bluuebunny/crossref_metadata_2025 huggingface dataset:
Predicted bond energies in peroxides and disulfides by density functional methods
Authors: Fournier René , DePristo Andrew E.
Published: July 2002

We have performed self-consistent linear combination of Gaussian-type orbitals-density functional calculations for the molecules YY, RY, RYY, RYR′, RYYR, and RYYR′ with Y=O,S and R,R′=H,CH3. The structures were optimized within the local spin density approximation while the Y–Y, Y–C, and Y–H bond dissociation energies (BDE) were calculated with both a local exchange-correlation energy functional and a gradient corrected functional. Comparison of results obtained with the local and gradient corrected exchange-correlation functionals provides more experience on the successes and failures of gradient corrections. Trends in BDEs and the nature of bonding in oxygen and sulfur containing analog molecules are analyzed on the basis of two observations: (1) the O atom is more electronegative than S, C, and H atoms; (2) a S atom can have a valency larger than two and has a greater ability for multiple bonding than oxygen. Finally, comparison with a number of experimental results suggest that the C–S BDE in CH3S, the S–S BDE in CH3SSCH3, and the enthalpy of formation of CH3S should be reexamined.
Molecular dynamics of paraffins in the n-alkane/urea clathrate
Authors: Lee Kuei-Jen , Mattice Wayne L. , Snyder Robert G.
Published: July 2002

Simulations of the molecular dynamics of n-CNH2N+2 in the n-alkane/urea clathrate have been performed for N=13,23,33, for a period of 1 ns. Parallel simulations were performed starting from conformations in which all CH2–CH2 bonds were in the trans conformation, and starting from a conformation in which one of the terminal CH2–CH2 bonds was in a gauche conformation. These simulations suggest less than 3% of the terminal CH2–CH2 bonds would be in a gauche state at equilibrium. The gauche content is much smaller at the CH2–CH2 bonds further removed from the ends of the chain. No significant differences are seen in the simulations performed with n-alkanes with N=13,23,33. The mobility of the C–H bond vectors about the axis defined by the channel is greater at the ends than in the middle of the chain. The angular distribution is nearly randomized in 1 ns for the C–H vector in a methyl group, but a longer time scale would be required for randomization of this distribution for the C–H vector bonded to the central methylene group in C33H68.
A simple SU(2)-based approach to Coriolis-adapted vibrational states
Authors: Martens Craig C.
Published: July 2002

The representation of the two-dimensional harmonic oscillator by the unitary group SU (2) simple Coriolis-adapted vibrational basis states for the treatment of vibration–rotation interaction in polyatomic molecules. The vibrational part of the zeroth-order vibration–rotation Hamiltonian is expressed in terms of the generators (Sx,Sy,Sz) of the group SU(2), leading to a coupled angular momentum representation of the vibration–rotation Hamiltonian. In the prolate limit, this leads to an effective k-dependent zeroth-order vibrational Hamiltonian that is linear in the group generators. The problem can be solved exactly in this limit by a simple axis transformation in the vibrational ‘‘spin’’ space. Because of the underlying SU(2) structure, the transformation matrix elements and overlaps of basis states of different effective Hamiltonians corresponding to different values of k are given by simple expressions involving Wigner d matrices.
High resolution x-ray photoelectron spectroscopy study of Cr(CO)6 in the gas phase
Authors: Nilsson A. , Mårtensson N. , Svensson S. , Karlsson L. , Nordfors D. , Gelius U. , Ågren H.
Published: July 2002

The gas phase high resolution x-ray photoelectron spectra of the Cr(CO)6 metal complex are presented. The valence electron spectrum and the core level spectra including the associated satellite structures from the Cr, C, and O atoms are discussed. Comparison to free CO and to CO adsorbed on metal surfaces are made. An assignment of the valence levels is achieved using an intensity model. The anomalous dispersion of the narrow band in CO adsorbed on Cu seems to be related to the localization properties of the corresponding 7a1g level in Cr(CO)6. The C1s and O1s shake-up structures are assigned using the Z+1 approximation, electron energy loss data and data from shake-up spectra of free CO. The Cr core level binding energy shifts relative to Cr metal are shown to vary with the subshell. Electronic structure and vibronic calculations are carried out for the ground and C1s and O1s core ionized states. The results for the vibronic coupling constants and for the vibrational excitations confer more with the observations in the electron energy loss spectra of free CO rather than with the photoelectron spectra of free CO. This supports the notion of a, close to complete, charge transfer screening accompanying ligand core ionization of Cr(CO)6.
Electronic spectra of NaAr4 and NaAr6: Isomerization and melting
Authors: Tsoo Chiachin , Estrin Dario A. , Singer Sherwin J.
Published: July 2002

Absorption spectra of a sodium atom chromophore clustered with four or six argon atoms are calculated as a function of temperature. Certain spectral features are sensitive to isomerization and phase transitions, particularly the peak which arises from transitions to the middle of the three sodium 3P levels distorted by the cluster environment. Nonlocal pseudopotentials for the sodium core and argon solvent are used to reduce the electronic problem to that of one valence electron. Ground and excited Born–Oppenheimer states of the sodium chromophore are obtained during a molecular dynamics simulation via simulated annealing. Implications of our results for more complicated experiments are also discussed.
Coupled channel treatment of transitions between Na(2P1/2,mj) and Na(2P3/2,mj′) in collision with Na(2S1/2,mj″)
Authors: Kimura Misako , Katô Hajime , Someda Kiyohiko , Nakamura Hiroki
Published: July 2002

