bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular motions in lipid bilayers. II. Magnetic resonance of multilamellar and vesicle systems
Authors: Pace R. J. , Chan Sunney I.
Published: February 2003

The chain motion model developed in part I is shown to be consistent with a range of NMR data on Lα phase lipid multilayers and vesicles. A simple treatment of correlation effects within the intramolecular (crankshaft) rotations in the weak collision limit leads to a realistic T1 profile along the acyl chain, despite the assumption of a constant order parameter profile. The observed dependence of T1 on chain length is not reproduced however. The measured activation energy (∼13 kJ/mol) for T1 relaxation is consistent with the model, as are the ’’anomalously’’ low T1 and C–D order parameters observed on the methene groups in monounsaturated chains. A very small angular dependence for the 2H methylene T1 at high frequency (≳50 MHz) is suggested. This accords with observation and is a direct consequence of the crankshaft geometry. At temperatures sufficiently above Tm, the nematic fluctuation model of part I semiquantitatively predicts the components of multilayer T1 and T1ρ relaxation rates linear in ω−1/2. However, the description fails for ω∼108 and T∼Tm, in a manner possibly indicating nonrigid chain axis fluctuation on this time scale. A two-dimensional formulation of the nematic model provides a simple description of small vesicle 1H and 2H NMR linewidths, consistent with the experimental data.
Time dependent transport in a fluid with static traps
Authors: Kirkpatrick T. R.
Published: February 2003

The work of Bixon and Zwanzig on diffusion in the presence of a random distribution of spherical traps is extended to include more complicated dynamical processes that appear to lead to contributions of higher order in the trap density. It is shown that a class of these density corrections are divergent for long times and that when these divergent terms are resummed they modify the lowest order results of Bixon and Zwanzig.
Higher resolution photoionization study of acetylene near the threshold
Authors: Ono Y. , Osuch E. A. , Ng C. Y.
Published: February 2003

High resolution (0.14 Å FWHM) photoionization efficiency data for C2H2+ near the threshold (1059–1090 Å) have been obtained using the molecular beam method. The positions of the autoionization features resolved on the v2=0 vibrational step of C2H2+(X̃ 2Πu) were found to be consistent with those calculated from the two Rydberg equations νn=93 770−[R/(n−0.95)2] cm−1, νn=93 770−[R/(n−0.36)2] cm−1, where R and n are the Rydberg constant and the principal quantum number, respectively.
Analysis of nearly degenerate bands of spherical top molecules: Matrix elements of the vibrational-rotational Hamiltonian
Authors: Litvin Daniel B. , Fox Kenneth
Published: February 2003

We formulate a general method for calculating energy eigenvalues and eigenfunctions in a simultaneous analysis of nearly degenerate vibrational-rotational bands of spherical top molecules. The basis functions are products of rigid rotor and N-dimensional harmonic oscillator eigenfunctions. General explicit expressions are derived for the matrix elements of vibrational operators in the basis of N-dimensional harmonic oscillator eigenfunctions. Using these general expressions, the matrix elements of vibrational operators in the basis of five-dimensional harmonic oscillator eigenfunctions applicable, for example, in the analysis of the nearly degenerate ν2 and ν4 fundamental vibrational-rotational bands of tetrahedral XY4 molecules like CH4, are calculated explicitly.
The structure of chain molecules in the liquid state: Low-frequency Raman spectra of n-alkanes and perfluoro-n-alkanes
Authors: Snyder R. G.
Published: February 2003

A model, in which the random elements of the kinetic energy matrix are replaced by statistically weighted averages, is used to explain the principal features of the low-frequency Raman spectra of a series of liquid n-alkanes and perfluoro-n-alkanes. The spectra of the longer chains of the n-alkanes are dominated by an intense band that is associated with the longitudinal-acoustic mode observed in the crystalline state. For the liquid, the frequency ν and the number of carbon atoms n are found to be related as ν = ν0+B/n2. This relation, which can be derived from the weighted-average model, implies that the vibration exhibits frequency dispersion and has the character of an optic mode. The dependence of the width of this band on the number of carbons can also be accounted for in terms of the model. The intercept frequency ν0 and the slope B are functions of the energy difference Eg between gauche and trans CC bond states. For the perfluoro-n-alkanes, Eg was estimated from the observed value of the slope. The value derived 1050 cal/mol is roughly twice that reported for the polymethylene chain and is near the value reported by Bates and Stockmayer [J. Chem. Phys. 45, 2321 (1966)] from dipole moment measurements.
Core state vibrational excitations and symmetry breaking in the CK and OK emission spectra of CO2
Authors: Nordgren J. , Selander L. , Pettersson L. , Nordling C. , Siegbahn K. , Ågren H.
Published: February 2003

The CK and OK emission spectra of gaseous CO2 have been recorded in high resolution. From a Franck–Condon fit to the vibrational structure of the spectra, it is shown that C1s ionization leads to a bond length shortening of 2.0 pm, while O1s ionization mainly results in a displacement of the carbon atom of 5 pm, thereby breaking the D∞h symmetry of the molecule. The high resolution allowed the natural lifetime width of the O1s vacancy state to be determined to 0.07 eV and by the use of an accurate calibration procedure the adiabatic x-ray transition energies were determined. These energies were used to obtain accurate binding energies of the core electrons. The O1s binding energies in CO2 were determined to 297.651 and 541.07 eV, respectively.
ENDOR study of an irradiated crystal of L-alanine: Environment of the stable CH3ĊHCO2− radical
Authors: Kuroda Shin-ichi , Miyagawa Ichiro
Published: February 2003

