bluuebunny/crossref_metadata_2025 huggingface dataset:
Ferromagnetic and antiferromagnetic intermolecular interactions of organic radicals, α-nitronyl nitroxides. II
Authors: Awaga Kunio , Maruyama Yusei
Published: July 2002

The temperature dependence of the magnetic susceptibilities and the magnetizations for the three kinds of α-nitronyl nitroxides, 2-R-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy 3-oxide [with R=phenyl (I), 3-nitrophenyl (II), 4-nitrophenyl (III)] have been measured. It is found that the intermolecular spin interaction is ferromagnetic in the crystal of III, while it is antiferromagnetic in I or II. UV–visible absorption spectra suggests the enhancements of the π-electron delocalization and the spin polarization in III in comparison with the others. The difference in the magnetic coupling between I or II and III is interpreted based on these two effects.
One- and two-dimensional 31P cross-polarization magic-angle-spinning nuclear magnetic resonance studies on two-spin systems with homonuclear dipolar coupling and J coupling
Authors: Kubo Atsushi , McDowell Charles A.
Published: July 2002

The effect of dipolar interactions on the solid state 31P cross-polarization magic-angle-spinning (CP-MAS) nuclear magnetic resonance (NMR) line shapes for the coupled two-spin systems, sodium pyrophosphate decahydrate, Na4P2O7⋅10H2O, and tetraphenyl diphosphine-1-oxide, (C6H5)2PP(O)(C6H5)2, has been investigated. The one-dimensional (1D)CP-MAS spectra of Na4P2O7⋅10H2O shows spinning frequency dependent sideband splittings. A theory was developed to permit the calculation of the MAS NMR line shapes of the dipolar and J-coupled two-spin systems. An exact solution of the periodic Hamiltonian was obtained by the use of Floquet Hamiltonian theory. The experimental spectra of Na4P2O7⋅10H2O are well reproduced by the theoretical ones calculated from the present theory, in which the homonuclear dipolar interaction between the two 31P nuclei in the P2O74− group was taken into consideration. Our Hamiltonian also leads to calculated spectra which are in good agreement with the experimental observations even at low rotor spinning speeds. The two-dimensional J-resolved experiments, with rotationally synchronized acquisition in the t1 dimension, were performed for both Na4P2O7⋅10H2O and (C6H5)2PP(O)(C6H5)2. These experiments were found to be useful in distinguishing between the different mechanisms of the rotational sideband splitting of 1D spectra, as well as the dipolar interactions between spins with the same isotropic chemical shift, but different orientations of chemical shift tensors. These studies also allowed the identification of splittings caused by homonuclear J-coupled interactions, because the resolution of the 2D J-resolved spectra was greater than that of the 1D spectra.
Mass spectra and time-of-flight distributions of helium cluster beams
Authors: Buchenau H. , Knuth E. L. , Northby J. , Toennies J. P. , Winkler C.
Published: July 2002

Liquid helium clusters are produced by expanding gaseous 4 He into a vacuum from a cold source with temperatures between 5 and 20 K at stagnation pressures from P0 =8 to 20 bar and are studied by time-of-flight (TOF) and mass spectrometry. At low temperatures, T0 <12 K, the mass spectra show several anomalies which can be attributed to pick-up of residual gases. At T0 <10K, there is evidence for a very intense peak at m=16 amu which is attributed to He+4 . Depending on the temperatures, the TOF spectra reveal ions with three different velocities. These TOF observations are analyzed using isentropic lines in the known phase diagram of 4 He, which take into account deviations from ideal gas behavior. Three qualitatively different expansion regimes are identified: (I) the expansion proceeds through a region on the high temperature side of the critical point, (II) the expansion passes through or near the critical point, and (III) the expansion passes through a region on the low temperature side of the critical point. The mass spectra, peak velocities and speed ratios, when analyzed with the aid of the phase diagram, indicate that (a) two of the TOF peaks are due to clusters, (b) the fastest cluster peak is due to clusters formed by condensation of gas phase atoms, and (c) the slowest cluster peak is due to either separation into two phases (regime II) or disintegration of a liquid phase (regime III). Measured conversions of initial enthalpy into free jet kinetic energy suggest that the cluster temperature undergoes a sharp drop to a very low temperature approaching 0 K at T0 <6.5 K where the expansion isentrope intersects the liquid–vapor line upstream from the source orifice.
H+O3 Fourier-transform infrared emission and laser absorption studies of OH (X 2Π) radical: An experimental dipole moment function and state-to-state Einstein A coefficients
Authors: Nelson David D. , Schiffman Aram , Nesbitt David J. , Orlando John J. , Burkholder James B.
Published: July 2002

The relative intensities of 88 pairs of rovibrational transitions of OH (X 2Π) distributed over 16 vibrational bands (v′≤9, Δv=−1,−2) have been measured using Fourier transform infrared (FTIR) emission/absorption spectroscopy. Each pair of transitions originates from a common vibrational, rotational, and spin–orbit state, so that the measured relative intensities are independent of the OH number density and quantum state distribution. These data are combined with previous v=1←0 relative intensity absorption measurements and v=0, 1, and 2 permanent dipole moments to determine the OH dipole moment function as a cubic polynomial expanded about re, the equilibrium bond length. The relative intensities provide detailed information about the shape of the OH dipole moment function μ(r) and hence the absolute Einstein A coefficients.

