bluuebunny/crossref_metadata_2025 huggingface dataset:
Crystal structures of symmetric Lennard-Jones mixtures
Authors: Vlot Margot J. , Huitema Hjalmar E. A. , de Vooys Arnoud , van der Eerden Jan P.
Published: July 2002

Monte Carlo simulations of binary Lennard-Jones crystals with mole fraction x=0.5 are performed at constant temperature and pressure. In our symmetric model, the interactions between equal particles are the same (εDD=εLL and σDD=σLL). The interaction between D and L particles is changed by εDL=eεDD and σDL=sσDD. The parameters e and s represent interaction strength and distance, respectively, as deviations from the Lorentz–Berthelot mixing rules. Gibbs energies were calculated to determine the stable crystal structure as a function of e and s, separately. This resulted in demixing for e<0.93 and solid solutions for e>1, with a weak transition to a substitutionally ordered fcc at e>1.8. Variation of s resulted in various crystal structures: a CsCl structure for 0.8<s⩽0.95, NaCl structure for 0.6⩽s⩽0.8, ZnS structures with gradual transition to a double fcc structure for s<0.6. We conclude that small variations in the interactions between unlike particles in a mixture suffice to change the crystal structure completely.
Nonadditive three-body dipoles of inert gas trimers and H2⋯H2⋯H2: Long-range effects in far infrared absorption and triple vibrational transitions
Authors: Li X. , Hunt K. L. C.
Published: July 2002

Nonadditive, three-body dipoles are detected experimentally in far-infrared absorption by inert gas mixtures and in triple transitions, the absorption of a single photon by three molecules, each of which becomes vibrationally excited. In this work, we use perturbation theory to derive the nonadditive, three-body dipole of molecules A, B, and C of arbitrary symmetry, interacting at long range. Our results include linear induction, hyperpolarization, dispersion, and concerted induction–dispersion effects, with direct overlap damping. We derive exact, new equations for the dispersion and induction–dispersion dipoles, in terms of the polarizabilities and hyperpolarizabilities of A, B, and C, integrated over imaginary frequencies. With these, we obtain accurate numerical results for the dipoles of inert gas trimers and trimers containing H2, isotropically averaged over the orientations of the molecular axis. For application to heavier systems, we develop a new constant-ratio approximation, relating three-body dipoles to the van der Waals interaction energy coefficients C6 and C9, the static polarizability α, and the static dipole2-quadrupole hyperpolarizability B. For the test cases in this work, this approximation gives the integrals appearing in the induction–dispersion dipoles with root-mean-square errors of 10–14 %, and the integrals in the pure dispersion dipoles with root-mean-square errors of 1–4 %. Our numerical results for the dipoles of inert gas trimers should be useful in molecular dynamics simulations of far-infrared absorption, and in extracting information on intercollisional interference from the absorption line shapes.
Theoretical studies of internal rotation for an asymmetric top
Authors: Pickett Herbert M.
Published: July 2002

For internal rotation in three-fold tops, the internal axis system (IAS) has been a great convenience because the vibrational angular momentum is constant and is directed along a single axis. A procedure for finding an IAS for the general asymmetric top–asymmetric frame case is given. This procedure is an existence proof that such an axis system can be found. However, such an IAS will have an orientation which is a function of the internal rotation coordinate. Once the IAS is found, details will be given on how the Matthieu equation can be solved and how the results can be described as a Fourier series. A useful alternative to the IAS is an axis system obtained by rotating away from the IAS by a constant amount (independent of the torsional angle) so as to position the frame in a more convenient orientation. For this rotated internal axis system (RIAS), a Matthieu equation can also be used as a basis except that the kinetic energy for the Matthieu solution only contains contributions from the a component of the vibrational angular momentum. Expectation values for other operators including contributions from the b component of the vibrational angular momentum are deferred to the full diagonalization of the rotation–vibration Hamiltonian.
Avoided curve crossing for the dissociation of the Rydberg NH4 radical into (NH3+H)
Authors: Park Jong Keun
Published: July 2002

Potential energy curves of the ground and low lying excited states for the dissociation of the Rydberg NH4 radical into (NH3+H) have been calculated using ab initio Hartree–Fock and singly excited configuration interaction methods with a large basis set including Rydberg basis functions. In the ground correlation curve, the ground (NH4+)(e−)3s radical diabatically correlates to the [H3N(3A1; n→3s)+H(2S)] and [NH3+(2A2″)+H−(1S)] asymptotes. An avoided curve crossing between two attractive diabatic states emerging from [H3N(3A1)+H(2S)] and [NH3+(2A2″)+H−(1S)] and a repulsive diabatic state emerging from an antibonding interaction of [NH3(1A1)+H(2S)] is found near the equilibrium geometry of NH4. The potential energy barrier of 0.59 eV on the ground correlation curve is found at R(NH)≃1.4 Å. The potential well is shallowly bound. In the excited curves, the curve crossings between the dissociative diabatic excited states of [(NH4+)(e−)Rydberg] and the repulsive diabatic states from the antibonding interactions of [NH3(1A1)+H(2S)] and [NH3(3A1; n→3s)+H(2S)] are found around R(NH)≃2.0 and 6.0 Å. The potential energy curves of the first and second excited A12 states are shallowly bound, while that of the third excited state is widely bound. A maximum position of the potential energy barrier of the ground correlation curve is located out of line of those of the excited states.
Theory of coherent Raman scattering with quasi-cw noisy light for a general line shape function
Authors: Ulness Darin J. , Kirkwood Jason C. , Stimson Michael J. , Albrecht A. C.
Published: July 2002

