bluuebunny/crossref_metadata_2025 huggingface dataset:
Hot Electron Injection into Liquid Argon from a Tunnel Cathode
Authors: Silver M. , Kumbhare P. , Smejtek P. , Onn D. G.
Published: February 2004

Hot electrons from a tunnel cathode have been injected into liquid argon (99.998% pure) at 87°K. The current vs voltage characteristics indicate that the injected hot electrons thermalize very slowly, losing their energy only by elastic scattering processes and finally by capture by the dilute impurities. The deduced thermalization time and distance are very long compared with that in helium, where bubble formation is responsible for energy loss.
Ultrasonic Absorption in Triethylamine–Water Solution near Its Critical Solution Temperature
Authors: Yun S. S.
Published: February 2004

Ultrasonic absorption (3, 5, and 10 MHz) and velocity (5 MHz) in triethylamine–water binary liquid system have been investigated through the critical solution temperature by means of the pulse-echo method. The results are obtained for the variation of velocity with temperature and also for the behavior of the attenuation near the critical point. For each frequency investigated, the absorption exhibits a marked maximum at the critical solution temperature. The temperature dependence of the attuenation α / ν2 in the single phase of the solution is examined in terms of the Fixman theory. The result is in reasonable agreement with the theory with regard to functional conformity of α / ν2 in temperature and frequency. The temperature dependence of the velocity at 5 MHz is quite similar to the results obtained for 600 kHz, and no dispersion is observed.
Solubility of Anthracene in Compressed Methane, Ethylene, Ethane, and Carbon Dioxide: The Correlation of Anthracene–Gas Second Cross Virial Coefficients Using Pseudocritical Parameters
Authors: Najour G. C. , King A. D.
Published: February 2004

The solubility of anthracene in compressed methane, ethylene, ethane, and carbon dioxide has been measured over pressure ranges of 1–100 atm and temperature ranges of 63–185°C. Second cross virial coefficients representing anthracene–gas pair interactions have been evaluated from this data. It is shown that when allowance is made for the effect of the molecular quadrupole moment on the critical constants of carbon dioxide, ordinary combing rules produce pseudocritical parameters which reduce these anthracene–gas cross virial coefficients to a single function of reduced temperature. The functional behavior of these reduced virial coefficients is shown to be the same within experimental error as that exhibited by cross virial coefficients of phenanthrene with many of these same gases.
Isotope Effect in Electron Stimulated Desorption: Oxygen Chemisorbed on Tungsten
Authors: Madey Theodore E. , Yates John T. , King David A. , Uhlaner Carole J.
Published: February 2004

Electron stimulated desorption (ESD) studies of oxygen chemisorbed on tungsten have revealed a large isotope effect in the desorption of ions. The probability of desorption of 16O+ was found to be a factor of ∼1.5 greater than the probability of desorption of 18O+ upon bombardment of the adsorbed layer by 100-eV electrons. This observation is quantitatively consistent with the ESD mechanism proposed independently by Redhead and by Menzel and Gomer. On the other hand, the small isotope effect observed in ESD “total” desorption does not agree with this mechanism; possible reasons for this are discussed.
Small Gaussian Expansions of Atomic Orbitals: Second-Row Atoms
Authors: Stewart R. F. , Hehre W. J.
Published: February 2004

Gaussian expansions of Clementi's SCF atomic orbitals for the second-row atoms have been obtained by the method of least squares. Expansion lengths vary from two through six GTO's. Total energy evaluations of the least-squares expansions reveals a rather slow convergence rate with increasing number of GTO's to the Hartree–Fock limit. The GTO AO's in this work have optimal space filling properties and are useful for rapid evaluation of generalized x-ray scattering factors of second-row atoms.
Thallium Nuclear Magnetic Resonance in Thallous Iodide
Authors: Vaughan R. W. , Anderson David H.
Published: February 2004

Wide-line nuclear magnetic resonance techniques have been used to investigate the electron coupled (or indirect) spin exchange interactions and chemical shifts of the two thallium isotopes in thallous iodide. Thallous iodide undergoes a phase transition near 160°C from a layered orthorhombic structure to a cubic (CsCl) structure. An order-of-magnitude decrease in the estimated second moments of both thallium lines was observed in going from the orthorhombic to the cubic structure, and while the two thallium lines were the same width in the orthorhombic phase, the second moment of the 203Tl line was 50% greater than the second moment of the 205Tl line in the cubic phase. These results are interpreted in terms of a decrease in the “covalent character” of the bonding in the solid, with the transition to the cubic structure. This is consistent with the structural changes occurring at the orthorhombic to cubic transition which involve the lengthening of some unusually short thallium–iodine bonds to form a more symmetric solid with a higher density. The thallium chemical shift in the cubic phase was found to be − 15 × 10−4 (relative to a saturated TlF solution). An analysis of the chemical shifts of the broad lines in the orthorhombic phase was not conclusive as the shifts were field dependent.
Molecular Zeeman Effect of Dimethyl Ether and Dimethyl Sulfide; and Comparison of the Magnetic Susceptibility Anisotropies with Ethylene Sulfide and Ethylene Oxide
Authors: Benson R. C. , Flygare W. H.
Published: February 2004

