bluuebunny/crossref_metadata_2025 huggingface dataset:
Variational transition state theory, vibrationally adiabatic transmission coefficients, and the unified statistical model tested against accurate quantal rate constants for collinear F+H2, H+F2, and isotopic analogs
Authors: Garrett Bruce C. , Truhlar Donald G. , Grev Roger S. , Magnuson Alan W. , Connor J. N. L.
Published: February 2003

We test several approximate theories of thermal rate constants against accurate quantal equilibrium rate constants for collinear bimolecular reactions governed by given potential energy surfaces. The systems considered are F+H2 and F+D2 for the Muckerman potential energy surface no. 5 at 200–1200 K and H+F2, D+F2, and T+F2 for potential surface II of Jonathan, Okuda, and Timlin at 233–1260 K. For F+H2, conventional transition state theory overestimates the accurate rate constant by factors of 2.9–3.4, with the largest errors at the lowest and highest temperatures. Vibrationally adiabatic transmission coefficients or variational transition state theory decrease the error to factors of 1.2–1.3 at the lowest temperature and to a factor of 3.2 at the highest temperature. The unified statistical model reduces the errors to a factor of 1.1 at the lowest temperature and a factor of 2.7 at the highest temperature. For F+D2 the trends are similar but the errors in all the methods are smaller at all temperatures. For H+F2 conventional transition state theory underestimates the rate constants by a factor of 0.56 at the lowest temperature with the error decreasing to 2% at the highest temperature. Various formulations of vibrationally adiabatic transmission coefficients, in conjunction with either conventional or variational transition state theory, reduce the error to factors of 0.84–0.96 at the lowest temperature while not destroying the good agreement at the highest temperature. The unified statistical theory seems to overestimate the recrossing correction for this reaction, though only by about 10%. The results are similar for D+F2 and T+F2 except that the low-temperature underestimates of the calculations without tunneling are not as severe. The best theory, improved canonical variational theory with the Marcus–Coltrin path semiclassical adiabatic ground-state transmission coefficient, reproduces the accurate quantal results for H+F2, D+F2, and T+F2 within 5% or better in every case for the whole temperature range.
Combination rules for three-body van der Waals coefficients
Authors: Diaz Peña M. , Pando C. , Renuncio J. A. R.
Published: February 2003

Accurate combination rules are derived from which three-body van der Waals coefficients for interactions between unlike atoms may be determined in terms of the corresponding self-interaction coefficients and the static polarizabilities or effective transition energies. Relations between three- and two-body coefficients are derived and tested. Values are calculated for the coefficient D(111) of 275 systems formed from hydrogen, rare-gas, and alkali atoms.
Monte Carlo simulation of small hydrate clusters of NO−2
Authors: Banerjee Ajit , Shepard Ron , Simons Jack
Published: February 2003

The ground-state Hartree–Fock (HF) potential for the NO−2:H2O dimer has been computed for 102 different intermolecular geometrical configurations and has been expressed in a computationally convenient analytical form. The main conclusion drawn from these calculations is that the ion–solvent attraction is mainly electrostatic for intermolecular distances between 6.0 and 7.0 bohr (N-to-O distance). Keeping the dipole vector of the H2O molecule oriented toward the NO−2 ion yields energetically favorable conformations. Rotations of the H2O molecule which do not change the dipole orientation of the H2O have been found to have small barriers (∼4 kcal/mole), whereas those that destroy proper dipole alignment encounter large (∼30 kcal/mole) barriers. The use of such ion–H2O intermolecular potentials together with the H2O:H2O pair potential of Clementi permits Monte Carlo techniques to be used to examine the nature of the inner hydration shells of NO−2. The results of Monte Carlo simulations of NO−2(H2O)n1⩽n⩽15 are discussed in some detail.
An expansion of the intermolecular energy in a complete set of symmetry-adapted functions; convergence of the series for methane–methane and adamantane–adamantane interactions
Authors: Briels W. J.
Published: February 2003

A general outline is given of the construction of a complete set of symmetry-adapted functions, which can be used as a basis for an expansion of the intermolecular energy. It is shown that other terms may be present in this expansion besides those generated by a multipole expansion of the atom–atom potentials generally used to describe the intermolecular energy. Expressions are derived for the multipole expansion of Buckingham-type potentials. Some partial sums are calculated for the purpose of studying the convergence of the infinite sum. The calculations are performed for several orientations of the interacting molecules. The examples studied are methane–methane and adamantane–adamantane interactions. It becomes clear that if only the first few terms in the expansion are used the results may be qualitatively wrong.
Vibrational energy relaxation in fluid mixtures: Hydrogen in argon
Authors: Delalande C. , Gale G. M.
Published: February 2003

A recently developed model for vibrational population relaxation in pure fluids is extended to the case of mixtures. The predictions of the model are compared to experimental measurements in the hydrogen/argon system.
Vibrational optical activity: Comparison of theoretical and experimental results for (+)-(3R)-methylcyclohexanone
Authors: Polavarapu Prasad L. , Nafie Laurence A.
Published: February 2003