The theory of electronically nonresonant coherent Raman scattering (CRS) with quasi-cw noisy light (I(2) CRS) is developed for a general material response. The (Raman) resonant–resonant and resonant–nonresonant hyperpolarizability contributions to the I(2) CRS signal are interferometrically separable. It is found that, in general, the interferometric decay of each of these terms exposes the Raman line shape function in a different manner. Only for a Lorentzian line is their decay identical. Thus, in principle, I(2) CRS provides a new way to explore the line shape function that is analytically distinct from frequency domain and time domain methods. By way of illustration, the general theory is applied to three common line shapes: Lorentzian (as in the original I(2) CRS theory), Gaussian, and Voigt. The results are shown to be consistent with the principles of factorized time correlation diagram analysis.
Orientational correlation in liquid and amorphous carbon tetrachloride: A reverse Monte Carlo study
Authors: Jedlovszky Pál
Published: July 2002

Reverse Monte Carlo simulation of liquid and amorphous carbon tetrachloride has been performed. The structure of these systems is analyzed in detail in terms of partial pair correlation functions as well as angular and spatial correlation. Various orientational models suggested in previous investigations are tested in light of the present results. Among these models, face-to-face-type interlocked arrangements turn out to be preferred by nearest neighbors, whereas the head-to-tail-type orientations are clearly not preferred. The orientational correlation of the molecules is noticeably stronger in the amorphous than in the liquid phase. Considerable orientational correlation exists only in a rather short distance range in both phases and it vanishes almost completely within the first coordination shell. The molecules are forming a closely packed structure in both phases. The preferential location of the nearest neighbors is found to be around the face centers of each other.
Hindered rotation in ion-neutral molecular complexes: The ν1 vibration of H2–HCO+ and D2–DCO+
Authors: Olkhov Rouslan V. , Nizkorodov Sergey A. , Dopfer Otto
Published: July 2002

Infrared spectra of the mass selected ionic complexes H2–HCO+ and D2–DCO+ have been recorded in the vicinity of their ν1 vibrations (H2/D2 stretch) by means of photofragmentation spectroscopy. The anomalous rotational constants obtained by fitting the observed line positions to a semirigid Watson A-type Hamiltonian reflect the appreciable zero-point excursions of the H2/D2 molecule. Barriers for this internal motion are estimated utilizing a simple atom–diatom hindered rotor Hamiltonian. According to this one-dimensional model, the barrier increases by about 15% upon vibrational excitation which is mainly attributed to electrostatic effects.
The validity of the hard-sphere model in hydrogen bonded intermolecular interactions of HCN–HF
Authors: McIntosh A. , Gallegos A. M. , Lucchese R. R. , Bevan J. W.
Published: July 2002

Recently obtained spectroscopic constants for excited vibrational states of HCN–HF with up to five quanta in the ν7 intermolecular bending mode, which can be characterized as being mostly HCN hindered rotation, and one quanta in the ν4 intermolecular stretching mode were combined with earlier experimental data on this system to obtain a reduced intermolecular potential energy surface where the HF fragment was treated as a pseudoatom and the HCN fragment was treated as a rigid rotor. A functional form was assumed for the interaction potential and the vibrational dynamics was studied using a vibrational configuration interaction method. The parameters in the potential were varied to obtain the best fit to all available experimental data. The resulting interaction potential was found to have a strong coupling between the ν4 and ν7 vibrational modes. The locus of constrained minima on this surface was in excellent agreement with the hard-sphere assumption used in the Buckingham–Fowler model [A. D. Buckingham and P. W. Fowler, J. Chem. Phys. 79, 6426 (1983)] for weakly interacting molecular dimers.
WKB wave functions for a nonseparable Hamiltonian
Authors: Girard M. Fusco
Published: July 2002

The extended WKB method recently presented is applied to the construction of a high-lying semiclassical wave function, corresponding to the quantum numbers (30,30), for the Barbanis Hamiltonian, a well-known two-dimensional nonseparable system; the results are compared with the semiclassical wave function obtained, for the same state, by Davis and Heller, by means of the coherent Gaussians method. The extended WKB method shows that the semiclassical wave function is the sum of four wave functions, differing on parts of the classically allowed region, each of these separately satisfying the Einstein–Brillouin–Keller quantization rules. The results presented here show that our method, which is the direct extension to integrable nonseparable Hamiltonians of the usual WKB scheme, is suitable for the semiclassical quantization of high-lying states also, and enables a closer investigation of the fine structures of the semiclassical wave functions.
Ab initio molecular dynamics study of polyfluoride anions
Authors: von Rosenvinge Tycho , Parrinello Michele , Klein Michael L.
Published: July 2002