The rotational Zeeman effect has been observed in dimethyl sulfide and dimethyl ether. The molecular g values (gaa, gbb, and gcc), the magnetic susceptibility anisotropies [(2χaa − χbb − χcc) and (− χaa + 2χbb − χcc)], and the molecular quadrupole moments are obtained and listed below. The a and b axes are in the molecular plane. The b axis bisects the COC or CSC angle. The units of χ are 10−6 erg/G2·mole and the units of Qxx are 10−26 esu·cm2. CH3OCH3CH3SCH3gaa− 0.214 ± 0.0006− 0.0193 ± 0.0007gbb− 0.0093 ± 0.00040.0000 ± 0.0003gcc− 0.0210 ± 0.0006− 0.0083 ± 0.00032χaa − χbb − χcc− 10.4 ± 0.7− 4.2 ± 0.5− χaa + 2χbb − χcc+ 1.2 ± 0.6− 2.8 ± 0.5Qaa+ 3.3 ± 0.6+ 3.2 ± 0.5Qbb− 2.0 ± 0.5− 1.5 ± 0.5Qcc− 1.3 ± 1.0− 1.7 ± 0.8 The anisotropies in the second moment of the electronic charge distribution are also obtained. The susceptibility anisotropies are compared with those of propene, cyclopropene, ethylene sulfide, and ethylene oxide leading to the conclusion that nonlocal contributions to the susceptibility anisotropies in ring molecules have been underestimated.
Chemical Kinetics of Energetic Atoms. IV
Authors: Chapin D. M. , Kostin M. D.
Published: February 2004

An integrodifferential form of the Boltzmann equation is derived which includes the effects of the thermal motion of the scattering species and the effects of the rapid increase in the differential scattering cross section at small scattering angles. Solutions of the integrodifferential equation are presented for a system in which energetic tritium atoms are thermalizing and reacting in a mixture of helium, molecular hydrogen, and a scavenger species. Good agreement is found between solutions obtained by this method and those obtained independently by stochastic methods. In all cases treated, significant deviations of the energy distribution of tritium atoms from the Maxwellian distribution are found. The effects of varying the scavenger concentration on the value of the rate constant for an exchange reaction between a tritium atom and a hydrogen molecule are investigated.
Methylenecyclobutane: Ring-Puckering Potential Function from Mid-Infrared Combination Bands
Authors: Malloy Thomas B. , Fisher Farley , Hedges Richard M.
Published: February 2004

The mid-infrared spectra (4000–250 cm−1) of methylenecyclobutane, methylene-d2-cyclobutane-2,2-d2, and methylene-d2-cyclobutane-d6 have been examined closely for the occurrence of combination bands involving the ring-puckering vibration. A progression of sum bands for methylenecyclobutane and a progression of difference bands for methylene-d2-cyclobutane-2,2-d2 have been simultaneously fit by the one-dimensional potential function in the ring-puckering coordinate x: V(x) = 4.229 × 105x4 − 1.694 × 104x2. This potential function yields a barrier of 168 ± 10 cm−1 at the planar conformation, in good agreement with the barrier of 160 ± 40 cm−1 determined by other workers from the vibrational dependence of the rotational constants in the microwave spectrum. The detailed shape of the function does not agree as well and particularly the failure of the infrared function to account for the observed 0–1 splitting is noted. This had been noted earlier in the case of silacyclobutane and to a lesser degree, in the case of cyclopentene.
Molecular SCF Calculations for GaF, GaH3, GeH4, AsH3, and H2Se
Authors: Stevenson Philip E. , Lipscomb William N.
Published: February 2004

Exact minimum Slater basic sets have yielded LCAO–MO–SCF wavefunctions for the compounds GaF, GaH3, GeH4, AsH3, and H2Se. Chemical binding in these molecules is in accord with a very simple model of bond formation. Parameters for nonempirical molecular orbital (NEMO) calculations are computed from the Hamiltonian matrices of these molecules and from the previously published results on the model compounds ScH3NH3 and TiH3F. A planar geometry of the unobserved GaH3 is indicated by a NEMO calculation.
Single Vibronic Level Fluorescence. II. Vapor-Phase Fluorescence Spectra from the Zero-Point Level of B2u1 Benzene and Benzene-d6
Authors: Parmenter C. S. , Schuyler M. W.
Published: February 2004

Fluorescence spectra have been obtained at 300°K from C6H6 and from C6D6 vapor at 0.1 torr in which all emitting molecules are in the zero-point vibrational level of the B2u1 state. Analysis of the spectra are completed using selection rules appropriate for D6h symmetry of both the ground (1A1g) and excited (1B2u) states. The spectra are compared with the zero-point fluorescence obtained by the conventional method of matrix isolation of benzene at 77°K. The forbidden transitions seen in the latter are absent in isolated molecule gas-phase spectra. The zero-point spectra are also compared with that from a 300°K Boltzmann distribution of B2u1 vibronic levels.
Electron Scavenging in γ-Irradiated Liquid Methanol and Ethanol
Authors: Rzad Stefan J. , Fendler J. H.
Published: February 2004

Electron scavenging in liquid methanol and ethanol has been examined as a function of 14CH3Br and SF6 concentration in the region from 10−5 to 0.3 M. The empirical scavenging model previously proposed for hydrocarbons can be successfully extended to alcohols if a competitive first-order reaction of the solvated free electron is included. Solvated free-electron and geminate-electron yields have been determined to be 1.05 and 3.1. Analysis of the data from competitive experiments leads to an estimation of the absolute rate constants for reaction of solvated electrons with CH3Br, CH3Cl, SF6, N2O, and acetone in methanol and ethanol and to an estimation of the half-life of the solvated free electrons in both solvents. The present results in ethanol can be correlated with the observations of Thomas and Bensasson on the nanosecond decay of solvated electrons in terms of an ion-pair lifetime distribution function similar to that applicable to hydrocarbon radiolysis.
Infrared Reflectance Spectra of Carbon Monoxide Adsorbed on Nickel and Rhodium (110) Surfaces in High and Ultrahigh Vacua
Authors: Eckstrom Hartley C. , Possley Glen G. , Hannum Steven E. , Smith Wm. Hayden
Published: February 2004

