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  • 1
    Electronic Resource
    Electronic Resource
    Theoretical study of the electronic spectrum of magnesium-porphyrin (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 7202-7209 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Multiconfigurational self-consistent field (SCF) and second order perturbation methods have been used to study the electronic spectrum of magnesium-porphyrin (MgP). An extended ANO-type basis set including polarization functions on all heavy atoms has been used. Four allowed singlet states of E1u symmetry have been computed and in addition a number of forbidden transitions and a few triplet states. The results lead to a consistent interpretation of the electronic spectrum, where the Q band contains one transition, the B band two, and the N band one. The computed transition energies are consistently between 0.1 and 0.5 too low compared to the measured band maxima. The source of the discrepancy is the approximate treatment of dynamic correlation (second order perturbation theory), limitations in the basis set and the fact that all measurements have been made on substituted magnesium porphyrins. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478624
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  • 2
    Electronic Resource
    Electronic Resource
    A theoretical study of the 1B2u and 1B1u vibronic bands in benzene (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 2798-2809 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The two lowest bands, 1B2u and 1B1u, of the electronic spectrum of the benzene molecule have been studied theoretically using a new method to compute vibronic excitation energies and intensities. The complete active space (CAS) self-contained field (SCF) method (with six active π-orbitals) was used to compute harmonic force field for the ground state and the 1B2u and 1B1u electronic states. A linear approximation has been used for the transition dipole as a function of the nuclear displacement coordinates. Derivatives of the transition dipole were computed using a variant of the CASSCF state interaction method. Multiconfigurational second-order perturbation theory (CASPT2) was used to obtain absolute excitation energies (12 active π-orbitals). The results show that the approach works well. Vibrational progressions are well described in both bands and intensities, and energies are in agreement with experiment, in particular when CASPT2 derived geometries are used. One interesting result is that computed vertical energies fall about 0.1 eV on the high energy side of the band maximum. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480854
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  • 3
    Electronic Resource
    Electronic Resource
    The ozonide anion: A theoretical study (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 266-273 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Complete active space self-consistent field (CASSCF) and CASSCF second-order perturbation theory (CASPT2) methods have been used for the geometry optimization and calculation of harmonic and fundamental frequencies of the ozonide ion (O−3) and the ozonide lithium complex (Li+O−3). For O−3 harmonic frequencies have also been obtained using the coupled-cluster method, CCSD(T). Infrared intensities are computed from dipole moment derivatives at the CASSCF level. The predicted equilibrium geometry for O−3 is ROO=1.361 A(ring) and (angle)OOO=115.4°, and the fundamental frequencies are ν1=989 cm−1, ν2=556 cm−1, ν3=870 cm−1 [experimental values are ROO=1.36±0.02 A(ring), (angle)OOO=111.8±2.0°, ν1=975(50) cm−1, ν2=550(50) cm−1, ν3=880(50) cm−1]. Corresponding data for the lithium ozonide complex have also been obtained. The presented data contradict the previous interpretation of the IR and Raman spectrum obtained after codeposition of ozone and alkali atoms in N2, argon, or neon matrices. The presence of the lithium cation raises the asymmetric stretch frequency to about 940 cm−1, which is contradictory to assumptions made in the assignments of the matrix spectra. Calculations made in a dielectric medium strongly suggest that the effect of the matrix on the IR spectrum is small for O−3 itself. The dissociation and atomization energies of O−3 are found to be in agreement with experiment. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469639
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  • 4
    Electronic Resource
    Electronic Resource
    A theoretical study of the chemical bonding in M(CO)x (M=Cr, Fe, and Ni) (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6810-6821 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The equilibrium structure and bond energies of the transition metal complexes Ni(CO)x (x=1–4), Fe(CO)5, and Cr(CO)6 have been studied using the complete active space (CAS)SCF method and second-order perturbation theory (CASPT2). It is shown that the major features of the electronic structure are properly described by a CASSCF wave function based on an active space comprising the bonding and antibonding orbitals directly involved in the metal–ligand bond. Remaining correlation effects are dealt with in the second, CASPT2, step. The computed energies have been corrected for basis set superposition errors (BSSE) and relativistic corrections have been added. Resulting bond distances and bond energies are in agreement with experimental data, when available: Cr(CO)6, r(Cr–C)=1.91(1.91) A(ring), D0=148(153) kcal/mol; Fe(CO)5, rax(Fe–C) =1.79(1.81) A(ring), req(Fe–C)=1.80(1.83) A(ring), D0=133(137) kcal/mol; Ni(CO)4, r(Ni–C)=1.83(1.83) A(ring), D0=139(138) kcal/mol (experimental values within parentheses). Some excited states were computed for Fe(CO)5. The first charge transfer (CT) state was located at 4.8 eV in agreement with an intense band found at 5.0 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468309
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  • 5
    Electronic Resource
    Electronic Resource
    A theoretical study of the electronic spectrum of bithiophene (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 3580-3586 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electronic spectrum of bithiophene in the energy range up to 6.0 eV has been studied using multiconfigurational second order perturbation theory (CASPT2) and a basis set of ANO type, with split valence quality and including polarization functions on all heavy atoms. Calculations were performed at a planar (trans) and twisted geometry. The calculated ordering of the excited singlet states is 1Bu, 1Bu, 1Ag, 1Ag, and 1Bu with 0–0 transition energies: 3.88, 4.15, 4.40, 4.71, and 5.53 eV, respectively. The first Rydberg transition (3s) has been found at 5.27 eV. The results have been used in aiding the interpretation of the experimental spectra, and in cases where a direct comparison is possible there is agreement between theory and experiment. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468587