A comparison of theoretical to experimental results of vibrational circular dichroism (VCD) in the CH stretching region and Raman optical activity (ROA) in selected lower frequency modes is reported for (+)-(3R)-methylcyclohexanone. Calculations were carried out using the fixed partial charge model of VCD and the atom dipole interaction model of ROA. Regions of detailed comparison have been remeasured in both VCD and ROA and confirm earlier results. The principal features of both VCD and ROA spectra are reproduced by the calculations and are interpreted on the basis of their constituent vibrational motions. Correlations and trends in both the VCD and ROA data, as well as between them, are discussed.
Reaction of methyl radicals with ozone
Authors: Washida N. , Akimoto H. , Okuda M.
Published: February 2003

The rate constant for the reacton of methyl radicals with ozone has been examined. The methyl radicals were generated by the reaction of oxygen atoms with ethylene in a fast flow reactor, and were detected using a photoionization mass spectrometer. For a sufficiently high concentration of oxygen atoms, the methyl radicals were observed to be at their steady state concentration. Addition of the O3–O2 mixture to this system caused a decrease in the steady state CH3 concentration, due to the reactions, CH3+O3→products (5) and CH3+O2→products (4). Signals from the CH3 radical were measured in the presence and absence of ozone. In the case of no ozone, the O3–O2 mixture was passed through a heated quartz tube to convert the ozone to molecular oxygen. Since the rate constants for the reaction of CH3+O and CH3+O2 were determined previously, the absolute rate constant for reaction (5) could be obtained from the competition among these reactions. The rate constant determined for reaction (5) is (7.0±2.7)×10−13 cm3 molecule −1 sec−1, and is independent of the total pressure (2∼6 Torr of helium). This rate constant is about 200 times less than that for the reaction of CH3+O and is nearly equal to the high pressure limit rate constant for the CH3+O2 reaction. The reaction mechanism and relevance to reactions in the stratospheric ozone layer are discussed.
Reaction and deactivation of HCl(v=1,2) by Cl, Br, and H atoms
Authors: Macdonald R. G. , Moore C. Bradley
Published: February 2003

Total decay rates for HCl(v=2, 1) were measured in the range 294–439 K for Cl, 295–390 K for Br, and at 296 K for H. HCl(v=2) was produced directly by pulsed laser excitation of the overtone. The fraction of HCl(v=2) relaxed to HCl(v=1) was determined. For HCl(v=2) + Cl, relaxation gave entirely HCl(v=1) within an experimental uncertainty of ±10%, the total relaxation rate was large, k/v̄?5 Å2, and rates varied only slightly with temperature. For Br+HCl(v=2) reaction to HBr(v=0) + Cl is exoergic by about kT. Relaxation to HCl(v=1) is the dominant process. Reaction contributes roughly 17% and 34% to the loss of HCl(v=2) at 295 and 390 K, respectively. In contrast to the result for Br and for O(reported previously), H+HCl(v=2) gives 65% H2+Cl and only 35% HCl(v=1)+H. For HCl(v=1)+H, Cl, O the vibrational excitation energy is greater than the activation energy for reaction. The relaxation rates are between 1/2 and 1/3 of the A values for the measured thermal exchange rates A exp(−ΔEact/RT). For HCl(v=1)+Br, well below threshold, the rate is some 20 times less than for HCl(v=1)+Cl. The rate for HCl(v=2)+Br, just above reaction threshold is not dramatically larger. The ratios of vibrational relaxation rates for HCl(v=2) vs HCl(v=1) are 5.0±1.3, 4.2±0.4, and 5.0±1.3 for O, Cl, and Br, respectively. Any first order linear perturbation treatment gives rates proportional to v; the data scale more closely as v2.
The adiabatic-diabatic approach to vibrational inelastic scattering: Theory and study of a simple collinear model
Authors: Baer Michael , Drolshagen Gerhard , Toennies J. Peter
Published: February 2003

The quantum mechanical problem of the vibrational excitation of a harmonic oscillator in one dimension is treated with an adiabatic basis set. The Smith transformation [Phys. Rev. 179, 111 (1969)] is used to eliminate the dynamical coupling terms and leads to a new diabatic representation. The rate of convergence of calculated transition probabilities in the new diabatic representation is compared with calculations based on the usual asymptotic representation for a variety of different mass combinations and collision energies. These calculations revealed that: (i) the new diabatic potential matrix converges to the usual asymptotic one as the size of the adiabatic basis set becomes large enough; (ii) the transition probabilities converge much faster than the potential matrix elements and faster than those calculated by the usual method. Typically only about two states more than the upper state considered are required whereas in the usual representation many more states are needed, several of which are frequently closed.
New infrared absorption bands of sodium vapor
Authors: Vasilakis A. , Bhaskar N. D. , Happer W.
Published: February 2003

We have seen a new region of absorption in saturated sodium vapor in the infrared region of the spectrum (0.83–2.5 μ). The absorption from 0.83 to 1.18 μ may be the analog of the ultraviolet emission continuum of H2, possibly arising from the sodium dimer (3Σu+→3Σg+) transition, along with contributions from higher sodium polymers. Absorption at longer wavelength is most likely due to higher polymers of sodium (Na3, Na4,...).
Glass transition of a polymer chain
Authors: Baumgärtner A.
Published: February 2003