The intensity information is inverted through a procedure which takes full account of the strong rotation–vibration interaction and spin uncoupling effects in OH to obtain the dipole moment function (with 95% confidence limits): μ(r)=1.6502(2) D+0.538(29) D/Å (r−re)−0.796(51) D/Å2 (r−re)2−0.739(50) D/Å3 (r−re), 3 with a range of quantitative validity up to the classical turning points of the v=9 vibrational level (i.e., from 0.70 to 1.76 Å). The μ(r) determined in this study differs significantly from previous empirical analyses which neglect the strong effects of rotation–vibration interaction and spin uncoupling. The present work also permits distinguishing between the various ab initio efforts. Best agreement is with the dipole moment function of Langhoff, Werner, and Rosmus [J. Mol. Spectrosc. 118, 507 (1986)], but their theoretical predictions for higher overtone transitions are still outside of the 2σ experimental error bars. Absolute Einstein A coefficients from the present μ(r) are therefore presented for P, Q, R branch transitions for Δv=1, 2, 3, v′≤9, J′≤14.5, in order to provide the most reliable experimental numbers for modeling of near IR atmosphere OH emission phenomena.
Side-on binding of the nitrogen molecule to first-row transition-metal dimers
Authors: Siegbahn Per E. M.
Published: July 2002

Calculations including electron correlation have been performed for the systems M2N2, where M are the transition metals Ti, Cr, Fe, Co, and Ni. Rather surprisingly, it is found that these systems are well described by a single determinant. The spin is very high with a maximum of 11 for both Fe2N2 and Cr2N2. The optimal geometric structure of these side-on bonded structures is planar with N2 perpendicularly bridging the metal–metal bond. The electronic structure is best described as two M+ ionically bound to N2−2, with some back-donation occurring. With this simple picture, it is possible to explain the very irregular trend of the calculated dissociation energies with respect to N2 and free metal atoms. For example, the calculated binding energy difference between Ti2N2 and Cr2N2 is as large as 78.3 kcal/mol at the self-consistent-field level, whereas the simple prediction gives 79.2 kcal/mol. The implications of the present results for the catalytic dissociation of N2 by metal surfaces is discussed. A new explanation for the large effect on the N2 dissociation by alkali doping is suggested.
Multiphoton ionization studies of clusters of immiscible liquids. I. C6H6–(H2O)n, n=1,2
Authors: Gotch Albert J. , Zwier Timothy S.
Published: July 2002

Resonant two-photon ionization (R2PI) time-of-flight mass spectroscopy is used to record S0–S1 spectra of the neutral complexes C6H6–H2O, C6H6 –HDO, C6H6–D2O, C6H6–(H2O)2, and C6H6–(D2O)2. In C6H6–H2O, the lack of an S0–S1 origin transition and the presence of a splitting at 610 (which is absent in C6H6 –HDO) provide vibronic level evidence that the water molecule is on the sixfold axis undergoing internal rotation about that axis. Rotational band contour analysis of the 610 transitions of the isotopomers confirms this picture and also determines a ground state center-of-mass separation between C6H6 and D2O of 3.32±0.07 Å, very close to that predicted by ab initio calculations. R2PI scans of the van der Waals structure in the isotopic series C6H6–H2O, C6H6 –HDO, and C6H6–D2O provide tentative assignments for three of the six van der Waals modes in the complex. In C6H6–(H2O)2, rotational band contour analysis of the origin transition provides a best-fit structure in which the two water molecules reside on the same side of the benzene ring at a H2O–H2O separation close to that in the free water dimer.

Qualitatively, the structure of the 1:2 cluster is thus one which maximizes the strength of the water–water hydrogen bond at the expense of a somewhat poorer interaction of the second water molecule with the benzene ring in an off-axis geometry. Several intriguing features of the structure are suggested by our analysis, but are near the limit of our ability to distinguish from band contour fitting. Among these features are (i) the on-axis water molecule is pulled slightly in toward the ring from that in the 1:1 complex; (ii) the water dimer prefers an orientation bisecting a C–C bond in the benzene ring; (iii) the water–water separation is ∼0.2 Å less than that in the free water dimer; and (iv) the water dimer axis is tilted by about 10° relative to the plane of the benzene ring. Finally, the van der Waals structure in C6H6–(H2O)2 and C6H6–(D2O)2 suggests the possibility of large amplitude motion in these complexes as well. We postulate that this motion involves a hindered rotation of the on-axis water molecule.
Flow-induced gelation of living (micellar) polymers
Authors: Bruinsma Robijn , Gelbart William M. , Ben-Shaul Avinoam
Published: July 2002