Transitions between the Zeeman substates of Na(3p 2P1/2) and Na(3p 2P3/2) in collision with Na(3s 2S1/2) in thermal energy region are investigated by the close-coupling method. Rotational and spin–orbit couplings are taken into consideration, and potential energy functions generated by a quantum chemical calculation are employed. The cross sections strongly depend on the magnetic quantum numbers of the initial and final states. This dependence qualitatively coincides with an experimental observation in the presence of a weak magnetic field, although the magnetic field is not considered in the present calculation. Total cross section of the transition Na(2P3/2)→Na(2P1/2) is calculated to be 2.7×102 Å2 in good agreement with experiment. The following two mechanisms are found to dominantly contribute to the total cross section: (1) Transitions among the molecular states of Na2, 3Π2u↔3Π1u and 3Π1u↔3Π0u, induced by the spin–rotation coupling, and (2) the transition, 1Σ+0g↔1Π1g induced by the Coriolis coupling.
Brownian dynamics simulation of sediment formation of colloidal particles
Authors: Soppe W. J. , Janssen G. J. M.
Published: July 2002

The Brownian dynamics simulation method is used to study the sediment formation of colloidal particles by a process of irreversible single-particle accretion. The simulation method that is used is based on the algorithm of Ermak and McCammon with the incorporation of interparticle forces and hydrodynamic interactions on a two-particle level. The effect of the two-particle hydrodynamic interactions on the sediment structure is analyzed by means of comparison with simulations in which the hydrodynamic interactions are omitted. The colloidal interactions are described in terms of the Derjaguin–Landau–Verwey–Overbeek (DLVO) potential. The volume fractions of the sediments range from 0.09 to 0.37. It is shown that the process of sediment formation by colloidal particles is the result of a delicate balance of sedimenting field strength, DLVO interactions and hydrodynamic interactions.
Trends in inversion barriers. I. Group-15 hydrides
Authors: Schwerdtfeger Peter , Laakkonen Liisa J. , Pyykkö Pekka
Published: July 2002

Inversion barriers for the group-15 hydrides NH3, PH3, AsH3, SbH3 and BiH3 have been studied using ab initio self-consistent-field methods including electron correlation and relativistic effects. A modified symmetric inversion potential is introduced to describe the inversion from the minimum C3v arrangement through the D3h transition state. Tunneling rates and frequencies are calculated at the Hartree–Fock and Mo/ller–Plesset (MP2) level within the Wentzel–Kramers–Brillouin approximation. At the MP2 level the calculated 0+/0− ν2 frequency splitting of the vibronic ground state of NH3/ND3 (0.729 cm−1/0.041 cm−1) is in excellent agreement with the experimental values (0.794 cm−1/0.053 cm−1). The tunneling rate for PH3 suggests that previously published values are wrong by orders of magnitude. Correlation effects do not change the barriers significantly in accordance with Freed’s theorem. This has been studied in more detail for BiH3 at the quadratic configuration-interaction (QCI) level. Relativistic effects increase the barrier height of BiH3 by 81.6 kJ/mol at the QCI level. Nonrelativistic and relativistic extended Hückel calculations suggest that the a1 highest occupied molecular orbital, which is antibonding to the Bi 6s, relieves part of its antibonding character near equilibrium geometry due to the relativistic radial contraction of the 6s orbital and hence increases the barrier height. In the planar transition state this orbital is a nonbonding a″2. The increasing trend in barrier heights from NH3 to BiH3 can be explained by a second-order Jahn–Teller distortion of the trigonal planar geometry. Vibrational frequencies are predicted for BiH3.
High energy two-photon states in finite versus infinite polyenes
Authors: Mazumdar S. , Guo D. , Dixit S. N.
Published: July 2002

Optical nonlinearity in π-conjugated polyenes depends crucially on a single two-photon state that is located above the one-photon allowed exciton. In the case of the infinite polyene this single state should be thought of as a collection of states with very narrow bandwidth. The location of this two-photon state in the infinite polyene is of considerable interest. We present numerical data for finite chains and discuss in detail why extrapolation of finite chain data to obtain the energies of the high energy states in the infinite polyene requires considerable care. Based on upper and lower bounds that we put on the energy of the two-photon state in question, we predict it to be extremely close to the one-photon excition in the infinite chain limit. We present recent experimental data for a polydiacetylene that is consistent with our prediction.
Zinc(II) in liquid ammonia: Intermolecular potential including three-body terms and Monte Carlo simulation
Authors: Hannongbua Supot , Kerdcharoen Teerakiat , Rode Bernd M.
Published: July 2002

The failure of pair potentials in computer simulations in giving the correct coordination number for Zn(II) in liquid ammonia is investigated. Ab initio molecular orbital calculations for Zn(II)–(NH3)n complexes with n=1,2,3,4, and 6 have been performed. Pair potential and three-body correction function for Zn(II)/ammonia were constructed from the Zn(II)–NH3 potential surface and more than 1500 configurations of the Zn(II)–(NH3)2 complex. Monte Carlo simulations were performed with these functions for a single Zn(II) ion in liquid ammonia. Inclusion of the three-body effects leads to a lower coordination number and a shift of the first peak from 2.10 to 2.22 Å. A small peak containing three ammonia molecules, observed beyond the first solvation sphere in the simulation with conventional pair potential, disappears and the first solvation shell of Zn(II) is thus reduced to exactly six ammonia molecules.
Study of butterfly inversion of perfluoronaphthalene by laser-induced fluorescence in supersonic jet
Authors: Chakraborty Tapas , Nath Debnarayan , Chowdhury Mihir
Published: July 2002