The structure and dynamical behavior of liquid HF with dissolved KF have been investigated using the Car–Parrinello ab initio molecular dynamics scheme. Specifically, a system with stoichiometry KF⋅2HF was studied at temperatures of 400 and 1000 K. This system, which was started from a phase separated mixture, rapidly formed into solvated potassium ions and HnFn+1− polyfluoride anions with n=1, 2, 3, and 4. The resulting polyfluoride anions were classified, and their structures and dynamical behavior were compared with the known structures and spectra of crystalline compounds KF⋅xHF and with theoretical predictions of isolated gas phase species. In the present study we reveal dramatic frequency shifts in the H atom vibrational modes with variation in the HF coordination number of the polyfluoride anion. In particular, the FH wagging motion red shifts while the FH stretch blue shifts as n increases. The present calculations suggest that FH stretching motion infrared modes previously assigned as combination bands should likely be reinterpreted as fundamental modes of the polyfluoride anions.
Equilibrium charge distribution on weak polyelectrolytes
Authors: Berghold Gerd , van der Schoot Paul , Seidel Christian
Published: July 2002

Density functional theory and Monte Carlo simulations have been used to study the non-uniform equilibrium charge distribution on weak polyelectrolytes. For rigid rods, linearized density functional theory and simulation data show good agreement in a fairly wide parameter range. In the central part of the rod there appears a slight depletion of charge which is accumulated at the ends within a range of the order of the Debye length. For flexible chains, the simulations show an enhancement of the non-uniformity of the charge distribution.
The effect of angular momentum on the unimolecular dissociation HCO→H+CO
Authors: Yang Chao-Yie , Gray Stephen K.
Published: July 2002

Full dimensional wave packet calculations are performed to model the unimolecular dissociation HCO→H+CO on its ground electronic state. Results of calculations up to total (nuclear) angular momentum J=10 are reported. Most of our results are based on the recent potential surface of Werner, Keller, Schinke, and co-workers, although some comparative results with the earlier surface of Bowman, Bittman, and Harding are also presented. We focus on certain unimolecular resonances, and determine their positions, widths, and associated CO product distributions. We find good agreement with the available experimental results of Neyer et al. We also consider how rotation affects a group of states associated with the 2:1 stretch/bend resonance. Bowman’s adiabatic rotation model is found to provide a reasonable description of our results, and is also a useful tool for analysis.
Microscopic calculation of the third-order nonlinear optical susceptibility of DEANST crystal
Authors: Malagoli M. , Munn R. W.
Published: July 2002

The linear and nonlinear optical response properties of crystalline DEANST, N,N-diethyl-4-(2-nitroethenyl) phenylamine, have been computed rigorously, using a large set of input data including modified neglect of differential overlap (MNDO) and ab initio theoretical estimates of molecular polarizability α and hyperpolarizabilities β and γ, and experimentally measured refractive indices. The currently available microscopic expression for the third-order susceptibility tensor χ(3) of a molecular crystal has been corrected to include all possible frequency combinations in the cascading term. Calculations of linear response based on theoretical polarizabilities α fail to reproduce the experimental refractive indices of the crystal, and hence an effective polarizability tensor has been derived from the measured refractive indices and used in the computation of the local electric field. The calculation of χ(3) based on the MNDO hyperpolarizabilities is in tolerable agreement with the experimental measurements of third-harmonic generation. Environmental effects on the molecular response play a key role in the accuracy of the calculation.
On the ethene/HCl Van der Waals complexes observed in liquefied argon and liquefied nitrogen
Authors: Herrebout W. A. , Everaert G. P. , van der Veken B. J. , Bulanin M. O.
Published: July 2002

The mid-infrared (4000–400 cm−1) and far-infrared (300–10 cm−1) spectra of C2H4/HCl mixtures, dissolved in liquefied argon (93–125 K) and in liquefied nitrogen (80–118 K) are discussed. In all spectra, experimental evidence was found for the existence of 1:1 and 1:2 Van der Waals complexes. Using spectra recorded at different temperatures, the complexation enthalpy ΔH° for the 1:1 complex was determined to be −8.7(2) kJ mol−1 in liquid argon and −6.4(4) kJ mol−1 in liquid nitrogen, while for the 1:2 complex a ΔH° of −18.3(6) kJ mol−1 was found. For all species studied, the vibrational frequencies were obtained from ab initio calculations at the MP2/6−311+G** level. Using the SCRF/SCIPCM scheme to correct for the solvent influences, and using statistical thermodynamics to account for the zero-point vibrational and thermal contributions, approximate values for the dissociation energy were calculated from the complexation enthalpies. The resulting values, −12.7(5) kJ mol−1 for the 1:1 complex and −26.9(2) kJ mol−1 for the 1:2 complex are compared with the ab initio values. The transition dipole moment of the ν2 ethene mode induced in the 1:1 complex is determined to be 0.13 D Å−1. A multipole analysis shows that the main contribution is due to the quadrupole moment of the HCl moiety, closely followed by that due to the dipole moment, with much smaller contributions due to higher multipoles. In the 1:1 complex, the HCl stretching intensity is enhanced by a factor of 7.7 in comparison with the monomer. This increase is interpreted in the equilibrium charge–charge flux model using ab initio atomic polar tensors.
Master equation analysis of intermolecular energy transfer in multiple-well, multiple-channel unimolecular reactions. I. Basic theory
Authors: Venkatesh Prasana K. , Dean Anthony M. , Cohen Morrel H. , Carr Robert W.
Published: July 2002

