can be crossed only by streamlines perpendicular to it, according to the discussion
in Sect. 7.3. Orbits approaching this plane with an angle different from p=2 are
scattered. Actually, the phase portraits of two saddle SPs lying on Tr v are observed
in Fig. 7.44d.
A representation of the Dirac-Van Vleck vector model [16, 51, 52, 146] for the
experimental vicinal coupling in CH 3 –CH 3 is given by Fig. 7.44a. Within the
(a)
(b)
(c)
(e)
(f)
(d)
Fig. 7.44 Current density field induced via the Fermi contact interaction by two magnetic dipoles
at the vicinal protons of eclipsed ethane. The coupling pattern, for anti-parallel (parallel) dipoles, is
represented on the left (right). The streamlines are shown in (a) and (d); b and e are contour levels
for the intensity, in au; the values of the solid (dashed) lines increase (decrease) in steps of
5 Â 10
À2 from the 0-contour, up to 0.5 au; c and f are the corresponding 3-dimensional perspective
views. The position of the nuclei is marked by a cross; a corresponding symbol can be seen in the
contour maps
220
P. Lazzeretti