bluuebunny/crossref_metadata_2025 huggingface dataset:
Reinvestigation of the acetylenic C–H stretching fundamental of propyne via high resolution, optothermal infrared spectroscopy: Nonresonant perturbations to ν1
Authors: Kerstel E. R. Th. , Lehmann Kevin K. , Pate Brooks H. , Scoles G.
Published: July 2002

We present the high resolution spectrum of the ν1 fundamental of propyne near 3335 cm−1 obtained using a very warm free jet expansion in our optothermal detection spectrometer. By using a high concentration sample expanded at low backing pressures we have been able to observe transitions for K values up to K=6. The additional data available allow us to reinvestigate this vibrational band. We find an unusual perturbation pattern in this band where the individual subbands (rovibrational transitions for a single K value) appear to be completely unperturbed at the level of precision of our data (7.5 MHz), but the subband origin orderings are perturbed through nonresonant interactions. Attempts to account for the subband ordering using a two-state anharmonic interaction are unsuccessful indicating that the perturbations are of multistate origin. This type of nonresonant perturbation to the subband origins of symmetric top molecules should be a common feature of symmetric tops with large A rotational constants. As a result of this investigation we conclude that the previously reported value of αA, determined from a very cold expansion where only K=0 and K=1 were observed, is not a measure of the true (unperturbed) value of this constant. This conclusion is also supported by force field calculations presented here that use an empirical harmonic force field augmented by diagonal anharmonicities for the hydride stretches. These calculations, which reproduce measured values of αA and αB for lower energy bands quite successfully, also show that the previous determination of αA is too large and must be dominated by perturbation contributions. We have also measured the weak Fermi resonant band ν3+2ν09 which acquires its intensity through interaction with ν1. Again we find an anomalous subband ordering like that observed in ν1.
Monte Carlo study of associative polymer networks. II. Rheologic aspects
Authors: Groot Robert D. , Agterof Wim G. M.
Published: July 2002

The rheological behavior of associative polymer networks has been studied by direct Monte Carlo simulation. To describe the associations, an algorithm is used which for dilute monatomic systems reduces to the well-known mass action law. To describe the polymers, we adhere to the simplest possible model, namely hard beads connected by harmonic springs. On applying an external field the gel is stretched, and generally it starts to flow. When the flow velocity is plotted against the applied stress, we find Newtonian (weak gel) flow behavior if the association constant is small, and a gradual cross-over to nonlinear (strong gel) flow behavior when the association constant is larger. Furthermore, the applied flow is observed to induce gelation, hence for weak association constants we find shear thickening, whereas for strong association constants stress overshoot, thixotropic behavior and shear thinning are found. Thus all phenomena commonly observed in experiments seem to be reproduced qualitatively by the simple model. Furthermore, as a complement to existing analytic theories, the simulations are used to evaluate the chain-breaking rate function, and to measure the bond-length distribution under conditions of high shear.
Lattice dynamics of semirigid molecules
Authors: Kirin D.
Published: July 2002

A new approach to the lattice dynamics calculations for the crystals built up from semirigid molecules is presented. The rigid body approximation often used in the calculation of the lattice dynamics of molecular solid is extended to the molecules built from several rigid subunits (polyphenyles, etc.). The intermolecular force field is calculated from the atom–atom potential function. The intramolecular force field describing the dynamics of the rigid subunits within the molecule is calculated in the frame of the central force field model (CFF), i.e., assuming only the stretching coordinates between certain bonded and nonbonded atoms within the molecule. The parameters (force constants) are fitted in order to reproduce the observed low vibrational frequencies of the free molecule. The method is applied to the lattice dynamics of biphenyl, 2,2′-bipyridine and s-trans-bicyclopropyl. The possibility of the application of the present method in the calculation of Raman and coherent inelastic neutron scattering intensities is discussed.
Quantum rearrangement scattering calculations using the invariant imbedding method
Authors: Band Y. B. , Tuvi I.
Published: July 2002

We derive an algorithm to calculate quantum transmission and reflection amplitudes (S-matrix elements) for rearrangement reactions using the invariant imbedding method. Our approach can incorporate closed channels, treat complex potentials (to simulate decay), and can be used within an adiabatic representation (in which derivative coupling terms are present) or within a diabatic representation. The method can also be used to determine bound-state energies when only closed channels are present. The method is demonstrated on a rearrangement problem with two coupled potential surfaces (four channels) as a function of energy in the energy region from the lowest channel threshold through the highest channel threshold.
Coherent and dissipative wave packet dynamics in cyclic model systems with four equivalent potential minima
Authors: Brackhagen O. , Kühn O. , Manz J. , May V. , Meyer R.
Published: July 2002

The dynamics of cyclic systems with four equivalent potential minima is studied here from two different points of view. The solution of the time-dependent Schrödinger equation provides insight into the coherent wave packet motion. The resulting reaction mechanism involves relocalization between opposite, not neighboring potential minima. The inclusion of an environment within a density matrix description leads to dissipation and therefore to a transition from coherent to incoherent dynamics. The theoretical considerations are applied to a simple model of the cyclic motion of a proton in a molecular framework.
Far infrared laser magnetic resonance detection of CHD (X̃ 3A″)
Authors: Nolte J. , Temps F. , Wagner H. Gg. , Wolf M. , Sears T. J.
Published: July 2002

