bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational excitation of ozone and molecular fluorine reactions in cryogenic matrices
Authors: Frei Heinz , Fredin Leif , Pimentel George C.
Published: February 2003

Attempts have been made to induce under cryogenic conditions reactions between O3 and C2H2, C2H4, NO2, and NO, and between F2 and C2H4, by selective vibrational excitation of nearest neighbor reactant pairs. Samples in inert gas or nitrogen matrices at 12 K were irradiated with grating-tuned cw CO2 and CO lasers. Only linewidth narrowing due to thermal effects was observed in the first three systems with laser excitation of ν3 of ozone, ν7 and ν7+ν8 of ethylene, or ν3 of NO2. However, the dark reaction NO+O3→NO2+O2 in nitrogen matrices could be significantly enhanced by exciting the NO stretch of NO⋅O3 pairs at 1874 cm−1. No products were detected upon prolonged excitation of ν7 of C2H4⋅F2 reactive pairs at 953 cm−1 whereas excitation of ν7+ν8 of ethylene at 1896 cm−1 did promote reaction with an approximate quantum yield of 10−4. The products are trans- and gauche-1,2-difluorethane, vinyl fluoride, and HF. Kinetic studies revealed first order kinetics with linear dependence of the rate constant on laser intensity, which indicates a single photon photochemical process. The observed products are consistent with a molecular four center reaction mechanism, rather than the gas phase free radical mechanism. Isolated C2H4⋅F2 pairs produce predominantly the elimination products vinyl fluoride and HF, whereas an additional ethylene molecule in the reaction cage increases 100-fold the stabilization of the reaction adduct CH2F–CH2F°. The gauche/trans ratio of 1,2-difluorethane is near 2, close to the high temperature limit.
On the relation between the quantum scattering theory and the molecular orbital theories of chemical reactivity
Authors: Deguchi K. , Omae T. , Nishikawa K. , Aono S.
Published: February 2003

The reaction amplitude [or transition (T-)-matrix element] in the quantum scattering theory is expressed, under the several approximations, by two terms which we call here the electronic-transition (E) term and the nuclear-overlap (N) term, respectively. It is shown that the E and N terms are closely related to the correlation-diagram method and the perturbative method, both of which are familiar to chemists as the qualitative MO theories of chemical reactivity. We also show that as a merit of starting with the formal theory of scattering, the nonadiabatic effects are taken into account in our derivation, naturally and systematically. This fact enables us to extend our treatment to nonadiabatic chemical reactions and photochemical reactions.
Evidence for sequential reactions in the CO2 laser induced multiphoton dissociation of acetic anhydride and acetic acid
Authors: Grimley A. J. , Stephenson J. C.
Published: February 2003

The CO2 laser induced multiphoton dissociation of acetic acid and acetic anhydride has been investigated. We have observed the prompt formation of 1CH2 and OH by laser excited fluorescence and determined their nascent rotational energy distributions. The rotational energy of each product was the same, regardless of which starting material was photolyzed. This observation leads us to propose a mechanism in which both the 1CH2 and the OH are formed by sequential up-pumping of molecular intermediates. We have also determined the yield versus fluence curves for both the ã (0,0,0) and ã (0,1,0) levels of 1CH2. The relative yields of these two levels are found to change as a function of intensity.
Velocity dependence of rates for rotationally inelastic collisions in Na*2–Xe using velocity selection by Doppler shift
Authors: Smith Neil , Brunner Timothy A. , Pritchard David E.
Published: February 2003

We have measured the velocity dependence of rotational energy transfer (RET) rate constants for Na*2(A 1Σu+)–Xe collisions over an order of magnitude range in relative kinetic energy. Measurements were made for initial rotational levels ji=16, 38, and 66 for four different values of Δ=jf−ji. The present data are analyzed in terms of the energy corrected sudden (ECS) scaling approximation and determine the velocity dependence of both the ’’adiabatic correction’’ factor of the ECS theory, and the parameters a and γ which appear in the power law expression for the basis rates kl→0(vr) =a(vr)[l(l+1)]−γ(vr), an expression shown to be valid at all relative velocities studied. These measurements were made in a gas cell using the velocity selection by Doppler shift (VSDS) technique. Full details of analysis and deconvolution procedures used to isolate the relative velocity dependence are given.
Near resonant vibration → vibration energy transfer under single collision conditions
Authors: Brenner D. M.
Published: February 2003

Collisional energy transfer from energy levels ?1900 cm−1, produced by IR multiphoton absorption, C3H2O(v≳1)+M→C3H2O(v=1)+M°, is observed on a single collision basis by monitoring changes in the vibrational level populations of propynal as reflected in the laser excitation spectrum. It is concluded that vibration → vibration processes account for these observations and occur only with collision partners for which near resonant pathways exist. Under even the most favorable conditions (ΔE=40 cm−1), neither vibration → translation pathways nor multiple quantum changes involving nondriven modes are observed when ν6 is the pumped mode. Observations suggest that multiple quantum events involving the pumped mode occur, but comparison of rate constants from different levels is hindered at present by the identification of overtones in the visible spectroscopy.
Double quantum cross polarization via dipolar order
Authors: Reinhold M. , Brunner P. , Ernst R. R.
Published: February 2003

A scheme is proposed for the excitation and detection of multiple quantum coherence by cross polarization via dipolar order. It is applied to the recording of 14N double quantum spectra by observing proton resonance. The technique is suitable for systems with large quadrupolar splittings.
Emission spectra of KrXeCl*, KrXeBr*, KrXeI*, ArKrF*, and ArKrCl*
Authors: Brashears H. C. , Setser D. W. , Yu Y.-C.
Published: February 2003