We consider the effect of shear velocity gradients on the size (L) of rodlike micelles in dilute and semidilute solution. A kinetic equation is introduced for the time-dependent concentration of aggregates of length L, consisting of ‘‘bimolecular’’ combination processes L+L′ →(L+L′) and ‘‘unimolecular’’ fragmentations L→L′+(L−L′). The former are described by a generalization (from spheres to rods) of the Smoluchowski mechanism for shear-induced coalesence of emulsions, and the latter by incorporating the tension-deformation effects due to flow. Steady-state solutions to the kinetic equation are obtained, with the corresponding mean micellar size (L̄) evaluated as a function of the Peclet number P, i.e., the dimensionless ratio of flow rate γ̇ and rotational diffusion coefficient Dr. For sufficiently dilute solutions, we find only a weak dependence of L̄ on P. In the semidilute regime, however, an apparent divergence in L̄ at P≂1 suggests a flow-induced first-order gelation phenomenon.
Phase equilibrium in mixtures of flexible and stiff polymers studied by Monte Carlo simulation
Authors: Gauger Andreas , Pakula Tadeusz
Published: July 2002

We present computer simulations of dense polymer mixtures composed of flexible and semiflexible chains. It is found that a demixing transition takes place below a certain degree of flexibility, even without introducing repelling interactions between the two polymers except the excluded volume condition. Computing the equilibrium concentration of phases for various temperatures, a phase diagram for the binary mixture is found. The results show qualitative agreement with the Flory lattice theory of stiff chains.
ZGB surface reaction model with high diffusion rates
Authors: Evans J. W.
Published: July 2002

The diffusionless ZGB (monomer–dimer) surface reaction model exhibits a discontinuous transition to a monomer-poisoned state when the fraction of monomer adsorption attempts exceeds 0.525. It has been claimed that this transition shifts to 2/3 with introduction of rapid diffusion of the monomer species, or of both species. We show this is not the case, 2/3 representing the spinodal rather than the transition point. For equal diffusion rates of both species, we find that the transition only shifts to 0.5951±0.0002.
A comparison of a united atom and an explicit atom model in simulations of polymethylene
Authors: Yoon Do Y. , Smith Grant D. , Matsuda Tsunetoshi
Published: July 2002

We have carried out stochastic dynamics and molecular dynamics simulations of n-tridecane (C13H28) as isolated chains, in bulk melts, and in confined melts between solid surfaces, employing both a united atom (UA) model and an explicit atom (EA) model, in order to compare chain conformations, packing, orientational correlations, and self-diffusion predicted by the UA and EA models. The EA model, which explicitly takes into account all hydrogens, exhibits nearly identical results for chain conformations to those from the UA model. However, only the EA model, which shows considerably enhanced interchain packing and orientational correlations in the melts over those for the UA model, reproduces very closely the height and the width of the interchain peak in the experimental x-ray scattering profile. Dynamically, inclusion of explicit hydrogens decreases the self-diffusion constants in the melts by a factor of 6–8, resulting in reasonably good agreement with the experimental value. Moreover, in the melts confined between solid surfaces, the presence of explicit hydrogens leads to much more pronounced layering of both the monomer segments and the entire molecules, which are strongly oriented along the solid surfaces.
Vibrationally resolved inelastic scattering and charge transfer in H+–C2H4 collisions
Authors: Aristov N. , Maring W. , Niedner-Schatteburg G. , Toennies J. P. , Chiu Y.-N. , Köppel H.
Published: July 2002

Differential cross sections and time-of-flight spectra have been measured in a crossed molecular beam apparatus for inelastic scattering and charge transfer in collisions of 30 eV protons with ethylene. High resolution inelastic time-of-flight spectra reveal peaks corresponding to the fundamentals of at least two of the infrared active antisymmetric vibrational modes of ethylene. In the time-of-flight spectra of H atoms resulting from charge transfer energy loss, peaks corresponding to excitation of two electronic states of the ethylene molecular ion are resolved. The H-atom product angular distributions are consistent with a 2.0 eV potential well in the ground state potential energy surface of the charge transfer reaction. The observations for the antisymmetric vibrational modes are explained in terms of an ion-induced dipole mechanism. Excitation of the symmetric modes is attributed to a vibronic coupling mechanism. An appendix provides a detailed account of the vibronic distributions in idealized low energy, high symmetry collisions leading to stable C2H5+ complexes.
Dielectric and thermodynamic response of a generalized reaction field model for liquid state simulations
Authors: Alper Howard , Levy Ronald M.
Published: July 2002

A generalized reaction field (GRF) model for simulations of liquid water is presented. The method is based on the solution of the Poisson equation for the reaction field acting on a collection of charges in a spherical cavity which is immersed in a continuum dielectric medium. In this model, the reaction field potential arising from the charges in the spherical cavity can be expressed as an infinite series, each term of which is the reaction field arising from a corresponding multipole in the expansion of the charge distribution. The GRF model is used here in simulations of liquid water and of solutions containing one or two ions in water. The dielectric constant computed for liquid water with the generalized reaction field agrees with previous results obtained using the conventional dipolar reaction field. The water orientational correlation functions converge to within 5% of the correct value when the first three terms are included in the multipolar expansion of the reaction field. Thermodynamic results from simulations of a single ion in water in which the GRF was employed demonstrate that the hydration free energy of the ion agrees with the Born hydration energy when the ion is far from the boundary, but when the ion approaches the boundary, there are significant deviations in the free energy. The results of GRF simulations of two ions in water are also reported.
Electron paramagnetic resonance and relaxation study of copper (II) and silver (II) in CsCdF3 single crystals
Authors: Minner E. , Lovy D. , Bill H.
Published: July 2002

