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  • 1
    Electronic Resource
    Electronic Resource
    Theoretical assignment of the visible spectrum of singlet methylene (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 118-132 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The potential energy surfaces of the two lowest-lying singlet electronic states of methylene (CH2) are determined by internally contracted multireference configuration interaction calculations, using a full-valence reference space, with an extended Gaussian basis set. The rotation–vibration levels on these surfaces are calculated by diagonalizing the rovibrational Hamiltonian matrix in a contracted basis. The rovibronic mixing due to the strong Renner–Teller interaction in this system is treated through the Coriolis term in the kinetic energy operator, using geometry-dependent electronic angular momentum matrix elements calculated from ab initio wave functions. The agreement between experiment and this high-quality ab initio calculation is sufficiently good that the calculation can be used to assign the observed vibronic bands in this very complex spectrum, where 90% of the observed lines remain unassigned. Many of the previous vibronic band labels are found to be incorrect. Most of the K〉0 bands previously labeled b˜ 1B1 are actually predominantly a˜ 1A1 in character, and the vibrational numbering of their b˜ 1B1 components are also incorrect. This work demonstrates the importance of supplementing experimental data with good quality ab initio calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460385
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  • 2
    Electronic Resource
    Electronic Resource
    Erratum: Kinetic anharmonic coupling in the trihalomethanes: A mechanism for rapid intramolecular redistribution of CH stretch vibrational energy [J. Chem. Phys. 86, 6000 (1987)] (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3401-3401 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454767
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  • 3
    Electronic Resource
    Electronic Resource
    Bond-breaking without barriers. II. Vibrationally excited products (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1961-1969 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ketene is photolyzed in a supersonic jet, and the vibrationally excited singlet methylene CH2 (a˜ 1A1), produced is detected by laser-induced fluorescence. The appearance thresholds and yield curves of individual methylene rovibrational states are obtained by scanning the photolysis laser wavelength. As observed previously by probing the (0,0,0) state at lower photolysis energies, there are no barriers to dissociation and nuclear spin is conserved. Sharp steps are observed just above the energetic threshold in each of these photofragment excitation (PHOFEX) curves. This suggests that the rotational state distributions are given by phase space theory (PST). The quantum yield of the (0,1,0)101 rovibrational state is measured and the quantum yield for (0,1,0) inferred. These values are larger than predicted by PST, and are close to values predicted by variational Rice–Ramsberger–Kassel–Marcus (RRKM) theory and by the separate statistical ensembles (SSE) method. This indicates that near the (0,1,0) energy threshold the (0,0,0) yield is constrained, as by a tight transition state. The appearance of steps spaced by the energies of a free CO rotor in the PHOFEX curves close to the thresholds of each vibrational state probed indicates that the near threshold flux of vibrationally excited products is controlled by a loose "transition state'' on a vibrationally adiabatic surface. These observations are consistent with the variational RRKM theory for dissociations without barriers in which each product vibrational state evolves on its own vibrationally adiabatic potential surface and has its own transition state. As the energy increases above the threshold for a vibrational state, its transition state moves in along the reaction coordinate and tightens. Thus total rates increase less rapidly with energy than in PST and vibrational distributions are skewed towards higher levels than in PST.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459918
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  • 4
    Electronic Resource
    Electronic Resource
    The prediction of spectroscopic properties from quartic correlated force fields: HCCF, HFCO, SiH+3 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4965-4981 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Knowledge of a force field expanded through quartic displacements, together with a dipole field expanded through cubic displacements, yields all the harmonic and anharmonic molecular properties of interest to infrared spectroscopists. Such force fields may also explain much of the mechanism behind intramolecular vibrational energy redistribution. The ab initio quantum chemist can now calculate these fields, either at the self-consistent field level or with the inclusion of electron correlation effects. For accurate predictions, it is important to include electron correlations effects for at least the quadratic part of the force fields. Here we report studies using the second-order Møller–Plesset method for the full quartic fields. We examine the effects of using large basis sets. The quadratic force constants are calculated analytically; cubic and quartic constants are calculated using central differences of second derivatives in reduced normal coordinates. Three molecules are studied. HCCF, for which a large quantity of experimental data has been recently analyzed by Holland, Newnham, and Mills. The calculations are sufficiently accurate that errors in the experimental assignments became apparent. HFCO, where the theoretical anharmonic constants are helpful in understanding the highly excited vibrational states probed by Moore and co-workers. SiH+3, whose high resolution absorption spectra has just recently been detected by Davies and co-workers. The conclusions are that this straightforward way of calculating spectroscopic properties is an extremely valuable tool for the understanding of spectroscopy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458634
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  • 5
    Electronic Resource
    Electronic Resource