The research reported herein represents our efforts towards optimization of the experimental parameters for the purpose of obtaining infrared spectra of adsorbed species on clean, oriented metal films under reproducible and readily interpretable conditions. Spectra were obtained in the region 4000–50 cm−1. A narrow band due to physically absorbed or weakly chemisorbed CO on nickel (110) was found at 2056 cm−1, with a full width at half-maximum (fwhm) of about 10 cm−1 and a reflection maximum at 2038 cm−1. The corresponding isotopic (13C) CO absorption was located at 1998 cm−1. These absorptions could not be observed with certainty for pressures of CO below 0.5 torr. At lower pressures, reflectance changes indicated the presence of CO, but definite absorptions were not identified due to the lower signal-to-noise ratio and background variations. The fundamental of chemisorbed CO on rhodium (110) was observed as a doublet at 2078 and 2060 cm−1 (fwhm, 10 cm−1 for each component) with a reflection maximum at 2030 cm−1. A further weak absorption attributed to adsorbed CO was found at 1988 cm−1 on rearranged films. The bands remained undiminished upon evacuation at room temperature. These and other observations of the systems are discussed and compared with results found in the literature.
Flash Photolytic Production, Reactive Lifetime, and Collisional Quenching of O2(b 1Σg+, υ′ = 0)
Authors: Filseth S. V. , Zia A. , Welge K. H.
Published: February 2004

Collisional quenching of O2(b 1Σg+, υ′ = 0) at room temperature by O2 and a variety of foreign gases has been investigated with a pulsed lifetime measurement technique. O2(b 1Σg+, υ′ = 0) has been produced in a pulsed mode through flash photolysis of O2 in the vacuum uv and has been detected through the emission of the (0, 0) band at 7620 Å of the forbidden O2(b 1Σg+ → X 3Σg− transition. The (0, 0) band intensity has been measured as a function of time after the photolysis flash and as a function of the O2 and foreign gas pressures. Quenching rate constants are derived from the reactive lifetimes. The photolytic production of O2(b 1Σg+, υ′ = 0) from O2 and the quenching by O2 has been studied at O2 pressures from 0.02–100 torr. The observations at low O2 pressures from 0.02 to about 1 torr are consistent with the previously established fast O2(b 1Σg+) production mechanism, O2 + hν → O(1D) + O(3P), O(1D) + O2(X 3Σg−) → O(3P) + O2(b 1Σg+), in the Schumann–Runge continuum region. No emission of the (1, 1) and (2, 2) atmospheric bands has been observed indicating that under the conditions employed, O2(b 1Σg+, υ′ > 0) is either initially formed only to a relatively small degree in this mechanism or that O2(b 1Σg+, υ′ > 0) is relaxed or quenched by O2 within less than 103 collisions. From the (0, 0) band fluorescence decay rates measured at O2 pressures from 0.02 to about 0.5 torr a quenching rate constant of 4.5 × 10−16 cm3 molecule−1·sec−1 is derived. At higher pressures, the decay rate deviates from a linear dependence on the O2 pressure indicating that the reactive lifetime is influenced by some secondary process at these pressures. The decay rate measured for example at 20 torr, O2 would correspond to a quenching coefficient of 4.8×10−17 cm3 molecule−1·sec−1. Quenching of O2(b 1Σg+, υ′ = 0) by He, Ne, Ar, Kr, Xe, H2, N2, CO, CO2, SF6, NH3, H2O, CH4, C2H6, C2H4, NO, NO2, and N2O has been investigated by measuring reactive lifetimes at constant O2 pressures as a function of the added foreign gas pressures. Quenching rate constants are reported and compared with previous results.
Relaxation Behavior of the Freely Jointed Chain
Authors: Verdier Peter H.
Published: February 2004

A method is presented for treating the relaxation behavior of the freely jointed chain model of a random coil polymer. Exact results are exhibited for the relaxation of quantities linear in chain coordinates. For the treatment of quantities quadratic in chain coordinates, a numerical approach is employed and exemplified by obtaining the autocorrelation in the square of end-to-end length for chains of up to 16 beads. In both cases, the rapid approach of the behavior of the freely jointed chain of N beads to that of the Rouse model of N statistical segments is demonstrated.
Collision Induced Dissociation of Molecular Ions
Authors: Cheng M. H. , Chiang M. , Gislason E. A. , Mahan B. H. , Tsao C. W. , Werner A. S.
Published: February 2004

Velocity vector distributions of the fragment ion products of the dissociative collisions of O2+, N2+, NO+, and N2O+ with He have been determined, using projectile-target relative kinetic energies which are one to three times the bond energy of the molecular ion. The most probable dissociation event produces a fragment ion whose velocity is very nearly the same as that of the original projectile ion. Fragment ions also appear at smaller velocities and larger scattering angles, and the importance of these features increase with increasing initial relative energy. Three models for the dissociation are discussed, and it is concluded that a version of the stripping model is most nearly consistent with the data.
Far-Infrared and Raman Spectra of Phosphonium Chloride and Phosphonium Chloride-d4
Authors: Durig J. R. , Antion D. J. , Pate C. B.
Published: February 2004

