Caractéristiques géométriques de la séquence H α C α NH de l'isoquinuclidone‐3 examinée dans différents etats physiques influence sur la constante de couplage vicinal J H α H

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 NHOC 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 NH 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 CCl 4 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   H   α Hvicinal coupling constant, by nuclear magnetic resonance spectroscopy, afford a concentration‐dependent result in the case of CCl 4 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 CH 3 CN or CDCl 3 solutions. Then the 0.4‐Hz difference can be attributed to geometrical changes in the H α C α NH system when dimers are broken off and the valence angle C α NH 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 NH sequence, could yield overstimated corrections.