We present a full theoretical analysis of the master-equation formulation of the problem of intermolecular energy transfer in multiple-well, multiple-channel systems. It is shown that the master equation for chemical or thermal activation possesses a unique steady state, that corresponding to the trivial solution. Rate equations local in time and therefore time-independent rate coefficients for the dissociating processes may be obtained only if a state of secular equilibrium exists. For chemically-activated systems, a general state of secular equilibrium may exist which may contain within it a regime wherein there is a well-separated, nontrivial, least negative eigenvalue of the master equation kernel. The dynamics of thermally activated systems are similarly deduced by treating them as chemically activated systems with appropriate modifications to the inhomogeneous source term of the master equation. A degenerate and nondegenerate perturbation theory analysis of the case of rapid thermalization in the vicinity of the thermodynamic equilibrium state is also enunciated. The special case of negligible thermalization is analyzed. A classification of the ordering of the time scales of thermalization, isomerization, and dissociation is then given.
Hermiticity of the Hamiltonian matrix in a discrete variable representation
Authors: Tuvi Inbal , Band Y. B.
Published: July 2002

Application of a discrete variable representation (DVR) to Hamiltonians derived in a nonadiabatic basis or Hamiltonians obtained by making a nonlinear dependent-coordinate transformation, leads to a Hamiltonian matrix which (a) cannot be evaluated without explicitly performing summations, or (b) is not Hermitian. We develop a prescription to avoid these difficulties. Specific examples involving Fourier grid Hamiltonian DVRs are presented.
Polyelectrolyte solutions containing mixed valency ions in the cell model: A simulation and modified Poisson–Boltzmann study
Authors: Das T. , Bratko D. , Bhuiyan L. B. , Outhwaite C. W.
Published: July 2002

Monte Carlo simulations of linear polyelectrolyte solutions containing mixed valency simple ions in the cylindrical cell model are reported. The equilibrium distributions of the simple ions and the osmotic pressure of the solution are calculated at various concentrations of the monomer units of the polyelectrolyte. Specifically, the following systems are studied—monovalent counterions with added 2:2 salt, divalent counterions with added 1:1 salt, and systems containing mono- and divalent counterions only, and mono- and trivalent counterions only. The simulation results are compared with the corresponding predictions from the Poisson–Boltzmann and modified Poisson–Boltzmann theories applied to the cell model. It is seen that upto moderate concentrations of the polyion, the modified Poisson–Boltzmann theory provides a very good description of the systems with deviations occurring at higher concentrations. The theory also reproduces the charge reversal observed in the simulations when strongly correlated counterions overscreen the charge of the polyion. On the other hand, the classical Poisson–Boltzmann results begin to show discrepencies from the Monte Carlo results at relatively lower concentrations. Comparisons of the simulated osmotic pressures with available experimental results confirm the validity of the cell model in a spectrum of practical situations of interest.
Microwave spectroscopy of supercritical water
Authors: Okada K. , Imashuku Y. , Yao M.
Published: July 2002

Microwave spectroscopy that can be applied to study the dielectric relaxation of various fluids under high temperature and pressure has been developed in the frequency range up to 40 GHz. By utilizing this new technique, the dielectric relaxation of water has been measured in the temperature and pressure range up to 750 °C and 120 MPa, which corresponds to a density range between 0.05 and 1 g/cm3. The static dielectric constant ε(0) is deduced from the time required for a microwave signal to travel through the sample by means of the time domain analysis, and is in good agreement with the literature. The dielectric relaxation time τ is obtained by fitting the experimentally observed microwave transmission rate to the value calculated using the S-matrices on the assumption that the dielectric constant obeys the Debye relaxation. The results of τ at ambient pressure agree very well with previous data. Below about 350 °C, τ rapidly decreases with increasing temperature nearly independent of pressure, while above about 350 °C, τ changes little with temperature and increases rapidly with decreasing density. It is concluded that the most relevant parameter determining τ is the temperature at lower temperatures or higher densities, and it is the density d at higher temperatures or lower densities. A possible change in the nature of hydrogen bonding is suggested to explain the observed temperature and density dependence of τ.
On the phase equilibria of the He–H2O mixture: Perturbation theory results
Authors: Scalise O. H.
Published: July 2002

It is reported that the critical loci of the fluid He–H2O mixture exhibits maximum values in both temperature and pressure. This kind of behavior, never found before, was predicted by van der Waals for mixtures exhibiting immiscibility of the first kind. Results are obtained using a theoretical approach that takes into consideration the polarizability of the two components. Further, calculations for both isotherms and isobars show this kind of fluid phase behavior, as recently found experimentally. Since no temperature maximum was shown when using a nonpolarizable molecular model, the new result may be attributable to the polarizability of molecules.
Temperature dependence of vibrational lifetimes at the critical density in supercritical mixtures
Authors: Myers D. J. , Urdahl R. S. , Cherayil Binny J. , Fayer M. D.
Published: July 2002

