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  • 1
    Electronic Resource
    Electronic Resource
    Multiple lines of conical intersections and nondegenerate ground state in T⊗t2 Jahn–Teller systems (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 8470-8482 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have investigated the T⊗t2 Jahn–Teller problem with linear and quadratic vibronic coupling including a fourth-order term. First, numerical calculations of the lowest vibronic states were carried out by direct diagonalization of the Hamiltonian. The results show that the energy level of the ground vibronic state, which is triply degenerate T for small quadratic coupling g values, intersects the next A level with increasing g, thus realizing the nondegenerate ground state at sufficiently large g values. This result reverses the long-standing belief that the ground vibronic state for the T⊗t2 system has the same degeneracy and symmetry T as the initial electronic state. To explain these results in terms of Berry phase requirements and conical intersections, the adiabatic potential energy surface of the system is analyzed, and the relationships among the type and number of minima, conical intersections, and relevant tunneling paths are revealed. Depending on the vibronic coupling parameter values, there are four trigonal minima and six orthorhombic saddle points, which become minima at large g values, plus ten lines of conical intersections on the lowest potential energy surface. The barriers between the minima are significantly increased near the lines of conical intersections where the Born–Huang terms diverge. For small enough quadratic coupling, only four lines of conical intersections that originate from the highest symmetry point and proceed along the four trigonal directions are significant in determining the Berry phase, and the triply degenerate ground T state is obtained. By increasing the quadratic coupling parameter, the remaining six lines of conical intersections approach the point of highest symmetry, thus allowing for alternative tunneling paths and Berry phases which lead to the nondegenerate A ground state. This explanation of the origin of the nondegenerate ground state for some range of values of the vibronic coupling parameters is strengthened by model calculations of tunneling splitting. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481449
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  • 2
    Electronic Resource
    Electronic Resource
    Self-Consistent Changes of Geometrical Parameters: Experiment and Theory (1994)
    s.l. : American Chemical Society
    Accounts of chemical research 27 (1994), S. 242-249 
    Details
    ISSN: 1520-4898
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ar00044a005
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  • 3
    Electronic Resource
    Electronic Resource
    Vibrational energy levels of methyl fluoride (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 5088-5093 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Energy levels up to 6000 cm−1 above the zero point energy have been calculated for methyl fluoride using an ab initio potential function adjusted to reproduce the fundamental frequencies. The calculations were made using a novel variational procedure analogous to CI methods used in electronic structure theory. The calculated energy levels agree with the available experimental spectra with an rms error of 6.2 cm−1. In particular, the calculations reproduce the complicated resonances involving the CH stretching fundamentals. Some energy levels for CD2HF are also presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452652
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  • 4
    Electronic Resource
    Electronic Resource
    Vibrational energy levels of hydrogen cyanide (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 5838-5846 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Sixty-nine vibrational energy levels have been calculated for HCN, using an ab initio potential energy function adjusted to the fundamentals, and also an empirical potential function by Murrell et al. The energy levels were calculated variationally, using a novel direct CI procedure analogous to methods widely used in electronic structure theory. The present paper includes states with up to four quanta of excitation distributed among the four modes. Agreement with experimental spectra is good.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451545
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  • 5
    Electronic Resource
    Electronic Resource
    Local superdirect self-consistent field (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 7825-7832 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A "local superdirect self-consistent field (SCF)'' method has been developed which reduces the computational cost of a typical Hartree–Fock procedure from N4 to N2 more quickly than conventional SCF for a given molecular size. The program is based on the authors' modifications of density matrix formalism, the local space approximation, and a direct SCF method. Special features include more efficient integral upper-bound formulas as integral prescreening techniques and "superdirect'' prescreening of the projected Fock matrix elements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465661
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  • 6
    Electronic Resource
    Electronic Resource