The environment of the stable radical in an x-ray irradiated crystal of L-alanine CH3CH(NH3+)CO2− was studied at 77 K with the use of ENDOR technique. The analysis indicated that as the C–N bond ruptured and the stable radical was formed, the α carbon atom Cα shifted by a distance of 1.5 Å. As a result, for several protons in the radical environment the distance between the proton and Cα of the radical was found to be much shorter than the corresponding distance in the undamaged crystal; for example, 2.3 Å compared with 3.9 Å in the case of a methyl proton. The transferred N–H hydrogen was found to participate in exchange with the C–H hydrogens of this CH3ĊHCO2− radical.
ENDOR study of an irradiated crystal of L-alanine: Structure and the environment of the unstable CH3ĊHCO2− radical
Authors: Matsuki Kiyoshi , Miyagawa Ichiro
Published: February 2003

The unstable CH3ĊHCO2− radical and its environment in an x-ray irradiated crystal of L-alanine were investigated at 77 K with the use of ENDOR technique. It was found that upon radical formation, the atoms of the radical shifted substantially from their original positions in the undamaged crystal. Unusually strong positive ENDOR signals were detected from the methyl protons of the radical: some signals were found to be stronger than the ESR signal.
Optical spectra and Zeeman effect for Pr3+ and Nd3+ in LuPO4 and YPO4
Authors: Hayhurst T. , Shalimoff G. , Conway J. G. , Edelstein N. , Boatner L. A. , Abraham M. M.
Published: February 2003

Absorption spectra of Pr3+ and Nd3+ diluted in LuPO4 and YPO4 crystals have been measured from 4000 to 30 000 cm−1, at liquid He and N2 temperatures. Fluorescence was observed for Pr3+ and Nd3+ in YPO4, and Zeeman spectra were obtained in the visible region. The transitions were assigned and fit to a semiempirical Hamiltonian with adjustable parameters via a least squares procedure. Satisfactory fits and reasonable agreement between calculated and measured g values were obtained.
Vacuum ultraviolet spectroscopy of molecules using third-harmonic generation in rare gases
Authors: Miller John C. , Compton R. N. , Cooper C. D.
Published: February 2003

A variety of spectroscopic studies in the vacuum ultraviolet (VUV) region (100–200 nm) has been performed using tunable, narrow bandwidth (∼0.05 Å) light produced by third-harmonic generation in rare gases. These experiments include one-photon absorption and photoionization spectroscopy and photoionization threshold determinations. Two-photon ionization studies of NO, using the VUV light to probe Rydberg states and the fundamental light to ionize are also presented. These studies demonstrate the simplicity and utility of a laser-based, tunable high-resolution source for VUV spectroscopy.
Effects of jump diffusion rates on reaction kinetics in solids
Authors: Hwang Lian-Pin
Published: February 2003

Applying Torrey’s treatment of jump diffusion in solids to the study of reaction kinetics, a method is presented to describe the dynamics of caging processes in solids in which the reactants are confined to the cages. We put our attention to the study of three measures of time-dependent reactivity; namely, the observed reaction rate, cumulative reaction rate, and the fraction of correlated recovery. The consistency of the present approach is checked by comparison between Noyes’ particle pair model and diffusion model. Based on correlated recovery, a theoretical expression is obtained which describes the recovery and includes a parameter that characterizes the initial distribution function. Application of a jump diffusion model shows significant differences in the reactivity functions from those resulting from the Brownian model for the time region characterized by the average jump time.
Spectra and structure of small ring compounds. XLIII. Microwave spectrum of 2,3-dihydrothiophene
Authors: Durig J. R. , Little T. S. , Li Y. S.
Published: February 2003

The microwave spectrum of 2,3-dihydrothiophene CHCHSCH2CH2 has been recorded from 18.0 to 39.0 GHz and only a-type transitions were observed. The R-branch assignments have been made for the ground and first excited states of the ring puckering mode. The ground state rotational constants are: A = 6863.14±0.07, B = 4707.75±0.01, and C = 2953.43±0.01 MHz. It is shown that the quantity Ia+Ib−Ic = 9.87 U ⋅Å2 cannot arise totally from the four hydrogens on the two CH2 groups, but that the heavy atom skeleton contributes to it which indicates that the heavy atom skeleton is nonplanar. The rotational constants are shown to be consistent with reasonably assumed heavy atom structural parameters and a dihedral angle of puckering of 33°. The dipole moment components could not be accurately determined because of the near degeneracy of the ground and first excited state of the ring puckering mode but they were estimated to be ‖μa‖ = 1.5±0.2 D with ‖μb‖ and ‖μc‖ being relatively small. From relative intensity measurements, it is concluded that the second and third excited states are closely spaced with frequencies of 155±15 cm−1, whereas, the first excited state is nearly degenerate with the ground state. The origin of the difference band series associated with the ring puckering mode and the carbon–hydrogen stretch has been reassigned. Additionally,the first two ring puckering transitions 3←0, 2←1 have been observed in the far infrared spectrum of the gas at 167 and 152 cm−1, respectively. All of these results are consistent with a puckered ring governed by the following potential function: V(X) = 6.50×105X4−2.92×104X2, with a barrier to inversion of 328 cm−1. These results are compared to the corresponding quantities in similar molecules.
The most efficacious one-electron bases for determining and representing correlated molecular electronic wave functions. Unity in seemingly disparate electron correlation methods
Authors: Jasien Paul G. , Dykstra Clifford E.
Published: February 2003