Fluorescence excitation and dispersed emission spectrum of jet-cooled perfluoronaphthalene is reported. The origin region of the excitation spectrum exhibits a clear progression for a very low-frequency vibration which indicates that the molecule is highly floppy in the electronic excited state. The low-frequency progression has been assigned to the butterfly inversion of the fluorine atoms perpendicular to the ring plane. Barrier height of the butterfly inversion mode for the S1 electronic state, calculated by a quadratic Gaussian-type potential function, have been found to be 14 cm−1 only. A splitting if 7 cm−1 between the zero-point and the first vibronic levels indicates that butterfly inversion is active even in the zero-point level of the S1 electronic state. Simulated intensity distribution pattern over the cold progression fits well with the observed spectrum. Other vibrational frequencies of both the excitation and the dispersed emission spectrum have been tentatively assigned by correlating the observed frequencies with the values obtained from Raman and IR spectrum. Analysis of the dispersed fluorescence spectrum indicates that only totally symmetric modes are active in emission.
Pretransitional nematic surface order in the isotropic phase of phenylcyclohexanes
Authors: Immerschitt S. , Koch T. , Stille W. , Strobl G.
Published: July 2002

Pretransitional order at the free surface of the isotropic phases of two phenylcyclohexances (PCH5 and PCH7) was studied by reflection ellipsometry under variation of the angle of incidence. Data analysis shows a logarithmic divergence of the boundary layer thickness when the temperature approaches the isotropic-nematic transition point, indicating complete wetting behavior. The data are well represented by fits to a model based on a Landau–de Gennes approach, yielding order parameter profiles of the layer.
State-to-state cross section of rotational transition caused by dipole–quadrupole interaction between NH3 and nonpolar molecules
Authors: Uematsu Yoshihiko , Tachikawa Maki , Tanaka Yoshihito , Matsushima Fusakazu , Shimizu Tadao
Published: July 2002

Rotational transitions in NH3 induced by the dipole–quadrupole collisional interaction are studied by use of the supersonic molecular beam technique and the double-resonance spectroscopy. The measured state-to-state cross section agrees well with the cross section calculated nonempirically based on the Anderson–Tsao–Curnutte theory, which depends only on the dipole matrix element between the initial and final states of NH3. It is deduced that the state-to-all states cross section is independent of the molecular quantum states.
The rotational spectrum, internal rotation, and structure of H2O–NCCN and D2O–NCCN
Authors: Lee Seonghoon , Suni Ian I. , Klemperer William
Published: July 2002

The radio frequency and microwave spectra of H2O–NCCN and D2O–NCCN have been measured using molecular beam electric resonance spectroscopy. The spectrum is characteristic of a planar T-shaped asymmetric top in which the H2O subunit exhibits hindered twofold internal rotation. The spectroscopic parameters for H2O–NCCN are A=4692.11(2) MHz, A′=4689.45(25) MHz, B=4258.06(1) MHz, C=2219.34(1) MHz, and μa =2.06(3) D and for D2O–NCCN are A=4644.31(27) MHz, B=3822.76(10) MHz, C=2084.86(10) MHz, and μa =2.1617(10) D. The H2O subunit is bound to NCCN through the oxygen atom with a twofold barrier of 285(6) cm−1 hindering the internal rotation of H2O about the a axis of the complex.
Theoretical study of torsion potentials in t
r
a
n
s-stilbene and substituted t
r
a
n
s-stilbenes: Modeling torsions in poly(paraphenylene vinylene) and derivatives
Authors: Lhost O. , Brédas J. L.
Published: July 2002

We present ab initio and semiempirical quantum-chemical calculations as well as simple molecular mechanics calculations on the gas-phase torsion potentials of 1,1′-(trans-1,2-ethenediyl) bisbenzene, commonly known as trans-stilbene, and derivatives of trans-stilbene substituted by methyl or methoxy side groups: 1,1′-(trans-1,2-ethenediyl), bis-2,5-dimethylbenzene, and 1,1′-(trans-1,2-ethenediyl) bis-2,5-dimethoxybenzene. Our results are discussed in light of recent experimental data on poly(paraphenylene vinylene), poly(dimethyl paraphenylene vinylene), and poly(dimethoxy paraphenylene vinylene).
Modified Padé approximants and equation of state
Authors: Hu Zhe-Ming , Eu Byung Chan
Published: July 2002

A modified Padé approximant is proposed for a resummation of the virial expansion. It includes the Carnahan–Starling equation of state as a special case. The second Padé approximant is comparable in accuracy to the Carnahan–Starling formula, the Ree–Hoover [3,3] Padé approximant, and the computer simulation results. A generalized Carnahan–Starling formula is then obtained for the equation of state for the Lennard-Jones fluid by using the modified Padé approximant summing the virial expansion. The generalized Carnahan–Starling formula is comparable with computer simulation results and the Song–Mason equation of state down to the critical temperature, but its accuracy deteriorates below the critical temperature.
On the calculation of field-dependent relaxation times from the noninertial Langevin equation
Authors: Coffey W. T. , Kalmykov Yu. P. , Quinn K. P.
Published: July 2002