The far-infrared spectra of thin films of PH4Cl and PD3Cl have been recorded at −170°C. Raman spectra of polycrystalline samples of both compounds were also recorded at −170°C. One libration and three translations were observed as well as eight internal modes. Several combinations and overtones were assigned and the zone-edge wavenumber of a second libration has been inferred. The barrier restricting re-orientation of the PH4 and PD4 ions has been calculated from the observed librational wavenumbers to be 8.5 kcal/mole and forces contributing to the barrier are discussed. The lattice dimensions of PH4Cl were determined to be a = b = 5.82 Å and c = 4.22 Å from the x-ray diffraction pattern of a polycrystalline sample.
Electron Paramagnetic Resonance of (Agpy2)2+ in Nitric Solution at 77°K
Authors: Halpern T. , Phillips W. D. , McMillan J. A.
Published: February 2004

The paramagnetic resonance spectrum of (Agpy2)2+ in frozen nitric solution has been observed at 77°K. It exhibits silver hyperfine as well as superhyperfine structure, this latter due to interaction with two equivalent nitrogen nuclei. Due to a strong axial g anisotropy the principal values of the hyperfine and superhyperfine interaction tensors may be determined. Wavefunctions are calculated for the unpaired electron as well as its density in the metal ion and the ligands. The experimental values are consistent with the picture of a hole in the Ag d shell in an oblate tetragonal octahedron, which is partially transferred to an sp4 nitrogen hybrid. Coefficients for the wavefunctions are given and the ring C–N–C angle found in reasonable agreement with independently estimated values.
Comments on the Analysis of Atomic Correlation Energies
Authors: Gruninger John , öhrn Yngve , Löwdin Per-Olov
Published: February 2004

Analysis of correlation energies is advocated in terms of average values of electronic kinetic energy, electron–nuclear attraction energy, and electron–electron repulsion energy, rather than pair energies. Exploratory calculations on the two-electron and three-electron isoelectronic sequences are reported.
Ethereal Electrons
Authors: Glarum Sivert H. , Marshall James H.
Published: February 2004

Solutions of K, Rb, and Cs in dimethoxyethane consist primarily of metal cations and anions. The latter are ionized by light at wavelengths between 700 and 1000 nm to yield solvated electrons. Electron recombination occurs by a second-order mechanism involving an M·species, and rate constants have been estimated from ESR data. Photoelectrons emitted from Rb− are highly polarized, and an explanation combining spin–orbit and exchange interactions with an adiabatic dissociation is proposed.
Adsorption of Inert Gases on Tungsten: Measurements on Single Crystal Planes
Authors: Engel T. , Gomer R.
Published: February 2004

The adsorption of Ar, Kr, and Xe on the (110), (120), (100), (211), and (111) planes of a tungsten field emitter was studied under conditions of immobile adsorption and also under conditions of surface equilibrium. The relative dipole moments and heats of binding could be determined, and it was found that both these quantities were largest on the (110) plane. A discussion in terms of charge transfer bonding is presented to rationalize these results. It was found that chemisorption of oxygen prevented the relatively strong first layer adsorption observed on the clean surface, and also the dipole moment associated with it.
Simple and Accurate Approximation for the Centrifugal Factor in RRKM Theory
Authors: Waage E. V. , Rabinovitch B. S.
Published: February 2004

An approximate form for the low pressure centrifugal effect is derived for RRKM unimolecular rate theory calculations. Comparisons are made with earlier approximations and with a more exact result. The effects on the observed activation energy and extension to the region of falloff are described.
Identifying the Lowest Excited Singlet State of Biphenyl and Its Analogs
Authors: Berlman Isadore B.
Published: February 2004

From experimental evidence it is inferred that the broad structureless absorption band of biphenyl at about 2500 Å is composed of bands from three transitions, a weak transition similar to the A1 → 1Lb transition of benzene and two stronger transitions similar to the A1A → 1La and A1 → 1B transitions of benzene. Bands from these transitions are separated in rigid biphenyl analogs such as 9,10-dihydrophenanthrene and fluorene. Certain fluorescence characteristics of biphenyl (such as decay time τ and quantum yield Q) are distinctive of the weak and “hidden” transition and a dramatic change in these characteristics is observed when a crossover of the two lowest excited states is achieved in the analogs of biphenyl. Specific substituents, when positioned in the para position (such as a phenyl or vinyl group in p-terphenyl or 4-vinylbiphenyl, respectively), are particularly efficacious in producing a crossover of levels. In these cases, the lowest excited state is an allowed transition similar to one component of the degenerate A1 → 1B transition in benzene. Since the intense absorption bands are maximally shifted by substituents on the para position, the transition moment must be long-axis polarized as predicted theoretically. Bridging is particularly effective in producing a bathochromic shift because the phenyl rings can be constrained to lie in a relatively coplanar and linear configuration. The magnitude of the shift imparted to the various states depends on the bridging element. From the value of the ratio τ / Q, the assignment of the lowest excited singlet state can be determined: In fluorene, dibenzofuran, and 9,10-dihydrophenanthrene, it is La1; in carbazole and phenanthrene, Lb1; and in 2-phenylfluorene and 2-phenyl-9,2′-methylenefluorene, Bb1.
Lyman-α Fluorescence from the Photodissociation of H2
Authors: Mentall J. E. , Gentieu E. P.
Published: February 2004

Photodissociation and fluorescence cross sections for H2 have been measured from 860 to 700 Å by observing the Lyman-α photons emitted at 90° to an incident photon beam. The photodissociation quantum efficiency was found to be unity from 850 Å to the photoionization threshold at 804 Å and about 10% at shorter wavelengths. By observing the fluorescence radiation as a function of decreasing pressure, it was determined that most of the excited H atoms were formed in the metastable 2s state and that the Lyman-α emission results primarily from collisional quenching. The ratio of the collisional cross section with and without Lyman-α emission was measured for different relative velocities and was found to increase appreciably with increasing velocity.
Raman Spectra and Force Constants for OsO4 and XeO4
Authors: Huston John L. , Claassen Howard H.
Published: February 2004