Sensitized reactions of Xe and Kr with halogen donors in the presence of high buffer gas pressures of Kr and Ar, respectively, have led to the observation of five emission bands in the ultraviolet region of the spectrum. These broad structureless bands are assigned to the mixed rare gas–halide trimers KrXeCl, KrXeBr, KrXeI, ArKrF, and ArKrCl. Despite an extensive search, no emission corresponding to KrXeF could be found and this mixed trimer may be unstable because of interaction with a lower repulsive state. The positions and half-widths of the five emission bands are characterized, and the formation mechanism of the mixed trimers in these experiments is discussed.
A study of the absolute infrared intensity of ν3, absorption from vibrationally excited molecules, and the vapor pressure of UF6
Authors: Kim K. C. , Person W. B.
Published: February 2003

Results are presented from laser diode studies of ν3 transition of UF6 in the 16 μm region using a very long optical path absorption cell. The ν3 Q-branch and several individual rotational transitions have been recorded in the temperature range between 165 and 295 K. From the ν3 band contour analysis and the line intensity measurements, the vibrational band strength of the ν3 transition, S0V=710±120 km mole−1, was obtained. From absorption measurements of a UF6 sample in the cooled cell at pressures from 10−3 to 10−5 Torr, the vapor pressure curve of UF6 was extended to the temperature range down to 165 K. A new technique is described allowing us to obtain diode laser ’’survey’’ spectra over a range of 25 cm−1 by ’’mode scans.’’ This technique allows us to measure the changes in the band shape of ν3 as a function of temperature, and thus learn something about ’’hot band’’ absorption from vibrationally excited molecules.
Role of rotational thermal bath excitation on the EPR transient regime: A theoretical discussion
Authors: Ferrario Mauro , Grigolini Paolo
Published: February 2003

The excitation of rotational thermal baths can result in long-lived effects on spin relaxation in the slow motional region. An attempt at obtaining their quantitative evaluation is successfully made by enriching the Mori–Kivelson–Ogan (MKO) theory through comparison with the Kubo–Freed (KF) one and vice versa. This comparison, in turn, is allowed by suitably extending the MKO theory so as to deal with spin excitation by external radiation fields. The MKO, furthermore, is limited to the second-order Mori truncation while the KF one is related to the two-jump case. It is shown, then, that in the usual kind of steady-state experiments, the MKO theory provides the same result as the KF one. In contrast, when spin relaxation is monitored in the presence of a fluctuating force whose time average intensity is not vanishing, the two theories can result in quite different physical effects.
Crystal field analysis of Nd3+ and Er3+ in Bi4Ge3O12
Authors: Morrison Clyde A. , Leavitt Richard P.
Published: February 2003

The experimental optical spectra of Nd3+ and Er3+ in single-crystal Bi4Ge3O12 are analyzed with a crystal-field Hamiltonian Hx= Σnm B°nm Σi Cnm (ři) appropriate for C3 symmetry. The resulting best-fit Bnm (all in cm−1) are B20=−833, B40=−705, B43=1713, B60=594, RB63=−203, IB63=103, RB66=552, and IB66=−343 cm−1, with an rms deviation of 5.1 cm−1 for Nd3+, and B20=−732, B40=−920, B43=1626, B60=477, RB63=−33, IB63=−44, RB66=324, and IB66=−58 cm−1, with an rms deviation of 4.6 cm−1 for Er3+. Judd–Ofelt intensity parameters were calculated a priori from point-charge lattice sums. A set of smoothed Bnm are given for the entire lanthanide series.
Vibronic mechanisms in the two-photon spectrum of benzene
Authors: Rava Richard P. , Goodman Lionel , Krogh-Jespersen Karsten
Published: February 2003

Intensity measurements have been made on the single quantum bands of the active modes in the two-photon 1B2u←1A1g spectra of C6H6 and its isotopic homologs C6H5D, o-C6H4D2, C6HD5, p-C6H4D2, p-C6H2D4, s-C6H3D3, and C6D6. ν14 (b2u) and ν18 (e1u) (D6h symmetry) are found to be active in all the isotopic benzenes, and ν12 (b1u) in all except C6H6, s-C6H3D3, and C6D6 where it is identity forbidden, and p-C6H4D2 where it is weak. The results show that the band strength of 1410 is insensitive to deuterium substitution (confirming Duschinsky rotation of this mode), while 1210 and 1810 are strongly sensitive to the number and orientation of the deuterium atoms. INDO/S calculations of transition tensors at displaced nuclear coordinates, with and without doubly excited configurations, indicate that the ground state vibronic mechanisms usually neglected in one-photon spectra are important in the benzene two-photon spectra. The activity of ν14 can be attributed to vibronic perturbation of the final B2u state by the ground state, and that of ν18 by vibronic perturbation of the ground state by an excited state. The favored mechanism for the activity of ν12 in the spectra of the deuterium benzenes involves intensity borrowing from ν18 triggered by the kinematic perturbation of the deuterium atoms. The coupling states for ν12 are then E1u and E2g (D6h symmetry). Evidence is presented for Duschinsky rotation of the e1u modes.
The importance of nuclear spin diffusion as an angularly independent relaxation mechanism controlling the matrix ENDOR response of radicals in molecular crystals
Authors: Schlick Shulamith , Kevan Larry , Toriyama K. , Iwasaki M.
Published: February 2003