Far infrared laser magnetic resonance (FIR-LMR) spectra of the CHD radical in its X̃3A″ electronic ground state were observed and assigned. The radicals were generated in the reaction of Na atoms with CHDBr2. LMR spectra were observed using seven laser lines at wavelengths around 100 to 200 μm. The spectra were assigned to six different rotational transitions and the molecular parameters of CHD were determined by a least squares fit. A number of additional transitions, observed using a laser line at λ=184.3 μm but not yet assigned in detail, were attributed to a coupling between the accidentally almost degenerate NKaKc=505 and 413 rotational levels induced by the εab term in the spin–rotation Hamiltonian and by the off-diagonal components of the hyperfine coupling tensor.
Vibrational spectroscopy of NO and (NO)2 isolated in solid neon
Authors: Kometer R. , Legay F. , Legay-Sommaire N. , Schwentner N.
Published: July 2002

By highly resolved infrared absorption spectra the dependence of aggregation of NO in neon on concentration, annealing, and deposition temperature is studied in recording the intensities of monomers in two sites (1874.54 and 1877.56 cm−1), of cis-(NO)2 dimers in the symmetrical (around 1866 cm−1) and antisymmetrical (around 1780 cm−1) mode, of a special dimer around 1858 cm−1 and a series of monomer side bands shifted by about 0.3, 0.6, and 1.8 cm−1 due to coupling of molecules at different lattice sites. The dimer bands also exhibit a fine structure and a broad background caused by larger aggregates. The almost statistical size distribution at low concentration and condensation temperature changes to a preferential aggregation at higher concentration (≳2×10−3) and condensation temperature (≥7 K) and the irreversible aggregation by diffusion at elevated temperatures is followed on a time scale of hours. A reversible conversion of special dimers at 1778.67 and 1865.48 cm−1 to a dimer at 1857.93 cm−1 is accelerated by lowering the temperature and attributed to a martensitic hcp to fcc phase transition.
Inversion for dielectric relaxation spectra
Authors: Morgan Frank Dale , Lesmes David P.
Published: July 2002

A new least squares inversion algorithm is used to invert the real part of dielectric data for a spectrum of relaxation times. This inverse problem is inherently unstable; however, by transforming the linear inverse problem into log-space, the inversion iterates towards the true solution. Inversion of the theoretical distributions of Debye, Cole–Cole, and Davidson–Cole, show that this inversion method is stable, even when up to 5% of Gaussian noise is added to the data. Inversion of dielectric measurements on water, n-pentanol alcohol, and Morrison sandstone, illustrate the ability of this method to invert for relaxation-time distributions of arbitrary shape.
Solid state proton spin relaxation and methyl and t-butyl reorientation
Authors: Beckmann Peter A. , Al-Hallaq Hania A. , Fry Anne M. , Plofker Amy L. , Roe Brian A. , Weiss Jessica A.
Published: July 2002

We have measured the temperature T and Larmor frequency ω/2π dependence of the proton spin-lattice relaxation rate R in solid 1-hydroxy-2,4,6-tri-butylbenzene. The data is interpreted in terms of the rotational motion of the t-butyl groups and their constituent methyl groups. Our data is much more extensive than a previous report [J. Yamauchi and C. A. McDowell, J. Chem. Phys. 75, 1051 (1981)] resulting in a revised dynamical model and considerably larger rotational barriers. Interesting thermal history effects are discussed.
Three-dimensional analytical infinite order sudden quantum theory for triatomic photodissociation: Dependence on initial rotational and vibrational state and on thermal averages for NOCl dissociation on T1(1 3A″) surface
Authors: Grinberg Horacio , Williams Carl J. , Freed Karl F.
Published: July 2002

Our previously developed analytical infinite order sudden quantum theory of triatomic photodissociation is generalized to compute fragment internal energy distributions when the initial triatomic rotational state has K≠0. The dependence of product rotational energy distributions on initial rotational and vibrational state is illustrated through model computations for the direct NOCl photodissociation from the ground to the T1(1 3A″) potential energy surface. The calculations consider all J,K≤9 and employ a repulsive potential that is fit to ab initio computations. Comparisons of fragment rotational distributions with previous semiclassical approximations further elucidate the role of the mapping of the initial state bending wave function onto the fragment rotational distributions and the influence of parent rotations on this mapping. The infinite order sudden quantum-mechanical distributions exhibit a more complex structure, but upon thermal averaging they are already transformed at T=3 K into fairly broad rotational distributions. The present theory readily permits the calculations of energy distributions for initial states of high J and K.
Photodissociation of CH2: A test case for the light–heavy–light approximation
Authors: Kroes Geert-Jan
Published: July 2002