Raman spectra are presented for OsO4 vapor, and for XeO4 vapor and solid. Observed fundamental frequencies (in cm−1) for gaseous OsO4 are 965.2(a1), 333.1(e), 960.1(f2), and 322.7(f2). For XeO4 they are 773(a1) for the vapor, and 767.1(a1), 277(e), 867(f2), and 303-f2) for the solid. The Urey–Bradley constant F is 0.32 for OsO4, and −0.18 mdyn/Å for XeO4.
Microwave Spectrum and Intramolecular Hydrogen Bonding in 2-Fluoroethanol
Authors: Buckton Keith S. , Azrak Raymond G.
Published: February 2004

The microwave spectra of two isotopic species of 2-fluoroethanol (FCH2CH2OH and FCH2CH2OD) have been analyzed to yield some information about a proposed intramolecular hydrogen bond in the molecule. A molecular structure has been derived which reveals that the most stable rotamer is that which allows close approach of fluorine and hydrogen atoms. The F···H distance is 2.42 ± 0.02 Å; the dihedral angle FCCO is 62°12′ ± 1.0°, and the dihedral angle CCOH is 55°30′ ± 3°. The dipole moment components are μa = 0.38 ± 0.02 D, μb = 1.47 ± 0.01 D, and μc ≈ 0.0 D; these are consistent with bond moment calculations. A vibrational state attributed to the CC torsional motion has been assigned and determined to be 152 ± 10 cm−1 above the ground state. Our results suggest a hydrogen–fluorine interaction which does not grossly distort the normal molecular charge distribution.
Impulsive Reaction Model of Ion–Molecule Reactions: Angular Distributions
Authors: Chang Daniel T. , Light John C.
Published: February 2004

The direct reaction model of ion–molecule reactions proposed by Wolfgang to explain the velocity dependence of products of exothermic reactions is refined and extended to yield the angular distribution of products as well. The model consists of consecutive “two-body” trajectories for reactants and products with “reflection” of radial relative velocities at a reaction radius. It is found that a single value of the reaction radius yields velocity and angular distributions in substantial agreement with experiment for the Ar++D2 reaction over a substantial energy range. Some predictions of the model and some possible modifications are also discussed.
Calculation of the Effects of Hydration and Divalent Metal Ions on Nucleotide Base Pairs: Single G–C and Poly(G–C)
Authors: Rozsnyai B. F. , Ladik J.
Published: February 2004

SCF calculations have been carried out for the electronic energy levels and wavefunctions of single G–C and poly(G–C) in the presence of a neutral water molecule or Mg1+ ion. The Pariser- Parr–Pople (P–P–P) approximation was used for the single base pair, and its equivalent, the semiempirical SCF–LCAO–crystal-orbital (CO) approximation, for the polymer. Energy levels and energy bands are presented for four different cases. According to the results, the presence of the Mg2+ ion very strongly changes the band structure of the poly(G–C). The probable consequences of this on the conductive properties of DNA and the possibility of ab initio calculations on periodic DNA models are discussed.
Stochastic Theory of Vibrational Relaxation and Dissociation
Authors: Gelbart William M. , Rice Stuart A. , Freed Karl F.
Published: February 2004

The familiar theories of unimolecular reactions are based on the assumptions of strong collisions, a randomizing redistribution of internal energy, and an a posteriori choice of a single dissociation coordinate. In this paper we replace these starting points by introducing a model with a set of transition probabilities defining a Markov chain whose time evolution describes the individual bond breaking and the inter- and intramolecular energy transfer. We provide for the dissociation of all the bonds at all times and explicitly specify the couplings of the internal degrees of freedom. The differential-difference equations for the over-all relaxation process are transformed by the standard generating function technique to a single partial differential equation which can be solved exactly by the method of characteristics. This procedure displays the competing effects of the vibrational interactions and the individual dissociation energies and suggests that it is possible to determine a priori that bond which is most likely to break first. We treat in particular two fundamentally different classes of unimolecular reaction: (i) collisional activation and (ii) photochemical excitation at low pressures. In the former case we present a formal justification of the basic assumptions implicit in the familar theories of thermal dissociation. In the latter case, however, where the initial distribution of internal energy is highly nonuniform and the molecular relaxation processes occur on a time scale shorter than the mean time between collisions, we show that the details of the vibrational interactions and dissociation energies must be taken into account in order to properly describe the bond-breaking kinetics. Finally, the closed-form expressions obtained in the exact solution of our Markov chain allow for a simple discussion of the “random lifetime assumption.”
Molecular Orbital Interpretation of X-Ray Emission Spectra: Simple Hydrocarbons and Carbon Oxides
Authors: Manne Rolf
Published: February 2004

The carbon x-ray emission spectra of CH4, C2H2, C2H4, C2H6, C3H8, C6H6, CO, and CO2 obtained by Mattson and Ehlert are considered. The main parts of these spectra are shown to relate to the same ionized states as are observable by photoelectron spectroscopy, and generally may be interpreted with the help of molecular orbital theory. A simplified calculation of relative x-ray intensities based on extended Hückel wavefunctions gives good results for the compounds studied. Remaining structures on the high-energy side of the spectra are attributed to Wentzel–Druyvesteyn satellites.
Molecular Beam Kinetics: Reactions of Alkali Atoms with NO2 and CH3NO2
Authors: Herm R. R. , Herschbach D. R.
Published: February 2004

