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  • 1
    Electronic Resource
    Electronic Resource
    The low-lying electronic states of protonated C2, CCH+ (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8021-8028 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI) calculations have been performed on the energetically lowest lying electronic states of protonated C2 (CCH+), using extended Gaussian basis sets of the general contraction type.The 3Π state has been unequivocally identified as the ground state of CCH+, followed by 3Σ−, which is 0.34 eV higher in energy. The lowest linear singlet state is 1Π. However, this state is not stable towards Renner–Teller distortion. A bent structure of 1A' symmetry is 0.21 eV lower in energy than 1Π and represents the most stable singlet CCH+ cation, 0.91 eV higher than X 3Π. A 1Δ and two 1Σ+ states have also been studied. They are 1.13, 1.54, and 3.55 eV higher in energy than the 3Π ground state. The activation barriers of the degenerate hydrogen migration for ground state CCH+, which can occur on the 3A‘ or the 3A' surface, are computed to be 1.27 and 2.43 eV, respectively, with C2v transition structures. For the lowest singlet CCH+ cation, the 1A1 C2v structure is not a transition state, but represents a very shallow energy minimum, 0.26 eV higher than the 1A' minimum. The saddle point has Cs geometry and lies 0.30 and 0.04 eV above the 1A' and 1A1 minima. Our results are compared to earlier theoretical data, obtained at significantly lower levels of ab initio theory, and with the experimentally known translational energy spectrum of CCD+.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459331
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  • 2
    Electronic Resource
    Electronic Resource
    Theoretical investigations of small multiply charged cations. III. NeN2+ (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2464-2468 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: First-order configuration-interaction calculations based on 4–6 σ, 1–2 π complete active space self-consistent-field reference wave functions are reported for the potential-energy curves of the 21 lowest-lying electronic states of NeN2+ which dissociate into (2P)Ne++(3P)N+ or (2P)Ne++(1D)N+. Using the same complete active space self-consistent-field SCF reference function, second-order configuration-interaction wave functions have been calculated for the X2Π ground state of NeN2+. At this level the potential minimum lies 4.57 eV above the (3P)N++(2P)Ne+ dissociation limit and there is a barrier to dissociation of 0.94 eV. All other states of NeN2+ are either repulsive, or exhibit a flat potential curve at some closer interatomic distance. In addition, the X 3Σ− ground-state potential-energy curve of single charged NeN+ was computed at the complete active space SCF + second-order configuration-interaction level. The dissociation energy is predicted as De =0.47 eV at re =3.30 a0. Bonding in NeN2+ and NeN+ is discussed in terms of donor–acceptor interactions between Ne and N2+ or N+, respectively. [For paper II of this series see W. Koch and G. Frenking, J. Chem. Phys. 86, 5617 (1987).]
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457989
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  • 3
    Electronic Resource
    Electronic Resource
    Theoretical investigations of small multiply charged cations. II. CNen+ (1≤n≤4) (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 5617-5624 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Extended MCSCF calculations of the CASSCF type have been performed for the energetically lowest-lying electronic states of CNe+, CNe2+, CNe3+, and CNe4+. Whereas the mono- and tetracations exhibit essentially repulsive potential curves, the CNe2+ dication is predicted to be a kinetically stable ion. The computed barrier to dissociation for the X 1Σ+ state of CNe2+ amounts to 1.62 eV, and for the a 3Π state a potential well of 0.80 eV depth is calculated. The CNe2+ dication is therefore predicted to be experimentally observable in the gas phase. Also the CNe3+ trication is calculated to exhibit a small minimum in its X 2Σ+ state. Bonding in these unusual cations is discussed and compared to known isoelectronic species.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452538
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  • 4
    Electronic Resource
    Electronic Resource
    The silaformyl radical HSiO and its energetically lower-lying isomer SiOH (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 4585-4587 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio investigations with a double-zeta plus polarization basis set extended by configuration interation (single and double excitations) predict that the SiOH isomer is 11 kcal/mol lower in energy than HSiO. The barrier for hydrogen rearrangement is found to be substantial and both isomers should be observed experimentally. Two structures HSiO(A) and HSiO(B) were detected for HSiO with 2A' symmetry and nearly the same energy but different geometry at the DZ+P SCF level. However, HSiO(A) is clearly favored relative to HSiO(B) at higher levels of theory and might be the only true minimum for HSiO.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448715
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  • 5
    Electronic Resource
    Electronic Resource
