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  • 1
    Electronic Resource
    Electronic Resource
    Strained delocalized carbenoid ring systems — A theoretical investigation (1996)
    Springer
    Theoretical chemistry accounts 93 (1996), S. 17-33 
    Details
    ISSN: 1432-2234
    Keywords: Ring compounds ; Carbenes ; Quantum chemical methods ; CASSCF ; CASPT2
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A number of ring compounds containing a divalent carbon center (carbenes) have been studied usingab initio quantum chemical methods. The studied systems include: imidazol-2-ylidene, 4-pyranylidene, 9-xanthylidene, cyclohexa-2,5-dienylidene and 4-oxocyclohexa-2,5-dienylidene. Extended ANO type basis sets were used. Wave functions and energies were obtained with a multiconfigurational approach (CASSCF), where dynamic correlation effects are treated by using second-order perturbation theory (CASPT2). The singlet-triplet splitting has been found to depend linearly on the energy separation between the two carbene orbitals. All systems, where this splitting is larger than about 10 eV have been found to have a singlet ground state, while those with a smaller gap have a triplet ground state. A number of excited states have been characterized. Computed excitation energies are in agreement with experiment in cases where such information is available.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113855
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  • 2
    Electronic Resource
    Electronic Resource
    A CASPT2 study of the valence and lowest Rydberg electronic states of benzene and phenol (1995)
    Springer
    Theoretical chemistry accounts 91 (1995), S. 91-108 
    Details
    ISSN: 1432-2234
    Keywords: C6H6 ; C6H6O ; Electronic excitation ; ab initio ; CASPT2 ; Oscillator strength ; Rydberg states
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The valence excited states and the 3s, 3p, and 3d (united atom) Rydberg states of benzene and phenol have been obtained by the CASPT2 method, which computes a second-order perturbation correction to complete active space self-consistent field (CASSCF) energies. All non-zero dipole oscillator strengths are also computed, at the CASSCF level. For benzene, 16 singlet and 16 triplet states with excitation energies up to ca. 7.86 eV (63 400 cm–1) are obtained. Of these, 12 singlet and three triplet energies are experimentally known well enough to allow meaningful comparison. The average error is around 0.1 eV. The highest of these singlet states (21 E2g) is the highest valence ππ* state predicted by elementary π-electron theory. Its energy is then considerably lower than has been suggested from laser flash experiments, but in perfect agreement with a reinterpretation of that experiment. For phenol, 27 singlet states are obtained, in the range 4.53–7.84 eV (63 300 cm−1). Only the lowest has a well-known experimental energy, which agrees with the computed result within 0.03 eV. The ionization energy is in error by 0.05 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113865
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  • 3
    Electronic Resource
    Electronic Resource
    A quantum chemical study of the hydrogen bonding in the CO2⋯HF and N2O⋯HF complexes (1989)
    Springer
    Theoretical chemistry accounts 76 (1989), S. 173-185 
    Details
    ISSN: 1432-2234
    Keywords: Hydrogen bonding ; CO2-HF complex ; N2O-HF complex
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Quantum chemical ab initio calculations have been performed for the complex CO2⋯HF and N2O⋯HF. The interaction energies were computed through fourth order MBPT and were corrected for basis set superposition errors. Extended polarized basis sets were used which are constructed to give accurate values for electric moments and polarizabilities. The complex NNO⋯HF was found to be bent, while OCO⋯HF is linear, in agreement with experiment. The MBPT calculations give evidence for a second linear isomeric structure FH⋯NNO, a possibility which has also been suggested by recent experimental data. The computed binding energies are: 2.5 kcal/mol for OCO⋯HF, 2.4 kcal/mol for NNO⋯HF, and 3.0 kcal/mol for FH⋯NNO. At the SCF level, the FH⋯NNO complex is less stable than NNO⋯HF, but correlation has a large effect on the geometry and energetics of the latter complex. The NNO⋯HF complex seems to be a system where the positive intramolecular correlation correction prevails over the negative intermolecular component.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00527471
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  • 4
    Electronic Resource
    Electronic Resource
    SCF-CI studies of the equilibrium structure and the proton transfer barrier H3O 2 − (1976)
    Springer
    Theoretical chemistry accounts 42 (1976), S. 77-82 
    Details
    ISSN: 1432-2234
    Keywords: Configuration interaction ; Hydrogen bonding ; Ion hydration
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Large-scale configuration interaction (CI) calculations have been performed in order to study the effect of the correlation energy on the equilibrium geometrical structure, the stability, and on the energy barrier of the proton transfer reaction in the hydrogen bonded system HO− · HOH. An extended Gaussian basis set including polarization functions on each nuclear centre has been employed to approximate the molecular Orbitals. All possible single and double replacements resulting from a single determinant Hartree-Fock reference state have been taken into account in the CI wavefunction. Compared to the SCF results the equilibrium oxygen/oxygen distance has been obtained from the CI calculations to be smaller by about 0.08 Å and the correlation energy has been found to stabilize the composed system by 3.6 kcal/mole. An almost symmetric equilibrium structure with the hydrogen bonding H-atom midway between the two oxygen centres has been obtained in the CI treatment, whereas SCF calculations yield an asymmetric geometrical configuration with a small energy barrier of 1.4 kcal/mole for the proton transfer process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00548292