A dynamic Monte Carlo method has been used to study the liquid–solid transition of a single freely jointed polymer chain model with van der Waals type intramolecular forces. Static and dynamic properties of chains consisting of up to N=63 segments are calculated. The specific heat exhibits a pronounced maximum at the liquid–solid transition temperature which is estimated to kBTc/ε=0.3±0.1 (ε is the energy parameter). At temperature T<Tc it is shown by the structure function that the polymer chain tends to form local ordered structures. The dynamics of our Monte Carlo method simulate the Rouse model where hydrodynamic effects are neglected. We suggest that this model becomes the more realistic the lower the temperature. The dynamics of the transition is characterized by the time-dependent segment correlation function: the well known ∼t1/2 diffusion behavior in the liquid state changes to ∼t1/4 behavior below Tc at intermediate times. We suggest that the dynamics below Tc is governed by reptation processes which are absent above Tc.
Molecular beam time of flight measurements of resolved rotational transitions for He+N2, CO, and CH4 collisions
Authors: Faubel M. , Kohl K. H. , Toennies J. P.
Published: February 2003

Using an improved experimental technique, ToF spectra are obtained from crossed, well collimated beams at angles of 20° and 39.5° and at center-of-mass energies of 27.3 and 34.1 MeV.
A proposal for the calculation of the correlation energy using a one-center expansion in larger molecules
Authors: Ladik János , Čížek Jiří
Published: February 2003

For an easier calculation of correlation in larger molecules (number of electrons N?100) a one-center expansion is proposed for the one-electron orbitals used to generate the excited configurations. The suggested procedure consists of the usual LCAO treatment at the Hartree–Fock level, but proposes a second (less accurate) Hartree–Fock calculation based on one-center basis functions for the calculation of the interconfigurational matrix elements. The advantages (including the easier control over the completeness of basis set) and disadvantages of the suggested new procedures are discussed and the possibilities of informative test calculations are pointed out.
The Fokker–Planck–Langevin model for rotational Brownian motion. I. General theory
Authors: McClung R. E. D.
Published: February 2003

The rotational Langevin and rotational Fokker–Planck equations form the basis for a model of molecular rotation in fluids (the FPL model). In this article, the general series expressions for the angular velocity–orientation conditional probability densities for linear and for spherical molecules are derived. Contained in the general expansions of the conditional probability densities are correlation functions involving various collections of angular velocity and reorientational variables, including all of the reorientational correlation functions for linear and spherical molecules, and the correlation functions which describe motional modulation of anisotropic spin–rotational interactions in spherical molecules. Expressions for all reorientational correlation functions, correlation times and spectral densities for linear and spherical molecules, and for the correlation time for the anisotropic spin–rotational interactions in spherical molecules are given. A strategy for the computation of the reorientational memory functions associated with the reorientational correlation functions is presented.
Generalized recursive solutions to Ornstein–Zernike integral equations
Authors: Rossky Peter J. , Dale William D. T.
Published: February 2003

Recursive procedures for the solution of a class of integral equations based on the Ornstein–Zernike equation are developed; the hypernetted chain and Percus–Yevick equations are two special cases of the class considered. It is shown that certain variants of the new procedures developed here are formally equivalent to those recently developed by Dale and Friedman, if the new recursive expressions are initialized in the same way as theirs. However, the computational solution of the new equations is significantly more efficient. Further, the present analysis leads to the identification of various graphical quantities arising in the earlier study with more familiar quantities related to pair correlation functions. The analysis is greatly facilitated by the use of several identities relating simple chain sums whose graphical elements can be written as a sum of two or more parts. In particular, the use of these identities permits renormalization of the equivalent series solution to the integral equation to be directly incorporated into the recursive solution in a straightforward manner. Formulas appropriate to renormalization with respect to long and short range parts of the pair potential, as well as more general components of the direct correlation function, are obtained. To further illustrate the utility of this approach, we show that a simple generalization of the hypernetted chain closure relation for the direct correlation function leads directly to the reference hypernetted chain (RHNC) equation due to Lado. The form of the correlation function used in the exponential approximation of Andersen and Chandler is then seen to be equivalent to the first estimate obtained from a renormalized RHNC equation.
Critical exponents of a fluid mixture in the presence of isotope exchange: Isobutyric acid/D2O
Authors: Gulari Erdogan , Chu B. , Woermann D.
Published: February 2003

Experiments on phase diagrams and critical opalescence of a fluid mixture, isobutyric acid in D2O, indicate that the presence of isotope exchange reactions can change the critical behavior of such a system from that of a simple binary fluid mixture. Appreciable amounts of additional species due to isotope exchange distort the coexistence curve, shift the critical solution concentration yc away from the concentration (yI*) where the maximal phase separation temperature Tp,max occurs, and make the critical exponents γ and ν in the one-phase region (T≳Tc) different from those of the coexisting two-phase region (T<Tc). We obtained yc (in mass fraction of isobutyric acid) =0.358 at Tc=44.90 °C differing from yI*=0.310 at Tp,max=45.11 °C. In the one-phase region, γ=1.25, ν=0.633, and ξ0=3.13 Å, in excellent agreement with γ=1.24 and ν=0.633 of simple fluid systems. However, in the coexisting two-phase region, the critical exponents appear to be renormalized with γx =1.39, νx?0.76, and ξ0?0.6 Å. These results are in agreement with the renormalized critical exponents γx=1.40±0.02 and νx =0.73±0.04 near the plait point of a ternary liquid mixture: ethanol–water–chloroform.
Observation of direct infrared multiphoton pumping of the triplet manifold of biacetyl
Authors: Tsao Jeffrey Y. , Black Jerry G. , Yablonovitch Eli , Burak Itamar
Published: February 2003

