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  • 1
    Electronic Resource
    Electronic Resource
    A theoretical treatment of multiple quantum nuclear spin coherences in electron spin echo studies of polyacetylene (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 978-995 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a theoretical investigation of the origin of the unusual 3-pulse electron spin echo envelope modulation (ESEEM) pattern obtained from 13C enriched polyacetylene, which exhibits modulation at multiple harmonics of the nuclear Larmor frequency ωn. ESEEM signals have been calculated for different models of the electron–nuclear spin coupling. It is shown that the local hyperfine tensors observed in electron nuclear double resonance (ENDOR) experiments on (13CH)x cannot account for the ESEEM results. These tensors have isotropic components which are too large to give narrow peaks at ωn, and there are too few nuclei to produce the large number of harmonics observed. Simulations for a point electron coupled to nuclei on a spherical shell, the spherical model, give insight into the conditions required to obtain narrow lines at multiple harmonics of ωn@B: The electron must be weakly coupled to a very large number of nuclei. Analytical results show that for the spherical model in the limit of small coupling, the intensity profile of the Fourier transform of the three-pulse ESEEM will be a Gaussian of width k¯ N, where k¯ is a function of the magnitude of the electron–nuclear coupling and N is the number of nuclei. Simulations assuming that the unpaired electron is delocalized in one dimension along the polyene chain and dipolar coupled to the 13C nuclei on neighboring chains give multiple harmonic patterns, at a reduced interchain distance. We conclude that the observed ESEEM pattern arises from nuclei on neighboring polyacetylene chains and is closely related to the matrix/distant ENDOR line observed for (13CH)x.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458080
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  • 2
    Electronic Resource
    Electronic Resource
    The translational energy dependence of the F+C2H4 →H+C2H3F reaction cross section near threshold (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 275-284 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have carried out crossed molecular beam studies of the substitution reaction F+C2 H4 →H+C2 H3 F in the collision energy range 0.8–2.5 kcal/mol using a velocity selected F atom beam. The relative substitution cross section is found to decrease with increasing collision energy indicating that if there is a potential energy barrier to F atom addition to C2 H4 it is much less than 0.8 kcal/mol and that the transition state for addition occurs early along the reaction coordinate. These results agree well with ab initio calculations. Although the product translational energy distributions are similar to those obtained from earlier work at higher collision energies, the C2 H3 F angular distributions suggest that a wider range of reactant approach geometries lead to products at lower energies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458474
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  • 3
    Electronic Resource
    Electronic Resource
    Theoretical study of memory kernel and velocity correlation function for condensed phase isomerization. II. Velocity correlation function, barrier crossing rates, and generalized Smoluchowski equation (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6340-6346 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The time dependent friction (TDF) introduced in paper I for barrier crossing problems is used to derive a functional form of the angular velocity autocorrelation function (AVACF) associated with the reactive coordinate. This theoretical expression is then compared with AVACFs derived directly from molecular dynamics angular velocity data. A connection between this AVACF model and Zwanzig's waiting-time description can be made. The TDF functions from paper I can also be used to deduce barrier crossing rates using the Kramers–Grote–Hynes formalism. A special case leads to the "extended'' Kramers equation, which was found in previous work to be congruous with experimental barrier crossing results. Using methods introduced by Okuyama and Oxtoby, a generalized Smoluchowski equation can also be derived. This Smoluchowski equation permits an examination of free diffusion and of the approach to equilibrium. Throughout papers I and II the theme is raised concerning the effects of nonlinear coupling on the methodology for analyzing barrier crossing problems. Seemingly, the appealing Langevin approach can be retained providing the parameters in this formalism are considered strictly empirical.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456351
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  • 4
    Electronic Resource
    Electronic Resource
    Theoretical study of memory kernel and velocity correlation function for condensed phase isomerization. I. Memory kernel (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6335-6339 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A unified memory kernel function is proposed on the basis of molecular dynamics simulations and comparison with experimental data. A frozen-solvent-like friction arises from the near-neighbor solvent cage effect, while the interaction between the system and the disordered heat bath or "broken cage'' gives rise to a white-noise friction. On short time scales, the apparent friction is a "series addition'' of these two limits, each solute molecule experiencing either an intact cage or a broken cage environment. For longer time scale behavior, the solvent is able to evolve, and some system molecules can experience both types of environments during the course of their own dynamics: the effective friction for these molecules then appears as a "parallel addition'' of the same two contributions. This leads to a memory kernel having a simple exponential behavior. As the time scale for the system dynamics becomes still longer, the cage becomes indistinguishable from the heat bath, and a purely Markoffian relaxation with delta-function memory kernel ensues. This unified memory kernel model has been found to be applicable to ultrafast chemical reaction rates and diffusion properties both in molecular dynamics simulations and laboratory experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456350
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  • 5
    Electronic Resource
    Electronic Resource