In the generalized matrix ENDOR line shape model [P.A. Narayana et al., J. Chem. Phys., 67, 1990 (1977)] the ENDOR intensity is controlled by the nuclear spin lattice relaxation rate formulated as an angularly dependent electron-nuclear dipolar contribution and an angularly independent intrinsic bulk nuclear relaxation contribution. Comparison of this model with single crystal data [L. Kevan et al., J. Chem. Phys., 70, 5006 (1979)] showed that the angularly independent relaxation rate must be more rapid than the intrinsic bulk nuclear relaxation rate. Here we identify the angularly independent relaxation rate with nuclear spin diffusion and apply this interpretation quantitatively to data on methyl radicals in lithium acetate dihydrate crystals and to the radiation generated CH3CH(NH3)ĊOOH− radical in alanine at 77 K.
Nonempirical molecular orbital calculations for hydrogen bonded molecular solids: Molecular dipole and quadrupole moments for solid HF and HCl
Authors: Anderson S. G. , Santry D. P.
Published: February 2003

A b initio SCF calculations, using the SCF perturbative method proposed by O’Shea and Santry, with a 3-21G basis set are reported by solid HF and HCl. It is found that the convergence of the perturbative series is slightly better than that for the corresponding CNDO theory. The calculated changes in the molecular dipole and quadrupole moments, induced by the crystal interactions, are found to be about 20% of their free molecule values.
Phase transformation of wurtzite-type boron nitride at high pressures and temperatures
Authors: Onodera A. , Miyazaki H. , Fujimoto N.
Published: February 2003

Transformation behavior of boron nitride between the wurtzitic and zincblende forms has been studied under static pressures from 80 to 200 kbar (8–20 GPa) and at temperatures up to 1800 °C without planned addition of catalysts. A reaction boundary for this transformation has been delineated which exhibits a broad maximum in temperature around 120 kbar.
A Monte Carlo study on the size dependence in hydrophobic hydration
Authors: Okazaki Susumu , Nakanishi Koichiro , Touhara Hidekazu , Watanabe Nobuatsu , Adachi Yoshinori
Published: February 2003

Monte Carlo calculations have been carried out with the Metropolis method for two systems each containing 63 ST-2 water and one Lennard-Jones solute in NVT ensemble at 298.15 K and 18.07 cm3 mol−1. The solutes chosen approximately simulate ethane and pentane. The internal energy and various distribution functions have been calculated and their results enable us, when combined with the previous study on methane and isobutane solutions, to examine the solute size dependence of hydrophobic hydration. Although energetic stabilization and structuration of water due to the introduction of a nonpolar solute have been shown to exist for all of the four solutions studied, they do not change monotonically with the size of solute. A long MC run has also been done for an aqueous methane solution to check the convergence characteristics of the present calculation.
Thermodynamics of sulfur chemisorption on metals. II. Alumina-supported ruthenium
Authors: McCarty Jon G. , Wise Henry
Published: February 2003

Isosteric heats of chemisorption of sulfur on alumina-supported ruthenium have been determined by measuring the equilibrium ratio of H2S/H2 in contact with the solid in a closed gas-recirculation system. At low coverage the heat of chemisorption with respect to 1/2 S2(g) was found to be −215±15 kJ mol−1. With increasing coverage it decreased to −174±10 kJ mol−1, a value comparable to the heat of formation of bulk ruthenium sulfide. For equal coverage with sulfur, the thermodynamic chemical potential of alumina-supported ruthenium exceeds that of alumina-supported nickel by approximately one order of magnitude. The binding strength of adsorbed sulfur is 432±15 kJ mol−1 for Ru and 464±15 kJ mol−1 for Ni.
On the d
n4s2–d
n+14s1 separation for selected first row transition metal atoms
Authors: Bauschlicher Charles W. , Walch Stephen P.
Published: February 2003

The results of CI(SD) calculations performed with BIGGMOLI-NONAME presented.(AIP)
Matrix isolation study of D2Te and H2Te molecules in solid argon
Authors: Montano P. A. , Nagarathna H. M. , Newlin D. , Stewart G. W.
Published: February 2003

A careful synthesis of H2Te and D2Te and their isolation in solid argon are reported. Rare gas matrix isolated molecules of H2Te and D2Te were studied using Mössbauer spectroscopy. The gaseous species were identified using mass spectrometry. Mössbauer parameters were analyzed using extended Hückel calculations. A good agreement between the experimental and the calculated value of the quadrupole splitting requires a smaller quadrupole moment for the excited state of 125Te than that reported in the literature. A 10% increase in the QS for D2Te is explained as due to the enhancement of 〈1/r3〉 caused by a slight contraction of the D–Te bond.
Determination of the Na–Hg repulsive potential between 0.5 and 3 eV from the inversion of differential cross section data
Authors: Hüwel L. , Maier J. , Pauly H.
Published: February 2003

Differential scattering cross sections for the scattering of ground state sodium atoms by mercury have been measured in the energy range from 0.5 to 3 eV, using the seeded beam technique. Making use of the precisely known potential well of this system, the scattering data are inverted to the potential applying the method of Firsov. The results are compared with recent pseudopotential calculations.
Temperature dependence of O2(1Δ)+O2(1Δ) and I(2P1/2)+O2(1Δ) energy pooling
Authors: Heidner R. F. , Gardner C. E. , El-Sayed T. M. , Segal G. I. , Kasper J. V. V.
Published: February 2003

The electronic-energy pooling reactions forming O2(1Σ) are of critical importance to the overall mechanism of the O2(1Δ)–I atom transfer laser. In this study, we report temperature-dependent rate coefficient data for O2(1Δ)+O2(1Δ)→k2O2(1Σ)+O2(3Σ) and O2(1Δ)+I(2P1/2)→k3O2(1Σ)+I. These data were obtained using a temperature-controlled kinetic flow tube equipped with computer-controlled spectroscopic diagnostics. The fundamental data reported are k2(T)/k2(295) = (3.5±1.5)exp(−780/RT) and k3(T)/k2(T) = (5.5±1.0)×103. Both rate-coefficient ratios are reported for the temperature range T = 259 to 353 K. The results are compared with earlier measurements of these processes.
Ion–molecule reactions of SF6: Determination of I.P.(SF5), A.P.(SF5+/SF6), and D(SF5–F)
Authors: Babcock Lucia M. , Streit Gerald E.
Published: February 2003