The validity of the light–heavy–light (LHL) approximation for calculating absorption spectra and partial photodissociation cross sections has been assessed for photodissociation of H2O, CH2, and CD2 through their first absorption bands. For this purpose, results of full 3D (three-dimensional) calculations were compared with 3D LHL and 2D LHL results. The 3D LHL approximation works well for calculating absorption spectra and cross sections which are resolved with respect to the final rotational state of the fragment. However, the fragment vibrational distributions calculated using the 2D and 3D LHL approximations are too warm. For the v=0–3 partial cross sections, the LHL approximation gives good results for H2O and reasonable results for CH2, but breaks down when applied to CD2. Using time-independent perturbation theory, it is found that the colder vibrational fragment distributions obtained using full 3D dynamics arise mostly from the influence of one particular coupling term. This term acts to push the wave packet into the dissociation channel earlier, thereby decreasing the vibrational excitation which results from both oscillators being stretched simultaneously in the molecule’s motion towards the saddle point. The size of this term is proportional to the cosine of the bond angle divided by the mass of the central atom. Therefore, in assessing whether the LHL approximation will be valid for photodissociation of a particular molecule, both the mass of the heavy atom and the equilibrium bond angle should be considered, and CH2 is a ‘‘worse case LHL molecule’’ mostly because its ground state equilibrium bond angle (134°) is larger than that of H2O (104°).
Bound-state wave functions from coupled channel calculations using log-derivative propagators: Application to spectroscopic intensities in Ar–HF
Authors: Thornley Alice E. , Hutson Jeremy M.
Published: July 2002

A method for obtaining wave functions from coupled-channel bound state calculations using log-derivative propagators is presented. Bound states occur at energies for which the inward and outward log-derivative solutions match at a central point in the propagation; at such energies, the log-derivative matching matrix has one eigenvalue which is zero. The wave function at the matching point is the eigenvector corresponding to this zero eigenvalue. The wave function at other points can be obtained by back-substitution in the log-derivative propagation equations. The method is tested by calculating infrared intensities for the Ar–HF van der Waals complex, using the H6(4,3,2) potential.
Investigations into the common ion effect
Authors: Valdeavella C. V. , Perkyns John S. , Pettitt B. Montgomery
Published: July 2002

The molecular origins of the common ion effect and the salting out of nonpolar molecules from aqueous solutions are investigated. Thermodynamic stability criteria for a common ion mixture in a polar solvent are derived. Kirkwood–Buff statistical thermodynamics is used to make the connection with the microscopic pair correlation functions. The observed sensitivity of the compositional stability with respect to ionic strength indicates that a demixing transition is the primary cause of the instability for the common ion effect for our model Lennard-Jones plus Coulomb Hamiltonian.
An ab initio molecular orbital study of the structures and energetics of the neutral and cationic CuO2 and CuNO molecules in the gas phase
Authors: Hrušák Jan , Koch Wolfram , Schwarz Helmut
Published: July 2002

The interactions of neutral and ionic copper atoms with molecular oxygen and nitric oxide have been examined by using relativistic effective core potentials (ECP) and different ab initio MO methods up to the coupled cluster CCSD(T) approach. For neutral CuO2 the ECP calculations reproduce the results of all-electron calculations which prefer the ‘‘side-on’’ structure by 0.9 kcal/mol versus an ‘‘end-on’’ coordination. The binding energy for the ‘‘side-on’’ complex (2A2) has been calculated to be 9.7 kcal/mol. For the cationic end-on CuO+2 (3Σ+) an identical binding energy (9.7 kcal/mol) results from CCSD(T) calculations; the side-on complex (3B1) is 4 kcal/mol less stable. The singlet state (1A′) of the cation is 27 kcal/mol higher in energy and bound with 13.6 kcal/mol with respect to singlet oxygen (1Δ+g) and Cu+ (1S), while the side-on (1A1) state is 3.8 kcal/mol less stable than the 1A′ state. For the interaction of neutral Cu with NO the end-on CuNO coordination (1A′) with a bonding energy of 10.4 kcal/mol is preferred while the related triplet (3A″) is 5.5 kcal/mol less stable. In the cationic system [Cu,N,O]+, both isomers, i.e., CuNO+ and CuON+ represent stable species, with CuNO+ (2A′) being the more stable one. The bond dissociation energies of these two isomers are calculated to 19.3 and 11.4 kcal/mol, respectively.
Forces between polyelectrolyte coated surfaces in the presence of electrolyte
Authors: Woodward C. E. , Åkesson Torbjörn , Jönsson Bo
Published: July 2002

The interaction between charged surfaces bearing terminally attached polyelectrolyte chains is investigated using Monte Carlo computer simulations and a mean-field theory. In particular we focus on the role played by the addition of a simple electrolyte component. It is found that the force between the surfaces increases with increasing electrolyte concentration because of a repulsive osmotic pressure contribution due to the small ions, while the attractive bridging component is insensitive to added salt. Considering the approximate nature of the mean-field (polyelectrolyte Poisson–Boltzmann) theory it compares well with the computer simulations.
Comment on ‘‘Temperature programmed desorption of molecular hydrogen from a Si(100)-2×1 surface: Theory and experiment’’ [J. Chem. Phys. 99, 7038 (1993)]
Authors: Nachtigall P. , Jordan K. D.
Published: July 2002

The mechanism of H2 desorption from the Si(100)2×1 surface is discussed. Based on density functional calculations, it is concluded that the energy of the Si–Si surface dimer bond is about 37 kcal/mol, which is significantly larger than was assumed in a recent study [M. C. Flowers, N. H. B. Jonathan, Y. Liu, and A. Morris, J. Chem. Phys. 99, 738 (1993)] employing the lattice gas model. It is proposed that surface defects, in particular isolated surface Si atoms, play an important role in the desorption mechanism.
Response to ‘‘Comment on ‘Temperature programmed desorption of molecular hydrogen from a Si(100)-2×1 surface: Theory and experiment’ ’’ [J. Chem. Phys. 101, 2648 (1994)]
Authors: Flowers M. C. , Jonathan N. B. H. , Liu Y. , Morris A.
Published: July 2002