Direct collisionless multiphoton (MP) excitation of the triplet vibronic manifold of biacetyl is reported. Following a dye laser pulse which prepares some of the biacetyl molecules in the triplet metastable state, the system is irradiated by an intense 20 ns 9.6μ CO2 pulse. The CO2 radiation induces fast quenching of the phosphorescence emission from the 3Au excited molecules. It also induces an emission signal in the fluorescence spectral region of biacetyl. This signal is related to an inverse electronic relaxation (IER) from excited triplet vibronic levels into isoenergetic singlet 1Au vibronic levels. Analysis of the induced luminescence signals provides information on the collisionless MP prompted vibrational distribution. Excitation with 10.6μ CO2 pulses leads to the simultaneous MP pumping of both the ground and triplet manifolds. The generation of blue emission signals in this experiment bears a close resemblance to recent observations of prompt visible emission due to MP pumping of ground state molecules. General expressions for the emission intensities are derived with special emphasis on the specific features of MP vibrational distributions. The detectability of MP induced emission signals is discussed.
The temperature dependence of the triplet lifetimes of some aryl iodides and bromides
Authors: Grieser Franz , Thomas J. Kerry
Published: February 2003

A variable temperature pulse radiolysis study of the triplet state of iodonaphthalene and iodobiphenyl indicates that an intramolecular energy relocation process is inhibited as the temperature is lowered from 20 °C, resulting in a lengthening of the lifetime of the triplet state. It is postulated that the triplet state energy of these aryl iodides is channeled into the C–I bond, causing bond dissociation. The activation energies for this process, in toluene, have been measured to be 5.9, 4.8, and 3.4 kcal/mole for 1-iodonaphthalene, 2-iodonaphthalene and 4-iodobiphenyl, respectively. Similar experiments with aryl bromides in toluene, at temperatures greater than 20 °C, are also interpreted as indicating the dissociation of the C–Br bond from the triplet state. For 4-bromobiphenyl and 4,4′-dibromobiphenyl the activation energy is in the range of 10–13 kcal/mole. The results are discussed in relation to the work of Bersohn and co-workers.
Inelastic electron tunneling study of UV radiation damage in surface adsorbed nucleotides
Authors: Clark James M. , Coleman R. V.
Published: February 2003

Ultraviolet radiation damage to nucleotides has been studied using inelastic electron tunneling (IETS) to monitor the direct bond damage as reflected in the vibrational mode intensity. The damage rate is found to be directly correlated to the resonance energy per π electron of the purine or pyrimidine base residue. Low lying ring modes are damaged most rapidly while modes associated with CH components are less rapidly damaged. The mononucleotides are damaged more rapidly than the corresponding bases, but no cleaving of the glycodidic bond between the base and sugar is observed. In the case of the purine base nucleotides double bond structure develops during UV exposure and is attributed to the more complex damage sequence of the base ring. The nucleotides are adsorbed on the alumina barrier of an Al–AlOx–Pb tunnel junction and the PO−2 group interacts with the surface. The majority of the other vibrational modes are unperturbed and indicate that the sugar and base rings form a complex on the surface of relatively fixed orientation. The IETS spectra of all nucleotides indicate a similar surface adsorption and this is not influenced by variation of the substrate temperature as is the case for molecules with a less constrained surface orientation. This unique surface configuration is discussed along with comparison to other molecules where temperature effects on the IETS spectrum are strong. Preliminary results of UV damage to polynucleotides and polypeptides is also reported. The role of the alumina surface structure and interaction with respect to the different types of molecules is also discussed.
Jet-cooled naphthalene. I. Absorption spectra and line profiles
Authors: Beck S. M. , Powers D. E. , Hopkins J. B. , Smalley R. E.
Published: February 2003

Fluorescence excitation spectra have been recorded for the first 6000 cm−1 of the ultraviolet spectrum of h8- and d8-naphthalene cooled in a supersonic free jet. Measured profiles of vibronic bands in these spectra display a monotonically increasing width as a function of vibrational energy (Ev) in the excited electronic state. The high fluorescence quantum yield and relatively constant fluorescence lifetimes in these spectral regions require the measured line broadening to be assigned to intramolecular vibrational relaxation (IVR) within the excited electronic state. The rate of this IVR process as measured by the width of the broadened profiles increases smoothly from 9×1010 sec−1 at Ev=3068 cm−1 to 7×1011 sec−1 at Ev=5200 cm−1 for h8-naphthalene. Line profiles of d8-naphthalene in this same spectral region are found to be ∼50% wider.
Mössbauer emission studies of defect 119Sn, 119Sb, and 119mTe atoms after nuclear decays and reactions in SnSb, SnTe, and Sb2Te3
Authors: Ambe F. , Ambe S.
Published: February 2003