Experimental measurements are reported for the temperature dependence of the vibrational lifetime, T1, of the asymmetric CO stretching mode of tungsten hexacarbonyl in supercritical ethane at constant density from just above the critical temperature to substantially higher temperatures. T1 is found initially to increase with temperature along an isochore (reaching a maximum at about 70° above the critical point of ethane), and then subsequently to decrease. Using a recent classical theory of vibrational relaxation, we attempt to rationalize the T1 data. This behavior can be semiquantitatively reproduced by the theory if quantum corrections to the classical rate expressions are assumed to be temperature independent in the limit when the transition energy is much greater than thermal energy. In this case, the theory indicates that the initial increase in T1 with temperature arises because of a competition between properties of the solvent which are changing rapidly as the temperature is raised above the critical temperature. At sufficiently high temperature, properties of the solvent vary slowly with temperature, and the explicit temperature dependence of the vibrational relaxation dominates, producing a decrease in T1 with increasing temperature. The predictions of the theory are also examined when other postulated forms of the quantum correction factors are used, and the implications of these results for theoretical approaches to vibrational relaxation are discussed.
Long-time tail effect of the velocity correlation on diffusion-controlled reactions
Authors: Dong W.
Published: July 2002

The existence of the long-time tail in the velocity correlation function of a Brownian particle is first discovered from molecular-dynamics simulations and is now well established theoretically and experimentally. In this work, we ask the following question: does this long-time tail have any effect on the kinetics of diffusion-controlled reactions, and if there is any, how the reaction rate is affected, especially in the asymptotic region, t→∞? We will show that this long-time tail can be taken into account by the theory developed recently by Dong and André. The exact asymptotic solutions to the order of t−1/2 are found analytically with Smoluchowski and Collins–Kimball boundary conditions. This allows us to reveal that the long-time tail of the velocity correlation function contributes to the reaction rate an additional term of O(t−1/2) to the long-time limit of the classic Smoluchowski and Collins–Kimball theories.
Rotational energy analysis for rotating–vibrating linear molecules in classical trajectory simulation
Authors: Park Sang Tae , Moon Jeong Hee , Kim Myung Soo
Published: July 2002

A method has been developed to evaluate the rotational energy of a rotating–vibrating linear molecule in classical trajectory simulation. The method is based on our finding that the component of the angular momentum perpendicular to the figure axis which closely approximates the pure rotational angular momentum is a fairly good constant of motion. Classical kinetic energy of the system has been reorganized to separate the rotational and vibrational parts according to the above concept. Time evolution of the rotational energy thus evaluated shows much less irregular behavior than the ones evaluated with the previous methods over a wide range of rotational and vibrational energies. Combined with the method for mode-specific vibrational energy analysis reported previously, the present method allows a reliable separation of the total energy into each degree of freedom. In particular, the accuracy of the present method seems to be good enough for the rotational energy determination at an instantaneous configuration point along a trajectory, enabling the classical study of real time dynamics.
Diffusion-controlled reactions in an electric field: Effects of an external boundary and competition between sinks
Authors: Traytak S. D. , Tachiya M.
Published: July 2002

The effect of an external boundary and diffusive interaction between reactants on the rate constant for diffusion-controlled bulk reactions in an external electric field is considered. Appropriate boundary-value problems for “wall–sink” and “sink–sink” systems in an electric field are solved approximately by the method of reflections. Mainly we are interested in calculation of the deviation of the reaction rate constant from the value calculated by Smoluchowski approach. A thorough analysis is made of the time-dependent case of “wall–sink” system in absence of an electric field. It has been shown that the wall effect leads to a nonexponential long time tail for the bulk concentration of diffusing particles. Special attention is given also to the investigation of competitive effect between two sinks which is found to be of interest for many applications. The rigorous theoretical study of this problem provides a way of quantitative estimation of the shadow effect. Drift of diffusing particles in some arrays of ideal sinks is treated as well.
Zero-field electron magnetic resonance spectra of copper carboxylates
Authors: Delfs Christopher D. , Bramley Richard
Published: July 2002

The design of a new zero-field electron magnetic resonance spectrometer including cryogenic capabilities and a new tuning mechanism for sweeping over approximately two octaves is described. The tuning mechanism is based on a one-loop, two-gap resonator mounted on quartz rods in such a manner that rotating the rods changes the width of the gap and the resonant frequency of the resonator. The spectrometer was used to measure the zero-field emr spectra of several copper carboxylate dimers at 90 K and 70 K. The parallel hyperfine structure was well resolved in all of the spectra and this allowed accurate determination of D, E, and A‖. A⊥ was found to be poorly defined in general and least square refinements attached large errors to this term in the spin Hamiltonian. The zero-field terms in the spin Hamiltonian at 90 K were determined to be D=9.828 GHz, E=83 MHz, A‖=217 MHz for [Cu(CH3CO2)2(CO(NH2)2)]2⋅2H2O; D=9.985 GHz, E=43 MHz, A‖=221 MHz for [Cu(C2H5CO2)2(H2O)]2; D=10.107 GHz, E=49 MHz, A‖=218 MHz for [Cu(C2H5CO2)2(CO(NH2)2)]2; D=9.979 GHz, E=192 MHz, A‖=223 MHz for [Cu(C6H5CO2)2(C6H5CO2H)]2. A model which assumes A⊥=0 is discussed in detail as it was found that all of the features observed in the spectra could be interpreted using simple explicit expressions derived from the model.
A deuterium nuclear magnetic resonance investigation of the symmetry and orientational order of the nematic phase of 4-[3,4,5-tris(4-dodecyloxybenzyloxy)benzoyloxy]-4′- (4-dodecyloxybenzoyloxy)-1,1′-biphenyl. A biaxial nematic?
Authors: Hughes J. R. , Kothe G. , Luckhurst G. R. , Malthête J. , Neubert M. E. , Shenouda I. , Timimi B. A. , Tittelbach M.
Published: July 2002