Copper and silver, respectively, were introduced into single crystals of CsCdF3. Our detailed electron paramagnetic resonance (EPR) study showed that both elements enter the Cd lattice site—copper as Cu2+, silver as Ag+, which then was converted into Ag2+ by x raying the corresponding samples. Cu2+ and Ag2+ were shown to present in their ground state a pseudostatic Jahn–Teller effect. Motional effects were observed in the respective EPR spectra and studied in some detail for Cu2+ as they are seen over a wide temperature range. Predictions of a stochastic Kubo model [J. Phys. Soc. Jpn. 9, 935 (1954)] were compared with the temperature dependent linewidths of the motionally averaged EPR spectrum. A power law (Tn with n≂1.9) was determined for the temperature dependence of the reorientation frequency between 30 and 90 K.
On the infrared-active overtones nν3 of uranium hexafluoride
Authors: van der Merwe P. du T.
Published: July 2002

A perspective is provided of the infrared-active fundamental mode ν3 and the band structure of its overtones relevant for uranium hexafluoride. The treatment is based on nonlinear (Morse-type) creation and annihilation operators. The spectrum generating algebra provides ready access to anharmonic spectral features which incorporate dissociation characteristics.
Interpenetrations in polymer brushes
Authors: Chakrabarti Amitabha , Nelson Peter , Toral Raúl
Published: July 2002

We carry out Monte Carlo simulations in both lattice and off-lattice geometry, to study the configurational properties of a system of two polymer-coated plates. We compare our results with numerical and analytical self-consistent-field theories. For the short chain-lengths considered here, we find both compression and interpenetration of the polymer brushes as the plate separation is decreased, instead of the ‘‘classical-limit’’ picture of compression with no interpenetration.
An accurate quartic force field and vibrational frequencies for HNO and DNO
Authors: Dateo Christopher E. , Lee Timothy J. , Schwenke David W.
Published: July 2002

An accurate ab initio quartic force field for HNO has been determined using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), in conjunction with the correlation consistent polarized valence triple zeta (cc-pVTZ) basis set. Improved harmonic frequencies were determined with the cc-pVQZ basis set. Fundamental vibrational frequencies were determined using a second-order perturbation theory analysis and also using variational calculations. The N–O stretch and bending fundamentals are determined well from both vibrational analyses. The H–N stretch, however, is shown to have an unusually large anharmonic correction, and is not well determined using second-order perturbation theory. The H–N fundamental is well determined from the variational calculations, demonstrating the quality of the ab initio quartic force field. The zero-point energy of HNO that should be used in isodesmic reactions is also discussed.
The dissociation energy of CN and C2
Authors: Pradhan Atul D. , Partridge Harry , Bauschlicher Charles W.
Published: July 2002

The dissociation energies of CN and C2 are determined with multireference configuration- interaction calculations utilizing very large Gaussian basis sets. Valence level one-particle errors, such as basis set incompleteness and basis set superposition error, are essentially eliminated by extrapolation. The errors in the correlation, or n particle, treatment are considered in detail. The effect of 1s correlation is computed using the averaged coupled-pair functional approach. Our best estimate for C2 is in very good agreement with the recent experimental result by Urdahl et al. Our best D0 value for CN is 7.72±0.04 eV. This is larger than previous theoretical values and in good agreement with the recent experimental result (7.738±0.02 eV) of Huang et al. The origin of the differences between the current D0 and our previous theoretical value is discussed.
Laser spectroscopy of molecular ions: The A–X transition in the OCS radical cation
Authors: Weinkauf R. , Boesl U.
Published: July 2002

Spectroscopy of the OCS+ ion in its ground and first excited states has been performed over a wide energy range using one-photon dissociation spectroscopy. We used multiphoton ionization in the first step for state selective ion preparation in single well-defined vibrational and spin–orbit states. This simplifies the ion spectra of the transition to the first excited A state considerably and thereover delivers the information of the ion ground state by using hot ion preparation. For the stretching vibrations anharmonicities have been observed and for the first overtone of the bending vibration Fermi resonances have been found. Rotational constants could be determined for the vibrational ground states of the X and A ionic states in both spin–orbit components. A new double resonance technique was applied to measure vibrational frequencies in the ionic X ground state.
Protonated hydrochlorous acid (HOClH+): Molecular structure, vibrational frequencies, and proton affinity
Authors: Francisco J. S. , Sander S. P.
Published: July 2002

Protonated hydrochlorous acid (HOClH+) has been examined theoretically. Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for each of the parent and protonated structures at various levels of theory employing second-order Mo/ller–Plesset perturbation interaction theory (MP2), singles and doubles excitation configuration interaction theory (CISD), and coupled-cluster theory (CCSD). Our study has found that protonation of the oxygen of HOCl is favored over protonation at the chlorine site. Protonation of the oxygen leads to a pyramidal structure of Cs symmetry. There is a planar Cs structure which is the inversion transition state. The inversion barrier is 3.2 kcal mol−1. The proton affinity of hypochlorous acid, HOCl, is found to be 153.1 kcal mol−1 at 0 K.
Donor–acceptor interchange tunneling in HDO–DOH and the higher energy HDO–HOD isotopomer
Authors: Karyakin E. N. , Fraser G. T. , Lovas F. J. , Suenram R. D. , Fujitake M.
Published: July 2002

