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- Cremer, Dieter, 1944, et al.
(författare)
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Calculation and analysis of NMR spin-spin coupling constants.
- 2007
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Ingår i: Physical chemistry chemical physics : PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076. ; 9:22, s. 2791-816
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Tidskriftsartikel (refereegranskat)abstract
- The analysis of NMR spin-spin coupling leads to a unique insight into the electronic structure of closed-shell molecules, provided one is able to decode the different features of the spin-spin coupling mechanism. For this purpose, the physics of spin-spin coupling is described and the way how spin-spin coupling constants (SSCCs) can be quantum mechanically determined. Based on this insight, a set of requirements is derived that guide the development of a quantum mechanical analysis of spin-spin coupling. It is demonstrated that the J-OC-PSP (=J-OC-OC-PSP: Decomposition of J into orbital contributions using orbital currents and partial spin polarization) analysis method fulfills all requirements. J-OC-PSP makes it possible to partition the isotropic indirect SSCC J or its reduced analogue K as well as the four Ramsey terms (Fermi contact (FC), spin dipole (SD), diamagnetic spin orbit (DSO), paramagnetic spin orbit (PSO)) leading to J (or K) into Cartesian components (for the anisotropic Ramsey terms SD, DSO, PSO), orbital contributions or electron interaction terms. For the purpose of decoding the spin-spin coupling mechanism, FC, SD, DSO, and PSO coupling is discussed in detail and related to electronic and bonding features of the molecules in question. The myth of empirical and semiempirical relationships between SSCCs and bonding features is unveiled. It is found that most relationships are only of limited, partly dubious value, often arising from a fortuitous cancellation of terms that cannot be expected in general. These relationships are replaced by quantum chemical relations and descriptions that directly reflect the complex electronic processes leading to spin-spin coupling.
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- 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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- 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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