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  • 1
    Electronic Resource
    Electronic Resource
    An ab initio calculation of the stretching energies for the HF dimer (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 7432-7440 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: For the HF dimer we calculate the fundamental HF stretching vibration frequencies, and the fundamental and overtone frequencies of the intermolecular (HF–HF) stretching vibration, using an ab initio potential energy surface and the previously developed semirigid bender Hamiltonian. The ab initio surface used involves the addition of 459 nuclear geometry points to the 1061 reported in our earlier work. These extra points have been chosen to give more information on those parts of the surface that involve distortions of the HF bond lengths. We have fitted these 1520 points to an analytic expression, slightly modified from our previous work, that involves 39 adjustable parameters and one constrained parameter; the weighted standard deviation of the fit is 29.3 cm−1. To calculate the vibrational frequencies, and the tunneling splittings in these vibrationally excited states, we use the semirigid bender Hamiltonian to average over the trans-tunneling path. We also calculate from the ab initio surface the effect of the adiabatic corrections, for the HF stretching states, that arise from the separation of the tunneling mode. In the adiabatically corrected calculation we obtain ν1=3926 cm−1, and ν2=3875 cm−1, which are in good agreement with the experimental results (3930.9 and 3868.1 cm−1, respectively). We also predict ν4=146 cm−1, 2ν4=280 cm−1, and 3ν4=405 cm−1. The value obtained for ν4 enables us to explain the observed perturbation of the lower tunneling component of the K=4 ground state level as being due to interaction with the lower tunneling component of the ν4=1, K=3 level.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458229
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  • 2
    Electronic Resource
    Electronic Resource
    An ab initio semirigid bender calculation of the rotation and trans-tunneling spectra of (HF)2 and (DF)2 (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5154-5159 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a purely ab initio minimum energy path for the trans-tunneling motion in the HF dimer, the energy levels for the K-type rotation and trans-tunneling motion for (HF)2 and (DF)2 are calculated with a one-dimensional semirigid bender Hamiltonian and no adjustable parameters. The transition moments for rotation-tunneling transitions are calculated, using our ab initio value for the dipole moment of an isolated HF molecule, and we also calculate B¯ values. The energy levels we obtain are in close agreement with experiment; for example, the K=0 tunneling splitting in (HF)2 is calculated as 0.65 cm−1 compared to the experimental value of 0.658 69 cm−1. As well as showing that our ab initio minimum energy path is very good, the calculation demonstrates that the semirigid bender formalism is able to account quantitatively for the unusual K dependence of the rotational energies resulting from the quasilinear behavior, and that the trans-tunneling motion is separable from the other degrees of freedom. We use the results to predict the locations, and transition moments, of the ΔK=0 and ±1 subbands in the tunneling spectra of (HF)2 and (DF)2, many of which have not yet been observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457613
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  • 3
    Electronic Resource
    Electronic Resource
    The potential surface and stretching frequencies of X˜ 3B1 methylene (CH2) determined from experiment using the Morse oscillator-rigid bender internal dynamics Hamiltonian (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1327-1332 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Morse oscillator-rigid bender internal dynamics (MORBID) Hamiltonian [P. Jensen, J. Mol. Spectrosc. 128, 478 (1988)] has been used in a fitting to all extant rotation–vibration data for X˜ 3B1 methylene CH2. This fitting leads to an improved determination of the potential energy surface, and in particular to reliable predictions for the stretching frequencies. We predict ν1=2992 cm−1 and ν3=3213 cm−1 for 12CH2, and we hope that the new predictions will encourage the experimental search for these weak fundamentals. In the MORBID approach the rotation–vibration energies are obtained from the potential energy surface in a purely variational calculation, and consequently the present work is an improvement over previous determinations of the CH2 potential energy surface from experiment that used the nonrigid bender formalism [see P. R. Bunker et al., J. Chem. Phys. 85, 3724 (1986), and references therein]; this latter approach treats the stretching vibrations by second order perturbation theory. A fitting to the J=0 vibrational energy data for a˜ 1A1 methylene has also been made here using the MORBID Hamiltonian. Combining the results of these MORBID fittings to experimental data for the (X˜) and (a˜) states of CH2 we obtain the singlet–triplet splittings T0(a˜ 1A1)=3147 cm−1 (8.998 kcal/mol) and Te(a˜ 1A1)=3223 cm−1 (9.215 kcal/mol).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455184