The microwave and submillimeter spectra of the a-type K=0←0 and K=1←1, c-type K=1←0, and isotopically allowed b-type K=1←0 bands of the O–D bonded HDO–DOH water dimer isotopomer and the higher energy O–H bonded HDO–HOD isotopomer have been measured using molecular-beam electric resonance optothermal and pulsed-nozzle Fourier-transform microwave spectrometers. The present results obtained in He and He/Ne seeded molecular beams give the first evidence for the presence of the higher energy O–H bonded mixed protonated-deuterated water dimers. These species were not reported previously in studies using seeded Ar molecular beams. The donor–acceptor interchange tunneling splittings are found to be 1322.1019(43) and 5004.059(20) MHz for the HDO–DOH and the metastable HDO–HOD dimers, respectively. For both isotopomers, the donor-accepter interchange tunneling-state selections rules for the b- and c-type bands are consistent with tunneling pathways corresponding to geared partial internal rotation of the two subunits in double-minima potentials. The larger tunneling splitting in HDO–HOD is primarily the consequence of the smaller effective reduced mass for tunneling in this system compared to that in HDO–DOH. The presence of both b- and c-type K=1←0 bands allows the direct measurement of the largest tunneling splitting, that associated with the internal rotation about the O–H–O or O–D–O bond of the nonbonded proton/deuteron on the proton donating unit. For the K=0 state of HDO–DOH this splitting is 214 208.38(23) MHz, while for the K=0 state of HDO–HOD it is 117 440.97(17) MHz. A strong b-type Coriolis interaction is observed between the upper K=0 and lower K=1 states in HDO–DOH, similar to that observed previously in (H2O)2.
Effects of excitation migration and translational diffusion in the luminescence quenching dynamics
Authors: Jang Seogjoo , Shin Kook Joe , Lee Sangyoub
Published: July 2002

Luminescence decay in the presence of quencher is considered theoretically. Combined effects of molecular diffusion and excitation migration among energy-donors are treated within a unified framework. A set of general kinetic equations for describing the luminescence quenching dynamics is derived from a hierarchical system of many-body Smoluchowski equations for the reactant molecule distribution functions. Numerical and approximate analytic solutions to these equations for a simple model system show that the effective diffusion coefficient of donors upon a quencher can be considerably larger than the simple sum of translational diffusion coefficient and excitation-migration diffusion coefficient estimated for a system of fixed donor molecules. This finding is in accordance with a recent experimental result of Pandey et al. Another important finding is that the excitation migration diffusion coefficient should be proportional to the donor concentration C0D in the large C0D limit when the donor molecules are extremely mobile, while it varies with (C0D)4/3 for a system of immobile donors as is known. It is also shown that some well-known previous theories are incorrect at least in the fast excitation migration limit.
Two-photon absorption and third-harmonic generation of di-alkyl-amino-nitro-stilbene (DANS): A joint experimental and theoretical study
Authors: Beljonne D. , Brédas J. L. , Cha M. , Torruellas W. E. , Stegeman G. I. , Hofstraat J. W. , Horsthuis W. H. G. , Möhlmann G. R.
Published: July 2002

The one- and two-photon optical absorption spectra as well as the frequency dependent third-harmonic generation response of a side-chain polymer containing 4-dialkylamino- 4′-nitro-stilbene (DANS) as its side group have been measured over a wide range of frequency. A three-state model, based on an intermediate neglect of differential overlap/multireference double-configuration interaction description of the excited states, provides a coherent picture of the dynamic response of DANS, but fails in reproducing the red shift observed when going from the linear to the nonlinear absorption spectra; including vibronic coupling within the Franck–Condon approximation improves the fit.
Monte Carlo simulation of the conformational entropy of polymer chains
Authors: Zhao Delu , Huang Yun , He Ziru , Qian Renyuan
Published: July 2002

A Monte Carlo method, namely, the ‘‘statistical counting method’’ (SCM) has been proposed for simulating the conformational entropy of a single free or confined linear self-avoiding random walk chains on the simple cubic lattice. For a free linear chain with length 1081, it is found from the calculated results of 100 groups of 104 samplings that the maximum and the minimum values of the conformational entropy are 1670.2 and 1660.3, respectively, the deviations from the average value 1663.8 are only +0.39% and −0.2%. In the range of chain length 8–20 the calculated entropy data are found to be in agreement with their precise values obtained by M. F. Sykes, et al. [J. Phys. A 5, 653 (1972)] with deviation less than 0.05%. In the range of chain lengths up to 27 confined in a cube of side length 2, the entropy data are also consistent with their precise values obtained from the direct counting of conformation number with the deviations less than 0.6%. For a long chain with lengths up to 2101, the calculated entropy data have confirmed the prediction by the renormalization group theory very well and the deviation is less than 0.8%.
Structures, thermochemistry, and electron affinities of the PFn and PF−n series, n=1–6
Authors: Tschumper Gregory S. , Fermann Justin T. , Schaefer Henry F.
Published: July 2002

