Caractéristiques géométriques de la séquence Hα?Cα?N?H de l'isoquinuclidone-3 examinée dans différents etats physiques influence sur la constante de couplage vicinal J

The X-ray structure analysis of a crystalline sample of 2-azabicyclo-[2,2,2]-octanone-3 or 3-isoquinuclidone shows that the molecules of this compound are associated in centrosymmetrical dimers stabilized by two N? H? O?C hydrogen bonds in which the N,H,O atoms are nearly collinear. As a consequence of this interaction, the H atom is shifted from its usual position and the C^α? N? H angle is increased to 125°. Using infrared spectroscopy (ν_N–H frequency range), it is possible to demonstrate that 3-isoquinuclidone is mainly in a dimeric form when dissolved in an inert solvent such as CCl₄ and to observe the dimer-monomer equilibrium on dilution from saturation to a low concentration (0.005 mole/l.). On the contrary, dimers are broken off when operating in a polar medium (acetonitrile, deuterochloroform). In the same experimental conditions, measurements of the J vicinal coupling constant, by nuclear magnetic resonance spectroscopy, afford a concentration-dependent result in the case of CCl₄ solutions (increasing from 5.4 to 5.7 Hz when diluting from 0.5 to 0.005 mole/l.) and a constant one (5.8 Hz) in the case of CH₃CN or CDCl₃ solutions. Then the 0.4-Hz difference can be attributed to geometrical changes in the H^α? C^α? N? H system when dimers are broken off and the valence angle C^α? N? H consequently decreases from 125° to its standard value (about 115°). This experimental observation is consistent with the result of a theoretical analysis performed by the INDO method. Then it seems that the use of the formulas proposed by Karplus to account for the valence angle distorsions in ethane-like systems, in the case of the H^α? C^α? N? H sequence, could yield overstimated corrections.