Coefficient matrices and associated operator matrices are being used increasingly in various large-scale correlation methods. These matrices are used to find and represent the wave function directly in terms of one-electron basis functions. They eliminate serious redundancies in computation and provide for the use of different sets of nonorthogonal external orbitals to improve convergence. These features are shown to be independent of the choice of a one-electron basis, and illustrative calculations are presented for N2H2, HCN, and HNC.
Correlation of spectral and heat-capacity Schottky contributions for Dy2O3, Er2O3, and Yb2O3
Authors: Gruber John B. , Chirico Robert D. , Westrum Edgar F.
Published: February 2003

A self-consistent interpretation of the Raman and infrared spectra and the heat-capacity measurements on Dy2O3, Er2O3, and Yb2O3, which have the cubic (bixbyite) structure, is presented. New spectra recorded at 10, 80, and 300 °K on oriented single crystals of each oxide are compared with published spectral data and interpretations. Most of the data are consistent with earlier interpretations. However, several additional crystalline–electric-field split levels have been found for ions in the C3i sites that, together with the established CEF-split levels for ions in the C2 sites, now provide a total picture for the CEF splitting of the ground-state manifolds. These values allow calculation of the total Schottky contribution to the heat capacity. Experimental Schottky heat-capacity values are independently derived by the volume-weighted lattice-contribution scheme. Correlation between optical data and heat-capacity measurements is found to be excellent. Further confirmation of the analysis is found in the agreement (to within 3%) between observed and calculted CEF splitting for the lanthanide ions on C2 sites.
Voronoi polyhedra as a tool for studying solvation structure
Authors: David Elaine Eisler , David Carl Wolfgang
Published: February 2003

Voronoi polyhedra are employed to generate a coherent description of the immediate solvent sheath of complex solutes in water solution.
Numerical calculation of the self-diffusion, the shear viscosity, and the heat conductivity coefficient for a dilute hard sphere gas
Authors: Eder Otto J. , Lackner Thomas
Published: February 2003

The successive uncorrelated binary collision (SUBC) model is used to calculate numerically the self-diffusion coefficient, the shear viscosity, and the heat conductivity coefficient. It turns out that all three quantities can be represented as infinite sums over all possible successive uncorrelated binary collision sequences weighted by their probability of occurence. Depending on the accuracy required, a certain number of the elements of the infinite sums have to be evaluated. While the Chapman–Enskog procedure gives fast convergence and is therefore useful for numerical calculation, one looses insight in the physical meaning of the approximations. The SUBC model gives a clear picture of what is statistically going on in the sense of the underlying Markoffian nature of the SUBC model. It is briefly shown that the SUBC model and the infinite Chapman–Enskog approximation treats essentially the same problem, namely the solution of the linearized Boltzmann equation. The results obtained can be used not only to describe the dilute limit but also the moderately dense limit, since the higher density contributions, the cross term between the kinetic and the potential terms in the transport ccoefficients, can be expressed by the kinetic terms and δ(t) contributions in the approximate time correlation functions within the SUBC model.
The spin–orbit interaction in the π*→n phosphorescence of formaldehyde
Authors: Cooper David L. , Stutchbury Neil C. J. , Richards W. Graham
Published: February 2003

The spin-orbit interaction in the II*→n phosphorescence of formaldehyde is considered by using aw ab initio wave function and both the one-and two-electron parts of the L-S conpling operaton. (AIP)
On the relation between the quantum scattering theory and the molecular orbital theories of chemical reactivity. II. Justification for Hammond’s postulate
Authors: Deguchi Kisaburo
Published: February 2003

The connection between the simple molecular orbital theories of chemical reactivity and the transition T matrix element in quantum scattering theory is discussed. (AIP)
Beam maser measurements of HDO hyperfine structure
Authors: Fry H. A. , Kukolich S. G.
Published: February 2003

The 431←432, 532←533, and 321←414 transition in HDO were measured in a beam maser spectrometer. Accurate values for rotational transiton frequencies, the deuterium quadrupole coupling tensor, and hydrogen spin-rotation tensor were obtained from the present data combined with previous results on other transitions. Resonance line widths of 2 kHz were obtained for most of the data. Measured quadrupole coupling tensor elements eqQ(D)gg and spin-rotation tensor elements C(X)gg in kHz are as follows: eqQ(D)aa = 276.45±0.88, eqQ(D)bb = −110.97±1.46, eqQ(D)cc = −165.77±1.10, C(D)aa = −1.33±0.20, C(D)bb = −4.38±0.36, C(D)cc = −2.99±0.24, C(H)aa = −58.42±0.47, C(H)bb = −5.46±0.83, C(H)cc = −24.11±0.55.
Surface enhanced Raman scattering of pyridine on different silver surfaces
Authors: Seki H.
Published: February 2003