Magnetic and electric deflection analysis of the scattering of Cs + NO2 shows that the principal product is a paramagnetic, polar molecule. Magnetic analysis of the K + NO2 system indicates that the scattered signal is paramagnetic; a similar study of Na + NO2 shows a small yield of diamagnetic product. For the analogous reactions with CH3NO2, the product is diamagnetic and has a pseudo-first-order Stark effect. From these data and thermochemical arguments the principal alkali-containing products are identified as: for Cs + NO2, a 2Σ electronic state of CsO; for Na + NO2, probably a 2Π state of NaO; for M + CH3NO2, almost certainly MNO2 in a singlet state. The NO2 results indicate that the ground state of the MO molecule changes from 2Π for LiO (the only species which had been previously observed) to 2Σ for CsO. The usual differential surface ionization detection fails for Cs + NO2 and consequently only a very rough estimate of the scattering is obtained; this indicates that the total reaction cross section is ∼ 100 Å2. For the CH3NO2 reactions differential surface ionization is applicable. Again the reaction cross sections are ∼ 100 Å2 and increase as Na→K→Cs. The c.m. product angular distribution is broad, with about the same intensity in the forward and backward hemispheres. These results are discussed in terms of the electronic structure of the reactant and product molecules and contrasted with reactions of alkali atoms with halogen-containing molecules. Scattering of related molecules has also been studied briefly, including RONO and R′ONO2 (with R = i-C5H11 and R′ = C2H5), which give diamagnetic products with yields very similar to CH3NO2, and N2O and R″OOR″ (with R″ = t-C4H9), for which only paramagnetic species were observed.
Analytic Approach to Electron Correlation in Atoms
Authors: White Ronald J. , Stillinger Frank H.
Published: February 2004

A novel perturbative treatment of electron correlation in N-electron atoms is devised. The unperturbed starting point is a central-force “hydrogenic” problem in the full dN-dimensional configuration space (d = dimensionality). The central potential in this solvable “hydrogenic” problem is obtained by averaging the actual electron–electron and electron–nucleus potentials over all dN − 1 hyperspherical polar angles in the configuration space. The relevant projected Green's functions are computed for the ground states of the model one-dimensional two-electron atom (with delta function interactions), as well as for the real three-dimensional helium isoelectronic sequence. The corresponding first-order wavefunctions exhibit weakly singular logarithmic behavior (at three-particle confluence) of the type first advocated by Fock. Second-order energies are evaluated for both of these two-electron problems. The basic ingredients of our hyperspherical coordinate method for three-electron atoms are displayed, in preparation for later application. Explicit suggestions are made for inclusion of singular terms in high-accuracy atomic and molecular variational wavefunctions.
Reaction of the Excited Oxygen Atoms O(1D2) with Neopentane
Authors: Paraskevopoulos G. , Cvetanović R. J.
Published: February 2004

The reaction of O(1D2) atoms, formed by photolysis of N2O at 2139 Å, with neopentane, has been studied at room temperature in the gas phase at total pressures from 5 to 1300 torr. The main features of the reaction are similar to those observed previously in this Laboratory with propane and isobutane. The reaction proceeds mainly along two paths: (a) insertion of O(1D2) into the CH bonds of neopentane to form vibrationally excited (“hot”) neopentanol accounts for about 66% of the total reaction, and (b) abstraction of hydrogen from neopentane to form neopentyl radicals and OH accounts for about 25% of the reaction. The “hot” neopentanol is stabilized at pressures much lower than those required for stabilization of the “hot” butanols. The mean lifetime of the “hot” neopentanol has been determined and is found to be 4.2 × 10−9 sec. The reaction mechanism is discussed.
General Theory of Autoionizing States of Two-Electron Systems
Authors: Nazaroff George V.
Published: February 2004

The present theory is concerned with the bound-state components of autoionizing states. The bound and quasibound states of a two-electron system of a given symmetry type were grouped into clusters, and a wavefunction's natural eigenvalues (NEV's) were defined. The theory was used to show that the nth NEV of every approximate wavefunction can be made to yield an upper bound to the energy of any state in the nth cluster. The arguments for a two-electron system are extendable to an arbitrary many-electron system. Configuration-interaction (CI) results for several autoionizing states of an S-function helium atom were presented, and upper bounds to several autoionizing-state energies were obtained. The rather complicated structure of autoionizing-state CI roots was shown to result from the intersection of rather simple curves for the autoionizing states with CI roots belonging to states of lower clusters. The intersections are nonphysical and occur because an incomplete set of basis functions was used in the CI calculation.
Semiempirical Helium Intermolecular Potential. II. Dilute Gas Properties
Authors: Bruch Ludwig W. , McGee Ian J.
Published: February 2004

Parameter adjustments in a family of semiempirical potentials are reported. The result is a two-parameter potential incorporating theoretically determined dipole–dipole and dipole–quadrupole interactions which fits dilute helium gas data to within 6% over the temperature range 1–2000°K. The data fits near 50°K are not fully satisfactory, nor is the fit of viscosity data below 4°K. The sensitivity of the calculations to the inclusion of the dipole–dipole interaction for properties below 4°K is studied. The calculations using quantum mechanics show a greater sensitivity in the virial coefficient than the viscosity and differ from calculations using classical mechanics. The semiclassical limits of the calculations are discussed.
Far-Infrared Absorption in ortho- and meta-Dihalobenzenes
Authors: Mansingh Abhai
Published: February 2004