The problem of the dielectric relaxation of an assembly of noninteracting rigid dipoles subjected to a strong direct current field superimposed on which is a weak alternating field is treated by averaging of the noninertial Langevin equation regarded as a Stratonovich stochastic differential equation. The hierarchy of differential-difference equations so obtained is linearized in the alternating current (ac) field. It is then closed by assuming that the ratio of the Fourier–Laplace transforms of the ensemble averages of the first and second spherical harmonics may be replaced by its zero-frequency value. This allows one to obtain closed-form expressions in terms of the Langevin function for the relaxation times, complex susceptibility and loss tangent induced by the coupling between the direct current (dc) field and the weak ac field. The relaxation times produced by this closure procedure for ac fields applied parallel and perpendicular to the dc field are in good agreement with the results of a numerical solution of the secular equation of the problem and with available experimental data.
Vibronic theory of magnetic vibrational circular dichroism in systems with fourfold symmetry: Theoretical analysis for copper tetraphenylporphyrin
Authors: Pawlikowski M. , Pilch M. , Mortensen O. Sonnich
Published: July 2002

A vibronic theory which includes multimode and multistate effects is developed to study magnetic vibrational circular dichroism (MVCD) and absorption spectra for molecular systems with fourfold symmetry. The MVCD and absorption spectra of the e fundamental transition and its combinations with infrared inactive modes are discussed in some detail to demonstrate the importance of electric and magnetic anharmonicity arising from vibronic coupling. The theory is applied to experimental MVCD spectra for the CuTPP molecule.
Resonant two-photon ionization of atomic Fe: Electronic branching fractions and state-resolved photoelectron angular distributions
Authors: Hanton Scott D. , Noll Robert J. , Weisshaar James C.
Published: July 2002

Resonant two-photon ionization (R2PI) of atomic Fe is used to create low energy beams of Fe+ in well characterized state distributions, many of which are sharply peaked. In one-color R2PI schemes, we measure angle-resolved time-of-flight photoelectron spectra (TOF-PES). For ionization through nine different resonances, we present term or spin–orbit-level resolved Fe+ branching fractions and photoelectron anisotropy coefficients. Ionization through intermediate states of nominal 3d74p configuration creates remarkably pure beams of Fe+(3d7,4F). Several resonances produce Fe+ beams with ∼90% of the population in a single spin–orbit level. Ionization through nominal 3d64s4p intermediates yields Fe+ beams with 80%–90% of the population in 3d64s terms, but with little selectivity between the 6D and 4D terms. Both the configuration specificity and the lack of spin specificity are in accord with a direct ionization mechanism. In contrast with our earlier results for the V atom, in Fe some of the photoelectron angular distributions demand inclusion of both P2 (cos Θ) and P4 (cos Θ) terms in the fitting function. This indicates that alignment of the intermediate electronic state produced by ω1 is preserved on the time scale of absorption of ω2 for Fe, in accord with its nuclear spin of zero. We also describe two-color R2PI experiments that create pure beams of the 6D9/2 ground state of Fe+. By detecting Fe+ vs ω2 at fixed ω1, we observe two Rydberg series, one converging to 6D9/2 (quantum defect δ=0.81±0.12 modulo one) and the other to 6D7/2 (δ=0.64±0.09 modulo one).
Photoabsorption, photoionization, and neutral dissociation cross sections of dimethyl ether and ethyl methyl ether in the extreme-ultraviolet range
Authors: Kameta Kosei , Ukai Masatoshi , Kamosaki Tetsu , Shinsaka Kyoji , Kouchi Noriyuki , Hatano Yoshihiko , Tanaka Kenichiro
Published: July 2002

The absolute photoionization and neutral dissociation cross sections of dimethyl ether (CH3OCH3) and ethyl methyl ether (CH3CH2OCH3) in the 52–92 nm range have been obtained by measuring the absolute photoabsorption cross sections and photoionization quantum yields using a multistage ionization chamber and synchrotron radiation. A large deviation in the photoionization quantum yield of both molecules from unity, namely a noticeable enhancement of the neutral fragmentation cross sections, has shown that superexcited states exist densely in this wavelength range. Fluorescence excitation spectra of neutral dissociation fragments produced in the 35–100 nm photon impact on both molecules have also been measured by observing ultraviolet and vacuum ultraviolet emissions. A general appearance of the neutral dissociation cross sections correlates with the fluorescence excitation spectra from H(n=2), CH(A 2Δ), and CH3O(Ã). The superexcited states of dimethyl ether observed are shown to be purely repulsive states or bound states for which the lowest energy level achieved from the Franck–Condon region is higher than the dissociation limit. Dissociation of ionic satellites have also been observed in the fluorescence excitation spectra of both molecules.
Investigation of charge-transfer complexes by computer simulation. II. Iodine in pyridine solution
Authors: Danten Y. , Guillot B. , Guissani Y.
Published: July 2002

In this article we report molecular-dynamics (MD) simulation results for the thermodynamic, structural, and dynamic properties of a dilute solution of iodine in liquid pyridine (Py). The molecules of the simulated solution (10I2+246Py) interact through site–site intermolecular potentials for which the pyridine molecules are modeled by 12 sites (11 atomic sites plus a lone-pair site on the nitrogen atom) and the iodine molecules by four sites (two sites coinciding with the iodine atoms plus two extra sites). The site–site potentials contain isotropic terms to describe the electrostatic, dispersion and repulsion interactions between molecules. The potential parameters are determined from a Monte Carlo (MC) simulation of the iodine–pyridine complex in the gaseous phase where it appears that the most stable conformation is realized when iodine and pyridine are in a linear arrangement (the iodine molecule interacting with the nitrogen atom of the pyridine molecule). The MC study also shows that the complex becomes unstable near room temperature. On the contrary, in liquid pyridine our molecular-dynamics simulation predicts that iodine and pyridine molecules associate to form long-lasting (1:2) complexes, the latter ones (Py–I2–Py) adopting a nearly linear conformation. Concerning the intermolecular motions the simulation indicates that the stretching mode of the I–I⋅⋅⋅N bonds produces the predominant contribution to the far-infrared absorption spectrum of the iodine solution and is responsible of the sharp band observed on the experimental spectra reported in the literature.
Vibrational spectra of penta-atomic silicon–carbon clusters. I. Linear SiC4 in Ar at 10 K
Authors: Withey P. A. , Graham W. R. M.
Published: July 2002