SF6 reacts with CF3+ to produce SF5+ and CF4. Under conditions of large amounts of CF4, an equilibrium is established. From flowing afterglow data, not only is a forward rate constant kf = (2.5±0.2)×10−10 cm3 molecule−1 s−1 determined, but also an equilibrium constant of 5.9×103 (±50%) is deduced. Using this equilibrium constant and standard entropies for the various species, ΔH°298 = −0.17±0.02 eV is calculated. A thermochemical cycle then yields a value of A.P.(SF5+/SF6) = 14.62±0.09 eV. An examination of charge exchange reactions of SF5+ leads to establishment of the ionization potential of SF5 at 10.5±0.1 eV. Combining this with A.P.(SF5+/SF6) allows a calculation of the bond dissociation energy D°298(SF5–F) = 4.1±0.13 eV. In addition to these new values of A.P.(SF5+/SF6), I.P.(SF5), and D°298(SF5–F), the fragmentation pattern of SF6 reacting with He+ as well as rates of reaction of SF6 with several positive ions are reported.
Expansion approach to photodissociation dynamics. I. Theory
Authors: Takatsuka Kazuo , Gordon Mark S.
Published: February 2003

We propose a new formulation for a photodissociation process to which an expansional (or algebraic) quantum-variation-method of scattering is applicable. By solving a ’’full collision’’ problem which describes a multichannel process on the repulsive surface, the photodissociation scheme takes account of interchannel coupling from the outset. Our expression for the amplitude of the partial linewidth is similar to that of the ’’half-collision’’ approximation of Jortner et al. The present formalism differs in that the scattering wave functions take acount of interchannel coupling exactly. As a result, only on-the-energy-shell contributions appear in the partial linewidth.
EPR and a
b i
n
i
t
i
o SCF–MO studies of the Si⋅H–Si system in the E′4 center of α-quartz
Authors: Isoya J. , Weil J. A. , Halliburton L. E.
Published: February 2003

The E′4 center in irradiated α-quartz has been studied by single-crystal EPR. The unpaired electron is shared mainly by two silicon ions Si(1, 2) with the larger fraction on Si(2). The spin-Hamiltonian parameter matrices ḡ, Ā1H, Ā29Si(1, 2) at 40 K, as well as Ā29Si(1, 2) at 300 K, are reported. A model of the E4′ center, consisting of an O− vacancy between Si(1, 2), with a hydride ion bonded to Si(1), is presented. Ab initio SCF–MO calculations for a 15-atom cluster, yielding the minimum energy atom configurations, have supported this model by giving good qualitative agreement with the EPR data. The temperature dependence of the hyperfine matrices Ā29Si(1, 2) is explained by Boltzmann population of the vibrational levels in the asymmetric double-minimum potential of the hydrogen bond system Si(2) ⋅ H–Si(1).
Laser photoelectron spectroscopy of CH2−, and the singlet–triplet splitting in methylene
Authors: Engelking P. C. , Corderman R. R. , Wendoloski J. J. , Ellison G. B. , ONeil S. V. , Lineberger W. C.
Published: February 2003

Photodetachment of an electron from CH2− ions using visible light produces CH2 in both its ground 3B1 and lowest excited 1A1 states. In an earlier letter, this process was utilized to determine directly a singlet–triplet splitting of 19.5 kcal/mol. In this paper are reported the details of the early measurement and a number of additional investigations designed to test critically the earlier assignments. Data reported here indicate that one suggested problem with the early results, a misassignment of the 3B1 origin resulting from negative ion hot bands, is not likely. These new results do reveal the presence of an additional peak in the photoelectron spectrum which could possibly be the lowest level of the 3B1 state, allowing a singlet–triplet splitting of 19.5 or 23.2 kcal/mol. The results reported here do not indicate any other inconsistency in the original assignment. Finally, these data are discussed in light of the formidable body of theoretical and indirect thermochemical determinations suggesting a singlet–triplet splitting near 8 kcal/mol.
Low temperature intrinsic defects in x-irradiated hydroxyapatite synthetic single crystals
Authors: Close D. M. , Mengeot M. , Gilliam O. R.
Published: February 2003

ESR studies of radiation-induced defects have been conducted on synthetic calcium hydroxyapatite single crystals. For a room temperature x-irradiation a major defect (labeled A) was reported to be an O− ion. X irradiation at 6 K shows defect A, trapped atomic hydrogen, and a nonaxial holelike center (labeled I). These new centers are stable at 6 K but anneal near 77 K. Observations at 9 and 35 GHz indicate that the I center is a spin-1/2 defect located in six inequivalent sites. In the ab plane, spectra exhibit an isotropic hyperfine doublet (∼13 G splitting) and an anisotropic doublet (17–27 G splitting) in three symmetry-related sites. For other orientations additional site splitting and ’’forbidden transitions’’ make the spectra very complex. The hyperfine coupling tensor for the anisotropic doublet has diagonal elements −29.5, −19.2, and +3.11 G. The g tensor for this defect has diagonal elements 2.0068, 2.0032, and 2.0148. The sets of directional cosines associated with the minimum g value and the intermediate A value each indicate a direction corresponding approximately to that of the vector from an OH oxygen to a neighboring PO43− oxygen. The model proposed for the I center is a hole trapped by both an OH− and a neighboring PO43−. The anisotropic doublet is accounted for with 65% of the spin density on OH−. The remaining spin density is on a phosphate oxygen. This creates a PO2−4 defect with the isotropic coupling arising from hyperfine interaction with the 31P nucleus.
Raman spectroscopic study of the conformational order in hexadecane solutions
Authors: Wunder Stephanie L. , Merajver Sofia D.
Published: February 2003