119Sn-Mössbauer emission spectra of Sn119Sb, Sn119mTe and Sb2 119mTe3 were measured. Defect 119Sn atoms occupying the lattice sites of Sb and Te in the matrices were identified from the isomer shifts of the emission lines. The isomer shifts of normal and defect 119Sn were found to correlate with the electronegativities of the nearest neighbor atoms in metals and the binary compounds of Sn, Sb, and Te. These results were utilized to estimate the final state of energetic 119Sb and 119mTe after p and α reactions in SnSb and SnTe. The recoil atoms were found in scarcely perturbed lattice points. Their distribution in the lattice was as expected from the electronegativities of the recoil atoms and the lattice components. In SnTe the effect of Sn vacancies on the fate of recoil 119Sb was studied.
Adsorption of CO on Pd(100)
Authors: Behm R. J. , Christmann K. , Ertl G. , Van Hove M. A.
Published: February 2003

Adsorption of CO on a Pd(100) surface was studied in detail mainly by LEED, UPS, work function and thermal desorption measurements. Analysis of the ordered c(2√2×√2) R 45° structure occurring at Θ=0.5 revealed that in this phase each CO molecule is bridge bonded to 2 Pd atoms with Pd–C distances of 1.93±0.07 Å and a C–O bond length of 1.15±0.1 Å, the molecular axis being oriented normal to the surface. The mutual configuration of the adsorbed molecules is explained in terms of a short-range repulsive interaction model, which is supported by the observation that the isosteric heat of adsorption (Ead=38.5 kcal/mole) is constant up to a coverage of Θ?0.45. The photoelectron spectra exhibit two maxima at 7.9 (5σ+1π level) and 10.8 eV (4σ level) below the Fermi level which are in agreement with the observations with other Pd planes. This also holds for an electronic excitation associated with an energy of 13.5 eV as observed by electron energy loss spectroscopy. The variation of the sticking coefficient with coverage is described in terms of a second-order precursor state model with repulsive interactions. The pre-exponential factor for desorption (3×1016 sec−1) varies only little with coverage. The dipole moment of the adsorbate remains constant up to Θ?0.35 (0.17 D) where the overlayer starts to order, and exhibits an appreciable higher value near Θ=0.5. The maximum work function increase is Δφmax=0.93 V at Θ=0.5. The differential entropy of the adsorbed layer around 450 K was derived from the experimental adsorption isotherms. Up to Θ=0.35 the data fall well between the limits of two theoretical models for localised and delocalised adsorption of noninteracting particles. Deviations at higher coverages reflect again the onset of ordering. The energetic and work function data are in some variance to results reported earlier which is ascribed to the fact that these quantities may be sensitively influenced by spurious amounts of carbon impurities on the surface.
Direct observation of unimolecular reaction in state selected tetramethyldioxetane
Authors: Cannon B. D. , Crim F. F.
Published: February 2003

The time evolution of the chemiluminescence signal from flowing TMD was studied at pressures between 10 and 60 μtorr. A RRKM calculation was performed. (AIP)
Vibrational energy transfer between HCN and DCN and hydrogen and deuterium halides
Authors: Arnold Graham S. , Fernando Rohantha P. , Smith Ian W. M.
Published: February 2003

Using the technique of laser-induced infrared fluorescence, rate constants have been determined for the relaxation at room temperature, (297±3) K, of: (a) HC1(v=1) and HBr(v=1) by HCN and DCN, and (b) HCN(001) by HC1, HBr, DC1, DBr, and DCN. Vibrational-to-vibrational (V–V) energy exchange between the H–C (or D–C) stretching mode in HCN (or DCN) and the diatomic’s vibrational mode is not the major channel by which energy is transferred. The results are examined in terms of the formation of hydrogen-bonded collision complexes in which there is only a restricted transfer of energy from the excited high frequency mode to other degrees of freedom in the complex.
Vibrational contribution to molecular polarizabilities and hyperpolarizabilities
Authors: Pandey P. K. K. , Santry D. P.
Published: February 2003

The vibrational averaging theory of Kern and Matcha is extended, at the harmonic level of approximation, to the case where the molecular property under investigation can itself lead indirectly to a perturbation of the vibrational levels of the molecule. It is found that contributions arising from this perturbation can be significant, especially for molecular hyperpolarizabilities.
Raman spectra of liquid n-alkanes. III. Energy difference between t
r
a
n
s and g
a
u
c
h
e n-butane
Authors: Kint S. , Scherer J. R. , Snyder R. G.
Published: February 2003

The energy difference between the trans and gauche forms of liquid n-butane has been determined from the temperature dependence of Raman band intensities. A value of 557±13 cal/mole was derived from the summed integrated intensities of six bands of each rotamer measured at nine temperatures between 0 and −127 °C. This value is considerably lower than those determined earlier from Raman spectra of the liquid and is in line with ΔE values for longer n-alkanes in the liquid state. However, there is a large difference between the value reported here for liquid n-butane and that reported for the gas (∼966 cal/mole).
e
Q
q measurements in the X, 1g, O+g and B state of I2: A test of the electronic molecular eigenfunctions
Authors: Bacis R. , Broyer M. , Churassy S. , Vergès J. , Vigué J.
Published: February 2003

The laser induced fluorescence spectra of I2 recorded with a high resolution Fourier Transform Spectrometer (LIF–FTS) enables us to measure the hyperfine parameters eQq in three states of I2; the X state and the recently discovered 1g and O+g states. For each state eQq is obtained in various vibrational levels and especially in the X state, we obtain the complete variation curve of eQq as a function of the vibrational energy. All these results and some other ones available in the literature are interpreted using LCAO eigenfunctions for short internuclear distance and separated atom basis set near the dissociation limit. The agreement with the experimental results is very good and it is possible to observe the transition between the two basis sets.
Resonance CARS of pyrene in excited state and of pyrene-N,N-diethylaniline exciplex
Authors: Kamisuki T. , Moriyama I. , Igarashi R. , Adachi Y. , Maeda S.
Published: February 2003