A Fourier transform infrared (FTIR) study has resulted in the first identification of a fundamental vibration of SiC4, the ν1, C■C stretching mode at 2080.1 cm−1. SiC4 was produced by trapping in Ar at 10 K the products of the vacuum ultraviolet (VUV) photolysis of silane (SiH4) and 1,3-butadiene (C4H6) or the products from the evaporation of a mixture of Si and C powder. The vibrational assignment is confirmed by 13C -isotopic data for 20 isotopomers, which is in excellent agreement with the predictions of ab initio calculations.
Grand-canonical simulations of solvated ideal fermions. Evidence for phase separation
Authors: Alavi A. , Frenkel D.
Published: July 2002

A novel scheme to perform finite-temperature grand-canonical simulations of ideal fermions in arbitrary external potentials is introduced. This scheme is based on the evaluation of the grand-canonical function of lattice fermions. As an application, we present results on the phase behavior of a mixture of fermions and hard spheres. A simple analytic model of solvated fermions in a hard-sphere fluid is also studied. We address here the possibility of phase separation between a pure delocalized phase of fermions and a homogeneous solution of solvated fermions. These calculations indicate that the homogeneous phase is expected to be stable only at low fermion concentration and low thermal wavelengths. The fermion simulations indicate that such phase separation is a likely scenario.
Exact results on diffusion from a piecewise linear potential well
Authors: Samanta Alok , Ghosh Swapan K.
Published: July 2002

We obtain an exact solution for the Laplace transformed distribution function P̃(x,s) for a Brownian particle trapped in a piecewise linear potential well with a finite barrier on one side. Explicit analytical expressions for the rate of diffusion over the barrier are thereby derived.
Discretized model of proteins. I. Monte Carlo study of cooperativity in homopolypeptides
Authors: Kolinski Andrzej , Skolnick Jeffrey
Published: July 2002

A discretized model of globular proteins is employed in a Monte Carlo study of the helix–coil transition of polyalanine and the collapse transition of polyvaline. The present lattice realization permits real protein crystal structures to be represented at the level of 1 Å resolution. Furthermore, the Monte Carlo dynamic scheme is capable of moving elements of assembled secondary and supersecondary structure. The potentials of mean force for the interactions are constructed from the statistics of a set of high resolution x-ray structures of nonhomologous proteins. The cooperativity of formation of ordered structures is found to be larger when the major contributions to the conformational energy of the low temperature states come from hydrogen bonds and short range conformational propensities. The secondary structure seen in the folded state is the result of an interplay between the short and long range interactions. Compactness itself, driven by long range, nonspecific interactions, seems to be insufficient to generate any appreciable secondary structure. A detailed examination of the dynamics of highly helical model proteins demonstrates that all elements of secondary structure are mobile in the present algorithm, and thus the folding pathways do not depend on the use of a lattice approximation. Possible applications of the present model to the prediction of protein 3D structures are briefly discussed.
Multiconfigurational quadratic response functions for singlet and triplet perturbations: The phosphorescence lifetime of formaldehyde
Authors: Vahtras Olav , Ågren Hans , Jo/rgensen Poul , Jensen Hans Jo/rgen Aa. , Helgaker Trygve , Olsen Jeppe
Published: July 2002

A formalism is presented for the calculation of quadratic response functions of multiconfigurational self-consistent field reference wave functions. The formalism is general in the sense that it applies equally well to singlet and triplet perturbations and it does not assume any permutational symmetry in the integrals of the perturbational operators. This formalism can be used to derive expressions for various properties related to singlet or triplet quadratic response functions and their residues. We focus on the spin-forbidden dipole transitions between singlet and triplet electronic states responsible for the long lifetime of phosphorescent states. The singlet–triplet transition moments are evaluated as the residues of quadratic response functions. Sample calculations are presented for the formaldehyde molecule.
Studies of molecular Rydberg states by Schwinger variational-quantum defect methods: Application to molecular hydrogen
Authors: Stephens J. A. , McKoy V.
Published: July 2002

An ab initio electronic structure technique has been developed to study highly excited states of molecules by combining Schwinger variational methods of collision theory with generalized quantum defect theory. The technique exploits methods of scattering theory to study the region of highly excited Rydberg levels below and across ionization thresholds for molecules. The reaction matrix K, which describes the interaction of the Rydberg electron with the ionic core, is found at arbitrary negative electron energies by employing an unbounded Coulomb Green’s function in the Lippmann–Schwinger equation for the electronic wave function. Quantal conditions are imposed to obtain discrete molecular energy levels, associated Rydberg wave functions, and quantum defect functions, all as a function of the internuclear distance. Results within the static-exchange approximation for the 1,3Σ+u(1σgnσu) and 1,3Πu(1σgnπu) Rydberg states of H2, for n=2–20 and R=1.2–5.0 a0, are presented and discussed.
Para phenylenediamine radical cation structure studied by resonance Raman and molecular orbital methodsa)
Authors: Chipman Daniel M. , Sun Q. , Tripathi G. N. R.
Published: July 2002