The Raman spectrum of hexadecane (n-C16) has been analyzed in the high frequency C–H stretching region (2800–3100 cm−1). The intensity of the bands was studied as a function of the concentration of n-C16 when it was diluted in the low molecular weight, quasispherical solvent CDCl3. This solvent does not exhibit any Raman active bands in the region of interest. In order to characterize the influence of the concentration on the local order, parallel studies were conducted on both neat n-C16 and a very dilute solution of n-C16 in CDCl3 as functions of temperature. A comparison of these three series of spectra suggests that the neat liquid exhibits about 5% higher trans bond content than does the dilute solution.
Vibrational circular dichroism of 3-methylcyclohexanone. Fixed partial charge calculations
Authors: Singh R. D. , Keiderling T. A.
Published: February 2003

The vibrational circular dichroism (VCD) in the CH stretching region of 3-methylcyclohexanone (MCH) is analyzed with the fixed partial charge (FPC) model. First, an extensive search of the conformations of MCH using the consistent force field (CFF) led to five stable geometries of which the chair-equatorial (C-EQ) conformer was lowest in energy. Then, normal mode calculations were performed for these conformers with both the CFF and a modified valence force field. Good agreement with the observed absorption and VCD spectra was found for the C-EQ conformer in the FPC model using CNDO generated charges. The other conformers led to significantly different calculated VCD. Consistent with the above results, temperature variation measurements of the VCD indicate that only one conformer dominates the observed spectra. Mid-IR VCD results for the CH deformation of MCH are also qualitatively analyzed and discussed in terms of our FPC calculations. The FPC model appears to be predictive for those regions of the VCD spectrum that are dominated by interaction of well-defined local modes such as the CH stretches. However, it may be less useful where modes are highly mixed such as the CH2 deformations.
Infrared band intensities of ethylene oxide
Authors: Nakanaga Taisuke
Published: February 2003

The infrared absorption intensities of ethylene oxide and ethylene oxide-d4 were measured in the gas phase using the pressure broadening method. When a band is overlapped by other bands, it is separated by the simulation calculation. The obtained band intensities were analyzed by the least squares method with the aid of the results from the Coriolis analyses and the CNDO/2 calculations. The correct sign combination of (∂p/∂Qi) has been sucessfully determined. The atomic polar tensors and the local intensity parameters of the CH2 group were calculated. The CH2 local intensity parameters of ethylene oxide were found almost equal to those of cyclopropane, and different from those of C–CH2–C and CH2 = C systems.
High resolution solid state 13C NMR spectra of carbons bonded to nitrogen in a sample spinning at the magic angle
Authors: Naito A. , Ganapathy S. , McDowell C. A.
Published: February 2003

High resolution solid state 13C NMR spectra of amino acids and nitrile compounds have been obtained by a combination of cross-polarization, dipolar decoupling, and magic angle spinning techniques (CP–MAS). The resonances of Cα and cyano carbons in the observed 13C spectra show asymmetric doublet patterns. This problem has been treated by using the adiabatic approximation since the 14N quadrupole interaction is much larger than the spinning frequency. Theoretical spectra have been simulated for these carbons and they show very good agreement with experimental observations.
Identification and atomization energies of gaseous molecules ScC2, ScC3, ScC4, ScC5, and ScC6 by high temperature mass spectrometry
Authors: Haque R. , Gingerich K. A.
Published: February 2003

The high temperature Knudsen effusion mass spectrometric technique has been used to determine the thermodynamic properties of the gas phase species ScC2, ScC3, ScC4, ScC5, and ScC6, of which the second, fourth, and fifth are new molecules. The atomization energies Δ?a,0[ScC2(g)], ΔH°a,0[ScC3(g)], ΔH°a,0[ScC4(g)], ΔH°a,0[ScC5(g))] and ΔH°a,0[ScC6(g)] were obtained as 1185±14, 1779±22, 2455±22, 3139±29, and 3777±34 kJ mole−1 or 283.2±3.3, 425.2±5.3, 586.8±5.3, 750.2±6.9 and 902.7±8.1 kcal mole−1, respectively. The corresponding standard heats of formation ΔH°f,298 for gaseous ScC2, ScC3, ScC4, ScC5, and ScC6 were evaluated as 617±12, 735±20, 772±18, 798±24, and 874±28, kJ mole−1 or 147.5±2.9, 175.7±4.8, 184.5±4.3, 190.7±5.7, and 208.9±6.7, kcal mole−1, respectively. These results were derived from the enthalpies, ΔH°0, of the following reactions: The results for ScC2(g) and ScC4(g) are compared with the literature data. An upper limit for D°0(ScC)?440±21 kJ mole−1 was also obtained.
Critical points and tricritical points in liquid sulfur solutions
Authors: Wheeler John C. , Pfeuty Pierre
Published: February 2003

Equilibrium polymerization in a solvent can be described by the n→0 limit of a dilute n-vector model of magnetism in a small magnetic field. In the molecular field approximation the model becomes identical to the earlier theory of Scott for liquid sulfur solutions. The lower critical solution temperature in sulfur solutions is found to be intimately associated with a tricritical point in the magnetic model which accounts for the distinctive shape found by Scott for the high-temperature phase separation curve. The connection with the magnetic model also establishes a relationship between the phase diagrams predicted by Scott for sulfur solutions and those predicted by Blume, Emery, and Griffiths for He3xnHe4 mixtures. Modern theory of critical and tricritical phenomena suggests that incorporation of nonclassical critical behavior in the dilute n→0 vector model may help to resolve certain discrepancies between Scott’s mean field theory and experimental coexistence curves for sulfur solutions.
Structure of the liquid–vapor interface of molecular fluids: Homonuclear diatomic molecules
Authors: Thompson S. M. , Gubbins K. E.
Published: February 2003