A quantum mechanical study of the phosphorus fluorides and their singly charged anions was carried out. A range of density functional methods was used. Optimized geometries, adiabatic electron affinities, vertical electron affinities, vertical detachment energies, and stabilities toward the loss of a single fluorine atom or fluorine ion are reported. These properties were evaluated exhaustively using four exchange-correlation functionals: Becke’s 1988 exchange functional with the correlation functional of Lee, Yang, and Parr, Becke’s 1988 exchange functional with the 1986 correlation functional of Perdew, Becke’s three parameter Hartree–Fock/density functional hybrid exchange functional with the correlation functional of Lee, Yang, and Parr and Becke’s half-and-half Hartree–Fock/density functional hybrid exchange functional with the correlation functional of Lee, Yang, and Parr (BHLYP). These exchange-correlation functionals were used in conjunction with a double-ζ plus polarization basis and a double-ζ plus polarization basis set which was augmented with an even tempered set of s and p diffuse functions. Less complete examinations of the local spin density approximation, Becke’s 1988 exchange functional with the 1991 correlation functional of Perdew and Wang are also reported.

Results were compared to the limited experimental data to see which combination of functional and basis set, if any, reproduced known results and could be expected to make accurate predictions where experimental data is absent. This comparison shows that the BHLYP exchange-correlation functional reproduces the known experimental geometrical parameters quite well. From work on related systems, the BHLYP method appears to predict the most reliable molecular electron affinities. With the double-ζ plus polarization basis set augmented with s and p diffuse functions, the predicted BHLYP adiabatic electron affinities are 0.71 eV (PF), 0.75 eV (PF2), 0 (PF3), 3.17 eV (PF4), and 1.25 eV (PF5). These theoretical electron affinities are expected to lie somewhat above the true values. The PF6 molecule is predicted to be dissociative with respect to PF5 and F, but PF−6 is significantly bound with respect to either PF5+F− or PF−5+F.
A study of the vibronic structure in the HeI excited photoelectron spectrum of CO2 involving the X 2Πg and A 2Πu ionic states
Authors: Baltzer P. , Chau F. T. , Eland J. H. D. , Karlsson L. , Lundqvist M. , Rostas J. , Tam K. Y. , Veenhuizen H. , Wannberg B.
Published: July 2002

The HeI excited photoelectron spectrum of the CO2 molecule covering the X 2Πg and A 2Πu ionic states has been recorded at a resolution of better than 5 meV. Complex vibrational structures are resolved in both photoelectron bands. In the X 2Πg state, the ν2 and ν3 modes are observed to be excited in both an odd and even numbers of quanta in addition to the ν1 mode, whereas for the A 2Πu state the spectrum is dominated by excitations of the ν1 mode alone and in combinations with excitations of the ν2 mode in two quanta involving strong Fermi resonance. The observed spectrum has been assigned by comparison with optical spectra and with calculations of the vibrational fine structure including vibronic and spin–orbit coupling.
Time-dependent calculation of reactive flux employing complex absorbing potentials: General aspects and application within the multiconfiguration time-dependent Hartree wave approach
Authors: Jäckle A. , Meyer H.-D.
Published: July 2002

A novel modification of the flux operator formalism is introduced that combines the merits of the flux operator approach with those of complex absorbing potentials. The method is used to determine initial-state selected reaction probabilities for a broad energy range from a single appropriately chosen time-dependent wave packet. The propagation may be performed solely in the coordinates of the reagents arrangement channel. State-to-state transition probabilities can also be obtained when appropriate projectors are included. In contrast to similar methods the present one does not require the calculation of derivatives with respect to the reaction coordinate. More importantly, it avoids the need to (E,t)-Fourier transform the wave packet at every grid point on a dividing surface. The proposed formula, though completely general, is especially well suited to handle multiconfiguration time-dependent Hartree wave functions. As a check of the reliability initial-state selected reaction probabilities for the collinear H+H2→H2+H reaction are calculated and compared with (numerically) exact results. We also show that the initial wave packet may be placed close to the interaction region when its energy distribution is corrected for the mean potential energy.
Time resolved four- and six-wave mixing in liquids. II. Experiments
Authors: Steffen Thomas , Duppen Koos
Published: July 2002

Femtosecond four- and six-wave mixing is employed to study intermolecular motion in liquids, using CS2 as a working example. Nonresonant four-wave mixing yields the total spectral response associated with the low-frequency motions in the liquid. The results of optical Kerr effect and transient grating scattering experiments can be modeled equally well by homogeneously and inhomogeneously broadened intermolecular vibrations. Femtosecond nonresonant six-wave mixing, where two independent propagation times can be varied, contains a temporally two-dimensional contribution that provides information on the time scale(s) of these intermolecular dynamics. The six-wave mixing signal of CS2 shows distinctly different behavior along the two time variables. When the first propagation time is varied, both librational motion at short times and a picosecond diffusive tail are observed. Along the second propagation time, there is no sign of diffusive response and the signal is solely determined by the librational motions. Its shape depends on the first propagation time, when it is varied between 0 and 500 fs, but it is unaffected by further increase of that delay. This is a strong indication for a finite correlation time of the fluctuations in the intermolecular potentials. The interplay between the initial coherent motions and the diffusive behavior on longer time scales is far from clear. A widely used model in which these are treated as independent harmonic processes fails to describe the results.
Elimination of high order terms in multiple pulse nuclear magnetic resonance spectroscopy: Application to homonuclear decoupling in solids
Authors: Hohwy M. , Nielsen N. C.
Published: July 2002