The optical absorption in the region 20–100 cm−1 has been measured by Fourier transform spectroscopy for the pure liquids of the ortho and meta isomers of difluoro-, dichloro-, and dibromobenzene at a temperature of 20°C. The absorptions are very broad having half-widths of about 80 cm−1 and the peak value of the absorption coefficient is higher than that predicted by the Debye theory of dielectric relaxation for all the substances studied. The theories proposed for absorption in the far-infrared region are inadequate and do not explain the experimental results. It is concluded that the strong absorption is predominantly due to the rotational transitions of the molecule and that a similar phenomenon should occur in all polar liquids.
ESR Spectra from Paramagnetic Centers in Irradiated KClO4. III. Centers Appearing between 10 and 200°K
Authors: Byberg J. R. , Jensen S. J. K.
Published: February 2004

ESR spectra of KClO4 crystals irradiated with x rays at 10 and at 26°K contain three “families” of intense signals. The signals are assumed to derive from primary defects constituted by electrons and holes trapped on ClO4− ions. The families of signals are assigned to the species ClO4=, ClO4, and (ClO4)2−, each trapped in a number of distinct sites. ClO4= and ClO4 appear to be stabilized by intraionic configuration change, whereas interionic configuration change may stabilize (ClO4)2−. Thermal processes starting at 30°K result in formation of the secondary defects O3− from ClO4= and ClO2 from ClO4 and in destruction of (ClO4)2−.
Oxidation of 4a,4b-Dihydrophenanthrene. IV. Kinetic Isotope Effect of Deuterium in the Propagation and the Initiation Steps
Authors: Bromberg A. , Muszkat K. A. , Warshel A.
Published: February 2004

A kinetic isotope effect was found in one of the propagation steps of the reaction of perdeutero-4a,4b-dihydrophenanthrene (PH2) with O2. In this step, PH2+HO2·→PH·+H2O2, the kinetic isotope effect is 7.2 at − 10°C and 9.9 at − 31°C. The theoretical calculation by the previously described method predicts for this step an isotope effect of the correct magnitude. The success of the same method, using a similar theoretical model, for the treatment of reactions involving widely different extents of tunneling is a good indication to the validity of the isotope effect calculations and supports the previous conclusions about the exceptional importance of tunneling in the initiation step. As tunneling is relatively unimportant in the propagation step, the kinetic isotope effect is markedly reduced compared to its value in the initiation step. Hence, the kinetic isotope effect is significantly reduced compared to that in the initiation step. The ratio between the pre-exponential factors in the initiation step for the oxidation of the perhydro and perdeutero compounds is discussed. Bell's criterion for reactions involving tunneling is AD / AH≫1, while in the initiation step the ratio is close to unity. It is concluded that in reactions with a significant tunneling effect the ratio AD / AH may vary within limits, being less than, equal to, or greater than unity.
New Partitioning Perturbation Theory. II. Example of Almost Degeneracy
Authors: Certain Phillip R. , Dion David R. , Hirschfelder Joseph O.
Published: February 2004

A system of two coupled simple harmonic oscillators where many of the energy levels are almost degenerate doublets serves as an excellent “guinea pig” for testing many perturbation techniques including the modified Kirtman, Kirtman, and DE–FOP–VIM treatments discussed by Certain and Hirschfelder in a companion paper. These methods are compared with the “usual” approach to almost-degenerate problems which makes the zeroth-order energies of the doublet pair degenerate by a suitable choice of the zeroth-order Hamiltonian. In addition, four types of Rayleigh–Schrödinger expansions are considered. The best values of the energy are given by the Kirtman treatment. However, the DE–FOP–VIM and the modified Kirtman procedures give values almost as good, and have the added advantage that they give upper bounds to the energy of the states which are considered.
New Partitioning Perturbation Theory. III. Applications to Electron Exchange
Authors: Certain Phillip R. , Hirschfelder Joseph O.
Published: February 2004

A new partitioning perturbation technique provides a rigorous derivation and generalization of the Hirschfelder–Silbey formalism for treating electron exchange problems. A new method of solving the first-order perturbation equation is introduced. The first-order wavefunction is the sum of the polarization function which describes van der Waals correlations and an exchange function which introduces ionic terms. The exchange function is determined variationally for both the ground-state H+H+ and H+H interactions. In the many-electron case, the first-order equation reduces to a set of one- and two-electron equations.
Crystal Structures of the Three Modifications of Nitrogen 14 and Nitrogen 15 at High Pressure
Authors: Schuch A. F. , Mills R. L.
Published: February 2004

We have determined from x-ray diffraction photographs that high-pressure γ N2 is tetragonal with two molecules per unit cell in special position f of space group P42 / mnm. At an average pressure and temperature of 4015 atm and 20.5°K, respectively, the unit cell dimensions are a = 3.957 Å and c = 5.109 Å, giving a molar volume in good agreement with that from p–V–T measurements. The β N2 solid modification, which is contiguous with the melting curve, remains hexagonal up to at least 4125 atm and 49°K where the unit cell constants are a = 3.861 Å and c = 6.265 Å. Over the pressure range investigated, the c / a ratio is very close to the ideal value for closest packing of hard spheres. The atomic positions in hexagonal (β) N2, which are known to be highly disordered at low pressure, show no evidence of ordering at the highest pressures studied. The third allotrope, α N2, is cubic at 3785 atm and 19.6°K with four molecules in a unit cell 5.433 Å on a side. Both the Pa3 and P213 space groups, which have been reported at zero pressure, appear to be spatially possible at limiting high pressures for α N2. However, from our diffraction measurements on the cubic solid at all pressures, we were unable to prove the existence of the P213 structure. The measurements on pure 30N2 show that this isotope also exists in the same three solid modifications as 28N2 with cell dimensions that are similar for both isotopes. However, the transition to γ N2 at 20°K for the mass-15 isotope occurs 400 atm lower than that for the mass-14 isotope.
Ionic Collision Processes in Water Vapor
Authors: Ryan K. R.
Published: February 2004

