and Doi is extended to the electric field case. This theory relates the electric-field-induced form dichroism to the second moment of the structure factor. Excellent agreement is observed between the form dichroism calculated from measured scattering intensities and the dichroism measured by optical polarimetry. In addition, a possible mechanism of dipolar coupling between the large critical concentration fluctuations and the electric field is proposed. Similar simultaneous SALS and birefringence measurements confirm this model of dipolar interaction.

The nonadditive intermolecular potential for water revised

The results of an improved version of a nonadditive intermolecular model for water that explicitly includes the nonadditive polarization energy are reported. The original polarizable water potential model (POL1), upon which the improved version is based, was developed by Caldwell, Dang, and Kollman [J. Am. Soc. Chem. 112, 9144 (1990)]. To improve the POL1 model, we developed a new set of atomic polarizabilities that reproduce the experimental molecular polarizability for water using the atom–dipole interaction model (Applequist, Carl, and Fung [J. Am. Soc. Chem. 94, 2952 (1972)]). Using the new atomic polarizabilities, we optimized the Lennard-Jones parameters for O–O interactions to improve the model. As expected, the new model has improved the radial distribution functions and the average potential energy for liquid water as well as the density and the average total dipole moment. The model is then used to compute the binding energies of Cs+–water clusters. Without the need for three-body forces (ion–water–water interaction), the agreement between the results of molecular-dynamics simulations and experimental energies of cluster formation is very good.

Anomalous band shifts in the 14 μm infrared absorption spectra of rare gas–BF3 complexes

High resolution infrared absorption spectra of rare gas (Rg)–BF3 van der Waals complexes are studied in the 14 μm region near the ν2 band of BF3 monomer. Spectroscopic constants are determined for the 20Ne–11BF3, Ar–10,11BF3, 82–84,86Kr–11BF3, and 84Kr–10BF3 complexes. The observed redshifts from the monomer band origin correlate linearly with the rare gas polarizabilities. These shifts are about three times as large as those measured previously near the monomer ν3 band. This mode dependence of the shifts cannot be reproduced in a consistent manner with the instantaneous vibrational dipole–induced dipole interaction model, and indicates much greater enhancement of bonding energy by the excitation of ν2 vibration. The band shifts are discussed on the basis of electrostatic interaction between rare gas atom and point charges on BF3. The anomalous band shifts for the ν2 band are successfully accounted for by the interaction of vibrational dipole moment with the static induced dipole moment on the rare gas atom, which is parallel to the direction of vibrational motion. The isotope shifts observed for the Kr–BF3 complexes and the band shifts due to the modification of force field by vdW bonding are discussed with a linear triatomic molecular model.

Raman intensity of lattice vibrations in β‐Ba(OH)2 · H2O

AbstractIt is shown that the Raman intensity of translational lattice vibrations in β‐Ba(OH)2 · H2O can be explained satisfactorily in terms of bond polarizability parameters obtained from the atom dipole interaction model. The analysis was carried out on the basis of two models. Model A used only bond polarizability parameters for bond stretchings. Model B used bond polarizability parameters for bond stretchings and bond rotations, where the parameters were obtained using only three atomic or ionic polarizabilities. Model B resulted in excellent agreement with observed spectra.

Charge injection asymmetry: A new route to strong optical nonlinearity in poled polymers

Amorphous polymer films containing hyperpolarizable dye molecules are poled by an electric field applied parallel to the plane of the thin film. The resultant second harmonic generation is much stronger than predicted by the standard model of dipolar interactions, and furthermore indicates asymmetry both parallel and perpendicular to the poling field. These results are attributed to a new, efficient mechanism for the production of optical nonlinearity in poled films in which asymmetric charge injection into the polymer and its trapping by dye species sets up a charge gradient perpendicular to the direction of the external poling field.