Arguments are given for the validity of defining the Si(100):D surface in terms of four basic species and two equilibria. Differences between experimental results and theoretical calculations remain unresolved especially whether hydrogen desorption occurs directly from a dimer pair or via a site defect mechanism.
Molecular spectroscopy with light pulses of arbitrary pulse shape and field strength: A nonperturbative approach
Authors: Ebel Gernot , Schinke Reinhard
Published: July 2002

We present a novel approach to molecular spectroscopy with light pulses of arbitrary strength and duration. The key quantity is the frequency-resolved net energy transfer dE/dω which reveals at which frequencies energy is transferred from the field to the molecule (absorption) or from the molecule to the field (stimulated emission). It is shown that dE/dω can be expressed as the Fourier transform of the cross-correlation function of the molecular polarization P(t) and the time derivative of the applied field, dℰ/dt. In this sense, it is formally equivalent to the absorption cross section under weak-field conditions which, as commonly known, can be represented as the Fourier transform of the autocorrelation function S(t). The time-dependent polarization P(t) is determined by exact integration of the time-dependent Schrödinger equation including the light-matter interaction to all orders. It is shown that under weak-field conditions the expression for dE/dω reduces to the well-known cross section formula in the time-dependent picture of spectroscopy, multiplied by the spectral intensity of the light pulse. Therefore, we consider the expression for the frequency-resolved energy transfer, which is valid for arbitrary electric fields, as the natural extension of the absorption cross section in the weak-field limit. Furthermore, dE/dω is shown to be formally equivalent to the change of the spectral intensity, ΔI(ω), of an optical pulse after transmission through a sample, the latter being derived by solving Maxwell’s equations under well-known approximations. The theory is applied to a simple one-dimensional model with two electronic states and the frequency-resolved energy transfer is investigated as a function of the field strength. For sufficiently strong fields, dE/dω exhibits transitions between essentially all vibrational levels in the ground and all states in the excited electronic manifold. The new expression distinguishes between absorption and emission and that is clearly seen in the spectra.
Experimental investigation of vibrational radiative lifetimes: H2O+ and D2O+ ions in their ground electronic state (X 2B1)
Authors: Heninger M. , Lemaire J. , Mauclaire G. , Fenistein S. , Jullien S. , Marx R.
Published: July 2002

Radiative lifetimes of vibrationally excited H2O+ and D2O+ ions in their ground electronic state (X 2B1) have been determined using the monitor ion technique in a triple cell ion cyclotron resonance spectrometer with Fourier transform detection. The monitor reactions are proton or deuteron transfer from H(D)2O+ to CO2 and N2O. The lifetimes are corrected for collisional deactivation and reactions with the background gases occurring during the relaxation time of the ions. N2O probes all the excited vibrational levels of H2O+ and D2O+. For H2O+ only the bending modes (0,v≥1,0) contribute to the decay curve. The corresponding overall lifetime, 26.8±3 ms, is in very good agreement with the computer simulated overall lifetime including the theoretical lifetimes of Weis et al. [J. Chem. Phys. 91, 2818 (1989)] and estimated populations of the bending vibrational levels. For D2O+, the overall lifetime of the (0,v≥1,0) bending modes, 99.5±15 ms, and the lifetime of the (1,0,0) stretching mode, 27.5±4.5 ms, are observed, also in good agreement with the computer simulated and theoretical values, respectively. For both ions the overall lifetime of the (0,v≥1,0) levels may be considered as a good approximation for the radiative lifetime of the (0,1,0) level. The overall lifetimes determined with CO2 as a monitor may be attributed to the (0,v≥4,0) bending modes: 8.1±1 ms for H2O+ and 44±12 ms for D2O+. In this case, several levels having similar populations and lifetimes contribute to the decay curve, therefore the lifetimes of the individual levels cannot be determined. The agreement with computer simulated lifetimes is an indication for the validity of the theoretical lifetimes.
A theoretical study on the ionization of pyrrole with analysis of vibrational structure of the photoelectron spectra
Authors: Takeshita Kouichi , Yamamoto Yuichi
Published: July 2002

Ab initio calculations have been performed to study on the molecular structures and the vibrational levels of the low-lying ionic states (2A2, 2B1, 2A1, and 2B2) of pyrrole. The equilibrium molecular structures and vibrational modes of these states are presented. The theoretical ionization intensity curves including the vibrational structure of the low-lying two ionic states are also presented and compared with the photoelectron spectrum. A number of new assignments of the photoelectron spectra are proposed.
Cage-to-cage migration rates of Xe atoms in zeolite NaA from magnetization transfer experiments and simulations
Authors: Jameson A. Keith , Jameson Cynthia J. , Gerald Rex E.
Published: July 2002