The molecular dynamics method is used to simulate the liquid–vapor interface of fluids modeled on nitrogen and chlorine. Site–site Lennard-Jones potentials are used, with electrostatic interactions included in some simulations by means of point charges. The simulations yield the surface tension, surface thickness, density profile ρ(z), and density-orientation profile ρ(z, ϑ); the latter quantity gives the probability density for finding molecules whose axes are at an angle ϑ to the z axis at a height z in the interface, z being perpendicular to the interface. The calculated surface tensions are in good agreement with experiment. For the N2 models no detectable orientational correlations were observed, but for the Cl2 models such correlations were significant at low temperatures. In the absence of electrostatic forces the model Cl2 molecules on the liquid side of the Gibbs dividing surface showed a preferred orientation perpendicular to the surface, in qualitative agreement with first order perturbation theory. The addition of electrostatic forces decreased the tendency of the molecules to align perpendicular to the surface, and for very strong quadrupoles there appears to be a preference for a molecular alignment parallel to the surface. Simulations were made for systems of 216 and 1000 molecules to test the effect of system size. This increase in size caused the surface thickness to increase by about 23%, but had little effect on the other properties.
A direct experimental determination of the elastic contribution of chain entangling in a tightly cross-linked elastomer
Authors: Batsberg Walter , Kramer Ole
Published: February 2003

A polybutadiene with a 88% vinyl-content was studied in order to confirm previous results of the Langley and two-network methods.(AIP)
Laser production of supersonic metal cluster beams
Authors: Dietz T. G. , Duncan M. A. , Powers D. E. , Smalley R. E.
Published: February 2003

Cold beams of metal clusters are produced by combining a laser vaporization technique with pulsed supersonic nozzle technology. (AIP)
The direct photodissociation of the van der Waals molecule NO–Ar
Authors: Langridge-Smith Patrick R. R. , Carrasquillo M. Edwin , Levy Donald H.
Published: February 2003

Fluoresence spectra, due to dye laser excitation, are obtained for NO–Ar. (AIP)
Production of O2(a 1Δg) by oxygen atom recombination on a Pyrex surface
Authors: Black G. , Slanger T. G.
Published: February 2003

An oxygen atom afterglow was prepared by titrating nitrogen atoms with nitric oxide. After some of the atoms had recombined, principally by heterogeneous recombination on the Pyrex walls, the resulting concentrations of O2(X 3Σ−g) and O2(a 1Δg) were measured by absorption in the vacuum ultraviolet. Production of a large yield of O2(a 1Δg) was found. Implications of the finding for kinetic studies in oxygen atom afterglows are discussed.
Nonradiative decay of indoles under collison-free conditions
Authors: Glasser N. , Lami H.
Published: February 2003

Absorption spectra and fluorescence spectra of indile, 5-methoxyindole, 1-methyl indole, and 2,3-dimethyloudole are reported. The mean nonradiative decay rates were deduced from the measured lifelime and quantum yield.(AIP)
Chaos in the Belousov–Zhabotinskii reaction
Authors: Hudson J. L. , Mankin J. C.
Published: February 2003

Experimental results obtained with the Belousov–Zhabotinskii reaction in a continuous stirred reactor are analyzed in order to distinguish between periodic and chaotic behavior. Power spectra and stereoscopic three-dimensional state space trajectories are presented for both periodic oscillations and for chaos. A next amplitude map is constructed for the chaotic case and from it a Liapunov characteristic exponent of +0.62 is calculated. The rate of divergence of trajectories is illustrated by following the behavior as predicted from the next amplitude map of 10 points initially very close together.
Microwave, Raman, and far infrared spectra, barrier to internal rotation, and dipole moment of 2,2-difluoropropane
Authors: Durig J. R. , Guirgis G. A. , Li Y. S.
Published: February 2003

The microwave spectrum of 2,2-difluoropropane has been investigated in the region from 26.5 to 40.0 GHz. Only b-type transitions were observed and these were assigned on the basis of the rigid rotor model. The rotational constants in the ground vibrational state were found to be A = 5149.79±0.04, B = 4840.11±0.02, and C = 4805.62±0.02 MHz. Two excited states were also identified and assigned as the excited states of the two methyl torsional modes. From relative intensity measurements the frequencies for these two methyl torsional modes were determined to be ν(A2) = 217±15 and ν(B2) = 250±30 cm−1. From the Stark effect the dipole moment was determined to be ‖μb‖ = ‖μt‖ = 2.40±0.02 D. No splitting due to the internal rotations of the methyl groups were observed in the ground or first excited states. From a diagnostic least-squares adjustment to fit the three rotational constants, the following heavy atom structural parameters were obtained: r(C–F) = 1.352±0.013 Å; r(C–C) = 1.532±0.013 Å; ∢FCF = 106.09±1.44°; ∢CCC = 112.54±0.79°. These parameters are compard to the corresponding ones in some related molecules. The Raman spectra (0–3500 cm−1) were recorded for the gaseous, liquid, and solid states of 2,2-difluoropropane. The far infrared spectra (40–520 cm−1) of the gas and the solid were also investigated. A revised assignment is presented for the skeletal bending modes. The methyl torsions were observed in the Raman spectrum of the solid at 255 and 285 cm−1 from which a periodic barrier of 1675 cm−1 (4.79 kcal/mole) for the methyl internal rotation was calculated for the solid state. From combination and overtone bands of the methyl torsions, the barrier for the gaseous state has been determined to be 1207 cm−1 (3.45 kcal/mole). These results are compared to the corresponding quantities in related molecules.
Multiphoton ionization spectroscopy of the 3s Rydberg state in the deuterated acetaldehydes
Authors: Eichelberger T. S. , Fisanick G. J.
Published: February 2003