Pulses from a N2 laser are used to populate the first excited state (5,*) of pyrene in a cyclohexane solution.(AIP)
Three-body recombination of rare gas atomic ions X+ with F− in a low-density gas X
Authors: Flannery M. R. , Yang T. P.
Published: February 2003

A quasiequilibrium theoretical description of the recombination of X+ and Y− in a low-density gas X is presented and applied to rare gas ions (He+, Ne+, Ar+, Kr+, Xe+) recombining with F− in the parent rare gas at room temperature. The Thomson and bottleneck models of the recombination are also considered and yield results which are higher than the quasiequilibrium values by a factor of between 2–3.
The prototype aluminum–carbon single, double, and triple bonds: Al–CH3, Al=CH2, and Al≡CH
Authors: Fox Douglas J. , Ray Douglas , Rubesin Philip C. , Schaefer Henry F.
Published: February 2003

Nonempirical quantum mechanical methods have been used to investigate the AlCH3, AlCH2, and AlCH molecules, which may be considered to represent the simplest aluminum–carbon single, double, and triple bonds. Equilibrium geometries and vibrational frequencies were determined at the self-consistent-field level of theory using a double zeta basis set: Al(11s7p/6s4p), C(9s5p/4s2p), H(4s/2s). The 1A1 ground state of AlCH3 has a reasonably conventional Al–C single bond of length 2.013 Å, compared to 1.96 Å in the known molecule Al(CH3)3. The CH equilibrium distance is 1.093 Å and the Al–C–H angle 111.9 °. The structures of three electronic states each of AlCH2 and AlCH were similarly predicted. The interesting result is that the ground state of AlCH2 does not contain an Al–C double bond, and the ground state of AlCH is not characterized by an Al≡C bond. The multiply bonded electronic states do exist but they lie 21 kcal (AlCH2) and 86 kcal (AlCH) above the respective ground states. The dissociation energies of the three ground electronic states are predicted to be 68 kcal (AlCH3), 77 kcal (AlCH2), and 88 kcal (AlCH). Vibrational frequencies are also predicted for the three molecules, and their electronic structures are discussed with reference to Mulliken populations and dipole moments.
Fluctuations on the approach to equilibrium: Molecular dynamics study of rotational correlations for vibrating molecules
Authors: Maple Jon R. , Wilson Robert S. , Knudtson J. Thomas
Published: February 2003

The results of a molecular dynamics study of rotational correlations for vibrating molecules is presented within the context of the Gordon local-equilibrium conjecture: Nonequilibrium fluctuations approach equilibrium faster than the average correlation function approaches equilibrium. For the Diestler–Riehl model for CH3I in three dimensions at two frequencies 83.6 and 526 cm−1 and at a temperature of 300 °K the Gordon conjecture was verified. In the low frequency case a harmonic oscillator potential was used for each molecule but for the high frequency case an anharmonic potential was used. The decay times for the rotational correlation functions for the low and high frequency models, respectively, were τ=1.2 psec and τ=1.3 psec. For the root mean square fluctuations about the average rotational correlations the decay times were τ=0.57 psec and τ=0.55 psec for the low and high frequency models, respectively.
The transport coefficients for polyatomic liquids
Authors: Collings A. F. , McLaughlin I. L.
Published: February 2003

A square-well, rough sphere model of transport in dense polyatomic fluids is evaluated with accurate radial distribution functions and potential parameters which fit both second virial data and square-well fluid critical properties. We show that the model is a successful approximation to the transport properties for 14 polyatomic liquids, and particularly successful in predicting thermal conductivities.
Liquid state torsional dynamics of butane: The Kramers rate and the torsion angle correlation times
Authors: Knauss D. C. , Evans G. T.
Published: February 2003

The diffusion equation for hindered internal rotation of a butane molecule in the liquid state is solved for the Kramers-like rate for trans–gauche isomerization and the torsion angle correlation times. High accuracy solutions obtained by numerical quadrature and approximate asymptotic results are compared with previous molecular dynamics calculations of butane in several thermodynamic states.
Effects of solute–solvent attractive forces on hydrophobic correlations
Authors: Pratt Lawrence R. , Chandler David
Published: February 2003

A theory is presented for the effect of slowly varying attractive forces on correlations between nonpolar solutes in dilute aqueous solution. We find that hydrophobic correlations are sensitive to relatively long range slowly varying interactions. Thus, it is possible to make qualitative changes in these correlations by introducing small changes in the attractive forces. Several model calculations are presented to illustrate these facts. The contributions of the Lennard-Jones attractive forces to the computer simulation results of Pangali, Rao, and Berne are calculated. For this case it is found that the potential of mean force between spherical nonpolar solutes is hardly affected by inclusion of attractive forces. However, the osmotic second virial coefficient is dominated by the contributions of the attractive forces. For spherical solutes which provide a reasonable model for the methane molecule, inclusion of attractive forces produces a qualitative change in the methane–methane potential of mean force. The connection between these effects of slowly varying attractive forces and the enthalpic part of Ben-Naim’s δAHI is discussed.
Theory of phase transitions in solid methanes. XIII. The differential neutron scattering cross section in phase II of solid CH4
Authors: Ozaki Yoshiaki , Kataoka Yosuke , Yamamoto Tsunenobu
Published: February 2003

