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| 2. |
- Cremer, Dieter, 1944, et al.
(författare)
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The ozone-acetylene reaction: Concerted or non-concerted reaction mechanism? A quantum chemical investigation
- 2001
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Ingår i: Chemical Physics Letters. - 0009-2614. ; 347, s. 268-276
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Tidskriftsartikel (refereegranskat)abstract
- The ozone-acetylene reaction is found to proceed via an intermediate van der Waals complex (rather than a biradical), which is the precursor for a concerted symmetry-allowed [4+2] cycloaddition reaction leading to 1,2,3-trioxolene. CCSD(T)/6-311G+(2d,2p) and CCSD(T)/CBS (complete basis set) calculations predict the ozone-acetylene van der Waals complex to be stable by 2.2 kcalmol -1 , the calculated activation enthalpy for the cycloaddition reaction is 9.6 kcalmol -1 and the reaction enthalpy -55.5 kcalmol -1 . Calculated kinetic data for the overall reaction (k=0.8lmol -1 s -1 , A=1.71×10 6 lmol -1 s -1 , E a =8.6kcalmol -1 ) suggest that there is a need for refined kinetic measurements. © 2001 Elsevier Science B.V.
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| 3. |
- Gräfenstein, Jürgen, 1963, et al.
(författare)
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An accurate description of the Bergman reaction using restricted and unrestricted DFT: Stability test, spin density, and on-top pair density
- 2000
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Ingår i: Journal of Physical Chemistry A. - 1089-5639. ; 104:8, s. 1748-1761
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Tidskriftsartikel (refereegranskat)abstract
- DFT calculations provide a reliable description of the Bergman reaction of(Z)-hex-3-ene-1,5-diyne 1 provided the following are considered. (a) Restricted DFT (RDFT) calculations along the reaction path have to be replaced by unrestricted DFT (UDFT) calculations at those locations where the former description becomes unstable. This is the case in the region of the p-didehydrobenzene biradical 2, which possesses significant multireference character. (b) LSD and pure GGA functionals are more stable than hybrid functionals, which can be directly related to the composition of these functionals. With increasing instability, RDFT calculations lead to increasing errors in the S-T splitting and the geometry of 2 as well as in the energetics of the Bergman reaction. (c) LSD and CCA Functionals underestimate the energy barrier of the Bergman reaction of 1. This becomes obvious when the correct experimental barrier is considered, which was not clone in previous DFT investigations. (d) The best description of the Bergman reaction is provided by a mixed RDFT/UDFT description using the B3LYP functional (average error of 2.7 kcal/mol). Although the B3LYP functional is rather unstable, its semiempirical calibration helps to compensate for the typical underestimation of barriers by GGA functionals, which demonstrates chat the performance of a hybrid functional does not necessarily have to do with its stability. (e) Application of the sum formula to the UB3LYP energy of biradical 2 improves the description of the Bergman reaction so that the most reliable data are obtained at RB3LYP-UB3LYP(sum)/G-311+G(3df,3pd). Activation enthalpies at 470 K for forward and backward reaction are 29.9 and 21.4 kcal/mol, respectively (exptl values, 28.23 +/- 0.5 and 19.75 +/- 0.7 kcal/mol), while the calculated reaction enthalpy at 298 K is 8.5 kcal/mol (exptl value, 8.5 +/- 1.0 kcal/mol) in reasonable agreement with experiment. The calculated S-T splitting is 2.6 kcal/mol (after correction, 4.9 kcal/mol; exptl value, 3.8 +/- 0.5 kcal/mol at 298 K). It is shown that the UDFT description covers static correlation effects needed for the correct treatment of 2S. Total and on-top pair density reflect this, while Kohn-Sham orbitals and spin density have to be considered as physically not meaningful intermediates in line with the interpretation given by Perdew, Savin, and Burke (Phys. Rev. A 1995, 51, 4531).
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| 4. |
- Gräfenstein, Jürgen, 1963, et al.
(författare)
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Analysis of the paramagnetic spin-orbit transmission mechanism for NMR spin-spin coupling constants using the paramagnetic spin-orbit density distribution
- 2004
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 383:3-4, s. 332-342
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Tidskriftsartikel (refereegranskat)abstract
- For the analysis of the paramagnetic spin orbit (PSO) term of the NMR spin-spin coupling constant (SSCC), ring current density and PSO density distribution are derived and used to explain magnitude and sign of the isotropic PSO term. Decomposition of the PSO components into orbital contributions helps to identify those orbital pairs (occupied, virtual) dominating the PSO term. The induction of strong ring currents requires low excitation energies and complementing nodal properties of zeroth and first order orbitals. The PSO components for a typical triple, double, and single CC bond are explained. (C) 2003 Elsevier B.V. All rights reserved.