A novel approach to the design of solid state nuclear magnetic resonance multiple pulse experiments is described. Based on time dependent perturbation theory the pulse cycle decoupling principle is extended to fifth order. Furthermore, by analyzing symmetry and commutator relations for high order terms in the average Hamiltonian expansion we introduce so-called z-rotational decoupling accomplished by concatenation of phase shifted pulse cycles. These fundamental tools prove extremely useful for the development of multiple pulse techniques capable of eliminating undesired interactions to high order in the average Hamiltonian expansion. The applicability of the methods is demonstrated by construction of homonuclear multiple-pulse decoupling methods which suppress pure dipolar terms up to fifth order and cross terms between rf inhomogeneity and dipolar coupling to second order in the Magnus expansion. For the dipolar terms this represents an improvement by two orders of magnitude compared to previous homonuclear decoupling sequences. High order truncation decoupling sequences based on the BLEW-12 and magic sandwich pulse cycles are compared to state-of-the-art methods numerically and by preliminary experiments.
A variational approach to nonlocal exciton–phonon coupling
Authors: Zhao Yang , Brown David W. , Lindenberg Katja
Published: July 2002

In this paper we apply variational energy band theory to a form of the Holstein Hamiltonian in which the influence of lattice vibrations (optical phonons) on both local site energies (local coupling) and transfers of electronic excitations between neighboring sites (nonlocal coupling) is taken into account. A flexible spanning set of orthonormal eigenfunctions of the joint exciton–phonon crystal momentum is used to arrive at a variational estimate (bound) of the ground state energy for every value of the joint crystal momentum, yielding a variational estimate of the lowest polaron energy band across the entire Brillouin zone, as well as the complete set of polaron Bloch functions associated with this band. The variation is implemented numerically, avoiding restrictive assumptions that have limited the scope of previous assaults on the same and similar problems. Polaron energy bands and the structure of the associated Bloch states are studied at general points in the three-dimensional parameter space of the model Hamiltonian (electronic tunneling, local coupling, nonlocal coupling), though our principal emphasis lies in the understudied area of nonlocal coupling and its interplay with electronic tunneling; a phase diagram summarizing the latter is presented. The common notion of a “self-trapping transition’’ is addressed and generalized.
Local dynamics in a long-chain alkane melt from molecular dynamics simulations and neutron scattering experiments
Authors: Smith Grant D. , Paul Wolfgang , Yoon Do Y. , Zirkel Achim , Hendricks Joerg , Richter Dieter , Schober H.
Published: July 2002

We make a detailed comparison of the incoherent dynamic structure factors Sinc(q,ω) and Sinc(q,t) for n-C100H202 melts, as determined from time-of-flight (TOF) neutron scattering measurements, with results from atomistic molecular dynamics simulations. Quantitative agreement is seen between simulations and experiments over the dynamic range accessible by the TOF measurements. From detailed analysis of the molecular dynamics trajectories, it is determined that the decay of Sinc(q,t) that occurs between 0.1 and 10 ps is the result of a combination of torsional librations and jumps, with the latter becoming more important with decreasing q.
Optical properties of protein monolayers during assembly
Authors: Guemouri L. , Ogier J. , Ramsden J. J.
Published: July 2002

Protein monolayers were built up by allowing protein molecules in solution to deposit at the solid/liquid interface. The structural evolution of these layers was followed using optical waveguide lightmode spectroscopy (OWLS). The measured effective refractive indices were interpreted using the uniform thin film model. For a relatively rigid, spheroidal protein (transferrin), the thickness of the layer was already established at very low surface coverages, and the refractive index increased during deposition. For a highly elongated and flexible protein (fibronectin), the converse occurred. The former case is consistent with classical random sequential adsorption. The latter result implies constant conformational rearrangement during deposition producing a compact proteinaceous membrane.
Self-diffusion in fluid carbon dioxide at high pressures
Authors: Groß T. , Buchhauser J. , Lüdemann H.-D.
Published: July 2002

The self-diffusion coefficients D for neat CO2 have been determined in a wide p, T range by the Stejskal-Tanner NMR method applying pulsed field gradients. In our apparatus CO2 supercools readily and the data could be collected between 223 K and 450 K at pressures up to 200 MPa. The ρ, T dependence of D(CO2) shows significant deviation from hard-sphere fluid behavior. The rough hard sphere (RHS) model, as well as the simple hydrodynamic theory show a very unusual temperature dependence, when compared to other simple fluids.
Quantum wave packet dynamics with trajectories: Application to reactive scattering
Authors: Wyatt Robert E.
Published: July 2002

