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  • American Institute of Physics (AIP)  (6)
  • 1
    Electronic Resource
    Electronic Resource
    Intermolecular potential for thermal H2O–He collisions (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1401-1407 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical potentials for rotational excitation of H2O by He were constructed via several methods, all of which start with a large basis set SCF interaction. The semiempirical Hartree–Fock with damped dispersion (HFD) model adds a damped long-range attraction with parameters adjusted to fit experimental total differential cross sections. Purely ab initio potentials add correlation energies obtained via perturbtion theory (MP2 and MP4) or a variational method (ICF1). Scattering calculations were performed on all surfaces to compare with available beam scattering and pressure broadening data and to assess sensitivity of state-to-state rates to uncertainties in the potential. From comparison with the limited experimental data, the ICF1 surface appears to be marginally better than the MP4 surface. Thermal rates calculated from this surface should be accurate to better than 50%, at least for the larger, more important rates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455139
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  • 2
    Electronic Resource
    Electronic Resource
    Calculations of H2O microwave line broadening in collisions with He atoms: Sensitivity to potential energy surfaces (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1346-1359 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Broadening parameters for three microwave lines of water at 22.2, 183.3, and 380.2 GHz, in a bath of helium atoms, are calculated using accurate molecular scattering S matrices obtained from two theoretical potentials presented by Palma et al., J. Chem. Phys. 89, 1401 (1988). For the 22 GHz line results are in substantial agreement with values presented in that work, indicating the accuracy of approximate methods used there. The present work improves the potential energy surfaces, computed from perturbation theory (MP4) and variational interacting correlated fragments (ICF1) wave funcitons, by correcting them for basis set superposition error (BSSE), and recomputes the line broadening using a different procedure for fitting computed energy points. In addition, the entire set of calculations are repeated with a quite different basis set for orbital expansion to establish the reliability of the potential energy surface. We show that adjustments for superposition error are essential, and that broadening cross sections computed from the new surfaces are changed 10%–30% from the old, significantly improving agreement with experiment. The MP4 BSSE adjusted surface appears to be the most accurate, giving room temperature broadenings of 8.9, 11.8, and 10.0 A(ring)2 compared with experimental determinations of 12.2±1.2 , 11.9, and 11.2 A(ring)2 for the 22, 183, and 380 GHz lines, respectively. Thus, computed line to line variation is larger than observed. The ICF1 BSSE adjusted results for pressure broadening cross section parallel those from the MP4 BSSE calculations but are about 10% smaller. We believe our computed results are stable with respect to basis set for orbital expansion and that the scattering calculations are accurate. Any theoretical inadequacy has been pinpointed to too few points on the potential energy surface resulting in an inadequate description of the angle dependence. It is not clear whether the present discrepancy between computation and experiment stems from this or from errors in the experimental values, although we show some indication that additional information on the surface might decrease the computed broadenings, worsening agreement with experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459992
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  • 3
    Electronic Resource
    Electronic Resource
    Calculations on the rate of the ion–molecule reaction between NH+3 and H2 (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7842-7849 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rate coefficient for the ion–molecule reaction NH+3 +H2→NH+4+H has been calculated as a function of temperature with the use of the statistical phase space approach. The potential surface and reaction complex and transition state parameters used in the calculation have been taken from ab initio quantum chemical calculations. The calculated rate coefficient has been found to mimic the unusual temperature dependence measured in the laboratory, in which the rate coefficient decreases with decreasing temperature until 50–100 K and then increases at still lower temperatures. Quantitative agreement between experimental and theoretical rate coefficients is satisfactory given the uncertainties in the ab initio results and in the dynamics calculations. The rate coefficient for the unusual three-body process NH+3+H2+He→NH+4+H+He has also been calculated as a function of temperature and the result found to agree well with a previous laboratory determination.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460119
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  • 4
    Electronic Resource
    Electronic Resource
    Ab initio determination of mode coupling in HSSH: The torsional splitting in the first excited S–S stretching state (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5905-5909 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A mechanism for the enhanced splitting detected in the millimeter-wave rotational spectra of the first excited S–S stretching state of HSSH (disulfane) has been studied. The mechanism, which involves a potential coupling between the first excited S–S stretching state and excited torsional states, has been investigated in part by the use of ab initio theory. Based on an ab initio potential surface, coupling matrix elements have been calculated, and the amount of splitting has then been estimated by second-order perturbation theory. The result, while not in quantitative agreement with the measured splitting, lends plausibility to the assumed mechanism.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457459
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  • 5
    Electronic Resource
    Electronic Resource
    Is N-protonated hydrogen isocyanide, H2NC+, an observable interstellar species? (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 5194-5199 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio molecular orbital theory is used to examine the singlet and triplet potential energy surfaces for the CH2N+ system. The results confirm those of earlier studies which suggested that the singlet H2NC+ isomer could be formed via the corresponding triplet isomer. Also, it is shown that the reaction HCN++H2 might lead to this metastable isomer without invoking the triplet species. The best test of the hypothesis that this molecule can be formed by gas phase, ion molecule reactions and may be an important precursor in the interstellar synthesis of HCN and HNC is to search for it in space. To this end, theoretical predictions are made of its rotational frequencies and its vibrational frequencies and intensities to serve as a guide to laboratory spectroscopists and radioastronomers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451683
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  • 6
    Electronic Resource
    Electronic Resource
    Molecular orbital predictions of the vibrational frequencies of some molecular ions (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 333-341 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recent spectroscopic advances have led to the first determinations of infrared vibration-rotation bands of polyatomic molecular ions. These initial detections were guided by ab initio predictions of the vibrational frequencies. The calculations reported here predict the vibrational frequencies of additional ions which are candidates for laboratory analysis. Vibrational frequencies of neutral molecules computed at three levels of theory, HF/3-21G, HF/6-31G*, and MP2/6-31G*, were compared with experiment and the effect of scaling was investigated to determine how accurately vibrational frequencies could be predicted. For 92% of the frequencies examined, uniformly scaled HF/6-31G* vibrational frequencies were within 100 cm−1 of experiment with a mean absolute error of 49 cm−1. This relatively simple theory thus seems suitable for predicting vibrational frequencies to guide laboratory spectroscopic searches for ions in the infrared. Hence, the frequencies of 30 molecular ions, many with astrochemical significance, were computed. They are CH+2, CH+3, CH+5, NH+2, NH+4, H3O+, H2F+, SiH+2, PH+4, H3S+, H2Cl+, C2H+, classical C2H+3, nonclassical C2H+3, nonclassical C2H+5, HCNH+, H2CNH+2, H3CNH+3, HCO+, HOC+, H2CO+, H2COH+, H3COH+2, H3CFH+, HN+2, HO+2, C3H+, HOCO+, HCS+, and HSiO+.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448805
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