The vibrational and electronic structures of p-phenylenediamine radical cation have been studied using time-resolved resonance Raman spectroscopy and molecular orbital calculations. The Raman spectra in aqueous solution show striking variations in intensity profile when excitation is varied from 340 to 480 nm. Excitation in resonance with the 460–480 nm absorption shows enhancement of only totally symmetric vibrations. Most prominent are the δNH2 scissor mode at 1658 cm−1 and the Wilson modes ν8a (CC stretch) at 1644 cm−1, ν7a (CN stretch) at 1423 cm−1, and ν9a (CH bend) at 1184 cm−1. Also observed are ν1 (ring breathe) at 840 cm−1 and ν6a (CCC bend) at 467 cm−1. With excitation in the 340–410 nm region the ν1 band becomes relatively stronger and an additional band at 1524 cm−1 appears. This latter band, which dominates the spectrum at 360 nm, is assigned to the nontotally symmetric vibration ν8b (CC stretch) having b3g symmetry that gains intensity through vibronic coupling. Raman spectra of ring- and amine-deuterated radicals allow distinction of overtone and combination bands enhanced by Fermi resonance from fundamentals due to δNH2 and ν8a (CC stretch) modes in the 1600–1700 cm−1 region. The theoretical calculations show reasonable agreement with the observed vibrational frequencies and lead to detailed information on the bond structure and normal modes of the radical. The calculated CN bond lengths of 1.33 Å are intermediate between lengths typical of single and double bonds in aromatic amines. The calculations also allow interpretation of the experimental absorption spectrum. The broad medium intensity absorption peaked at 460 nm with a shoulder at 480 nm is assigned to a 2B3u ←2B2g transition, the very weak 360 nm absorption to 2Au ← 2B2g and the very strong ∼325 nm absorption to a higher 2B3u ← 2B2g.

Calculations with the simple relation Ik∝ (V′k)2/ωk, which requires evaluation only of vertical excited state gradients, are found to satisfactorily describe the dramatic changes in Raman intensity profile that are observed with different excitation wavelengths. The 460 and 480 nm Raman spectra are governed by scattering from the lowest 2B3u state. Spectra taken in the 410–340 nm range are governed mainly by preresonance scattering from the higher 2B3u state, which is vibronically coupled to the weak 2Au state via the b3g symmetry ν8b (CC stretch) vibration. The relative intensities of totally symmetric modes calculated for excitation into the two 2B3u excited states agree well with those in the two distinct patterns observed experimentally.
Integral equation theory for charged liquids: Model 2–2 electrolytes and the bridge function
Authors: Duh D.-M. , Haymet A. D. J.
Published: July 2002

The integral equation theory for a model 2–2 electrolyte is studied in detail. In this model electrolyte, the ions are assumed to be the same size, and interact via a continuous potential energy which behaves as the Coulomb potential at large distances and an inverse ninth power repulsion at short distances. The ions are embedded in a dielectric continuum of fixed dielectric constant, here taken to be 78.3 ε0 in order to model water at 25 °C. The bridge function for this model is studied as a function of concentration (a) for six proposed closures, and (b) via ‘‘exact’’ inversion of data from computer simulations. A proposed closure derived from examination of the inverted bridge function yields predictions in good agreement with computer simulations. We emphasize the importance of choosing an ‘‘optimized’’ long-range potential, as opposed to the traditional Coulomb choice. A simple functional form for the bridge function results from this optimized choice of long-range potential.
Solvation effects on the electronic structure of 4-N, N-dimethylaminobenzonitrile: Mixing of the local ππ* and charge-transfer states
Authors: Shang Quan-yuan , Bernstein Elliot R.
Published: July 2002

The effect of polar solvents acetonitrile and water on the electronic excited states of 4-N, N-dimethylaminobenzonitrile (DMABN) is studied through the optical spectroscopy of small clusters of DMABN/solvent. The clusters are created in a supersonic jet expansion. The results of mass resolved excitation spectroscopy (MRES), fluorescence excitation (FE), dispersed emission (DE), and photodepletion studies demonstrate that the solvent molecule can bind to DMABN at two distinct sites for the one-to-one cluster. Both DMABN (H2O)1 clusters generate small blue shifts for the S1←S0 cluster transition and evidence low-energy vibronic structure nearly identical to that found for the bare molecule. The DMABN (CH3CN)1 clusters behave quite differently. One cluster geometry induces a small blue shift of the S1←S0 electronic transition with little change in its vibronic structure and intensity pattern. We suggest this binding site involves the cyano end of the DMABN molecule. The second cluster geometry induces a large red shift (∼1000 cm−1) and significant broadening (>103 cm−1) of the lowest-energy transition. This red shifted transition is associated with a charge-transfer transition within the DMABN molecule lowered in energy due to the acetonitrile coordination with the DMABN aromatic ring. The lowering of the charge-transfer state in DMABN (CH3CN)n, n=1,...,5 clusters is supported by the following data: long wavelength emission from clusters with broad red shifted absorption; distinct lifetimes for emission at 350 nm (4.6 ns) and 400 nm (6.0 ns); broad red shifted absorption for one geometry of the DMABN (CH3CN)1 cluster. These results support the idea that the charge-transfer transition in DMABN is stabilized by short-range dipole–dipole interactions between DMABN and polar nonhydrogen bonding solvents.
Spectroscopic interrogation of heterocluster isomerization. I. Simulations of nuclear dynamics and electronic spectroscopy
Authors: Ben-Horin Narda , Even Uzi , Jortner Joshua
Published: July 2002