The quantum trajectory method (QTM) for time-dependent wave packet dynamics involves integration of the de Broglie–Bohm hydrodynamic equations for the evolving probability fluid [C. Lopreore and R. E. Wyatt, Phys. Rev. Lett. 82, 5190 (1999)]. The equations of motion for discretized elements of the probability fluid (particles) are integrated in the Lagrangian, moving with the fluid, picture. These fluid elements move under the influence of both the usual potential energy function and the quantum potential, which involves the curvature of the quantum amplitude. The quantum potential and the quantum force are evaluated using a moving weighted least squares algorithm. As a demonstration of applicability, the QTM is applied to a model collinear reaction with an activation barrier. The reaction probabilities are in good agreement with exact quantum results, even for a relatively small number of particles in the ensemble. The QTM accurately describes tunneling using only real valued trajectories. In addition to the reaction probability, plots are presented to show the probability density and the flux distributions at several time steps during the reaction.
On the connection between the reference interaction site model integral equation theory and the partial wave expansion of the molecular Ornstein–Zernike equation
Authors: Ten-no Seiichiro , Iwata Suehiro
Published: July 2002

We develop an integral equation theory based on the partial wave expansion of the molecular Ornstein–Zernike (OZ) equation. The theory provides a rigorous and transparent framework for multiple site treatments of molecular fluids. We examine free-energy functional and closure expressions with pilot calculations of homonuclear diatomic Lennard-Jones liquids.
Protein interactions and phase behavior: Sensitivity to the form of the pair potential
Authors: Rosenbaum D. F. , Kulkarni Amit , Ramakrishnan S. , Zukoski C. F.
Published: July 2002

A robust correlation between experimentally determinable parameter, B2/B2HS, and protein solubility is explored. Here B2 is the protein solution second virial coefficient and B2HS=2πσ3/3 is the hard sphere second virial coefficient of the protein whose core is described as a sphere with diameter σ. The origin of this correlation, which extends over three decades in solubility, is discussed in terms of the phase behavior of simple fluids composed of particles experiencing short range attractions. For a given solubility B2/B2HS values cluster in a narrow range for a variety of protein solution conditions. However, the value of B2/B2HS at the metastable fluid/fluid transition remains as sensitive solution conditions. We discuss this sensitivity in terms of phase behavior predicted for particles interacting with the three parameter, square well and Yukawa potentials where metastable critical points are sensitive to the range of attraction. The position of this critical point relative to the solubility boundary and the ease of crystal nucleation are found to depend on solution conditions.
Phase separation of weakly ionized polymer gels during shrinking phase transition
Authors: Bai Gang , Suzuki Atsushi
Published: July 2002

We have investigated the shrinking phase transition of weakly ionized poly(N-isopropylacrylamide) gels prepared in a cylindrical shape with submillimeter diameter. The macroscopic conformation changes were obtained on heating processes in two different methods. One is a continuous heating process with a constant temperature drift rate, and the other is an isothermal process after a steplike temperature increase beyond the transition point. It was found that the macroscopic behavior can be characterized by several conformation changes; the phase coexistence, the grain pattern, the bubble pattern, and the opaque phase. On a continuous heating process, the phase transition can be characterized by the phase coexistence of completely collapsed and swollen states for the smaller temperature drift rates; the selected portions on the surface can start to collapse at the transition point, which develops with time and finally becomes a completely collapsed phase. For the larger temperature drift rates, the phase transition starts many places on the surface, and the whole gel with surface bubble pattern gradually shrinks with time. These different processes can be clearly observed in the latter measurements, which depend on the degree of the super-heating (quench); for the shallow quenches, the number of the completely collapsed states correspondingly increased with increasing the super-heating. For the deep quenches, the gel becomes opaque, and the transparent surface skin (collapsed phase) develops with time. The stability of the phase coexistence and the relationship with the transition velocity were qualitatively discussed in terms of the classical phase-separation model based on the nucleation and the spinodal decomposition.
Generalized Gaussian moment thermostatting: A new continuous dynamical approach to the canonical ensemble
Authors: Liu Yi , Tuckerman Mark E.
Published: July 2002

A new method for generating the canonical ensemble via continuous dynamics is presented. The new method is based on controlling the fluctuations of an arbitrary number of moments of the multidimensional Gaussian momentum distribution function. The equations of motion are non-Hamiltonian, and hence have a nonvanishing phase space compressibility. By applying the statistical mechanical theory of non-Hamiltonian systems recently introduced by the authors [M. E. Tuckerman, C. J. Mundy, and G. J. Martyna, Europhys. Lett. 45, 149 (1999)], the equations are shown to produce the correct canonical phase space distribution function. Reversible integrators for the new equations of motion are derived based on a Trotter-type factorization of the classical Liouville propagator. The new method is applied to a variety of simple one-dimensional example problems and is shown to generate ergodic trajectories and correct canonical distribution functions of both position and momentum. The new method is further shown to lead to rapid convergence in molecular dynamics based calculations of path integrals. The performance of the new method in these examples is compared to that of another canonical dynamics method, the Nosé–Hoover chain method [G. J. Martyna, M. L. Klein, and M. E. Tuckerman, J. Chem. Phys. 97, 2635 (1992)]. The comparison demonstrates the improvements afforded by the new method as a molecular dynamics tool. Finally, when employed in molecular dynamics simulations of biological macromolecules, the new method is shown to provide better energy equipartitioning and temperature control and to lead to improved spatial sampling over the Nosé–Hoover chain method in a realistic application.