Ultra high vacuum (UHV) experiments have been carried out in which different types of Ag substrates are prepared side-by-side in situ. One of the Ag substrates in a thin island film formed by depositing Ag on heated substrate. The other is prepared be depositing a thicker continuous film onto a cooled substrate. By depositing the pyridine simultaneously on the different Ag surfaces a direct comparison of the surface enhanced Raman scattering (SERS) behavior on these different Ag substrates have been made. The results indicate that some of the significant differences in the spectral features of the SERS phenomenon such as peak positions and relative intensity of the various vibrational modes observed by various laboratories for pyridine molecules condensed on Ag substrates are real. The differences involve microscopic features of the surface morphology and cannot be easily explained in terms of classical electromagnetic theory alone. It is proposed that there is a variety of ’’active sites’’ that can be formed on the Ag surface to which a pyridine molecule can be attached giving rise to a characteristic SERS behavior for that site. The density distribution of the various active sites on the surface of an Ag substrate varies depending on its preparation and its history.
Criteria for model evaluation in the case of deconvolution calculations
Authors: Ameloot M. , Hendrickx H.
Published: February 2003

The time-resolved fluorescence relaxation obtained by a pulse method can be described by a convolution product of the measured excitation profile and the fluorescence response of the sample. In this paper, the behavior of this convolution model, with respect to the parameters, is examined. The model happens to be nearly linear in the neighborhood of the least-squares estimates, according to an adapted nonlinearity measure. As a consequence, the lack of fit-test and the extra sum of squares-test from the theory of linear models can be applied to decide how many components should be included in the fluorescence response function. The proposed method can be easily extended to other areas involving exponential relaxation.
Laser induced fluorescence studies of the charge transfer reactions of N2+ with Ar and N2
Authors: Mahan Bruce H. , Martner Cecilia , O’Keefe Anthony
Published: February 2003

The charge transfer reactions of N2+ with Ar and N2 have been studied using laser induced fluorescence to monitor N2+. The ions were formed initially within a quadrupole ion trap by electron impact of neutral N2. The experimental conditions were varied such that the trapped N2+ could be examined under collision free conditions or after experiencing a controlled number of collisions with selected background gas. The ion–molecule collisions rapidly altered the internal energy distribution of the trapped N2+ and by monitoring these changes charge transfer cross sections for individual initial vibrational states of the N2+ with Ar and N2 have been estimated. The results show a strong vibrational dependence of the charge transfer cross section in the N2+–N2 system and no measureable dependence in the N2+–Ar system.
Quasiclassical trajectory investigation of the reaction O(1D)+H2
Authors: Whitlock P. A. , Muckerman J. T. , Fisher E. R.
Published: February 2003

A theoretical investigation of the energetics and dynamics of the O(1D)+H2 reaction is reported. Two different valence bond diatomics-in-molecules potential energy surfaces were used which differed only in the presence or absence of a small barrier to the C2v approach. The results of quasiclassical trajectory calculations were completely different on the two surfaces and showed that the dynamics of this exothermic reaction are sensitive to features of the surface at large internuclear distances. Reactions were found to occur by two possible mechanisms. In one mechanism, the oxygen atom abstracts a hydrogen atom from the end of the molecule in a direct reaction. Alternatively, the oxygen atom inserts into the H–H bond to form a collision complex which subsequently breaks apart. At all collision energies, the vibrational distribution of products from insertion/decomposition events is statistical while that of abstraction events is centered about v′ = 2. An inversion in the total populations of the levels v′ = 1 and v′ = 2 is observed at a collision energy of 5.0 kcal mol. Insertion/decomposition reactions lead to hot rotational distributions of products. A transition from cold rotational distributions at low collision energies to a hot distribution at the highest collision energy is observed for abstraction reactions. At low collision energy, the differential cross section for insertion/decomposition reactions indicates a collision complex is formed whose lifetime is less than the rotational period. As the collision energy increases, a transition to the formation of a long-lived complex occurs.
Kinematic (mass) effects in reactions of the type H+H
L→H
H+L
Authors: Siegel Augustin , Schultz Arnulf
Published: February 2003

To get a better understanding of reactions such as Ba+HCl→BaCl+H 3D trajectory calculations of the type H→HL→HH+L (H = heavy, L = light) were performed on a number of different model potentials, at different collision energies. The study was mainly devoted to a systematic search for kinematic effects caused by the special mass combination. In (H→HL) reactions, kinematics play a dominant role in the reaction probability and in the impact parameter dependence of the product molecule energies. It is therefore possible to draw further information from direct experimental results. All kinematic effects found can be simply explained by the relatively small moment of inertia of the light particle. Some insight into the dynamics of such reactions is also obtained.
Deuteron quadrupole coupling in hydrogen bonded molecules. V. Relationship of experiment to theory
Authors: Clymer J. W. , Goldstein N. , Ragle J. L. , Reed E. L.
Published: February 2003

