Abstract:Systematic investigations of conformational effects of the hydroxyl group on carbon or hydrogen chemical shifts of different types of alcohols reveal that, besides stereoelectronic effects, hyperconjugation with lone pairs may have a strong influence. In view of the growing use of chemical shifts in probing the structure of biologic molecules, we employed DFT/GIAO/NBO calculations at the B3LYP level for conformers obtained from partial optimization of structures resulting from 30°variations of the COCOXOH (XAO… Show more
“…The contribution of LP(2) to the chemical shift of C 3 is much larger, deshielding this nucleus by almost 2.5 ppm at 60° and 270°. Thus, the angular dependence of non-Lewis contributions of both lone pairs on oxygen is in good agreement with our previous interpretations based on hyperconjugative interactions. , Those of LP(2) to the chemical shifts of C 1 and C 3 are stronger, probably owing to its better overlap with the antibonding σ* orbital of the adjacent C−C bond. 7 Variations of non-Lewis contributions from oxygen lone pairs, LP(1) and LP(2), respectively, to natural chemical shieldings of C 1 and C 3 in 2- endo -norborneol with the C 3 −C 2 −O−H dihedral angle.…”
Section: Resultssupporting
confidence: 91%
“…4 In this region, the C 1 -C 2 bond reaches its maximum length at a C 3 -C 2 -OH dihedral angle of 180°( Figure 6). In this conformation, overlap between the lone pair in a p orbital corresponding to LP(2) and the antibonding orbital on C 1 is most favorable and σ* backdonation is strongest, lengthening the C 1 -C 2 bond.…”
This paper represents an extension of our work on the (1)H and (13)C NMR chemical shifts of norbornane and 2-endo-norborneol. NCS-NBO analysis was employed to probe contributions of bond orbitals and orbitals of lone pairs to nuclear shielding in conformers of the alcohol generated by rotation of the C-O bond. Variations in (1)H and (13)C chemical shifts with the dihedral angle are discussed in terms of Lewis and non-Lewis partitioning and their respective importance is evaluated. In addition to hyperconjugation of the lone pair in a p orbital of oxygen that was previously reported, a sizable participation of the lone pair which is in an sp orbital is also observed and their combined effect dominates the carbon chemical shifts of the C(1)-C(2)-OH and C(3)-C(2)-OH fragments. Both lone pairs on oxygen also contribute to localized, though-space effects on nuclei in the vicinity, these effects answering for the largest deviations in hydrogen chemical shifts on rotation around the C-O bond. On the other hand, for conformers in which nonbonded repulsions lead to distortions in the molecular framework, variations in chemical shifts may be attributed to angular effects.
“…The contribution of LP(2) to the chemical shift of C 3 is much larger, deshielding this nucleus by almost 2.5 ppm at 60° and 270°. Thus, the angular dependence of non-Lewis contributions of both lone pairs on oxygen is in good agreement with our previous interpretations based on hyperconjugative interactions. , Those of LP(2) to the chemical shifts of C 1 and C 3 are stronger, probably owing to its better overlap with the antibonding σ* orbital of the adjacent C−C bond. 7 Variations of non-Lewis contributions from oxygen lone pairs, LP(1) and LP(2), respectively, to natural chemical shieldings of C 1 and C 3 in 2- endo -norborneol with the C 3 −C 2 −O−H dihedral angle.…”
Section: Resultssupporting
confidence: 91%
“…4 In this region, the C 1 -C 2 bond reaches its maximum length at a C 3 -C 2 -OH dihedral angle of 180°( Figure 6). In this conformation, overlap between the lone pair in a p orbital corresponding to LP(2) and the antibonding orbital on C 1 is most favorable and σ* backdonation is strongest, lengthening the C 1 -C 2 bond.…”
This paper represents an extension of our work on the (1)H and (13)C NMR chemical shifts of norbornane and 2-endo-norborneol. NCS-NBO analysis was employed to probe contributions of bond orbitals and orbitals of lone pairs to nuclear shielding in conformers of the alcohol generated by rotation of the C-O bond. Variations in (1)H and (13)C chemical shifts with the dihedral angle are discussed in terms of Lewis and non-Lewis partitioning and their respective importance is evaluated. In addition to hyperconjugation of the lone pair in a p orbital of oxygen that was previously reported, a sizable participation of the lone pair which is in an sp orbital is also observed and their combined effect dominates the carbon chemical shifts of the C(1)-C(2)-OH and C(3)-C(2)-OH fragments. Both lone pairs on oxygen also contribute to localized, though-space effects on nuclei in the vicinity, these effects answering for the largest deviations in hydrogen chemical shifts on rotation around the C-O bond. On the other hand, for conformers in which nonbonded repulsions lead to distortions in the molecular framework, variations in chemical shifts may be attributed to angular effects.
“…Therefore, this should be dependent mainly on the distances and orientations of the two species. The intermolecular interaction calculated in the present case is on the same magnitude of intramolecular interactions associated to hyperconjugation that we have calculated elsewhere [40].…”
Section: Resultsmentioning
confidence: 89%
“…The very small residual energetic and charge contributions in saturated systems are largely due to delocalized, non-covalent interactions between bonding and antibonding orbitals of the NBO approach. This noncovalent bonding-antibonding interaction gives the quantitative description of hyperconjugation [40]. In terms of the NBO approach this is expressed by means of the secondorder perturbation interaction energy (E (2) ) involving neighboring orbitals.…”
“…For ethylamine, the use of the switching function causes the potential energy to overshoot the MM3 energy of the dissociated radical by about 7 kJ/mole. While it is possible that use of larger a in the switching function would force the PES curve down, it may also be that the evolving radical is resonance-stabilized by the lone pair on the nitrogen atom [31,32]. In any case, it appears likely that a more accurate model representation could be obtained by adjusting the forcefield parameters to provide a better description of the radical species.…”
Section: Testing Transferability Of the Modelmentioning
RMDff is a new forcefield that smoothly couples the reactive intersections of potential energy surfaces to model chemical reactions. The method uses switching functions to accomplish a smooth transition from reactant to product atom types. This paper demonstrates and tests RMDff for homolytic scissions. The reaction networks are described by localized events involving only a few atoms, so that the complex mechanisms employed in conventional kinetics modeling are not needed. Unlike quantum chemical calculations, which are feasible only for small molecules, this new valence-bond forcefield can be coupled with Reactive Molecular Dynamics to describe chemical reactions in large domains.
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