Xenon trapped in the alpha cages of zeolite NaA exhibits distinct NMR signals for clusters Xe1, Xe2, Xe3,..., up to Xe8. Using multisite magnetization transfer experiments, we have measured the rate constants kmn for the elementary processes that are involved in the cage-to-cage transfer of Xe atoms in the zeolite NaA, that is, for a single Xe atom leaving a cage containing Xen to appear in a neighboring cage containing Xem−1, thereby forming Xem. In a random walk simulation, these rate constants reproduce over a hundred magnetization decay/recovery curves that we have measured in four samples of Xe in zeolite NaA at room temperature, in selective inversion, and complementary experiments for all the significantly populated clusters. The simulations also lead to the correct experimental equilibrium distributions, that is, the fractions of the alpha cages containing Xe1,Xe2,...,Xe8.
Ab initio all-electron Dirac–Fock–Breit calculations for ThF4 using relativistic universal Gaussian basis set
Authors: Malli Gulzari L. , Styszyński Jacek
Published: July 2002

Ab initio all-electron fully relativistic Dirac–Fock–Breit calculations are reported for ThF4 assuming the experimental tetrahedral geometry with the Gaussian nuclear model for the Th and F nuclei. The calculations were performed with our relativistic universal Gaussian basis set, which has been shown to be of Dirac–Fock accuracy for all atoms. The calculated relativistic correction to the total electronic energy of ThF4 is −2150.5 hartrees (−58 518 eV) which is about 9% of its total Hartree–Fock energy. There are also major relativistic corrections to the binding energies of the molecular orbitals, especially for the inner (core) orbitals of ThF4. The magnetic part of the Breit interaction is calculated to be 38.8 hartrees (1056 eV) for ThF4. The results of our ab initio all-electron relativistic calculations, predict the molecule ThF4 to be bound with respect to dissociation into one Th and four F Dirac–Fock atoms. The dissociation energy predicted by our relativistic calculations for ThF4 of 19.34 eV is 70% of the experimental value (27.7 eV) reported by Lau et al. [J. Chem. Phys. 90, 1158 (1989)]. This result is quite remarkable in view of the fact that it was obtained by using single configuration Dirac–Fock self-consistent field wave function for the tetrahedral ThF4. Our NR HF calculations for the tetrahedral ThF4 also predict the molecule to be bound with the predicted dissociation energy of 19.11 eV, which is only 0.23 eV less than that predicted by our relativistic wavefunction. Therefore, although the relativistic correction to the total electronic energy of ThF4 is very significant, its contribution to the binding energy of the molecule is almost negligible (0.23 eV). This is due to the cancellation of the relativistic corrections for the ThF4 molecule and its constituent atoms.
Intersystem crossing dynamics in the spin–crossover systems [M:Fe(pic)3]Cl2⋅Sol (M=Mn or Zn, Sol=MeOH or EtOH)
Authors: Vef Andreas , Manthe Uwe , Gütlich Philipp , Hauser Andreas
Published: July 2002

The intersystem crossing rates from the high-spin (5T2) to the low-spin (1A1) state in the spin–crossover systems [M:Fe(pic)3]Cl2⋅Sol, M=Zn or Mn, Sol=MeOH or EtOH have been measured in the temperature range between 5 and 170 K. Within the series of investigated compounds, the low-temperature rates are strongly correlated to the transition temperatures T1/2 of the thermal transition. The temperature dependence of the relaxation rates can only be understood in terms of a nonadiabatic multimode multiphonon process involving lattice vibrations. The relaxation rates are controlled by the zero point energy difference ΔE0, the Huang–Rhys parameters Siop and Sak for intramolecular and lattice vibrations, respectively, and the frequencies ωi of the intramolecular vibrations.
Effect of an adsorbing surface on the phase behavior of a confined semiflexible liquid crystal
Authors: Dadmun M. D. , Muthukumar M.
Published: July 2002

Results of a study of the nematic to isotropic transition of a liquid crystal near an adsorbing surface in a thin film using Monte Carlo simulation are presented. The order parameter profile, heat capacity and overall order parameter are calculated to characterize the system as it undergoes the transition. We show that the strength of the adsorption energy has a large influence on the phase behavior of the system. At low adsorption energies a boundary layer is present which undergoes a transition from a nematic to a partially ordered phase at the reduced temperature τ1 and then undergoes a transition to an isotropic orientation at the reduced temperature τ2 which is separate from the bulk. The bulk undergoes an order–disorder transition at a temperature τ3 between the two orientation transitions τ1 and τ2 of the boundary layer. For strong adsorption energies, the boundary layer increases to the thickness of the sample. In this regime, the complete system undergoes the nematic–isotropic transition simultaneously and the temperature of this transition increases linearly with the adsorption energy. The case of intermediate adsorption energies corresponds to a crossover between the above two limiting situations. Here, the boundary layer is more disordered than the bulk and, yet, the entire sample undergoes the nematic–isotropic transition simultaneously. The observed phase behavior is explained in terms of the influence of the boundary layer on the order of the system. The present situations suggest the existence of a critical adsorption energy above which there is only one transition but below which several transitions occur due to the intervention of the boundary layer.
Breaking in polymer chains. I. The harmonic chain
Authors: Doerr T. P. , Taylor P. L.
Published: July 2002

The concept of the breaking strength of a polymer chain is analyzed by means of a study of the dynamics of a rectilinear chain of monomers connected by Hookian bonds. A formalism is then developed whereby the average time to breaking of the chain can be calculated as a function of temperature and strain. An approximation to the exact solution of the resulting equations is discussed. It is concluded that correlations in space and time in the motion of the chain lead to breaking times that are not simple functions of the chain length. The predicted breaking times are appreciably smaller than those that would be found in a chain in which the thermal motions of the monomers were uncorrelated.
The electron affinity of CF
Authors: Xie Yaoming , Schaefer Henry F.
Published: July 2002