The first compound for which the previously predicted biaxial nematic phase was claimed is 4-[3,4,5-tris(4-dodecyloxybenzyloxy)benzoyloxy]-4′-(4-dodecyloxybenzoyloxy)-1,1′-biphenyl (I). This assignment was based on the observation of the optical texture and the x-ray diffraction pattern. To confirm this identification of the biaxial nematic phase, we have studied the deuterium nuclear magnetic resonance (NMR) spectra of nonuniformly aligned samples with the deuterons located specifically in the mesogen itself or in the disklike solute, hexamethylbenzene-d18, dissolved in the mesogen. These experiments allow us to determine the biaxiality in the partially averaged quadrupolar tensor. For both systems, the biaxiality parameter is found to be zero within the experimental error which is estimated to be less than ±0.08 for the pure mesogen and ±0.06 for the solute. The orientational order parameters determined from the quadrupolar splitting change discontinuously at the nematic-isotropic transition thus confirming the first order character of the transition and in accord with the uniaxial symmetry of the nematic phase. The order parameters determined for the two groups of equivalent deuterons in the specifically deuteriated oxymethylene links allow us to comment on the conformations of these links.
Polarizability tensor theory by Green’s function matrix: Circular dichroism and ultraviolet absorption band shape calculations of cyclo(Gly-Pro-Gly-D-Ala-Pro) having β- and γ-turns
Authors: Ito Hirotoshi
Published: July 2002

In a recent paper, we represented the partial polymer polarizability tensor equation in terms of partial monomer polarizability tensors and intermonomer unit dipole–dipole interaction tensors. Here, we represent the same equation in terms of a partial polymer Green’s function matrix equation being constructed from partial monomer Green’s matrix elements and intermonomer interaction energies. We also show that the circular dichroism (CD) and ultraviolet (UV) absorption band shape functions bear a strong resemblance to DeVoe’s scalar representation of the band shape functions, which we have rewritten, using our terminology, in terms of the three terms describing the dipole–dipole coupling, the electromagnetic coupling, and the intrinsic optical activity. The latter is capable of incorporating the one-electron mechanism. The present tensor theory offers five advantages. First, for describing physical phenomena, the tensor form may be more convenient than the scalar forms. Second, in both the partial polarizability tensor theory and DeVoe’s partial scalar polarizability theory, it is possible to place different transition dipoles within each monomer in different positions, whereas this is not possible in the usual Fano–DeVoe types of polarizability theories. Third, the intermonomer interaction energy can be represented readily either by the dipole approximation or by the monopole approximation. Fourth, various types of intermonomer interactions can be taken into account flexibly by making use of the Green’s matrix equation. Fifth, an efficient algorithm can be derived by reducing the matrix size by the factor 3×3 as compared with the partial polymer polarizability matrix tensor equation. By employing the dipole positions already known or discussed by other authors for the two nπ* and ππ* transitions within each amide monomer, the theory has been applied to the semiempirical calculations of the CD and UV absorption band shapes of a cyclic pentapeptide, cyclo (Gly-Pro-Gly-D-Ala-Pro), with comments on the adaptability of simplified parameterization.
Effective Hamiltonian analysis of fluid criticality and application to the square–well fluid
Authors: Brillantov Nikolai V. , Valleau John P.
Published: July 2002

The critical behavior of fluids is analyzed in terms of an effective Hamiltonian which is obtained by a Hubbard–Schofield transformation that maps the fluid Hamiltonian onto a Wilson–Fisher Ising–type Hamiltonian. Within this approach we find an approximate relation between critical temperature and critical density, depending on the parameters of the interaction potential, and compare it with simulation results. In the framework of the effective Hamiltonian we examine the critical data for the square–well fluid obtained using thermodynamic-scaling Monte Carlo and discuss the impact of finite–size effects on the Monte Carlo studies of the fluid criticality.
Effects of chain branching on the structure of interfacial films of decane isomers
Authors: Wang Jee-Ching , Fichthorn Kristen A.
Published: July 2002

This work is a molecular dynamics simulation study of the influence of chain branching on the structures of molecular films physically adsorbed on solid surfaces. The model systems considered are free-standing films of three C10H22 isomers: n-decane, i-decane (2-methylnonane), and t-butyl-hexane (2,2-methyloctane), on a Pt(111) surface. Proper potential models for the t-butyl group were developed and verified. All of these films exhibit layering and in-layer ordering near the surface and we have quantified these features. In general, layered molecules tend to lie with their molecular axes and backbone planes parallel to the surface. In-layer ordering is observed for molecules in layers near the surface. Neighboring molecules within ordered layers tend to align with their long molecular axes parallel to each other. n-Decane, a symmetric chain molecule, exhibits the strongest layering and in-layer ordering. The structures of i-decane films are very similar to those of n-decane films and show only slightly less order. t-Butyl-hexane films have a novel pillared layered structure, in which a few randomly distributed molecules orient themselves with the t-butyl end near the surface and the alkyl tail perpendicular to the surface. These molecules are surrounded by parallel, layered molecules. We discuss the implications of our findings for solvation forces in confined thin films of these molecules.
Nanosecond time-resolved FTIR emission spectroscopy: Monitoring the energy distribution of highly vibrationally excited molecules during collisional deactivation
Authors: Pibel Charles D. , Sirota Egor , Brenner Jerrell , Dai Hai-Lung
Published: July 2002