The two-photon resonant n→3s Rydberg transitions in CH3CHO, CH3CDO, CD3CHO, and CD3CDO have been studied by multiphoton ionization in a parallel plate cell with 0.4 cm−1 resolution. Vibrational analysis confirms that the molecule remains planar in the resonant state, and that the methyl group torsional barriers have values of 750±25, 770±25, 750±25, and 780±25 cm−1 in the respective molecules compared to ground state values of 413, 413, 410, and 408 cm−1. The methyl group has an s–cis configuration in both the ground and excited states. Experiments using a supersonic expansion of the sample show that hot bands other than those arising from the low frequency methyl torsion are readily suppressed. Evidence for saturation effects in the MPI spectrum are also presented.
Vibrational relaxation in jet-cooled phenylalkynes
Authors: Powers D. E. , Hopkins J. B. , Smalley R. E.
Published: February 2003

Absorption and dispersed fluorescence spectra are reported for a variety of 1-phenylalkynes cooled in a supersonic free jet. The 1B2(ππ)←1A1 spectra of these compounds is characterized by a near perfect cancellation of transition strength sources for the totally symmetric vibronic bands including the origin. Two skeletal distortion modes are found to dominate the spectra, inducing strong transitions by vibronic coupling to a higher state of 1A1 symmetry. These two modes are rotated in the 1B2 state relative to their configuration in the ground state. The nature of these modes is found to be largely unaffected by changes in the length of the alkyl chain, but vibrational relaxation into the chain still appears to proceed on a subnanosecond time scale.
High-field laser magnetic resonance spectra of NF2 (ν1) tuned by the avoidance of Zeeman-level crossings
Authors: Hakuta Kohzo , Uehara Hiromichi
Published: February 2003

The high-field laser magnetic resonance spectra of NF2 (ν1) tuned by the avoidance of Zeeman-level crossings were observed by using a CO2 laser. The analysis has yielded the band origin of the ν1 vibration–rotation band and the rotational and spin–rotation constants of the excited vibrational state.
NMR investigation of orientationally disordered NaCN and KCN
Authors: Wasylishen R. E. , Pettitt B. A. , Jeffrey K. R.
Published: February 2003

Nitrogen-14 NMR spin–lattice relaxation times are reported for the cubic phases of NaCN and KCN. The 14N T1 values of both compounds increase as temperature increase with apparent activation energies of 1.40±0.10 and 0.78±0.05 kcal/mole, respectively. The measured 14N T1 values indicate that T1≫T2 and that the rotational correlation times for cyanide reorientation are at least two orders of magnitude shorter than indicted in previous 14N studies. Carbon-13 T1 values for KCN decrease with increasing temperature with an apparent activation energy of 1.38±0.2 kcal/mole and are interpreted assuming that the spin rotation mechanism dominates. Reduced correlation time τ2  and τJ  derived from the 14N and 13C data, respectively, are compared with those calculated using the J-diffusion model which is quite successful for liquids.
Collisional linewidths of the EPR spectrum of molecular oxygen
Authors: Gardiner W. C. , Pickett H. M. , Proffitt M. H.
Published: February 2003

Pressure broadening of the X-band EPR spectrum of molecular oxygen was studied in an apparatus designed to minimize systemtic and random errors in the collisional linewidth. Results are reported for 16 lines with rotational quantum number N ranging from 1 to 11. No explicit M dependence of linewidth at the 1% accuracy level was found except that the N = 1, J = 1 line with initial M value of 0, which is at a substantially higher field than the other N = 1 lines, was found to be significantly broader than the other N = 1 lines. Otherwise, a slight but definite decrease of width with increasing field was found, presumably due to magnetic alignment of O2. Over the 0.5 to 2.3 Torr pressure range studied there was a remarkable variation of width with pressure, implying a small negative linewidth at zero pressure, for which we were unable to find a satisfactory explanation in theory or systematic measurement errors.
Laser induced fluorescence studies of UI
I produced by photoionization of uranium
Authors: Chen Hao-Lin , Borzileri C.
Published: February 2003

Multiphoton ionization of uranium atoms followed by laser-induced fluorescence detection of uranium ions is used to study the initial electronic state distribution of photon-produced ions and the degree of ionization within the uranium vapor. Our experimental data clearly demonstrate that photoionization of uranium through the 50 701 cm−1 excited even levels using a broad band pulsed dye laser of 5915 Å produces roughly 50% ground state ions and 50% metastably excited (289 cm−1) ions. The effective photoionization cross section is about 4.7×10−17 cm2. The subsequent dispersion of UII after photoionization appears to be caused by ambipolar expansion of ions and electrons in the plasma. The gA values for the 4050 and 3859 Å transitions of UII are measured to be 0.86×108 and 2.8×108 s−1, respectively.
Rate constants for the collisional dissociation of N2O5 by N2
Authors: Viggiano A. A. , Davidson J. A. , Fehsenfeld F. C. , Ferguson E. E.
Published: February 2003