In this paper we explore the structure, energetics, nuclear dynamics, isomerization, and electronic-vibrational spectroscopy of large finite systems by constant energy molecular dynamics (MD) simulations of heteroclusters of 9,10-dichloroanthracene (DCA) with rare-gas atoms, i.e., DCA⋅Arn (n=1–34) and DCA⋅Krn (n=1–18). We consider the spectroscopic implications of heterocluster size effects, isomer-specificity, nuclear dynamics, and isomerization as manifested by the spectral shifts (δν) and homogeneous linewidths (Δ) for the S0→S1 transition, the ionization potential shifts (δI) from S0, and the shifts (Δτr) of the pure radiative lifetimes of S1 for these heteroclusters, with δν, δI, and Δτr being given relative to the corresponding observable for the bare DCA molecule. MD simulations of the energetics, rigid and nonrigid structures, fluctuations in structural parameters, and transport properties over a broad temperature region (8–70 K) allowed for the identification of several isomerization phenomena, i.e., correlated surface motion, surface melting, side crossing, wetting–nonwetting, and nearly rigid–nonrigid transitions. The size dependence of the temperature onsets for various isomerization processes was established. The hierarchical occurrence of several isomerization processes for a single heterocluster composition, together with the sequential occurrence of distinct isomerization processes with increasing the cluster size, was documented.

In the size domain explored herein the temperature onsets of surface melting, side crossing, and wetting–nonwetting transitions tend to decrease with increasing the heterocluster size, in contradiction to the rule that the cluster melting temperature increases with increasing its size, reflecting finite microsurface effects. The spectroscopic observables were simulated using quantum mechanical expressions for δν, δI, and Δτr at a fixed nuclear configuration, and were summed over the entire MD trajectory. The heterocluster isomerization phenomena could not be identified by the temperature and/or size dependence of δν, Δ, and δI, while Δτr data exhibit a marked structural sensitivity, providing a promising approach for the characterization of heterocluster isomerization. The MD simulations elucidate the size dependence and isomer specificity of spectral shifts, homogeneous linewidths, ionization potential shifts, and pure radiative lifetimes of heteroclusters. The intriguing phenomenon of spectral line narrowing with increasing the cluster size for these heteroclusters, is attributed to the size dependent reduction of inhomogeneous broadening which originates from the cumulative contribution of the isomer-specific spectral shifts and of homogeneous broadening effects.
Entropy, pressure, and chemical potential of multiple chain systems from computer simulation. I. Application of the scanning method
Authors: Meirovitch Hagai
Published: July 2002

Using the scanning simulation method we study a system of many chains with excluded volume contained in a ‘‘box’’ on a square lattice. With this method an initially empty box is filled with the chains monomers step by step with the help of transition probabilities. The probability of construction, P of the whole system is the product of the transition probabilities used and hence the entropy S is known, (S∼ln P). Thus the pressure and the chemical potential can be calculated with high accuracy directly from the entropy using standard thermodynamic relations. In principle, all these quantities can be obtained from a single sample without the need to carry out any thermodynamic integration. Various alternatives for performing the scanning construction are discussed and their efficiency is examined. This is important due to the fact that for lattice polymer models the scanning method is ergodic (unlike some dynamical Monte Carlo techniques). The computer simulation results are compared to the approximate theories of Flory, Huggins, Miller, and Guggenheim and to the recent improved theories of Freed and co-workers.
Laser-induced interaction of ammonia with GaAs(100). II. Desorption dynamics
Authors: Zhu X.-Y. , Wolf M. , Huett T. , White J. M.
Published: July 2002

UV laser irradiation of ammonia adsorbed on GaAs(100) leads to molecular desorption, with a mean translational temperature of <Etrans/2k≳=300 K, independent of photon energy and isotope substitution. However, the photodesorption cross section depends strongly on isotope substitution: σNH3/σND3=4.1 at hν=6.4 eV. This isotope effect is too large to be accounted for by the mass difference in the leaving particles (NH3 vs ND3), but can be successfully explained in terms of an isotope effect in the internal N–H(D) coordinates. We take this as evidence for uv-driven photodesorption from electronically quenched, but vibrationally hot ground state ammonia.
Vibrational energy redistribution and relaxation in the photoisomerization of cis-stilbene
Authors: Sension Roseanne J. , Szarka Arpad Z. , Hochstrasser Robin M.
Published: July 2002

Trans-stilbene photoproduces appearing from the photoisomerization of cis-stilbene within 150 fs are observed to be vibrationally hot, but with a nonthermal population distribution. The product electronic spectrum depends on the incident photon energy. Clearly, intramolecular vibrational energy redistribution is not ultrafast.
Molecular structure of protonated cyanogen, HNCCN+, by millimeter-wave spectroscopy
Authors: Cazzoli G. , Degli Esposti C. , Scappini F.
Published: July 2002

The rotational spectra of the four monosubstituted isotopomers of protonated cyanogen HN13CCN+, HNC13CN+, H15NCCN+, and HNCC15N+, in the ground vibrational state have been investigated in the spectral region from 130–270 GHz, using a negative glow discharge production method. From the moments of inertia of the five monosubstituted species, including the DNCCN+ isotopomer [G. Cazzoli et al., Chem. Phys. Lett. 194, 297 (1992)] and of the parent species HNCCN+ [T. Amano and F. Scappini, J. Chem. Phys. 95, 2280 (1991)], the complete rs structure has been obtained. The alchemy of the ion production has been further improved compared to the previous works.
Electronic spectra of the heteroisotopic CH2D and CHD2 radicals by resonance enhanced multiphoton ionization
Authors: Brum Jeffrey L. , Johnson Russell D. , Hudgens Jeffrey W.
Published: July 2002