The 10−8 second time resolution in infrared emission spectroscopy has been demonstrated using a Fourier Transform spectrometer paired with a fast HgCdTe detector. The rapid time response of this system has enabled us to measure, with subcollisional period time resolution, the emission spectrum of highly vibrationally excited NO2 molecules during collisional deactivation by room temperature NO2. The greatly improved time resolution of the spectra allows the determination of N(E,t), the instantaneous energy distribution of the ensemble of excited molecules, with virtually no distortion due to collisional averaging. In addition, an improved procedure for extracting optimized N(E,t) from the spectral data makes no prior assumptions about the shape of the energy distribution. It is found that the distribution is well approximated as the sum of a Gaussian function at high vibrational energies and a population at low energies resulting from V–V transfer to bath NO2 molecules. The observation of a Gaussian-like function for the highly excited molecules is consistent with the widely invoked assumption that the step-size function of energy transfer per collision is exponential.
Predissociation of the Rydberg states of CO: State specific predissociation to the triplet channel
Authors: Okazaki Akihiro , Ebata Takayuki , Sutani Takumichi , Mikami Naohiko
Published: July 2002

Photofragment yield spectra of the 3dπL′ 1Π(v=1) and 4pπL 1Π(v=0) Rydberg states have been observed by monitoring the fragment atoms generated by the predissociation. The Rydberg states of CO were prepared by two-color double resonant excitation through the 3sσB 1Σ+ state, and both C(3P and D1) and O(3P) atom fragments were observed by two-photon resonant three photon (2+1) ionization. In particular, it has been found that the C(1D) fragmentation takes place in these Rydberg states, which is the first evidence of the predissociation to the triplet channel. Predissociation rates to the triplet channel were estimated, indicating that the rates are comparable to those to the singlet channel.
The effects of vibrational resonances on Renner–Teller coupling in triatomic molecules: The stretch–bender approach
Authors: Duxbury Geoffrey , McDonald Barry D. , Van Gogh Marcel , Alijah Alexander , Jungen Christian , Palivan Horatiu
Published: July 2002

A vibration–rotation Hamiltonian for a symmetric triatomic molecule, based upon a stretch–bender reference frame has been derived. This frame is chosen so that as the molecule bends the reference geometry follows the minimum in the potential energy surface, thus minimizing the size of the displacements required to reach the instantaneous axis geometry. This may be regarded as an extension of methods based upon the rigid–bender reference frame approach developed by Hougen, Bunker, and Johns [J. Mol. Spectrosc. 34, 136 (1970)]. This new stretch–bender Hamiltonian is combined with the Barrow, Dixon, and Duxbury [Mol. Phys. 27, 1217 (1974)] and the Jungen and Merer [Mol. Phys. 40, 25 (1980)] methods of solving the Renner–Teller coupling problem in which molecules execute large amplitude nuclear motion, producing a compact method for the variational calculation of the energies of such a system. The ã 1A1 and b̃ 1B1 states of the methylene radical, CH2, are used to demonstrate the use of this method for the analysis of the behavior of strongly coupled electronic and vibrational states.
Quantum harmonic transition state theory—Application to isomerization of stilbene in liquid ethane
Authors: Gershinsky Gidon , Pollak Eli
Published: July 2002

A harmonic quantum transition state theory, suggested recently by Pollak and Gershinsky [in Lectures on Stochastic Dynamics, edited by W. Lutz and T. Poeschel, Lecture Notes in Physics (Springer Verlag, New York, 1997)], is applied for the first time to a realistic reacting system. The isomerization of trans-stilbene in the gas phase and in the presence of dense liquid ethane solvent is investigated. We find that the overall quantum effect at room temperature is rather small. The quantum correction to the classical reaction rate estimate is approximately 23% for gas phase stilbene at room temperature. The addition of the dense solvent lowers the correction down to 13%, thus making the reacting system even more “classical,” justifying the extensive use of classical molecular dynamics in investigating this reaction.
The vibrational frequency of nitrogen near the fluid–solid transition in the pure substance and in mixtures
Authors: Michels J. P. J. , Kooi M. E. , Schouten J. A.
Published: July 2002