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| 5. |
- Gräfenstein, Jürgen, 1963, et al.
(författare)
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Analysis of the spin-dipole transmission mechanism for NMR spin-spin coupling constants using orbital contributions, spin polarization, and spin-dipole energy density distribution
- 2004
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 387:4-6, s. 415-427
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Tidskriftsartikel (refereegranskat)abstract
- For the analysis of the spin-dipole (SD) term of the indirect NMR spin-spin coupling constant, the SD spin-polarization distribution and the SD energy density distribution are derived and used to explain magnitude and sign of the SD term. Orbital pairs (occupied, virtual) are identified that are most important for the SD spin-spin coupling mechanism. The induction of strong SD spin-polarization requires low excitation energies and complementing nodal properties of zeroth and first-order orbitals. The SD components for (1)J(CC) of typical CC bonds are analyzed. The SD term is a sensitive antenna for detecting the pi-character of a bond. (C) 2004 Elsevier B.V. All rights reserved.
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| 6. |
- Gräfenstein, Jürgen, 1963, et al.
(författare)
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Can density functional theory describe multi-reference systems? Investigation of carbenes and organic biradicals
- 2000
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 2, s. 2091-2103
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Tidskriftsartikel (refereegranskat)abstract
- Kohn-Sham (KS) density functional theory (DFT) in its present approximate form cannot be applied to electron systems with strong multi- reference character. Various ways are discussed to develop DFT for multi- reference systems. The restricted open-shell singlet (ROSS) formalism is a modification of the conventional open-shell KS formalism, which makes a reliable, but still economical treatment of open-shell singlet biradicals possible. The complete-active-space DFT (CAS-DFT) method combines an explicit treatment of multi-configurational character with the DFT treatment of dynamic electron correlation effects. Due to the flexibility in the choice of the active space, this method is in principle appropriate for all kinds of multi-reference problems. Based on sample calculations for carbenes and organic biradicals, the advantages and limitations of ROSS-DFT and CAS-DFT are discussed. Some possibilities for future improvements of DFT methods for multi-reference problems are pointed out.
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| 7. |
- Gräfenstein, Jürgen, 1963, et al.
(författare)
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Can unrestricted density-functional theory describe open shell singletbiradicals?
- 2002
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Ingår i: International Journal of Molecular Sciences. - : MDPI AG. - 1422-0067. ; 3, s. 360-394
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Tidskriftsartikel (refereegranskat)abstract
- Unrestricted density functional theory (UDFT) can be used for the description of open-shell singlet (OSS) biradicals provided a number of precautions are considered. Biradicals that require a two-determinantal wave function (e.g. OSS state of carbenes) cannot be described by UDFT for principal reasons. However, if the overlap between the open-shell orbitals is small (the single electrons are located at different atomic centers) errors become small and, then, the principal failure of UDFT in these cases is not apparent and may even be disguised by the fact that UDFT has the advantage of describing spin polarization better than any restricted open shell DFT method. In the case of OSS biradicals with two- or multiconfigurational character (but a one-determinantal form of the leading configuration), reasonable results can be obtained by broken-symmetry (BS)-UDFT, however in each case this has to be checked. In no case is it reasonable to lower the symmetry of a molecule to get a suitable UDFT description. Hybrid functionals such as B3LYP perform better than pure DFT functionals in BS-UDFT calculations because the former reduce the self-interaction error of DFT exchange functionals, which mimics unspecified static electron correlation effects, so that the inclusion of specific static electron correlation effects via the form of the wavefunction becomes more effective.
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| 8. |
- Gräfenstein, Jürgen, 1963, et al.
(författare)
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Decomposition of nuclear magnetic resonance spin-spin coupling constants into active and passive orbital contributions.