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  • 4
    Electronic Resource
    Electronic Resource
    An ab initio calculation of the intramolecular stretching spectra for the HF dimer and its D-substituted isotopic species (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6266-6280 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have carried out an ab initio calculation of the intramolecular stretching spectra (wave numbers and transition moments) of (HF)2, (DF)2, and HFDF involving v1+v2≤3, where v1 and v2 are the local mode quantum numbers for the two intramolecular (HF or DF) stretches. The ab initio surface used as a basis for these calculations has already been published [M. Kofranek, H. Lischka, and A. Karpfen, Chem. Phys. 121, 137 (1988); P. R. Bunker, P. Jensen, A. Karpfen, M. Kofranek, and H. Lischka, J. Chem. Phys. 92, 7432 (1990)], but in the present work we have extended the 1520 nuclear geometry points previously available with 198 points in order to explore further the variation of the intramolecular stretching energies and the dipole moment along the minimum energy (trans tunneling) path. We compute the intramolecular stretching energies and transition moments by making an adiabatic separation of the intramolecular stretching motion and the other vibrational motions of the molecules, and we use the semirigid bender Hamiltonian to average over the trans-tunneling motion. For HFHF, we obtain the fundamental level corresponding to the "free-H'' stretch ν1 at 3925 cm−1 and that corresponding to the "bound-H'' stretch ν2 at 3874 cm−1, in very good agreement with the experimental results of 3930.9 and 3868.1 cm−1, respectively [A. S. Pine, W. J. Lafferty, and B. J. Howard, J. Chem. Phys. 81, 2939 (1984)]. For the higher excited states, we obtain the 2ν1 energy level at 7674 cm−1 (7700±20 cm−1), 2ν2 at 7570 cm−1 (7555±15 cm−1), 3ν1 at 11 259 cm−1 (11 260 cm−1), and 3ν2 at 11 085 cm−1 (11 060 cm−1), where the experimental values [K. von Puttkamer and M. Quack, Chem. Phys. 139, 31 (1989)] are given in parentheses.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458996
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  • 5
    Electronic Resource
    Electronic Resource
    An ab initio calculation of ν1 and ν3 for triplet methylene (X˜ 3B1 CH2) and the determination of the vibrationless singlet–triplet splitting Te(a˜ 1A1) (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 2166-2169 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio values of ν1, ν3 and the zero point energy for triplet methylene (X˜ 3B1 CH2) have been calculated. Complete second order configuration interaction calculations for the six valence electrons, with a Davidson-type unlinked cluster correction, were done at 37 selected nuclear geometries. We have fitted an analytic potential through these points and used the nonrigid bender Hamiltonian, and a variational procedure, to calculate the rotation–vibration energies. We have obtained ν1=2985±20 cm−1, ν3=3205±20 cm−1, and the zero point energy as 3710±20 cm−1. The uncertainties represent our best estimate of the full uncertainties in the calculations. From the experimental data for singlet methylene (a˜ 1A1) we have determined the zero point energy of that state to be 3620±20 cm−1. Combining these two zero point energies with the experimental value of T0(a˜ 1A1)=3156±5 cm−1 [Bunker et al. J. Chem. Phys. 85, 3724 (1986)] we obtain the vibrationless singlet–triplet splitting Te(a˜ 1A1) =3246±30 cm−1 (9.28±0.1 kcal/mol).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453141
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  • 6
    Electronic Resource
    Electronic Resource
    The potential surface of X˜ 3B1 methylene (CH2) and the singlet–triplet splitting (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 3724-3731 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The data in the two immediately preceding papers, when combined with the extant microwave, infrared, and photodetachment spectroscopic data, provide 152 rotation and rotation-bending energy level separations in X˜ 3B1 methylene (involving 12CH2, 13CH2, and CD2). In the present paper we fit all this data using the two nonrigid bender Hamiltonians NRB1 and NRB2. The more refined model (NRB2) leads to the following results for triplet methylene: re=1.0766±0.0014 A(ring), αe=134.037°±0.045°, and the barrier height to linearity=1931±30 cm−1 (the uncertainties are three times the standard errors). Rotation-bending energy levels for CH2, CD2, and CHD are calculated for v2≤4 and N≤6. The determination of the rotation-bending energy levels in CH2 leads to an improved determination of the singlet–triplet splitting T0(a˜1A1) in methylene as 3156±5 cm−1 (9.023±0.014 kcal/mol, 0.3913±0.0006 eV). Although the rotation-bending energy levels are accurately predicted it is not possible to predict the stretching frequencies of CH2 very accurately, since the data to hand are not very sensitive to the stretching force field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450944
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  • 7
    Electronic Resource
    Electronic Resource