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  • 6
    Electronic Resource
    Electronic Resource
    A Theoretical Study of the (Cyclobutane)diazadivanadium Complex (1995)
    s.l. : American Chemical Society
    Organometallics 14 (1995), S. 63-69 
    Details
    ISSN: 1520-6041
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/om00001a014
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  • 7
    Electronic Resource
    Electronic Resource
    A CASSCF-CCI study of the valence and lower excited states of the benzene molecule (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 1458-1466 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio complete active space (CAS) SCF and contracted CI calculations have been carried out for all valence and the lower Rydberg states of the benzene molecule. The CASSCF active space comprised 12 π-type molecular orbitals and the basis set included both polarization functions and diffuse functions in order to describe properly both valence and Rydberg type orbitals. Resulting excitation energies for the Rydberg states are in close agreement with experiment. CASSCF results for the valence states give errors ranging from 0.0 for the covalent states up to more than 1.0 eV for the most ionic states. Inclusion of σ–π correlation effects reduces the errors in the ionic states to less than 0.6 eV. The 1E1u state is computed to lie 7.4 eV above the ground state with a transition moment of 1.70 a.u., experimental values are 7.0 eV and 1.61 a.u., respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452235
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  • 8
    Electronic Resource
    Electronic Resource
    The nature of the bonding in XCO for X=Fe, Ni, and Cu (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 354-364 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The bonding in the 5,3Σ− and 3Δ states of FeCO, the 3,1Σ+, 3Δ, and 3 Π states of NiCO, and the 2Σ+ state of CuCO are analyzed using the constrained space orbital variation (CSOV) technique for both (CASSCF) and SCF wave functions. The bonding is discussed in terms of σ repulsion between the metal 4s and the CO 5σ, CO to metal σ donation when there is an empty or partly occupied dσ orbital and metal to CO 2π * backdonation. The bonding is compared for the different metals and between the different states.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451610
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  • 9
    Electronic Resource
    Electronic Resource
    A CASSCF study of the potential curves for the X 1Σ+, A 1Π, and C 1Σ+ states of the AlH molecule (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 5032-5042 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The complete active space SCF (CASSCF) method was applied to compute the potential curves for the X 1Σ+, A 1Π, and C 1Σ+ states of the AlH molecule. Molecular constants were obtained from the potential curves by solving the radial Schrödinger equation. The results obtained for the ground state are in excellent agreement with experiment, and show that the intershell correlation effects only have a very small effect on the properties of AlH. Calculated values for ΔGv+1/2 differ from experiment within 4.4–5.6 cm−1for the five known band origins (v=0–4). The calculated dissociation energy (De) is 3.11 eV (experiment, 3.16 eV). The A state has a shallow minimum followed by a maximum. Two bound vibrational states have been found, the upper predissociative. The calculated value for De is 0.12 eV, which is 0.12 eV less than the experimental value. All deviations from experiment can be derived from this error. Rotational constants are in excellent agreement with experiment. Radiative lifetimes for the two bound states have been computed to be 62 (v=0) and 102 ns (v=1). The corresponding experimental values are 66±4 and 83±6 ns, respectively. The C state exhibits a double minimum in the potential curve, with the outer minimum lower in energy, in contrast to the corresponding state in BH.The dissociation energy (D0), 0.61 eV agrees well with the value 0.64 eV derived from experimental data. The second minimum is located at R=3.76 A(ring) and has a depth of 1.16 eV. Transition moments from the six lowest levels of the ground state to any of the outer minimum levels are all very small and this minimum is not expected to be seen in absorption. The rotational levels for the second vibrational band in the inner minimum are heavily perturbed by interaction with outer minimum levels, and a normal analysis of the spectrum in terms of Bv and Dv is not possible.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452679
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  • 10
    Electronic Resource
    Electronic Resource
    Theoretical characterization of the lowest-energy absorption band of pyrrole (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 7526-7536 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The lowest-energy band of the electronic spectrum of pyrrole has been studied with vibrational resolution by using multiconfigurational second-order perturbation theory (CASPT2) and its multistate extension (MS–CASPT2) in conjunction with large atomic natural orbital-type basis sets including Rydberg functions. The obtained results provide a consistent picture of the recorded spectrum in the energy region 5.5–6.5 eV and confirm that the bulk of the intensity of the band arises from a ππ* intravalence transition, in contradiction to recent theoretical claims. Computed band origins for the 3s,3p Rydberg electronic transitions are in agreement with the available experimental data, although new assignments are suggested. As illustrated in the paper, the proper treatment of the valence–Rydberg mixing is particularly challenging for ab initio methodologies and can be seen as the main source of deviation among the recent theoretical results as regards the position of the low-lying valence excited states of pyrrole. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1465406
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