    Relative energies of the C2H2S2 isomers 1,2-dithiete and dithioglyoxal: Peculiar basis set dependencies of density functional theory and ab initio methods (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 8430-8433 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio calculations at the levels of Hartree–Fock (HF), second order Møller–Plesset perturbation theory (MP2), coupled-cluster theory with singles, doubles, and estimated triple excitations [CCSD(T)], and density functional theory (DFT) using the functionals B3LYP and B3PW91 of the relative energies of the C2H2S2 isomers 1,2-dithiete (2a), and dithioglyoxal (2b) show a peculiar dependence of the results on the f-type polarization functions. The ab initio calculations with 6-31G(nd) basis sets with n=1–3 incorrectly predict that 2a is higher in energy than 2b. The relative energies at the MP2 and CCSD(T) levels change by more than 6 kcal/mol in favor of 2a if the basis set is augmented by one set of f functions. The DFT calculations also give a higher stability of 2a relative to 2b if f functions are included in the basis sets, but the change in the relative energy is only ∼2 kcal/mol. The large change in the relative energies which are calculated at MP2 and CCSD(T) are mainly due to the functions at sulfur, while the effect of the f functions in the DFT calculations is mainly due to the f functions at carbon. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1318749
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  • 6
    Electronic Resource
    Electronic Resource
    Theoretical investigations of small multiply charged cations. I. SiH2+ (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 2703-2706 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In excellent agreement with experiments, high levels ab initio MO calculations of the multireference CI (MRD-CI) and complete active space SCF (CASSCF) type show a deep potential well for the X 2Σ+ ground state of the diatomic SiH2+ dication with a barrier of 1.06 eV (CASSCF) and 1.07 eV (MRD-CI) for the charge separation reaction. This result, which is in distinct contrast to the findings for the valence isoelectronic CH2+ dication, is explained by the charge polarization character of the SIH2+ ground state. Also the first excited A 2Π state was found by MRD-CI to be weakly bound.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450344
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  • 7
    Electronic Resource
    Electronic Resource
    The low lying electronic states of O−3 (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 1271-1277 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The energies of the three lowest lying excited states of the ozonide anion (1 2B2, 1 2A1, 1 2A2) at the optimized geometry of the X 2B1 ground state are theoretically predicted at the MRCI-SD level of theory using large atomic natural basis sets. The calculated vertical excitation energy Tv=2.85 eV for the 1 2A2←X 2B1 transition, which has a large transition moment, is in good agreement with the experimental results for the isolated O3− anion in host matrices and solution between 2.69–2.81 eV. The state symmetry forbidden transition 1 2B2←X 2B1 is calculated with Tv=2.26 eV. The excitation energy for the 1 2A1←X 2B1 transition is theoretically predicted with Tv=2.28 eV and a very low transition moment. The theoretical results are also discussed in comparison with recently observed low lying absorption bands of O3−.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465371
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  • 8
    Electronic Resource
    Electronic Resource
    Use of canonical orbital energy derivatives for closed-shell self-consistent-field wave functions (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 8749-8760 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The characteristics of the first and second derivatives of the canonical orbital energies for the closed shell self-consistent-field (SCF) wave function have been studied. The first derivatives of the orbital energies were determined analytically and the second derivatives by the finite difference method. These derivatives were transformed from the Cartesian coordinate system to the normal coordinate system for a variety of stationary points. It is demonstrated that the first and second derivatives of the orbital energies in terms of the normal coordinates provide very useful chemical information concerning the molecular structures and unimolecular reactions for a range of chemical species. Similarities and differences between the present approach and Mulliken–Walsh method are also discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464483
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  • 9
    Electronic Resource
    Electronic Resource
    A Møller–Plesset study of the electron affinities of the diatomic hydrides XH (X=Li, B, Be, C, N, O) (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 3224-3229 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electron affinities for LiH, BeH, BH, CH, NH, and OH are computed at the MP2, MP3, and full MP4(SDTQ) order of perturbation theory employing the basis sets 6-31G(d,p), 6-311G(d,p), 6-311++G(d,p), and 6-311++G(3df,3pd), using geometries optimized at MP2/6-31G(d,p). At the MP4(SDTQ)/6-311++G(3df,3pd)//MP2/6-31G(d,p) level including zero-point corrections, electron affinities are predicted with a maximum deviation of 0.2 eV from experiment. At the same level of theory, dissociation energies of the anions are computed to an accuracy of 0.11 eV. The energy results reported here are in most cases in better agreement with experiments than previous theoretical results. Vibrational frequencies and atomic distances are determined at the MP2/6-31G(d,p) level indicating longer bonds and lower frequencies as a consequence of electron attachment to the neutral hydrides.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450832
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