Unexpected Geometrical Effects on Paramagnetic Spin–Orbit and Spin–Dipolar ²J_FF Couplings

Abstract

The second-rank tensor character of the paramagnetic spin-orbit and spin-dipolar contributions to nuclear spin-spin coupling constants is usually ignored when NMR measurements are carried out in the isotropic phase. However, in this study it is shown that isotropic (2)J(FF) couplings strongly depend on the relative orientation of the C-F bonds containing the coupling nuclei and the eigenvectors of such tensors. Predictions about such effect are obtained using a qualitative approach based on the polarization propagator formalism at the RPA, and results are corroborated performing high-level ab initio spin-spin coupling calculations at the SOPPA(CCSD)/EPR-III//MP2/EPR-III level in a model system. It is highlighted that no calculations at the RPA level were carried out in this work. The quite promising results reported in this paper suggest that similar properties are expected to hold for the second-rank nuclear magnetic shielding tensor.

An efficient DFT method of predicting the one-, two- and three-bond indirect spin–spin coupling constants involving a fluorine nucleus in fluoroalkanes

“…Calculations show, however, that the 2 J(F,F) parameter is poorly conformation-dependent. This observation is in accord with the nding of Tormena et al 67 who have shown that in CF 2 group the geminal F-F coupling constant depends mainly on the angle between C-F bonds involved.…”

Revisiting NMR Through-Space JFF Spin–Spin Coupling Constants for Getting Insight into Proximate F---F Interactions

“…It is noted that such steric compression does not change the NBO occupancies of both LP 2 (F) lone pairs. This suggests that in halogen atoms the LP 2 s % character can be employed to gauge semiquantitatively the steric compression exerted on it. The large calculated PSO term for J F1F8 SSCC in compound 1 is notably contributed from the “geometric effect” reported previously The isotropic value of the diamagnetic shielding tensor, σ D (F) is notably insensitive to substituents placed at the “para” position.…”

“…The three PSO tensor eigenvectors contributing to 4 J F1,F8 SSCC are shown schematically at the right-hand side of Figure . The “local system”, shown at the left-hand side of Figure , is defined by the right-handed Cartesian orthogonal system defined by X , along the LP 2 lone pair, Y along the LP 3 lone pair, and the Z axis along the C–F bond. , It is observed that the right-handed system determined by the three PSO tensor eigenvectors differs from the “local” coordinate system just by a β angle rotation around the eigenvector perpendicular to the aromatic plane. From the above considerations it is obvious that the “geometric effect” is nil when the PSO 1,8 tensor eigenvectors coincide with the “local” coordinate system.…”

Influence of stereoelectronic effects on the 1JC─F spin–spin coupling constant in fluorinated heterocyclic compounds

“…An inspection of Table indicates that the SD and PSO terms are responsible for such a behavior, since these terms contribute more positively to 1 J C‐Fax than to 1 J C‐Feq . Considering n J FF SSCCs, the isotropic part of the SD and PSO tensors has been shown to be strongly dependent on the C─F bond orientations . Accordingly, such spatial dependence of SD and PSO regarding the 1 J C─F SSCC is more clearly expected and straightforward.…”

“…Considering n J FF SSCCs, the isotropic part of the SD and PSO tensors has been shown to be strongly dependent on the C─F bond orientations. [20] Accordingly, such spatial dependence of SD and PSO regarding the 1 J C─F SSCC is more clearly expected and straightforward. For compound 5, in which the fluorine substituent is positioned between the endocyclic nitrogen atom and the carbonyl group (thus a real ketone, where the above-mentioned resonance is not possible), an increase in both | 1 J C-Fax | and | 1 J C-Feq | values relative to 1-4 can be observed, indicating the same effect of the vicinity of the C─F bond to these groups on the 1 J C─F SSCC.…”