Theoretical study of the differential neutron cross section in phase II of solid CH4 is carried out with special emphasis upon the dependence of the incident energy of the neutron and the scattering angle. Spin–rotational wave functions given by T. Yamamoto, Y. Kataoka, and K. Okada [J. Chem. Phys. 66, 2701 (1977)] are employed, which are obtained on the basis of the extended James–Keenan model. The interference effect of spin-dependent scattering by a tetrahedral four-proton system is explicitly taken into account according to Hama and Miyagi’s formalism [Prog. Theor. Phys. 50, 1142 (1973)]. The effect of inter- and intramolecular vibrations is included in terms of a Debye–Waller factor. The proton spin-dependent part of the cross section for an unpolarized neutron beam is calculated in the case of an equilibrium mixture of three nuclear spin species in a polycrystalline sample at around liquid helium temperature. The results are compared with available experimental data and the main features of observed spectra are reproduced by theory. Some predictions are made for future experimental work to deepen our understanding of solid methane.
Photoreaction in aromatic crystals: EPR studies of anthracene doped fluorene
Authors: Furrer R. , Fujara F. , Petersen J.
Published: February 2003

Light absorption in fluorene single crystals doped with anthracene induces a photoreaction. One hydrogen atom from a fluorene host gets transferred to an anthracene guest molecule over a distance of several angstroms. The resulting triplet state radical pair is investigated by EPR. Fine- and hyperfine structure analyses are given. The photoprocess is similar to that found earlier in fluorene crystals doped with acridine or phenazine. Where the nitrogen lone pair orbitals of acridine or phenazine were suitable acceptors for the transferred hydrogen atoms with anthracene, the hydrogen has to enter a tetrahedral sp3 configuration at the guest molecule. Somewhat higher crystal temperatures are required for comparable intensity EPR-lines of the photoproduct.
Mode selective laser chemistry of ethylene
Authors: de Hemptinne X. , De Keuster D.
Published: February 2003

The action of 29 rotational lines of the CO2 laser on ethylene at low pressure (1 to 2 Torr) and subsequent relaxation are investigated by means of a time of flight device, coupled to a mass spectrometer. In all cases activation is sufficient to convert ethylene efficiently into its triplet state. Two different behaviors of laser excited ethylene are discussed. They depend on which line the laser is tuned to. Considering that two IR active modes extend their rotational manifolds over the range covered by CO2 laser radiations (ν7 and ν10), it is believed that the different laser lines energize selectively either one of them.
Progress in the application of classical S-matrix theory to inelastic collision processes
Authors: McCurdy C. William , Miller William H.
Published: February 2003

Methods are described which effectively solve two of the technical difficulties associated with applying classical S-matrix theory to inelastic/reactive scattering. Specifically, it is shown that rather standard numerical methods can be used to solve the ’’root search’’ problem (i.e., the nonlinear boundary value problem necessary to impose semiclassical quantum conditions at the beginning and the end of the classical trajectories) and also how complex classical trajectories, which are necessary to describe classically forbidden (i.e., tunneling) processes, can be computed in a numerically stable way. Application is made to vibrational relaxation of H2 by collision with He (within the helicity conserving approximation). The only remaining problem with regard to applying classical S-matrix theory to complex collision processes has to do with the availability of multidimensional uniform asymptotic formulas for interpolating the ’’primitive’’ semiclassical expressions between their various regions of validity.
Electromagnetic theory of enhanced Raman scattering by molecules adsorbed on rough surfaces
Authors: Gersten Joel , Nitzan Abraham
Published: February 2003

A theory for surface enhanced Raman scattering (SERS) is developed. Effects due to realistic surface geometry and dielectric properties are included. Three sources of enhanced Raman scattering are noted: the image dipole enhancement effect, the increase of local field (’’lightning rod’’ effect), and the resonant excitation of surface plasmons. The surface is modeled as a hemispheroid protruding from a conducting plane, although other models are considered. The spherical limit is discussed in some detail and molecular orientation effects are considered. Cross sections for Mie, Rayleigh, and Raman scattering are derived.
Absorption and laser induced fluorescence spectra of C6F6+ inert gas matrices
Authors: Bondybey V. E. , Miller Terry A.
Published: February 2003

The vibrational structure of the ? 2E1g ↔? 2A2u electronic transition of C6F6+ in solid inert gas matrices is examined. Three kind of spectra are observed: (i) laser excitation, (ii) laser excited, wavelength resolved emission, and (iii) simple absorption. The laser excitation spectrum shows regular, assignable ? 2A2u vibrational structure for more than 3000 cm−1 above the origin. Similar structure is revealed by the absorption spectra which gives, as well, reliable values for the oscillator strengths of the transitions. The vibrational structure of the ground state is determined from the emission spectrum to be very irregular and complicated. However, it can be analyzed by recourse to the sophisticated Jahn–Teller theory. The spectra obtained in solid Ne are found to be essentially the low temperature limit of the gas phase spectra. Various observations are made concerning matrix shifts and perturbations in solid Ar and Kr.
A critical test of vibrational dephasing theories in solids using spontaneous Raman scattering in isotopically mixed crystals
Authors: Marks S. , Cornelius P. A. , Harris C. B.
Published: February 2003