The rate constants for the collisional dissociation of N2O5 by N2 are measured over the pressure and temperature ranges of 10 to 800 Torr and 285 to 384 K, respectively. The measurements are carried out in flow-through reaction cells of different volumes, the N2O5 being detected by selected ion–molecule reactions in a flowing afterglow apparatus. The present rate constants are somewhat smaller than those of earlier studies and also suggest that the transition from second- to first-order kinetics occurs at lower pressures than was earlier thought. The data are fitted to theoretical models of varying sophistication. The recommended values for use in extrapolation for the limiting low-pressure and high-pressure rate constants corresponding to the first-order Troe extension of the Lindeman–Hinshelwood model for Fsccent = 0.572 are k0 = 1.15×10−5 exp (−19.70 kcal/mol/RT) cm3 s−1 and k00 = 1.21×1017exp(−25.41 kcal/mol/RT) s−1. Other forms yield similar results.
Vibrational relaxation in jet-cooled phenoxyalkanes
Authors: Hopkins J. B. , Powers D. E. , Smalley R. E.
Published: February 2003

Results of vibrational relaxation of phenoxyalkanes is presented.(AIP)
On departures from the Stern–Volmer law for fluorescence quenching in liquids
Authors: Baird James K. , Escott Scott P.
Published: February 2003

Fluorescence quenching with solutions of fluorophores and quenchers in inert solvents in considered.(AIP)
A classical spectral theorem in bimolecular collisions
Authors: Pollak Eli
Published: February 2003

A proof of a classical spectral theorem recently conjectured by P. Brumer, D. E. Fitz, and D. Wardlaw [J. Chem. Phys. 72, 386 (1980)] is provided. The theorem states that the amount of time spent by a set of classical trajectories in phase space is proportional to the volume in phase space swept out during that time. Implications of the theorem are briefly discussed.
Determination of cross sections for formation of parent and fragment ions by electron impact from SO2 and SO3
Authors: Smith O. I. , Stevenson J. S.
Published: February 2003

Partial ionization cross sections are measured for the formation of parent and fragment ions from SO2 and SO3. For SO2, the SO+2, SO+, and S+ and/or O+2 ions are observed for ionizing electron energies between the ionization potential and 40 eV at temperatures in the interval 295–865 K. For SO3, SO+3, SO+2, SO+, and S+ and/or O+2 the ions are observed between the ionization potential and 30 eV over the temperature range 496–870 K. Total ionization cross-section curves are formulated for both species by appropriate summing of the partial cross sections. Appearance potentials for formation of various ions are identified and discussed in terms of elementary processes which may be responsible. The fragmentation patterns of both SO2 and SO3 are found to be insensitive to temperature over the range studied.
Nonresonant collision-induced absorption in Xe/Cl2 mixtures
Authors: Wilcomb B. E. , Burnham R.
Published: February 2003

Mixtures of Xe and Cl2 have been irradiated at 193 nm with a pulsed ArF laser. The collision-assisted laser-induced absorption results in emission of XeCl (B and C) at 308 and 340 nm, respectively. The intensity of XeCl emission is found to vary linearly with ArF laser power, indicating the occurrence of a single photon-assisted collision-induced process. The temporal characteristics of the emission vary substantially with the number density of each reagent showing fast formation and decay rates at high densities, but slower rates at lower densities. A kinetic mechanism is proposed in which a (XeCl2) transient complex is initially formed and that complex subsequently dissociates to yield XeCl.
The a
b i
n
i
t
i
o potential energy surfaces of some low-lying states of acetylene
Authors: Nomura Okio , Iwata Suehiro
Published: February 2003

Excitation energies, fluorescence wave numbers, vibrational frequencies, and ionization potentials of singlet valence states of acetylene were calculated by means of MRBW (multireference Brillouin–Wigner) perturbation, CI (configuration interaction), and GF matrix methods with minimal and extended basis sets. The behavior of the main configurations with respect to the CCH bending angle was elucidated with the π electron approximation. The fluorescence of acetylene recently found with strong Stokes shift was assigned to 1Au. The mechanism of the fluorescence in acetylene and propyne was explained with the use of potential energy maps. The vibrational structure of the acetylene cation in the lowest 2Πu state was well simulated with the potential energy curve and by the harmonic oscillator approximation. The breakdown of the Koopman’s theorem was analyzed by the CI method.
Hydrodynamic interaction effects on local motions of chain molecules
Authors: Pear M. R. , McCammon J. A.
Published: February 2003

The librational motions of a butane molecule in water are simulated by Brownian dynamics with and without inclusion of hydrodynamic interactions. Inclusion of the hydrodynamic interactions is found to increase the torsional correlation times by 35% to 46%, depending on the isomeric state of the butane molecule, and to decrease the rate constants for gauche–trans isomerization by about 28%. A critical discussion of the approximations involved in such simulations is presented.
Dynamic NMR in liquid crystalline solvents—ring inversion of s-trioxane
Authors: Luz Z. , Naor R. , Meirovitch E.
Published: February 2003

The proton magnetic resonance spectrum of s-trioxane in nematic liquid crystalline solvents was studied over a wide range of temperatures. The spectra exhibit striking line shape variations with temperature which are attributed to the ring inversion of s-trioxane. At high temperatures (≳100 °C) the spectra correspond to an average Hamiltonian due to the two fast interconverting chair forms of the molecule. As the temperature is lowered the resonance lines undergo selective broadening due to incomplete averaging of the two conformers. In order to obtain quantitative kinetic results the line shape theory for dynamic NMR in anisotropic liquids is formulated and is used to calculate theoretical spectra for s-trioxane undergoing ring inversion. Comparison of the experimental and theoretical spectra yields kinetic parameters for the reaction in the temperature range at which the liquid crystalline solutions were studied, i.e., −20 to +110 °C. The proton NMR of s-trioxane was also studied in the isotropic solvent freon in the temperature range −80 to −40 °C. Analysis of these spectra yield essentially the same kinetic parameters as in the anisotropic solvents. The final overall results are ΔH≠ = 11.2 kcal/mole, ΔS≠ = +3.1 e.u., and 1/τ (300 °K) = 2.0×105 sec−1. The advantages and limitations of dynamic NMR in liquid crystalline solvent are discussed.