    Dynamics of endoergic substitution reactions. I. Br+chlorinated aromatic compounds (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6226-6237 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The endoergic substitution reactions Br+R–Cl→Cl+R–Br (R=o-, m-, and p-CH3C6H4, C6H5, C6F5; ΔH°(approximately-equal-to)15 kcal/mol) have been studied using the crossed molecular beams method in the collision energy (Ec ) range 20–35 kcal/mol. The CH3C6H4Br and C6F5Br products were found to be mostly forward scattered with respect to the incident Br beam indicating that the lifetimes of the Br–R–Cl collision complexes are short compared to their rotational periods. The product translational energy distributions and excitation functions for these reactions are well reproduced by statistical calculations that assume that only a few vibrational modes in the collision complexes participate in intramolecular energy redistribution prior to Cl elimination. Ring substituents are found to affect both the extent of energy redistribution in the complexes and the probability of Br addition. For example, no substitution product was observed with m-CH3C6H4Cl or C6H5Cl. The relative magnitudes of the cross sections are explained in terms of possible features of the potential energy surfaces along their reaction coordinates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455439
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  • 6
    Electronic Resource
    Electronic Resource
    Collisional relaxation of DF(v=1) and HF(v=1) by the DF dimer (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 1340-1347 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Overtone vibration-laser double resonance measurements determine the vibrational relaxation rates of DF(v=1) and HF(v=1) by the DF dimer. Both monomers are efficiently relaxed by the dimer at a rate that is 20% of the gas kinetic rate. The similarity of the rate constants for the two systems, which have very different energy defects, indicates that the relaxation occurs by collision complex formation and energy redistribution, rather than direct vibration-to-vibration energy transfer from the monomer to the dimer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452222
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  • 7
    Electronic Resource
    Electronic Resource
    State-to-state vibrational energy transfer in DF(v=1–3) (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3207-3214 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Overtone vibration–laser double resonance studies of DF(v=1–3) energy transfer yield self-relaxation rate constants for v=1, 2 and 3 of k1=(0.37±0.06)×10−12 cm3 mol−1 s−1, k2=(22.0±2.0)×10−12 cm3 mol−1 s−1, and k3=(17.0±1.8)×10−12 cm3 mol−1 s−1, respectively. The approach also directly measures the relative importance of vibration-to-vibration (V–V) and vibration-to-translation-and-rotation (V–T,R) energy transfer. The fraction of DF(v) molecules relaxing by V–V energy transfer is 1.1±0.1 and 0.72±0.10 for v=2 and v=3, respectively. Essentially all of the vibrational energy transfer in v=2 occurs via the V–V mechanism. The slower relaxation of DF(v=3) compared to DF(v=2), in contrast to simple scaling law predictions, reflects the decreasing influence of the V–V mechanism, even though it is still the primary relaxation pathway for DF(v=3). Comparison with HF self-relaxation qualitatively indicates that V–R energy transfer is important in V–T,R relaxation of DF(v=1).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458853
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  • 8
    Electronic Resource
    Electronic Resource
    Direct detection of very slow two-jump processes by saturation recovery electron paramagnetic resonance spectroscopy (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 2478-2488 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theory of saturation recovery EPR has been developed to explore the very slow motional regime using the two-jump model. The EPR signal is predicted to be a biexponential curve for this motional model. From the two exponential decay constants λ1 and λ2 the exchange rate constant k can be calculated by the simple formula: k= 1/2 (λ2−λ1). Experimental results obtained from slowly tumbling 15N-TEMPOL in sec-butylbenzene have been fitted to the two-jump model. Even though this is a crude attempt, good agreement has been observed between the two-jump rate constant (k=0.043 μs−1) and the isotropic Brownian diffusion constant calculated from the hydrodynamic Debye expression (D=0.051 μs−1).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453087
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  • 9
    Electronic Resource
    Electronic Resource
    A photoemission study of the surface carbonate species on Ag{110} (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 8592-8599 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electronic structure of the surface carbonate species adsorbed on Ag{110} has been studied with angle-resolved photoemission using synchrotron radiation. Despite the low symmetry of the surface complex (Cs) and more than one azimuthal orientation of the vertical symmetry plane, it has been possible to assign the adsorbate-induced levels. Recent semiempirical quantum chemical calculations have proved to be of considerable assistance in this process. The resonant behavior observed for the 3a‘ level is discussed in the context of photoelectron data for the free BF3 molecule, which is isoelectronic with CO2−3. The Cs symmetry pertaining in the present experiment is compared with the effective D3h symmetry observed in x-ray-absorption spectroscopy of the same system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460041
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  • 10
    Electronic Resource
    Electronic Resource
    Individual cross sections for 1D2 sublevels (ML=0, ±1, ±2) in the alignment-dependent process: Ca(4p2 1D2)+Rg→Ca(3d4p 1F3)+Rg as a function of rare gas (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 5260-5269 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Ca(4p2 1D2) state is prepared in a two-step excitation with linearly polarized lasers. Two different angular wave functions are selected, Y2,0 or (Y2,−1−Y2,1)/, by using parallel or perpendicular laser polarizations, respectively. Subsequent collision with a rare gas atom (He, Ne, Ar, Kr, or Xe) populates the near-resonant Ca(3d4p 1F3) state. The dependence of the collisional energy transfer process is measured as a function of the alignment of the initial 1D2 state wave function with respect to the average relative velocity vector. The laser-selected Y2,0 and (Y2,−1−Y2,1)/ angular wave functions display dramatically different alignment dependences, which are understood by an analysis of the rotation properties of these wave functions. The relative contributions to the cross section of the individual 1D2 sublevels, ML=0, ±1, and ±2, are extracted, and these vary considerably depending on the rare gas. For He, the ML=±2 sublevel (asymptotic Δ molecular state) contributes the most to the total cross section, while for all the other rare gases, the ML=0, ±1 sublevels (asymptotic Σ and Π molecular states, respectively) are more important. The contribution of the ML=0 sublevel increases smoothly with increasing mass of the rare gas collision partner, becoming the largest contributor for Xe.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458532
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