At high densities intramolecular vibrations are strongly dependent on the interactions with the surrounding molecules. In this paper a study is made of the consequences of these interactions on the Raman Q-branch of nitrogen. In particular the difference between a disordered and an ordered surrounding is surveyed. For this purpose, high-resolution Raman spectroscopy has been performed at room temperature on pure nitrogen as well as on a dilute mixture of nitrogen in argon, around the fluid–solid phase transition of these systems, which occur at ≈2.5 GPa and at ≈ 1.3 GPa, respectively. Going from the liquid to the solid phase, a positive jump in the line shift and a dramatical drop in the linewidth are seen in both systems at the transition pressure. For a better understanding of the underlying mechanisms, molecular dynamical simulations have been performed on corresponding model systems. The results of these calculations are in fair agreement with the experimental data and reveal the reasons for the discontinuities. Although the average distance of the nearest neighbor molecules around the nitrogen molecule increases, the distance to the nearest neighbor molecules in line with the molecular axis of the nitrogen decrease at the phase transition. This results in a positive jump in the frequency. Further, the time-autocorrelation function of the vibration frequency has a long persisting positive tail in the fluid phase. This behavior is absent in the solid phase. Even more important is that this function has negative values during a substantial time interval in the solid phase. As a result, the correlation time is greatly reduced at the phase transition, which results in an important reduction of the linewidth as well. Finally, it is proven that also in the solid phase the nitrogen is really dissolved in argon.
Generalized molecular mechanics including quantum electronic structure variation of polar solvents. I. Theoretical formulation via a truncated adiabatic basis set description
Authors: Bursulaya Badry D. , Kim Hyung J.
Published: July 2002

A theoretical framework to describe the evolving electronic charge distributions of bulk solvent that can be incorporated into the computer simulation algorithms is considered. By using a truncated adiabatic basis set representation, an effective quantum mechanical description for the electronic polarizabilities of the constituent molecules is constructed. It is found that there is a near one-to-one correspondence between the theory developed here and gas-phase quantum chemistry; with the substitution, electrons → molecules, many known results of the latter are transferable to the former with some modifications. The ground-state solution for the solvent system is studied with the analogs of the self-consistent field (SCF) and second-order Mo/ller–Plesset (MP2) methods of many-body perturbation theory. It is found that the states with one of the molecules electronically excited are not directly coupled to the SCF ground state; this is very similar to Brillouin’s theorem in the gas-phase electronic structure theory. As a result, they do not contribute to the MP2 energy correction; only the two-molecule excited states make nonvanishing contributions. The ground-state electronic properties, e.g., total and single-molecule polarizability tensors, are analyzed at the SCF level. The inclusion of electronic relaxation in the excited state calculations is briefly considered within the context of the configuration interaction method. The incorporation of the theory into the molecular dynamics computer simulation algorithms via the interaction site model description is also discussed.
Ab initio study of the van der Waals interaction of NH(X 3Σ−) with Ar(1S)
Authors: Kendall Rick A. , Chałasiński Grzegorz , Kłos Jacek , Bukowski Robert , Severson Mark W. , Szczȩśniak M. M. , Cybulski Sławomir M.
Published: July 2002

The potential energy surface for the Ar(1S)+NH(X 3Σ−) interaction is calculated using the supermolecular unrestricted Mo/ller–Plesset (UMP) perturbation theory approach and analyzed via the perturbation theory of intermolecular forces. The global minimum occurs for the approximate T-shaped geometry with Ar skewed toward the H atom at about Θ=67° and R=6.75 a0. Our UMP4 estimate of the well depth of the global minimum is De=100.3 cm−1 and the related ground state dissociation energy obtained by rigid-body diffusion quantum Monte Carlo calculations (RBDQMC) is D0=71.5±0.1 cm−1. These values are expected to be accurate to within a few percent. The potential energy surface also features a wide plateau in the proximity of Ar-N-H collinear geometry, at ca. 7.0 a0. RBDQMC calculations reveal nearly a free rotation of the NH subunit in the complex.
The distribution of density matrices over potential-energy surfaces: Application to the calculation of the far-wing line shapes for CO2
Authors: Ma Q. , Tipping R. H.
Published: July 2002

Within the formalism developed previously for the calculation of the far-wing line shape for molecular systems, most of the computer resources were used to diagonalize anisotropic potential-energy matrices whose sizes are determined by the number of states included. As this number is increased, one expects the results to converge. However, for some systems of atmospheric interest, e.g., CO2, the convergence is so slow that one is unable to obtain converged results within reasonable computer limitations. In the present paper, a new formalism is presented in which the eigenfunctions of the orientations of the system, not the states themselves, are chosen as the complete set of basis functions in Hilbert space. In this case, the diagonalization procedure is unnecessary and one can include as many states as desired. The main computational task is transformed from a diagonalization procedure to the carrying out of multidimensional integrations over the continuous orientational variables. In practice, the integrals are approximated by multidimensional summations over discrete values, the number of which is determined by the resolution required so that the approximated integrals are close to their true values. By choosing reasonable resolutions based on the smooth functional behavior of the integrands, one is able to evaluate the required integrations within reasonable computer time. Furthermore, by introducing weighting functions which are the distribution of the density matrices over potential-energy surfaces, one can reduce the multidimensional integrations to two-dimensional ones. The calculation of the weighting functions can also be carried out with reasonable CPU time and furthermore needs only to be done once for a given molecular system at a specified temperature. Using these as input data, the remaining calculations of the line shapes and corresponding absorption for given potential parameters become straightforward. The formalism is applied in the present paper for linear molecular systems and sample calculations for CO2–CO2 and CO2–N2 are presented. To our knowledge, these are the first, first-principle calculations for the far-wing line shape of CO2 except for the much simpler CO2–rare gas systems.