Some ionic collision processes in water vapor have been studied by the techniques of high-pressure single-source mass spectrometry. Rate coefficients for reactions resulting from collisions of H2O+ with H2O, and of OH+ with H2O have been studied as a function of primary-ion translational energy over the energy range of 0–8 eV. For purposes of calibration and comparison, the reaction leading to CH5+ production in methane gas has also been studied over the same primary-ion energy range. The rate coefficient for H3O+ production from H2O+ / H2O collisions was found to drop from a value of 26 × 10−10 cm3 molecule−1·sec−1 for thermal ion reactions to a value of less than half of this for ions with a maximum energy of 4 eV. H3O+ production from OH+ / H2O collisions was found to proceed with a rate coefficient of 20 × 10−10 cm3 molecule−1·sec−1 for ions of thermal energy, but this coefficient had dropped to zero as the maximum ion energy was increased to 0.8 eV. Evidence is presented to support the contention that although H3O+ production decreases as the OH+ energy increases, the rate of removal of OH+ continues with high cross section The results indicate that in the ion energy region above 0.8 eV, OH+ is removed exclusively by charge transfer to H2O.
Effect of Pressure on Naphthalene-d8 Phosphorescence in Durene Crystals
Authors: Beardslee R. A. , Offen H. W.
Published: February 2004

The phosphorescence lifetime of C10D8 in durene mixed crystals has been studied in the temperature range 77–320°K and under pressures of 0–30 kbar. Matrix compression to 30 kbar shortens the triplet lifetime at 77°K about 20%. At higher temperatures where thermally activated quenching is evident, the triplet decay is very sensitive to pressure and becomes nonexponential with increasing pressure. Compression decreases the activation energy and high concentrations of defects increase the pre-exponential factor. The phosphorescence quenching is ascribed to charge-transfer interaction with the host molecules at structural defects in the crystal lattice.
Gol'danskii–Karyagin Effect in Dimethyl Tin Difluoride
Authors: Herber R. H. , Chandra Subhas
Published: February 2004

The temperature dependent asymmetry in the intensity of the two components of the Mössbauer resonance doublet in dimethyl tin difluoride can be related to the mean-square amplitudes of vibration parallel and perpendicular to the cylindrical molecular symmetry axis, and hence to the anisotropy of the Mössbauer–Lamb faction (the Gol'danskii–Karyagin effect). With an assignment of the Δm = 1 and Δm = 0 transition in the Mössbauer spectrum based on a simple bond character description, an asymmetry ratio A = Mπ / Mσ of 0.78 ± 0.06 is derived from the Mössbauer data. This is in good agreement with the value A = 0.72 derived from precision x-ray diffraction data on (CH3)2SnF2 and constitutes an independent check on the presence of the Gol'danskii–Karyagin effect in such solids.
Correlation Effects on ππ* Transition Energies in the Series of Linear Polyenes. III Dynamical Correlation Effects
Authors: Denis Armelle , Malrieu J. P.
Published: February 2004

An optimization of the 2pz atomic orbitals is rederived for the ground-state energy of conjugated systems. Within some reasonable assumptions, this optimization (analogous to that proposed by Silverstone and Joy) allows the orthogonalized 3pz atomic orbitals to be considered as v rtual orbitals in the CI calculation when one starts from a SCF calculation performed in the minimal basis set of optimized n = 2 atomic orbitals. The reorganization of the π* MO in the ππ* excited state leads to a spatial expansion in the singlet state and a spatial contraction in the triplet state. But this effect is less and less important when the dimension n of the π system increases, and the corresponding energetic correction decreases as n−2. A less pronounced effect is obtained on the π singly occupied orbital; the doubly occupied MO's are not significantly modified in the singlet state. The role of the 3pz atomic orbitals in the correlation effects on the transition energies is much more important. It is not mainly due to the π→3pz monoexcited configurations (which only give a correction proportional to n−1) but to the doubly and triply excited states: Their correction tends towards a constant value which has been estimated.
Vibrational Spectra and Structure of Oxalyl Chloride in the Crystalline and Fluid States
Authors: Durig J. R. , Hannum S. E.
Published: February 2004

The vibrational spectrum of oxalyl chloride, [COCl]2, has been recorded between 33 and 4000 cm−1. The infrared spectra of the oxalyl chloride molecule have been examined in the solid, liquid, and gaseous states. The Raman spectra of the solid and liquid have also been observed, and qualitative depolarization ratios have been measured. The vibrational spectrum of the crystalline material has been interpreted on the basis of C2h molecular symmetry and D2h15 space group symmetry. All 12 of the normal vibrations have been assigned, and the barrier to internal rotation around the C–C bond has been obtained. Nine bands appear in the infrared and Raman spectra of the liquid/gaseous states which do not appear in the spectra of the solid. In light of depolarization data and the frequencies for the vibrations of the trans isomer, these bands are interpreted as arising from a cis rotamer. A temperature study was made in order to determine the relative stability of the rotamers. The trans isomer was found to be 2.2 kcal/mole more stable than the cis form. A rough estimate of the equilibrium mixture at room temperature based on intensities and an assumed equilibrium constant gives a value of 15%–20% for the cis isomer.