The purpose of this article is to examine experimental data for deuterium quadrupole coupling in a number of hydrogen bonded systems in the context of approximate, but moderately accurate, ab initio calculations of the field gradient, and to suggest a quantitative interpretation of experimental data in terms of the modification of the local electronic structure on hydrogen bonding. Systems discussed include HF, dimers of HF in the solid configuration and in the geometry of the gas phase complex, HCl, dimers and clusters of HCl in the geometry of the orthorhombic solid, formic acid, maleic acid, and several other carboxylic acids, and salts of pyridinium ion. For the case of DF, for which a plethora of calculations is available, computations of the deuterium field gradient at the STO-3G, −4/31G, −6/31G*, and −6/31G** are presented in the context of previous work. Hydrogen bonding has, according to the calculations, a substantial effect upon the magnitude of the field gradient and a much smaller effect upon its principal axis directions. According to these calculations, the deuterium coupling constant in the gas phase dimer in the interior position should be depressed by ∼ 5% from the monomer value, while the depression of the coupling constant at the exterior position should be less than 1%. In a model dimer at the solid geometry, the former is depressed by ∼ 10%. In solid DCl, these effects are somewhat smaller, of the order of 5%, but observable. For the other species examined in this investigation, effects of hydrogen bond formation on coupling constant are also substantial and always in the direction of decreasing field gradient. It is found that although the absolute values of the field gradient are strongly basis dependent, differences are remarkably constant. Calculations at STO-4/31G seem to predict the changes observed on hydrogen bonding to good precision. The partition of these changes into variation with bond length and with charge distribution is discussed. Cross-sectional charge density difference maps are presented for a variety of cases and discussed in terms of the changes which occur at constant distance. A normal feature which results from hydrogen bonding is depletion of charge at the hydrogen, accompanied by a strong accretion in the bond region. A very small buildup of charge is also found in the hydrogen bond region and other manifestations of polarization are evident in the density difference maps as well.
The photodissociation cage effect in van der Waals complexes: Fluorescence spectra of I2 B(3Π0+u) from the hindered photodissociation of I2Ar at 488 nm
Authors: Valentini James J. , Cross Jon B.
Published: February 2003

The B→X fluorescence from I2 produced in the hindered photodissociation of I2Ar at 488 nm has been resolved. The spectra show several vibrational progressions with low rotational energy, the most prominent of these is a (v′,0) progression extending from v′ = 49 to at least v′ = 23. The I2Ar complexes were excited and the I2 fluorescence was observed under collision-free conditions in a supersonic free-jet expansion. We attribute these observation to one-atom photodissociation cage effect in the I2Ar comples, in which the molecular iodine, although excited more than 400 cm−1 above B state dissociation limit, is prevented from dissociating by energy transfer to the argon atom, resulting in the breaking of the van der Waals bond: I2Ar→I2(B)+Ar, leaving the iodine molecule in a bound level of the B electronic state, 386–1889 cm−1 below the dissociation limit. (AIP)
Identification of the many-line visible emission spectrum of BaO
Authors: Reuther James J. , Palmer Howard B.
Published: February 2003

The identity of the electronic transitions giving rise to the low-pressure (0.1–10 mTorr) ’’many-line’’ visible emission produced by the reactions of Ba(g)+N2O or O3 → BaO*+N2 or O2 has been elucidated by direct spectroscopic observations. Experiments that provided strong evidence regarding the origin of this complex, headless emission consisted of spectrally resolving BaO* (* denotes electronic excitation) visible chemiluminescence (4100–5500 Å) at pressures of 5–10 mTorr as the barium vapor–triatomic oxidant diffusion flame stoichiometry was switched from metal-rich to oxidant-rich. Under oxidant-rich conditions, excess N2O partially relaxed the barium monoxide emission to reveal that the nascent excited products of the reaction included both the A′ 1Π and A 1Σ+ electronic states of BaO. Identification of specific A′ → X 1Σ+ and A → X 1Σ+ vibrational transitions and analysis of their relative intensity distributions led to the conclusion that the complexity of the many-line spectrum is primarily the result of direct bimolecular reaction of barium vapor with nitrous oxide or ozone to produce two chemiluminescent electronic states, one (A 1Σ+ → X 1Σ+) intense and rotationally hot, and the other (A′ 1Π → X 1Σ+) weak in intensity and partially rotationally relaxed.
The millimeter and submillimeter spectrum of NO2: A study of electronic effects in a nonsinglet light asymmetric rotor
Authors: Bowman Wayne C. , De Lucia Frank C.
Published: February 2003

425 new lines of 14N16O2 have been observed by high resolution microwave techniques in the spectral region between 100 and 600 GHz. These data have been analyzed and fit to within experimental uncertainty (∼0.05 MHz). This analysis provides accurate predictions of the results of a number of previous experiments as well as an accurate map of the ground state energy level structure of 14N16O2 over a wide range of states.
The unimolecular reaction of isolated CF3CN: The influence of laser fluence/intensity on the rovibronic excitation of CN(X 2Σ+) produced via infrared multiple photon dissociation
Authors: Reisler H. , Kong F. , Wittig C. , Stone J. , Thiele E. , Goodman M. F.
Published: February 2003

