Transmission mechanisms of NMR long-range J(13C,19F) scalar couplings in 1-F,4-X-cubanes: A DFT and experimental study
In order to get insight into transmission mechanisms of the title spin-spin coupling constants, nJ(C, F<inf>1</inf>) couplings (n=4, 5) were calculated at the DFT-B3LYP-6-311-G/EPR-III level, and 5J(Cα, F<inf>1</inf>) couplings were measured in 9 members of the series 1-F,4-X...
Guardado en:
| Autor principal: | |
|---|---|
| Publicado: |
2006
|
| Materias: | |
| Acceso en línea: | https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00268976_v104_n4_p485_Contreras http://hdl.handle.net/20.500.12110/paper_00268976_v104_n4_p485_Contreras |
| Aporte de: |
| Sumario: | In order to get insight into transmission mechanisms of the title spin-spin coupling constants, nJ(C, F<inf>1</inf>) couplings (n=4, 5) were calculated at the DFT-B3LYP-6-311-G/EPR-III level, and 5J(Cα, F<inf>1</inf>) couplings were measured in 9 members of the series 1-F,4-X-cubanes, where the α atom of the X group is a carbon atom. Hyperconjugative interactions were evaluated within the NBO approach, using the same level of theory as that employed for calculating spin-spin coupling constants. The unusual 4J(C<inf>4</inf>, F<inf>1</inf>) spin-spin coupling constants known in 1-F,4-Xcubanes are rationalized in this work as originating mainly due to two factors, namely, (i) the strong σ-hyperconjugative interactions involving as donors the cage C-C bonds; (ii) the particular arrangement of cage bonds not involving either the C<inf>1</inf> or the C<inf>4</inf> carbon atoms, C<inf>i</inf> and C<inf>j</inf>. For linear X substituents, the direction F<inf>1</inf>-C<inf>1</inf> ...C<inf>4</inf>-X is a three-fold symmetry axis and there are six C<inf>i</inf>-C<inf>j</inf> equivalent cage bonds, which are involved in (C<inf>i</inf>-C<inf>j</inf>)→(C<inf>1</inf>-F<inf>1</inf>)and (C<inf>i</inf>-C<inf>j</inf>)→(C<inf>4</inf>-X)hyperconjugative interactions. This means that coupling pathways involving such interactions are amplified six times, rendering the substrate very efficient for transmitting 'trans-cage' coupling constants. The large halogen substituent effects observed for 4J(C<inf>4</inf>, F<inf>1</inf>) spin-spin coupling constants in 1-F,4-X-cubanes (X = halogen atom) are rationalized in terms of interactions between σ-hyperconjugative (involving C-C bonds) and negative hyperconjugative interactions involving the halogen lone-pairs. © 2006 Taylor & Francis. |
|---|