- 2004
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Ingår i: The Journal of chemical physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 120:21, s. 9952-68
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Tidskriftsartikel (refereegranskat)abstract
- The theory of the J-OC-PSP (decomposition of J into orbital contributions using orbital currents and partial spin polarization) method is derived to distinguish between the role of active, passive, and frozen orbitals on the nuclear magnetic resonance (NMR) spin-spin coupling mechanism. Application of J-OC-PSP to the NMR spin-spin coupling constants of ethylene, which are calculated using coupled perturbed density functional theory in connection with the B3LYP hybrid functional and a [7s,6p,2d/4s,2p] basis set, reveal that the well-known pi mechanism for Fermi contact (FC) spin coupling is based on passive pi orbital contributions. The pi orbitals contribute to the spin polarization of the sigma orbitals at the coupling nuclei by mediating spin information between sigma orbitals (spin-transport mechanism) or by increasing the spin information of a sigma orbital by an echo effect. The calculated FC(pi) value of the SSCC (1)J(CC) of ethylene is 4.5 Hz and by this clearly smaller than previously assumed.
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| 9. |
- Gräfenstein, Jürgen, 1963, et al.
(författare)
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Effect of the self-interaction error for three-electron bonds: On the development of new exchange-correlation functionals
- 2004
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). ; 6:6, s. 1096-1112
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Tidskriftsartikel (refereegranskat)abstract
- The dissociation behavior as well as the equilibrium properties of radical cations with three-electron bonds, namely He-2(+.), N2H6+., O2H4+., F2H2+., and Ne-2(+.) are investigated using standard and self-interaction-corrected density functional theory (SIC-DFT) in connection with a variety of pure and hybrid exchange-correlation (XC) functionals. The impact of the self-interaction error (SIE) on the results of standard DFT is analyzed considering the individual orbital contributions to the SIE, the dependence of the SIE on the separation distance between the dissociation fragments, and its impact on the equilibrium properties of 2-6. A local analysis of the SIE in terms of exact and DFT exchange holes reveals that the SIE mimics not only non-dynamic but also an increasing amount of dynamic electron correlation effects as the number of valence electrons is enlarged. Standard DFT describes the dissociation of three-electron bonds qualitatively incorrectly. This can be traced back in the first instance to the SIE of the bonding beta electron, which mimics a spurious long-range correlation with a non-existing delocalized alpha electron in the same bond. A comparison of the covalent (symmetric) and ionic (symmetry-broken) state of radical cations 2-6 at large interaction distances provides further insight in the inconsistencies of the DFT description: (i) Not only the SIE but also the approximate description of the interelectronic exchange contributes to the incorrect description of the dissociation. (ii) Dissociating three-electron bonds show a specific form of long-range correlation effects, which is neither accounted for by standard DFT, SIC-DFT nor Hartree-Fock theory. Indeed, SIC-DFT provides a qualitatively better description of the dissociation of radical cations, however in general a poor performance when describing equilibrium properties. There is no need for SIC-DFT methods. Instead, there is need for XC functionals with exact exchange and long-range correlation effects (e.g. mimicked by the exchange SIE) absorbed in the correlation functional. Implications of our findings for the construction of new density functionals are discussed.
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| 10. |
- Gräfenstein, Jürgen, 1963, et al.
(författare)
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Investigation of the pi character of a C-C bond with the help of the diamagnetic and paramagnetic spin-orbit term of the NMR spin-spin coupling constant
- 2004
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Ingår i: Journal of Physical Chemistry A. ; 108:20, s. 4520-4535
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Tidskriftsartikel (refereegranskat)abstract
- The NMR spin-spin coupling constant (SSCC) (1)J(C-C) is a sensitive antenna for investigating the electronic structure in hyper- and pi-conjugated hydrocarbons. The magnetic dipole field of the perturbed nucleus generates orbital currents, which are experienced by the responding nucleus, thus leading to the paramagnetic spin-orbit (PSO) coupling mechanism. The strength of the orbital currents depends on the availability of high-lying occupied and low-lying virtual orbitals with distinct p(pi) or p(sigma) character, the nodal properties of these orbitals, and the overlap between zeroth- and first-order orbitals. For C-C bonds in unsaturated hydrocarbons, the value of (PSO)-P-1(C-C) depends on opposing effects such as the p(sigma) and the p(pi) character of the C-C bond (i.e., the degree of hybridization at the C atoms and the degree of pi delocalization in the conjugated system). This dependence leads to the overall cubic behavior of the PSO term when given as a function of the C-C bond character and the strong scattering of PSO values for a given bond type. Under certain conditions, the degree of hybridization can be determined with the help of the PSO term. Also, the PSO term helps to distinguish hyperconjugation and pi-conjugation effects. For a given C-C bond type with fixed or similar hybridization at the C atoms, the PSO term can be given as a function of the C-C bond order, thus representing a sensitive descriptor for the degree of pi delocalization.
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