Measurements of nascent CN(X 2Σ+) rovibronic state distributions following the unimolecular reaction CF3CN‡→CF3+CN are reported. Excitation under collision free conditions is provided by IR multiple photon excitation using the focused output from a CO2 TEA laser at fluences 3–150 J cm−2. At fluences 3–20 J cm−2, the CN(X 2Σ+,v′′ = 0) rotational temperature increases monotonically from 500 to 1200 K with increasing fluence, while at fluences 30–150 J cm−2, the rotational and vibrational temperatures do not change and are TR = 1200±100 K and TV = 2400±150 K. At low fluences, the increase in V, R, T excitations with increasing fluence reflects an increase in the vibrational excitation of the dissociating parent molecules. The ultimate level of excitation that the parent molecules attain depends both on the fluence and the intensity of the laser, and the appearance time of the CN fragments decreases with increasing fluence, since molecules excited significantly above dissociation threshold decompose more rapidly than those near reaction threshold. At fluences ≳30 J cm−2, the sample is depleted before the CO2 laser output reaches its peak intensity, thus causing the observed ’’saturation’’ effect in product excitation level. A straightforward model which includes effects due to laser pulse shapes can account for the experimental observations.
Anisotropy of the nuclear spin coupling in PH−2, PH3, and PH+4 molecules
Authors: Lazzeretti Paolo , Rossi Elda , Taddei Ferdinando , Zanasi Riccardo
Published: February 2003

The anisotropy of the nuclear spin–spin coupling tensor, relative to P, H and H, H interacting nuclei, has been investigated by means of extended ab initio calculations in the molecules PH−2, PH3, and PH+4. The coupled Hartree–Fock perturbed scheme has been employed retaining the nonrelativistic Ramsey Hamiltonians. The most important contributions to the anisotropy in the P, H coupling arise from the Fermi contact-spin-dipolar cross term, which is partially counterbalanced by the spin-orbit effect. The discrepancies emerging between computed and experimental J(P,H) in phosphine and phosphonium are interpreted in terms of neglected electron correlation contributions. A dramatic sensitivity of the Fermi contact term of the molecular geometry has been found for PH2−. The calculations show that the P–H coupling is more anisotropic than N–H, which is seemingly due to the enhanced nonsphericity of electron distribution.
Energetic considerations of the vibrational potential function in the effective nuclear charge model. I
Authors: Ohwada Ken
Published: February 2003

The properties of the vibrational potenital function assumed in the effective nuclear charge (ENC) model are investigated in detail with a view to predict the total molecular energies of polyatomic molecules. Especially, the R-independent term (R: internuclear distance) in the potential function, which plays an important role in determining the total molecular energies, is analyzed with the use of the March–Parr scaling function and its analytic form is derived. Some numerical tests for diatomic as well as polyatomic molecules are made on the energetic equations taking into account this analytic form.
Theoretical studies of surface diffusion: Self-diffusion in the fcc (111) system
Authors: Doll J. D. , McDowell H. K.
Published: February 2003

Surface self-diffusion constants for the fcc (111) Lennard–Jones system were calculated using classical molecular dynamics methods over the reduced temperature range 0.11<kBT<0.22. In this range the diffusion constant exhibited Arrhenius behavior with pre-exponential factor and activation energy given by D0(m/εσ2)0.5 = 0.359 and Ea/ε = 0.398, respectively. These results are compared with available experimental data for rhodium.
Dynamics of liquid state chemical reactions: Modeling of R-dependent correlation functions
Authors: Brooks C. L. , Adelman S. A.
Published: February 2003

Methods for the simplified construction of those reagent configuration point r0-dependent static and dynamic solvent correlation functions required to implement the molecular time scale generalized Langevin equation (MTGLE) approach to problems in liquid state chemical reaction dynamics [S. A. Adelman, J. Chem. Phys. 73, 3145 (1980)] are presented. These methods permit one to bypass straightforward but laborious construction of the correlation functions via molecular dynamics simulations of the solvent motion about the solute for many solute configurations r0. The methods are based on two approximations: (i) The matrix r0-dependent friction kernel of the reagents β1(t;r0) is modeled via a matrix generalization of the conventional Gaussian approximation for scalar time correlation functions. This modeling permits one to reduce the problem of constructing β1(t;r0) to an equilibrium calculation which may be carried out by Monte Carlo as opposed to molecular dynamics techniques. (ii) The equilibrium r0-dependent solvent correlation functions required to construct the Gaussian approximation to β1(t;r0) are calculated using the superposition approximation. This permits one to construct β1 (t;r0), and hence the MTGLE chain models, from equilibrium solute–solvent and solvent–solvent pair correlation functions. For simple liquids, the pair correlation functions may be obtained as solutions to the Percus–Yevick integral equation. The simplified methods are applied to construct the MTGLE chain models for three Lennard-Jones systems designed to simulate infinitely dilute solutions of molecular iodine in: liquid carbon tetrachloride, dense gaseous ethane, and liquid ethane. The results are tested against molecular dynamics for the I2 in CCl4 model solution.
Phosphorescence and the interphenyl interactions in the tetraphenyl group IV B compounds
Authors: Saigusa Hiroyuki , Azumi Tohru
Published: February 2003

The high resolution phosphorescence spectra are observed at 4.2 and 1.2 K for a series of tetraphenyl group IV B compounds Ph4X (X = C, Si, Ge, Sn, and Pb). The phosphorescence bands are split into doublet or triplet structures as a result of the interaction among the four phenyl groups in a molecule. The intensities of the multiplet components differ vastly with temperature. The analysis of this temperature dependence indicates that the populations of the emitting levels are governed by the Boltzmann distribution law. The decay of the phosphorescence is, in every case, expressed as a single exponential function. All of these experimental results indicate that the relaxation among the intramolecular Davydov states is quite efficient. This conclusion differs from the notion previously supposed by several authors. The high resolution phosphorescence spectra are observed at 4.2 and 1.2 K for a series of tetraphenyl group IV B compounds Ph4X (X = C, Si, Ge, Sn, and Pb). The phosphorescence bands are split into doublet or triplet structures as a result of the interaction among the four phenyl groups in a molecule. The intensities of the multiplet components differ vastly with temperature. The analysis of this temperature dependence indicates that the population of the emitting levels are governed by the Boltzmann distribution law. The decay of the phosphorescence is, in every case, expressed as a single exponential function. All of these experimental results indicate that the relaxation among the intramolecular Davydov states is quite efficient. This conclusion differs from the notion previously supposed by several authors.
Polyatomic Raman scattering for general harmonic potentials
Authors: Tannor David J. , Heller Eric J.
Published: February 2003