    An analytical six-dimensional potential energy surface for (HF)2 from ab initio calculations (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 3002-3007 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have developed an analytical expression for the 6D potential energy surface of (HF)2 and fitted it to 1061 ab initio points covering an energy range of up to 25 000 cm−1 above equilibrium. The ab initio calculation used the coupled pair functional approach with an extended polarized basis set. We have adjusted 42 parameters (and constrained 7 others) in the fitting of the analytical function to the points; the standard deviation of the weighted fitting is 26.8 cm−1 . The minimum energy path from the linear saddle point (345 cm−1 above equilibrium), through the equilibrium point, to the C2h saddle point (332 cm−1 above equilibrium) has been determined, and several cuts through the potential are presented. We plan to use this potential surface in reaction path and close coupling calculations of rotation–vibration term values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455007
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  • 8
    Electronic Resource
    Electronic Resource
    Analysis of the laser photoelectron spectrum of CH−2 (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 4866-4876 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have simulated the photoelectron spectrum of CH−2 using the model described previously [Sears and Bunker, J. Chem. Phys. 79, 5265 (1983)]. The optimization of the fit of the simulated spectrum to the recently observed spectrum of Lineberger and co-workers [J. Chem. Phys. 81, 1048 (1984) and preceding paper] has enabled us to determine the rotation-bending energy levels of triplet CH2 over an energy range of more than 1 eV. It has also enabled us to determine that the rotational temperature of the CH−2 in the experiment is 220 K and that, for v2=1, the vibrational temperature is 680 K. For CH−2 we determine that ae=103° and that ν2=1230 cm−1. The singlet–triplet splitting in methylene is determined to be 3150±30 cm−1 (0.3905±0.004 eV, 9.01±0.09 kcal/mol) from the photoelectron spectrum, in excellent agreement with the more accurate value previously obtained from LMR spectroscopy [McKellar et al., J. Chem. Phys. 79, 5251 (1983)] of 3165±20 cm−1 (0.3924±0.0025 eV, 9.05±0.06 kcal/mol), and the electron affinity of triplet CH2 is determined to be 0.652±0.006 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449747
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  • 9
    Electronic Resource
    Electronic Resource
    Potential barriers, tunneling splittings, and the predicted J=1←0 spectrum of CH+5 (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 3649-3653 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It has been established from earlier ab initio calculations that in its equilibrium configuration the CH+5 molecular ion consists of an H2 moiety bound to the apex of a pyramidal CH+3 group; the H2 group is approximately perpendicular to the C3 axis of the CH+3 group, and the binding energy is about 15000 cm−1. Two internal motions, the torsion and the flip, provide connections with low barriers between all 120 symmetrically equivalent minima on the potential energy surface so that all proton permutations are feasible. We present the results of new high level ab initio calculations of the parts of the potential energy surface associated with these two motions, and in particular we determine the continually optimized structure, and associated electronic energy, for the CH+5 molecular ion as it undergoes the flip motion. For the flip motion we numerically integrate the one-dimensional Schrödinger equation for the tunneling to determine the splitting. Since this splitting is small (1.4 cm−1) we can incorporate it into a 120×120 matrix treatment of the simultaneous torsion-flip dynamics to determine the energy level splitting pattern in the J=0 and 1 states, in the approximation of neglecting other tunneling pathways, and we calculate the positions of the lines in the J=1←0 millimeter wave spectrum. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472210
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  • 10
    Electronic Resource
    Electronic Resource
    Diode laser absorption spectroscopy of D3O+: Determination of the equilibrium structure and potential function of the oxonium ion (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 2676-2685 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High resolution measurements of the two components of the ν2 (umbrella) fundamental mode of the fully deuterated oxonium ion (D3O+) are reported. The spectra were obtained by diode laser absorption in a plasma containing D2 and D2O. Analysis of the observed spectra yields precise estimates of the two band origins and molecular parameters describing the states involved. By combining these data with the available high resolution data for H3O+, using the nonrigid invertor Hamiltonian, we are able to determine the equilibrium structure and the vibrational potential function; also we predict the inversion spectrum of D3O+ and the 2v2–v2 hot band spectra of H3O+ and D3O+.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449270
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