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  • 5
    Electronic Resource
    Electronic Resource
    SCF-CI studies of correlation effects on hydrogen bonding and ion hydration (1975)
    Springer
    Theoretical chemistry accounts 36 (1975), S. 249-274 
    Details
    ISSN: 1432-2234
    Keywords: Configuration interaction ; Hydrogen bonding ; Ion hydration
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Previous single-determinant Hartree-Fock studies on the equilibrium structures and stabilities of H2 O, H3 O+ as well as of the monohydrated ionic systems Li+ · H2O, F− · H2O and the hydrogen bonded water dimer, H2 O · HOH, are extended by large scale configuration interaction calculations including all the possible single and double excitations arising from the canonical set of Hartree-Fock molecular orbitals. The correlation energy effects on the equilibrium geometrical parameters of the systems under consideration are found to be quite small. The contributions of the correlation energy to the total binding energies of the weakly interacting composed systems are obtained to be of the order of 1 kcal/mole, leading to a considerable increase of the hydrogen bond strength in F− · H2O and H2O · HOH and to a small decrease of the binding energy in Li+ · H2 O. The observed strengthening of the hydrogen bonding interaction due to correlation is shown to be partly compensated by the change in the vibrational zero-point energy of the composed systems compared to the non-interacting subsystems. Approximate force constants corresponding to the intersystem vibrations in Li+ · H2O, F− · H2 O, and H2O · HOH are deduced from the calculated potential curve data on the SCF and the CI level of accuracy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00549690
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  • 6
    Electronic Resource
    Electronic Resource
    A CASPT2 study of the valence and lowest Rydberg electronic states of benzene and phenol (1995)
    Springer
    Theoretica chimica acta 91 (1995), S. 91-108 
    Details
    ISSN: 0040-5744
    Keywords: Key words: C6H6 ; C6H6O ; Electronic excitation ; ab initio ; CASPT2 ; Oscillator strength ; Rydberg states
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  The valence excited states and the 3s, 3p, and 3d (united atom) Rydberg states of benzene and phenol have been obtained by the CASPT2 method, which computes a second-order perturbation correction to complete active space self-consistent field (CASSCF) energies. All non-zero dipole oscillator strengths are also computed, at the CASSCF level. For benzene, 16 singlet and 16 triplet states with excitation energies up to ca. 7.86 eV (63 400 cm-1) are obtained. Of these, 12 singlet and three triplet energies are experimentally known well enough to allow meaningful comparison. The average error is around 0.1 eV. The highest of these singlet states (21E2 g) is the highest valence ππ* state predicted by elementary π-electron theory. Its energy is then considerably lower than has been suggested from laser flash experiments, but in perfect agreement with a reinterpretation of that experiment. For phenol, 27 singlet states are obtained, in the range 4.53–7.84 eV (63 300 cm-1). Only the lowest has a well-known experimental energy, which agrees with the computed result within 0.03 eV. The ionization energy is in error by 0.05 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113865
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  • 7
    Electronic Resource
    Electronic Resource
    Strained delocalized carbenoid ring systems – A theoretical investigation (1996)
    Springer
    Theoretica chimica acta 93 (1996), S. 17-33 
    Details
    ISSN: 0040-5744
    Keywords: Key words: Ring compounds ; Carbenes ; Quantum chemical methods ; CASSCF ; CASPT2
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract.  A number of ring compounds containing a divalent carbon center (carbenes) have been studied using ab initio quantum chemical methods. The studied systems include: imidazol-2-ylidene, 4-pyranylidene, 9-xanthylidene, cyclohexa-2,5-dienylidene and 4-oxocyclohexa-2,5-dienylidene. Extended ANO type basis sets were used. Wave functions and energies were obtained with a multiconfigurational approach (CASSCF), where dynamic correlation effects are treated by using second-order perturbation theory (CASPT2). The singlet–triplet splitting has been found to depend linearly on the energy separation between the two carbene orbitals. All systems, where this splitting is larger than about 10 eV have been found to have a singlet ground state, while those with a smaller gap have a triplet ground state. A number of excited states have been characterized. Computed excitation energies are in agreement with experiment in cases where such information is available.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113855
    Location Call Number Expected Availability
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    Paper (German National Licenses)
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