Lee and Heller’s time-dependent theory of resonance Raman scattering is reviewed. This theory is formally identical to the traditional Kramer–Heisenberg–Dirac (KHD) theory but, in its wave packet interpretation, the time-dependent theory has distinct calculational and conceptual advantages over the KHD sum-over-states method. For polyatomics with large Franck–Condon displacements and Duschinsky rotations, where typically the KHD sum is over 1010 states with complicated Franck–Condon factors, these advantages are most pronounced. Wave packet propagation on general harmonic potential surfaces (Franck–Condon displacement, frequency shifts, and Duschinsky rotation) is exact. Formulas for the propagated wave packet are given for various levels of harmonic sophistication. The role of symmetry in the wave packet dynamics is discussed and explicit formulas are derived for the overlap of the moving wave packet ‖φi(t)〉 with the final state of interest ‖φf〉. The half Fourier transform of this overlap gives the Raman amplitude α. The transform method of Tonks and Page, relating absorption and Raman excitation profiles, is shown to arise naturally in our approach. We show excitation profiles calculated by the time-dependent theory for multidimensional harmonic potential surfaces with and without Duschinsky rotation. For the no-Duschinsky cases, we compare our results with the profiles of Myers and Mathies and of Champion and Albrecht, which were calculated by a sum-over-states; we then discuss some discrepancies between the latter’s results and ours.
Molecular beam photodissociation of trimethylamine
Authors: Kawasaki M. , Kasatani K. , Sato H. , Shinohara H. , Nishi N. , Ibuki T.
Published: February 2003

A molecular beam of trimethylamine was photodissociated at 193 nm and both the time-of-flight and angular distributions of photofragments were measured. The isotropic angular distribution observed indicates that the dissociative lifetime is much longer than the molecular rotation period. The broad time-of-flight distribution is well represented by a statistical distribution of unimolecular decomposition. The averaged translation energy was 13% of available energy. The photodissociation of trimethylamine at 193 nm is shown to be dissociative through the hot ground state.
On determining conditions for RRKM behavior in conservative dynamical systems
Authors: De Leon Nelson , Berne Bruce J.
Published: February 2003

It is shown that a simple linear analysis on certain elliptic fixed points can be used to determine when a system will not exhibit a rate constant that is well approximated by RRKM theory. This method is used to locate those regions in parameter space where RRKM theory is expected to be valid.
Nonexponential spin-lattice relaxation from hindered rotations in dipolar solids
Authors: Pratt J. C.
Published: February 2003

A general formalism is presented for the computation of the longitudinal magnetization in a broad class of molecular crystals allowing for the effects of cross correlations between the motions of different spin–spin vectors. These effects will manifest themselves, in general, in a departure from exponentiality of the relaxing magnetization as a function of time. The molecular motion concerned is that of random, thermally-activated. hindered reorientations about the available symmetry axes. Any combination of available reorientations is covered by the theory, provided only that the motion preserves the complete equivalence of all the rij. The present work is essentially concerned with outlining the principles of the calculation, but to demonstrate the formalism, one simple example, amenable to analytic solution, is examined in detail.
Collision induced absorption spectra of the fundamental band of gaseous deuterium: Overlap parameters of D2–D2 collision pairs
Authors: Penney R. J. , Prasad R. D. G. , Reddy S. Paddi
Published: February 2003

The collision induced absorption of the fundamental band of D2 in the pure gas was recorded for a number of gas densities up to 60 amagat at 77 and 196 K with a 2 m absorption cell and at 298 K with a 2.1 m absorption cell. Binary and ternary absorption coefficients of the band have been determined from the measured integrated absorption coefficients. An analysis of the absorption profiles was carried out by assuming appropriate line shapes and the halfwidth parameters δd and δc of the short-range overlap induced transitions and δq of the long-range quadrupole induced transitions have been determined. The profile analysis also made it possible to separate the contributions of these two types of transitions to the total intensity of the band. For the D2–D2 collision pairs, the overlap parameters λ and ρ, which represent the magnitude and range, respectively, of the overlap induced dipole moment, and μoverlap(σ), the overlap induced moment at the Lennard-Jones intermolecular diameter σ, have been determined; these are λ = 4.0×10−3, ρ = 0.21 Å, and μoverlap(σ) = 2.2×10−2 ea0 and are compared with those of the H2–H2 and HD–HD pairs derived from earlier work in our laboratory [S. P. Reddy et al., Phys. Rev. A 15, 975 (1977); S. P. Reddy and R. D. G. Prasad, J. Chem. Phys. 66, 5259 (1977)].
On self-trapping of CO2 (ν3) fluorescence
Authors: Caledonia G. E. , Green B. D. , Murphy R. E.
Published: February 2003

