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- Cremer, Dieter, 1944, et al.
(author)
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Calculation and analysis of NMR spin-spin coupling constants.
- 2007
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In: Physical chemistry chemical physics : PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076. ; 9:22, s. 2791-816
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Journal article (peer-reviewed)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, et al.
(author)
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Some thoughts about bond energies, bond lengths, and force constants
- 2000
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In: Journal of Molecular Modeling. - 1610-2940 .- 0948-5023. ; 6:4, s. 396-412
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Journal article (peer-reviewed)abstract
- The bond energy (BE) of a polyatomic molecule cannot be measured and, therefore, determination of BEs can only be done within a model using a set of assumptions. The bond strength is reflected by the intrinsic BE (IBE), which is related to the intrinsic atomization energy (IAE) and which represents the energy of dissociation under the provision that the degree of hybridization is maintained for all atoms of the molecule. IBE and BE differ in the case of CC and CH bonds by the promotion, the hybridization, and the charge reorganization energy of carbon. Since the latter terms differ from molecule to molecule, IBE and BE are not necessarily parallel and the use of BEs from thermochemical models can be misleading. The stretching force constant is a dynamical quantity and, therefore, it is related to the bond dissociation energy (BDE). Calculation and interpretation of stretching force constants for local internal coordinate modes are discussed and it is demonstrated that the best relationship between BDEs and stretching force constants is obtained within the model of adiabatic internal modes. The valence stretching force constants are less suitable since they are related to an artificial bond dissociation process with geometrical relaxation effects suppressed, which leads to an intrinsic BDE (IBDE). In the case of AX(n) molecules, symmetric coordinates can be used to get an appropriate stretching force constant that is related to the BE. However, in general stretching force constants determined for symmetry coordinates do not reflect the strength of a particular bond since the related dissociation processes are strongly influenced by the stability of the products formed.
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- Cremer, Dieter, 1944, et al.
(author)
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The ozone-acetylene reaction: Concerted or non-concerted reaction mechanism? A quantum chemical investigation
- 2001
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In: Chemical Physics Letters. - 0009-2614. ; 347, s. 268-276
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Journal article (peer-reviewed)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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