Ab initio prediction of the adiabatic electron affinity of CF have been carried out at the self-consistent-field, configuration interaction, and coupled-cluster levels of theory. The most accurate result is 0.40 eV, from the single, double, and the perturbatively connected triple excitation coupled cluster method with Dunning’s augmented correlation consistent polarized valence quadruple ζ basis set. This result disagrees with the earlier experimental report (3.2±0.4 eV), but agrees qualitatively with one of the published density functional results (0.53 eV).
Structures of perylene complexes with long-chain alkanes and alkyl halides examined by rotational coherence spectroscopy
Authors: Troxler Thomas , Stratton John R. , Smith Philip G. , Topp Michael R.
Published: July 2002

Fluorescence excitation spectra of jet-cooled van der Waals complexes of the planar aromatic hydrocarbon perylene with the n-alkanes pentane, hexane, octane, and decane show in each case a single structural form. Rotational coherence transients observed for these species, spaced by 2.5–4.4 ns, are consistent with the n-alkane chain oriented parallel to the long axis of perylene and placed 3.6 Å above its surface. In contrast, the 1-chloropentane and 1-fluoropentane complexes of perylene both exhibit three conformational isomers (α,β,γ) in the electronic ground state. Rotational coherence experiments have measured the structures of these different species in the S1 state, via 000 excitation, proving the existence of three distinct isomers in each case. Dispersed emission spectra following vibronic excitation at 355 cm−1 (A10) indicate in each case that the γ structure relaxes to the α form. Knowledge of the structures of the different forms provides a basis to identify the photoisomerization trajectories. Rotational coherence spectroscopy (RCS) was also used to examine the structures following vibronic excitation. The n-alkane complexes each revealed at least one prominent recurrence transient, showing that vibronic excitation and subsequent vibrational redistribution processes do not necessarily cause rapid rotational dephasing. Similarly, an appreciable degree of rotational coherence persisted into the nanosecond domain for the α and β isomers of the fluoropentane and chloropentane complexes. On the other hand, loss of the γ form was confirmed via the RCS traces, since there were no recurrences for 355 cm−1 excitation. In this way, RCS measurements following vibronic excitation can provide a means to probe conformational stability.
The validity of the rotational isomeric state model for short alkyl chains with large substituents: A maximum entropy analysis of the 1H dipolar couplings within the trimethylene group of a liquid crystal molecule
Authors: Drobny G. P. , Shilstone G. N. , Catalano D. , Veracini C. A. , Zimmermann H.
Published: July 2002

Deuterium decoupled proton spectra of the chain region of a partially deuteriated liquid crystal have been taken. The liquid crystal is a trimethylene chain with perdeuteriated cyanobiphenyloxy groups on either end: 1,3-bis(4,4′-cyanobiphenyloxy-d8) propane (BCBO3-d16). The spectra have been analyzed to give the dipolar couplings within the propyl chain at various temperatures in the nematic phase of the liquid crystal. These couplings are the most sensitive measurements of molecular flexibility that have yet been taken of an alkyl chain in a pure liquid crystal molecule and give a unique insight into rotational potentials of the flexible component essential to a liquid crystal molecule. The couplings have been analyzed using a maximum entropy method designed for a two-rotor system; in our case the two C–C bonds in the propyl chain. The analysis was also performed by adding to the data set the deuterium quadrupolar splittings of the chain hydrogen atoms, previously measured on the same molecule but deuteriated in the chain. The method picks out the flattest distribution consistent with the data. This also happens to be the most likely distribution given only the data used in the analysis and no other constraints.

The results for BCBO3-d16 show a probability maximum for a partially eclipsed conformation (twisted approximately 86° in opposite directions about each C–C bond from the all-trans conformation), a smaller maximum for the all-trans conformation itself, and no significant probability for any of the gauche conformations. The possible motions, that are suggested by these results, therefore, are concerted librations (±86°) between the all-trans position and the partially eclipsed conformations, and full (360°) rotations. These results are in stark contrast to the assumptions made in the rotational isomeric state (RIS) model that the probability distribution can be well approximated by constraining the molecules to exist only in trans and/or gauche states with fast isomerization between such states. In fact the analysis rules out a rotational isomeric state distribution altogether, since our data could not be reproduced by restricting the distribution in this way. Instead it is shown that other distributions are not only possible, but also more likely than any kind of constrained trans–gauche model.
An approach to fluctuation and elasticity in polymer networks
Authors: Sung W.
Published: July 2002

The fluctuation of strands and junctions in fully crosslinked polymer networks under deformation is investigated along with the deformation free energy and elasticity. Nonergodic, disordered system as it is, statistical properties of a polymer networks need to be described on the basis of average over the partial configuration space accessible to the chains. In this work, we reconsider the ensemble average for the bulk and single-chain properties and devise a response theory to relate the fluctuations of the single strand with an external deformation. In the light of this formulation, we assess the ‘‘cooperative network’’ theory of Ronca and Allegra (RA), which represents a statistical mechanical theory with no adjustable parameters yielding good agreements with experiments. Their formulation of the fluctuation under strains is rather ad hoc, since it is adjusted to meet the empirically expected behaviors in the affine and phantom network limits. We find their free energy can be put into a form much more significant and suggestive, which combines with our approach to yield an alternative theory. An approximation within our approach yields better agreements with the experiments on elasticity.
Collision induced isomerization of a semirigid bender hydrogen cyanide
Authors: Lan Boon Leong , Bowman Joel M.
Published: July 2002

