Rotane: crystal structure, X-X difference electron density, and phase transition.
A newly redetd. x-ray crystal structure of rotane (I) provides structural parameters in close agreement with theor. calcns. Compared to cyclopropane, a shortening of the vicinal bonds at the spiro carbon atoms and a lengthening of the distal bonds is obsd. The latter is apparently due to the increased angular strain at the spiro carbon. In a series of spiro[2.n]alkanes the angle in the (n+1)-membered ring correlates inversely with the length of the distal s bond in the cyclopropane group. The detd. X-X difference electron d. for I strongly corroborates the theor. predicted surface delocalization of electrons in 3-membered rings; this adds an essential argument in favor of the concept of s aromaticity. Upon cooling, the crystal of I undergoes a reversible phase transition at 108.2 K. Most probably the increasing no. of van der Waals contacts in the low-temp. form provides the driving force for the phase transition.
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