The Experts below are selected from a list of 87 Experts worldwide ranked by ideXlab platform
Janet E Del Bene - One of the best experts on this subject based on the ideXlab platform.
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19f 19f spin spin Coupling Constant surfaces for hf 2 clusters the orientation and distance dependence of the sign and magnitude of jf f
Journal of Chemical Physics, 2004Co-Authors: Janet E Del Bene, Jose Elguero, Ibon Alkorta, Manuel YanezAbstract:Ab initio calculations using the equation-of-motion coupled cluster method have been carried out to investigate 19F–19F spin–spin Coupling Constants for a pair of HF molecules. The overall features of the JF–F Coupling surface with respect to the F–F distance and the orientation of the pair of HF molecules reflect those of the Fermi-contact (FC) surface, although the FC term may not be a good quantitative estimate of JF–F. The hydrogen-bonded HF dimer exhibits unusual behavior compared to other hydrogen-bonded complexes, since both the FC term and 2hJF–F exhibit variations in sign and magnitude as the F–F distance changes and the linearity of the hydrogen bond is destroyed. The FC term for F–F Coupling is relative small and negative for the equilibrium dimer. At the dimer F–F distance, the maximum negative value for the FC term is found for the linear arrangement F–H⋯H–F, while the maximum positive value is found for the linear H–F⋯F–H arrangement, despite the fact that neither of these structures is boun...
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what determines the sign of the fermi contact contribution to the nmr spin spin Coupling Constant
Chemical Physics Letters, 2003Co-Authors: Janet E Del Bene, Jose ElgueroAbstract:The Dirac vector model for the sign of the spin–spin Coupling Constant ðJ Þ between a pair of atoms states that onebond Coupling Constants are positive, two-bond negative, three-bond positive, etc. However, this rule is often violated. In an attempt to understand the sign of J , we propose a new model, the Nuclear magnetic resonance triplet wavefunction model (NMRTWM) which is based on the phases of excited triplet state wavefunctions, and the response of magnetic nuclei to these phases. The model demonstrates that, (1) the excited states which contribute to the Coupling Constant do so in a regular manner; (2) the sign of an individual contribution is determined by the nodal character of the excited-state wavefunction and the response of the nuclei to the phases of this function; and (3) the sign of the total Coupling Constant is the result of competing positive and negative contributions from various states. NMRTWM provides a fundamental explanation for both the successes and failures of the Dirac vector model, and can be used to gain insight into some puzzling results for the orientation dependence of F–F Coupling between two HF molecules. 2003 Elsevier B.V. All rights reserved.
Jose Elguero - One of the best experts on this subject based on the ideXlab platform.
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19f 19f spin spin Coupling Constant surfaces for hf 2 clusters the orientation and distance dependence of the sign and magnitude of jf f
Journal of Chemical Physics, 2004Co-Authors: Janet E Del Bene, Jose Elguero, Ibon Alkorta, Manuel YanezAbstract:Ab initio calculations using the equation-of-motion coupled cluster method have been carried out to investigate 19F–19F spin–spin Coupling Constants for a pair of HF molecules. The overall features of the JF–F Coupling surface with respect to the F–F distance and the orientation of the pair of HF molecules reflect those of the Fermi-contact (FC) surface, although the FC term may not be a good quantitative estimate of JF–F. The hydrogen-bonded HF dimer exhibits unusual behavior compared to other hydrogen-bonded complexes, since both the FC term and 2hJF–F exhibit variations in sign and magnitude as the F–F distance changes and the linearity of the hydrogen bond is destroyed. The FC term for F–F Coupling is relative small and negative for the equilibrium dimer. At the dimer F–F distance, the maximum negative value for the FC term is found for the linear arrangement F–H⋯H–F, while the maximum positive value is found for the linear H–F⋯F–H arrangement, despite the fact that neither of these structures is boun...