The isomerization of a hydrogen cyanide molecule from a localized HCN state to a localized HNC state is formulated as a bimolecular scattering process. The scattering partner is Ar, and the intermolecular interaction potential is written as a pairwise summation of Lennard-Jones potentials. The parameters of this potential are optimized to yield the correct Ar–hydrogen cyanide van der Waals structure. The intramolecular isomerization potential is based on an approximate reaction path through a three-dimensional ab initio potential, and HCN/HNC is treated as a semirigid bender. Eighty molecular eigenstates are calculated, and used as a coupled-channel basis in a scattering calculation in which the hydrogen cyanide rotation is treated in the infinite order sudden approximation. Transition probabilities and cumulative reaction probabilities to HNC states from initial HCN states are focused on; however, some results to final delocalized states are also presented. The bimolecular isomerization rate constant is presented over a wide temperature range. The energy transfer between Ar and initial HCN states is also briefly examined, as a function of the initial relative translational energy.
Application of generalized quantum defect theory to van der Waals complex bound state calculations
Authors: Fourré Isabelle , Raoult Maurice
Published: July 2002

Bound states of atom–diatom van der Waals complexes are calculated in the generalized quantum defect theory framework (GMQDT). Due to very strong interchannel couplings, the diabatic GMQDT formalism is extended in order to account for the strongly closed channels (associated with potential curves lying above the total energy E) in the asymptotic analysis of the wave function. An alternative GMQDT treatment combining diabatic and adiabatic representations is also presented. The influence of these strongly closed channels on the level positions and oscillator strengths is analyzed.
Accurate electron affinities of small carbon clusters
Authors: Watts John D. , Bartlett Rodney J.
Published: July 2002

Coupled-cluster calculations with large basis sets have been used to calculate the electron affinities of Cn (n=1–5). Accurate geometries have been calculated, and correlation and basis set effects have been analyzed. The most complete calculations yield results in excellent agreement with experiment, and are a significant improvement over previous calculations. The results suggest the assignment of the photodetachment data to adiabatic electron affinities, rather than vertical electron detachment energies.
Laser spectroscopy of the Ã 2Π–X̃ 2Σ+ transition of SrOH: Deperturbation analysis of K-resonance in the v2=1 level of the Ã 2Π state
Authors: Presunka Paul I. , Coxon John A.
Published: July 2002

Doppler limited laser excitation and sub-Doppler intermodulated fluorescence spectra of the (010)κ 2Σ(−), μ 2Σ(+), 2Δ3/2←(000)2Σ+ and (010)2Δ5/2←(010)2Π subbands of the Ã 2Π–X̃ 2Σ+ system of SrOH have been recorded and rotationally analyzed. A strong perturbation due to a K-resonance crossing of the μ 2Σ(+) and 2Δ3/2 vibronic components is observed. All measured line positions have been included in a complete deperturbation analysis of the v2=1 level of the Ã 2Π state that takes account explicitly of Renner–Teller, spin–orbit, Λ-type doubling, and l-type doubling interactions. The results include the first reliable estimate of the Renner–Teller parameter for SrOH (ε=−0.0791). Dispersed fluorescence from the Ã 2Π state has been recorded using a digitally interfaced scanning monochromator. Strong intensity interference effects in the fluorescence from the Ã(010) and Ã(000)2Π3/2 levels are observed and give rise to branches with anomalous intensities or branches that are entirely absent.
Liquid–liquid equilibrium in a slit pore: Monte Carlo simulation and mean field density functional theory
Authors: Kierlik E. , Fan Y. , Monson P. A. , Rosinberg M. L.
Published: July 2002

Monte Carlo simulations and mean field density functional theory have been used to study the behavior of a model liquid mixture in a slit pore. The mixture is symmetric with a weakened attractive interaction between the unlike species and exhibits liquid–liquid equilibrium with an upper critical solution temperature. We have studied the influence of confinement on this phase equilibrium. The components have the same interaction with the pore walls, but a lowering of dimensionality in the pore causes selective adsorption of the dilute component. The critical temperature is lowered by the confinement in a manner which is qualitatively consistent with earlier work on the Ising model between walls. The effects are also similar to those seen in recent experimental and theoretical work on thin polymer blend films.
Density functional approach to the solvent effect on the dynamics of nonadiabatic electron transfer reactions
Authors: Samanta Alok , Ghosh Swapan K.
Published: July 2002

A density functional approach is proposed for calculating the activation free energy in a nonadiabatic electron transfer reaction where the molecular nature of the solvent is incorporated in a self-consistent manner. The overall electron transfer rate constant is obtained by taking into account the static as well as dynamic aspects of solvent effects. Illustrative numerical results are presented for both the activation barrier and the electron transfer rate constant.
Diffusion-controlled reaction rate to an active site in an external electric field
Authors: Traytak S. D. , Tachiya M.
Published: July 2002