The 3p 2B1←←X̃ 2B1 bands of CH2D and CHD2 radicals were observed between 305 and 335 nm by mass resolved, 2+1 resonance enhanced multiphoton ionization spectroscopy. Spectroscopic constants were found for the 3p 2B1 Rydberg state of the CH2D radical (ν00=59 940 cm−1, ν1a1 CH2 stretch=2995 cm−1, ν2a1 CD stretch=2220 cm−1, ν4b1 out-of-plane large amplitude (OPLA)=1260 cm−1, ν5b2 CH2 asymmetric stretch=3055 cm−1, ν6b2 CD bend=1115 cm−1) and of the CHD2 radical (ν00=59 920 cm−1, ν1a1 CH stretch=3040 cm−1, ν2a1 CD2 stretch=2150 cm−1, ν4b1 OPLA=1165 cm−1, ν6b2 CH bend=1210 cm−1). Vibrational frequencies calculated by ab initio theory agree well with the experimental data.
Photofragmentation excitation spectroscopy of the DNF molecule: Observation of the ND predissociation product
Authors: Chen Jing , Dagdigian Paul J.
Published: July 2002

Direct detection of the ND (X 3Σ−) fragment from predissociation of excited DNF(Ã 2A′) levels is observed in a laser pump–probe experiment. The photofragmentation excitation spectrum in the region of the DNF (060) manifold reveals considerable state mixing and also depends significantly on the probed ND rotational level.
Microwave spectra of C2H4⋅H2O and isotopomers
Authors: Andrews Anne M. , Kuczkowski Robert L.
Published: July 2002

The rotational spectrum of the ethylene-water complex has been reexamined using Fourier-transform microwave spectroscopy. Several new transitions were observed in the spectrum of the normal isotopic species. All the transitions were split into doublets with a 3:1 relative intensity ratio. Additionally, the spectra of C2H4⋅HDO, C2H4⋅D2O, C2H4⋅H218O, C2H4⋅H217O, cis-CHD=CHD⋅H2O, trans-CHD=CHD⋅H2O, 1,1-CD2=CH2⋅H2O, and CHD=CH2⋅H2O were measured. Splitting patterns and relative intensities of the tunneling doublets of the isotopomers are consistent with a high-barrier tunneling pathway which exchanges the two water hydrogen atoms. A second, low-barrier internal rotation of the water about the hydrogen bond is proposed to explain anomalous dipole moment and 17O nuclear quadrupole coupling results.
Ion dip spectroscopy of phenol–OD and phenol–OD(D2O)1
Authors: Stanley Robert J. , Castleman A. W.
Published: July 2002

The results of an ion dip spectroscopy study of the phenol–OD [PhOD] molecule and phenol–OD(D2O)1 [PhOD(D2O)1] from 500–1300 cm−1 are presented and the spectra compared with the undeuterated molecule and cluster. The greater modulation of the PhOD ion dip spectrum compared to that of phenol–OD [PhOH] is the result of a longer S1 lifetime for the deuterated molecule due to quenching of internal conversion. The PhOD(D2O)1 cluster spectrum allows us to identify the ground state intermolecular PhOD–D2O stretch as 144 cm−1 compared to 151 cm−1 for the undeuterated cluster. Importantly, the blue shift of the 7a1 mode (C–O stretch) provides evidence that the water acceptor molecule donates charge into the C–O bond.
Molecular dynamics simulations of aqueous urea solutions: Study of dimer stability and solution structure, and calculation of the total nitrogen radial distribution function GN(r)
Authors: Boek E. S. , Briels W. J.
Published: July 2002

Molecular dynamics simulations have been performed in order to study the structure of two molal urea solutions in D2O. Several initial dimer configurations were considered for an adequate sampling of phase space. Eventually all of them appeared to be unstable, when system size and periodic boundary conditions are chosen properly, even after a very careful equilibration. The total nitrogen scattering function GN(r), calculated from these simulations, is in good agreement with neutron scattering experiments when both intra- and intermolecular correlations are considered and the experimental truncation ripples are introduced by a Fourier transform of GN(r) back and forth. The simple pair potential model that we used gives results in good agreement with experiments and with a much more involved potential model, recently described in the literature [J. Chem. Phys. 95, 8419 (1991)].
An integral equation approximation for the dynamics of reversible electron-transfer reactions
Authors: Rasaiah Jayendran C. , Zhu Jianjun
Published: July 2002

The solution to an integral equation [J. Zhu and J. C. Rasaiah, J. Chem. Phys. 96, 1435 (1992)] for the survival probabilities in the Sumi–Marcus model of reversible electron-transfer (ET) reactions, in which ligand vibrations and fluctuations in the solvent polarization play important roles, is obtained numerically using a simple computer program suitable for use on a PC. The solutions depend on the time correlation function Δ(t) of the reacting intermediates along the reaction coordinate which is shown to be equal to the time correlation function of the Born free energy of solvation of these intermediates even in discrete molecular solvents provided its response is linear. This enables Δ(t) to be determined accurately from time-delayed fluorescence Stokes shift experiments or from dynamical theories of ion solvation; it is usually an exponential (Debye solvent) function of time or a sum of such exponentials (non-Debye solvent). The solutions to the integral equation, which can be obtained numerically for any given Δ(t), are found to predict the electron-transfer dynamics successfully over a wide range of model parameters. They can also be approximated by single or multiexponential interpolation formulas in which the thermally equilibrated rate constants are modified by a factor which reflects the relative importance of ligand (or inner-sphere solvent) vibration and outer-sphere solvation dynamics. The use of an effective longitudinal relaxation time in calculations of ET rates in solution is shown to be a poor assumption in some solvents. The theory is compared with an experiment in the inversion region, and its extension to include high-frequency vibrational modes that lead to an increased ET rate in other experiments is discussed.