Spectrally resolved measurements are presented for ν3 band fluorescence of CO2. A pulsed 30 KeV electron beam is used to excite the fluorescence of CO2 in a low pressure chamber. The fluorescence is viewed at right angles. A comparison between measured spectrum and predicted spectrum is presented. (AIP)
Scaling theory and enthalpy of mixing for binary fluids
Authors: ten Brinke G. , Karasz F. E.
Published: February 2003

According to a classical result for binary mixtures one can, from the existence of lower and upper critical solution temperatures, draw resonable conclusions concerning the signs of the enthalpy of mixing. A crucial assumption in the various derivations of this result is the possibility of an expansion of the free energy in a Taylor series at the critical point. We give an alternative derivation based on the modern theory of critical phenomena.
The nature of dilute solutions of sodium ion in water, methanol, and tetrahydrofuran
Authors: Chandrasekhar Jayaraman , Jorgensen William L.
Published: February 2003

Monte Carlo statistical mechanics simulations have been carried out for dilute solutions of Na+ in water and tetrahydrofuran (THF) at 25 °C and 1 atm. The intermolecular interactions were described by Lennard-Jones and Coulomb terms in the TIPS format including the TIPS2 parameters for water–water interactions. In conjunction with previous simulation results for Na+ in methanol, the present study provides detailed insights into the nature of ionic solvation by dipolar protic and aprotic solvents. In agreement with x-ray data, the coordination number of Na+ in water is six, identical to the value obtained for Na+ in methanol. However, the coordination number of Na+ in THF fluctuates between five and six. Consistently, the ion–solvent interaction is less exothermic in THF compared to the other two solvents. However, the heat of solution which is the sum of two large opposing contributions, viz., the ion–solvent energy and the solvent reorganization energy, does not differ much for the three solvents. In all three cases, the formation of the first solvation shell accounts for all the solvent structure breaking. In particular, the number of hydrogen bonds formed by water and by methanol beyond the first shell rapidly attains the value in the pure solvent. The computed radial distribution functions and bonding and dimerization energy distributions provide additional support for this analysis.
Chain ordering and molecular orientational order in liquid crystals
Authors: Samulski Edward T. , Dong Ronald Y.
Published: February 2003

We have extended the scheme for computing deuterium quadrupolar splittings of a flexible solute in a nematic solvent [Ferroelectrics 30, 83 (1980)] to study chain ordering in the 4-n-alkyl-4′-cyanobiphenyl liquid crystals (nCB). All possible configurations {φ} of the nCB molecule are generated by the rotational isomeric state model of Flory. Each {φ} has a separate ordering matrix whose diagonal elements are assumed to scale with the shape anisotropy of the molecule as prescribed by the principal moments of inertia of the configuration I{φ}. Two models for identifying the molecular-fixed-frame are considered: (1) The frame diagonalizes I{φ}; (2) the frame is stationary on the nCB core. The variation of deuterium quadrupolar splittings along the alkyl chain can be satisfactorily explained by the calculations, however, the detailed behavior of the computed splittings are sensitive to the assumptions about the placement of the molecular-fixed-frame.
The shock induced electrical polarization of water
Authors: Harris Paul , Presles Henri-Nöel
Published: February 2003

Experiment and theory for shock wave induced electrical polarization signals in water have now been carried out. The consistent interpretation of the results is (i) a shock front rise time of ∼ 10−12 s throughout the shock pressure range of 100 to 35 kbar (3.5 Pa) and probably down to at least 6 kbar, (ii) a transition in the source of the dominant polarization voltage to the alignment of unbonded dipole moments at ∼ 35 kbar, and (iii) in the unbonded region, the average contributing dipole is perfectly aligned in the direction of shock propagation. Additionally, in the unbonded pressure range, the calculated concentration of unbonded molecules is approximately equal to the hydrostatic equivalent concentration of ions.
Dielectric relaxations in a supercooled liquid and glassy smectic phase
Authors: Johari G. P. , Goodby J. W.
Published: February 2003

The real and imaginary parts of the complex dielectric permittivity of the supercooled smectic G phase of N-(4′-n-pentyloxybenzylidene)-4-n-butylaniline have been measured from 50 Hz to 200 kHz and 77 to 260 K. The glass transition temperature of the smectic phase is 181 K. Two relaxation regions were observed; one above and the second below Tg, designated as α and β relaxations, respectively. The half-width of the spectrum for both relaxations increases on cooling. The rate of α relaxation follows the Vogel–Fulcher–Tamman equation, but that of β relaxation an Arrhenius equation with an activation energy of 14 kJ mol−1. The increase in static dielectric permittivity on cooling is consistent with Curie law and indicates no change in the orientational correlation with temperature. The dielectric relaxation in the smectic phase is similar to that observed in amorphous polymers, rigid molecular glasses, nematic and cholesteric glasses, and the glasslike state of orientationally disordered crystals. This suggests that the molecular mechanism of relaxations in the glassy state is less dependent upon the presence of internal degrees of freedom of molecules or the degree of order in their arrangement than is generally considered.
Theory of the refractive index of fluids
Authors: Ho/ye J. S. , Stell G.
Published: February 2003

The Ho/ye–Stell statistical mechanical treatment of dielectrics is extended to yield a quantitatively useful theory of the refractive index and related aspects of light scattering.