    Coupling of CH stretching and bending vibrations in trihalomethanes (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 5994-5999 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The vibrational spectra of HCCl3, HCF3, HCCl2F, and HCClF2 have been measured in the vapor from the CH stretching fundamental through to the fifth overtone (i.e., v1=6, where v1 is the number of quanta in the CH stretching mode), using FTIR and photoacoustic spectrometers. Instead of a single strong CH overtone progression, additional strong bands attributable to Fermi-resonant combination tones are prominent in the spectra. The CH stretch is found to be most strongly coupled to the CH bending mode and much less strongly coupled to the heavy atom motions. The resolved band structures are analyzed in the following paper to give quantitative stretch–bend coupling constants for these CH oscillators.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452486
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  • 6
    Electronic Resource
    Electronic Resource
    Kinetic anharmonic coupling in the trihalomethanes: A mechanism for rapid intramolecular redistribution of CH stretch vibrational energy (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 6000-6011 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The coupling of CH stretching and CH bending vibrations in trisubstituted methanes is analyzed. Improved spectroscopic constants, especially the cubic anharmonic stretch–bend coupling constants, are extracted from Fermi resonances in the overtone spectra of HCF3 and HCCl3. Both harmonic oscillator and Morse oscillator basis functions are used in the analysis and the results compared. That part of the coupling which arises from the kinetic energy as expressed in curvilinear coordinates is calculated and compared to the coupling calculated using the more conventional rectilinear treatment. Use of curvilinear coordinates is found to provide significant advantages. The formalism for curvilinear normal coordinates is clarified and generalized. From these calculations and the spectral analysis, one of the cubic anharmonic constants of the potential energy surface is extracted for comparison with ab initio calculations. The curvilinear model of the CH stretch–bend interaction tested for these isolated CH chromophores is expected to be useful in understanding CH bonds and vibrational energy flow in larger hydrocarbons.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452487
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  • 7
    Electronic Resource
    Electronic Resource
    Bond breaking without barriers: Photofragmentation of ketene at the singlet threshold (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 314-328 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ketene (CH2CO) in a supersonic free jet was photodissociated by a tunable pulsed laser in the frequency range just above the threshold for production of singlet methylene, CH2 (a˜ 1A1). CH2 was detected by laser-induced fluorescence (LIF). The appearance threshold and yield curve of individual 1CH2 rotational states were obtained by scanning the photolysis laser frequency with a fixed LIF probe laser frequency. The dissociation occurs on the ground electronic state potential energy surface. The threshold for CH2CO→1CH2+CO is found to be 30 116.2±0.4 cm−1. By varying the delay between the photolysis and probe pulses, a lower bound of 7×107 s−1 was set for the dissociation rate on the triplet surface at the singlet energy threshold. The yield curves, or photofragment excitation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotational term values. The experimental data provide a rigorous test of theoretical models of photofragment dynamics. The data clearly show that nuclear spin is conserved through the photodissociation. PHOFEX curves calculated from phase space theory (PST) are in excellent agreement with the experiment and show that there is no barrier along the reaction coordinate. The singlet/triplet branching ratio as a function of photolysis laser frequency is inferred from PST fits to the PHOFEX data. Comparisons with the statistical adiabatic channel model (SACM) are also presented. The data show that the only dynamical constraints on product state rotational energy distributions are conservation of energy, angular momentum, and nuclear spin.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455472
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  • 8
    Electronic Resource
    Electronic Resource
    Anharmonic corrections to vibrational transition intensities (1990)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 94 (1990), S. 5608-5616 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100377a038
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  • 9
    Electronic Resource
    Electronic Resource
    A perturbation theory guide to open-shell complexes: OH–Ar(X 2Π) (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 2573-2584 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Perturbation theory is used to understand the experimentally observed stimulated emission spectra of OH–Ar(X 2Π). A useful zero-order Hamiltonian for an open-shell van der Waals complex is presented, and the most important perturbation terms are identified: rotational decoupling ( j⋅s), Renner–Teller coupling (Vˆ2), and a Coriolis interaction (J⋅j). This treatment reveals those parts of the Hamiltonian which are responsible for various unusual features in the spectra of open-shell complexes, such as the large parity splittings in certain vibrational bands and spin–orbit–induced predissociation of the OH–Ar(X 2Π) complex. The magnitude of the parity splitting is shown to be directly proportional to the change in the intermolecular potential when the odd electron in the free radical lies in or out of the O–H–Ar plane, the A' and A‘ surfaces. The measured splitting is used to infer the magnitude of the difference between the A' and A‘ potential-energy surfaces (∼12 cm−1) in the region sampled by the first excited bend.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462008
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  • 10
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    POETRY (1986)
    Madison, N.J., etc. : Periodicals Archive Online (PAO)
    The Literary Review. 29:3 (1986:Spring) 291 
    Details
    ISSN: 0024-4589
    Topics: Linguistics and Literary Studies
    URL: http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pao:&rft_dat=xri:pao:article:5170-1986-029-03-000020
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