A series of experiments have been conducted in order to evaluate the relative importance of several recent theories of vibrational dephasing in solids. The theories are discussed briefly, and are used to interpret the temperature dependence of the C–H and C–D stretch bands in the spontaneous Raman spectra of h14- and d14-1,2,4,5-tetramethyl benzene (durene). The infrared spectra of these same molecules are also reported in the region of the combination bands involving C–H (or C–D) stretches and low-frequency modes. The results support the applicability of the model of Harris et al., [C. B. Harris, R. M. Shelby and P. A. Cornelius, Phys. Rev. Lett. 38, 1415 (1977); Chem Phys. Lett. 57, 8 (1978); R. M. Shelby, C. B. Harris, and P. A. Cornelius, J. Chem. Phys. 70, 34 (1979)], based on energy exchange in anharmonically coupled low-frequency modes. This theory is then used, in connection with Raman spectra obtained in isotopically mixed samples of durene, to elucidate the vibrational dynamics underlying the dephasing. It is found that the results are consistent with the hypothesis that some low-frequency modes in this molecule are significantly delocalized or ’’excitonic’’ in character, and that this delocalization may be studied by means of Raman spectroscopy on the low-frequency modes themselves, as well as by exchange analysis of the coupled high-frequency modes. These conclusions represent a generalization and extension of the previously published exchange model [R. M. Shelby, C. B. Harris, and P. A. Cornelius, J. Chem Phys. 70, 34 (1979)].
Counterion and solvent perturbations to the electronic structure of substituted pyrylium cations: A CNDO/S study
Authors: Bigelow Richard W.
Published: February 2003

The combined effects of solvation and specific counterion pairing schemes on the electronic structure of a series of closed-shell substituted pyrylium cations are examined within the context of semiempirical molecular orbital theory. Bonding characteristics are elucidated through a comparison of free-cation, cation/anion (supermolecule), and empirically dressed supermolecule models. The dressed models are constructed by introducing electrostatic interaction terms as perturbations to the electronic structure of the appropriate supermolecule to account for solvent screening of localized charge centers. The dressed models are shown to exhibit shifts in the transition energies of hypothetical free-cation models which produce excitation spectra in excellent correspondence with experiment. Electrochemical properties are also discussed in terms of counterion and solvent perturbations to the electronic structure of the free cation.
Potential energy characteristics and energy partitioning in chemical reactions: A
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o MO study of four-centered elimination reaction CH3CH2F→CH2=CH2+HF
Authors: Kato Shigeki , Morokuma Keiji
Published: February 2003

The mechanism of energy disposal along the reaction pathway was discussed in relation to the characteristic features of potential energy surface. A simplified theoretical model was constructed in terms of the intrinsic reaction coordinate (IRC) and normal coordinates perpendicular to it. Ab initio MO calculations with the split-valence basis set were carried out for the four-centered elimination reaction CH3CH2F→HF+CH2CH2. The energy gradient method was used to optimize the stationary points on the potential surface and to trace IRC as well as to calculate the force constants. The origin of vibrational enhancement of HF in the reaction product was interpreted as the result of interchange of the main components of IRC in a local region where the IRC curvature is large.
Theory of the chemical bond. V. Bond polarities of post-transition hydrides
Authors: Matcha Robert L. , King Stephen C. , Pettitt B. M.
Published: February 2003

A classical derivation of a dipole moment model derived earlier by a quantum mechanical implicit perturbation technique is given. This model is used to determine the bond polarities of the post transition (Groups IIIA–VIIA) hydrides. The polarity measures the extent of charge transfer in a bond. Polarities of alkali halides and posttransition hydrides are used to illustrate the gradual change in bond polarities with nuclear charge across the periodic table.
Van der Waals complexes of iodine with hydrogen and deuterium: Intermolecular potentials and laser-induced photodissociation studies
Authors: Kenny Jonathan E. , Russell Timothy D. , Levy Donald H.
Published: February 2003

A supersonic free jet expansion has been used to produce the van der Waals complexes of I2 with ortho- and para-H2 and D2. The results of fluorescence excitation and dispersed emission spectroscopy were combined to deduce the parameters of the isotropic part of the intermolecular potential. Using a Morse potential these were found to be De=122 cm−1 and ωe=106 cm−1. Observations were made which also indicate the presence of anisotropy in the potential: different absorption frequencies for complexes containing ortho and para forms of H2 and D2, and different product state distributions when ortho and para H2I2 vibrationally predissociate. Approximate measurements of vibrational predissociation lifetimes of I2H2 and some I2–rare gas complexes are reported here, and their dependence on various molecular and intermolecular potential parameters is discussed in light of current theoretical treatments, namely,the energy gap law of Beswick and Jortner and the momentum gap law of Ewing. In the case of o-H2I2, the relatively high van der Waals stretching frequency (ωe?100 cm−1) is well-matched to that of the I2 stretching frequency (ωe?128 cm−1 in the B 3Π state), resulting in good coupling between the initial bound state (prepared by laser excitation) and the final dissociative state. Hence, a comparatively short vibrational predissociation lifetime of ∼18 psec is observed.