The effect of an external electric field on the rate constant for diffusion-controlled reactions between asymmetric reactants described by the model of Solc and Stockmayer is investigated. We propose an easy method to calculate the rate constant with any necessary accuracy and derive some analytical formulas which approximate it well for any angular size of the active site or either weak or strong electric fields. We also calculate the rate constant by applying the constant-flux method of Shoup, Lipari, and Szabo and compare it with our results. It is shown that in the case of a weak electric field the rate constant slightly depends on the active site location but this dependence becomes significant for a strong electric field.
Relativistic Hartree–Fock by means of stationary direct perturbation theory. II. Ground states of rare gas atoms
Authors: Ottschofski Edgar , Kutzelnigg Werner
Published: July 2002

The combination of relativistic Hartree–Fock theory with stationary direct perturbation theory presented in paper I of this series is applied to the ground states of the rare gas atoms He, Ne, Ar, Kr, Xe, Rn, in STO basis sets with finite nuclei. The error of the truncation of the perturbation expansion at O(c−6)—with the nonrelativistic reference O(c0)—is only ≊1% for Rn and <0.1% for Xe. The corresponding error for truncation at O(c−2) is <1% for Ar. An approximation in which c−2E(2) is calculated exactly, but in which two electron contributions to c−4E(4) and c−6E(6) are neglected, performs almost as well as rigorous perturbation theory up to c−6E(6) but is computationally much cheaper.
Time reversible and phase-space conserving molecular dynamics at constant temperature
Authors: Winkler Roland G. , Kraus Volker , Reineker Peter
Published: July 2002

Algorithms for constant temperature molecular dynamics simulations are presented. The basic equations of motion, keeping the temperature constant, are derived by an extension of phase space. Time reversible integration algorithms are obtained for these equations of motion by a factorization of the classical Liouville propagator. In particular an algorithm is derived which includes the phase-space conservation property of the equations of motion. This algorithm is compared with the Nosé–Hoover approach for systems of a single degree of freedom. It is demonstrated that the derived equations of motion generate canonical distributions. In addition, a comparison with various isothermal integration algorithms for the Nosé–Hoover equations is presented for a system of Lennard-Jones particles.
Nuclear modulation effects in ‘‘2+1’’ electron spin-echo correlation spectroscopy
Authors: Raitsimring Arnold , Crepeau Richard H. , Freed Jack H.
Published: July 2002

This work represents the synthesis of the 2+1 pulse sequence for the study of electron spin-echo envelope modulation (ESEEM) with the technique of spin-echo correlated spectroscopy (SECSY), which has previously been used to study nuclear modulation by two-dimensional Fourier transform ESR methods. This example of ‘‘pulse adjustable’’ spectroscopy, wherein the pulse width and pulse amplitude of the second pulse in a three pulse sequence are introduced as adjustable parameters, leads to enhanced resolution to the key features of the nuclear modulation that are important for structural studies. This is demonstrated in studies on (i) a single crystal of irradiated malonic acid and (ii) a frozen solution of diphenylpicrylhydrazyl in toluene. In particular, it is shown for (i) how the nuclear modulation cross peaks can be preferentially enhanced relative to the autopeaks and to the matrix proton peaks, and also how the autopeaks can be significantly suppressed to enhance resolution for low-frequency cross peaks. For (ii) the low-frequency 14N nuclear modulation could be suppressed leaving just the high-frequency matrix 1H modulation. Additionally, the T2 homogeneous linewidth broadening in the f1 frequency direction is removed in 2+1 SECSY. These features significantly improve resolution to the modulation decay, which is the main observable utilized for distance measurements by ESEEM, compared to SECSY. A simple example of a distance measurement to matrix protons is presented for (ii). It is shown that a major advantage of the 2D format is that the full spin-echo shape is collected, which permits one to study how the effect of the nuclear modulation upon the echo varies with evolution time t1 after the first pulse, and thereby to detect the important modulation features. Additionally it allows for correlating the modulation cross peaks in making spectral assignments. A detailed quantitative theory for 2+1 SECSY is also presented. In general, very good agreement between experiment and theoretical simulations is obtained.
State-resolved rotational energy transfer in open shell collisions: Cl(2P3/2)+HCl
Authors: Zhao Zhong-Quan , Chapman William B. , Nesbitt David J.
Published: July 2002

Time- and frequency-resolved infrared (IR) laser absorption methods are used to probe hot atom energy transfer in open shell interactions of Cl(2P3/2)+HCl(J) in the single collision regime. The Cl(2P3/2) atoms are prepared by 308 nm laser photolysis of Cl2, and suffer collisions at Erel̄∼3500 cm−1 with a room temperature HCl distribution in a fast flow cell. Selective collisional excitation of final HCl(Jf) states is monitored by transient IR absorption on R(J≥4) branch lines in the HCl(v=1←0) band, while depletion of the initial HCl(Ji) states is monitored by transient bleaching of the room temperature Doppler profiles. Analysis of the J dependent Doppler profiles permits extraction of rotational loss [σloss(Ji)=∑fP(Ji)⋅σf←i] and gain [σgain(Jf)=∑iP(Ji)⋅σf←i] cross sections, as a function of initial and final J states, respectively. Absolute transient concentrations of the HCl(Ji) and HCl(Jf) are measured directly from absorbances via Beer’s Law, and used to extract absolute collisional cross sections. The results are compared with quasiclassical trajectory (QCT) calculations on a hybrid ab initio/LEPS surface of Schatz and Gordon, which prove remarkably successful in reproducing both the J dependent trends and absolute values of the state-resolved gain and loss collision cross sections.