    Ab initio and experimental quartic centrifugal distortion constants of acetone, pyrazole, and γ-pyrone (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 6698-6707 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The complete harmonic force field of acetone has been computed at the Hartree–Fock level with the 4–21 basis set of Gaussian orbitals. Seven scale factors optimized for related molecules were used to scale the computed force field. The mean deviation from the experimental vibrational frequencies is only 9.7 cm−1, except for the two methyl torsions. In order to obtain the best possible force field, a new set of seven scale factors was derived by fitting the computed spectrum of acetone to the observed spectrum. The only major change was for the C–C–C bending scaling factor. The final mean deviation was only 5.2 cm−1. The ground state rotational spectra of pyrazole and γ-pyrone have been reinvestigated in the millimeter-wave range. Thanks to millimeter-wave spectroscopy, accurate rotational and centrifugal distortion constants have been determined for acetone, pyrazole, and γ-pyrone. The experimental quartic centrifugal distortion constants are compared with the values calculated from ab initio force fields. A good agreement is found, even for the two-top molecule acetone for which the differences between the experimental and calculated T constants are smaller than 10%.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460246
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  • 7
    Electronic Resource
    Electronic Resource
    Theoretical study of anharmonicity in the vibrational spectra of the cyclopropenyl cation and its deuterated analogs (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 4320-4329 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Completely theoretical calculations for the structure, anharmonic force field up to partial sixth order, and vibrational frequencies of C3H+3, C3H2D+, C3HD+2, and C3D+3 have been carried out. The procedure included ab initio calculation of the vibrational energy surface at the 6-311 G4* + local MP4(SDQ) level and anharmonic vibrational configuration-interaction (CI) calculation using a variational expansion in a large, well-selected harmonic oscillator basis. The geometric parameters of C3H+3 are optimized as 1.3632 A(ring) for the C–C bond length and 1.0795 A(ring) for the C–H bond length. The fundamental vibrational frequencies of C3H+3 are 3193.5, 1622.1, 1015.5, 3148.8, 1297.5, 916.2, 756.8, and 1002.7 cm−1; those of C3H2D+, (3190.6, 3144.8), 2405.3, 1580.3, 1274.7, 913.4, 1001.2, 3146.3, 1295.2, 995.4, 717.8, 916.7 and 663.6 cm−1; those of C3HD+2, are 3164.7, 2434.0 (1541.6, 1506.8), 1276.0, 672.1, 824.3, 2361.6, 1261.3, 966.7, 762.3, 960.3, and 603.9 cm−1; and those of C3D+3 are (2536.7, 2451.8), 1487.4, 837.5, 2358.1, 1256.9, 671.9, 562.6, and 823.5 cm−1. The frequencies enclosed in parentheses are for Fermi resonance pairs, the prediction of which is handled automatically by this procedure. The theoretical vibrational frequencies agree with the experimental data available from both Raman and infrared spectra with 0.5% relative mean deviation. This agreement is as good as the experimental data allow, since the experiments were carried out in condensed phases and in the presence of counterions. The first overtone vibrational frequencies of C3H+3 are also predicted. The general applicability and present limitations of the method are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456643
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  • 8
    Electronic Resource
    Electronic Resource
    Theoretical study of anharmonicity in the vibrational spectrum of the perfluorocyclopropenyl cation (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1066-1071 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Complete theoretical calculations for the structure, anharmonic force field up to partial sixth order, and anharmonic vibrational frequencies of C3F+3 have been carried out. The procedure included ab initio calculation of the vibrational potential-energy surface at the 6-311G plus double polarization plus local MP4(SDQ) level, followed by a complete, 12-mode variational solution of the vibrational Schrödinger equation using a configuration-interaction (CI) expansion from a large, well-selected harmonic oscillator basis. The geometric parameters of C3F+3 (D3h) were optimized as 1.3796 A(ring) for the CC bond lengths and 1.2615 A(ring) for the CF distances. Localized orbitals show strong π-bond character for the CF linkages, conjugated with the ring π orbital. The fundamental vibrational frequencies of C3F+3 are 1999.5, 742.8 (a'1); 767.3 (a2); 1596.9, 998.9, 271.3 (e'); 250.0 (a‘2); and 664.1 (e‘). These frequencies agree with condensed phase measurements of Raman and infrared spectra with a ±11.5 cm−1 average deviation, the largest difference appearing for the CF bending and wagging frequencies. The first vibrational overtone frequencies of C3F+3 are also predicted. This completely ab initio calculation of anharmonic frequencies also leads to detailed bond characterization and full information on all intermode interactions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457231
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  • 9
    Electronic Resource
    Electronic Resource
    Mass Spectrometry for Structure Determination. Simple Nitrogen Heterocycles (1964)
    s.l. : American Chemical Society
    The @journal of organic chemistry 29 (1964), S. 2065-2066 
    Details
    ISSN: 1520-6904
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/jo01030a538
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  • 10
    Electronic Resource
    Electronic Resource
    Combination of theoretical ab initio and experimental information to obtain reliable harmonic force constants. Scaled quantum mechanical (QM) force fields for glyoxal, acrolein, butadiene, formaldehyde, and ethylene (1983)
    s.l. : American Chemical Society
    Journal of the American Chemical Society 105 (1983), S. 7037-7047 
    Details
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ja00362a005
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