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what determines the sign of the fermi contact contribution to the nmr spin spin Coupling Constant
Chemical Physics Letters, 2003Co-Authors: Janet E Del Bene, Jose ElgueroAbstract:The Dirac vector model for the sign of the spin–spin Coupling Constant ðJ Þ between a pair of atoms states that onebond Coupling Constants are positive, two-bond negative, three-bond positive, etc. However, this rule is often violated. In an attempt to understand the sign of J , we propose a new model, the Nuclear magnetic resonance triplet wavefunction model (NMRTWM) which is based on the phases of excited triplet state wavefunctions, and the response of magnetic nuclei to these phases. The model demonstrates that, (1) the excited states which contribute to the Coupling Constant do so in a regular manner; (2) the sign of an individual contribution is determined by the nodal character of the excited-state wavefunction and the response of the nuclei to the phases of this function; and (3) the sign of the total Coupling Constant is the result of competing positive and negative contributions from various states. NMRTWM provides a fundamental explanation for both the successes and failures of the Dirac vector model, and can be used to gain insight into some puzzling results for the orientation dependence of F–F Coupling between two HF molecules. 2003 Elsevier B.V. All rights reserved.
Jose G Napolitano - One of the best experts on this subject based on the ideXlab platform.
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on the configuration of five membered rings a spin spin Coupling Constant approach
Chemistry: A European Journal, 2011Co-Authors: Jose G Napolitano, Jose A Gavin, Celina Garcia, Manuel Norte, Jose J Fernandez, Antonio Hernandez DaranasAbstract:Five-membered rings are clearly among the most common structural motifs found in chemistry and biology. Nevertheless, the configuration of conformationally mobile five-membered rings is often difficult to assign from nuclear magnetic resonance (NMR) data. A simple, reliable, and efficient approach for the stereochemical analysis of five-membered rings based on the measurement of NMR Coupling Constants is presented. Density functional theory calculations using representative conformations of the full conformational space available to rings with different substitution patterns were used to identify differences between the accessible Coupling Constant values for cis and trans relative orientations of the substituents. The calculations were assessed experimentally using NMR data obtained from a number of models. This approach can be easily used to analyze different five-membered rings, such as oxolanes, cyclopentanes, furanosides and pyrrolidines, and their relative configuration can be determined without the need for making further conformational considerations.
Antonio Hernandez Daranas - One of the best experts on this subject based on the ideXlab platform.
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on the configuration of five membered rings a spin spin Coupling Constant approach
Chemistry: A European Journal, 2011Co-Authors: Jose G Napolitano, Jose A Gavin, Celina Garcia, Manuel Norte, Jose J Fernandez, Antonio Hernandez DaranasAbstract:Five-membered rings are clearly among the most common structural motifs found in chemistry and biology. Nevertheless, the configuration of conformationally mobile five-membered rings is often difficult to assign from nuclear magnetic resonance (NMR) data. A simple, reliable, and efficient approach for the stereochemical analysis of five-membered rings based on the measurement of NMR Coupling Constants is presented. Density functional theory calculations using representative conformations of the full conformational space available to rings with different substitution patterns were used to identify differences between the accessible Coupling Constant values for cis and trans relative orientations of the substituents. The calculations were assessed experimentally using NMR data obtained from a number of models. This approach can be easily used to analyze different five-membered rings, such as oxolanes, cyclopentanes, furanosides and pyrrolidines, and their relative configuration can be determined without the need for making further conformational considerations.
B Maraviglia - One of the best experts on this subject based on the ideXlab platform.
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product operator formalism in i 1 2 and s 3 2 j coupled nuclear spins application to j assessment via sedor experiment
Journal of Chemical Physics, 1998Co-Authors: Silvia Capuani, F De Luca, B MaravigliaAbstract:Product-operator formalism has been extended to weakly coupled I⩾1/2 and S⩾3/2 nuclear spins to describe the result of nuclear SEDOR (spin echo double resonance) experiments for such spin numbers. By SEDOR modulation, the indirect nuclear spin–spin Coupling Constant JIS can be measured also when the quadrupolar broadening hides the high resolution evaluation of JIS and the method based on scalar relaxation of the second kind cannot be utilized because of the low quadrupolar relaxation efficiency. Furthermore, the nuclear SEDOR modulation can be performed in low intensity and homogeneity magnetic field. The theory has been applied to estimate JIS of the coupled nuclear spins I=1/2 (F19) and S=9/2 (Nb93) in NbF6− (octahedral symmetry) and of I=1/2 (H1) and S=3/2 (B11) in Na2B1211H11SH (icosahedral symmetry).