Molecular, Crystal, and Valence Structures of Iso-B18H22

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Abstract
A three‐dimensional single‐crystal x‐ray diffraction study has established the formula and molecular structure, including all H‐atom positions, of i‐B18H22. The boron arrangement has the geometry of two B10 units from B10H14 which share a common B9–B10 edge so that the molecule has a twofold axis, and so that the two halves open up in opposite directions. Thus the geometry is similar to that in n‐B18H22, in which the two halves are related instead by center of symmetry. There are four molecules of i‐B18H22 in a unit cell having parameters a=9.199, b=13.180, c=12.515 Å, and β=109°35′. The space group is P21/c. The value of R=Σ‖ F0 |—| Fc‖/Σ| F0 |=0.108 for the 3195 observed diffraction maxima. In a study of the 6992 two‐center and three‐center resonance structures the charge distribution has been approximated in order to predict the nucleophilic and electrophilic sites of chemical activity. The shared B9 atom is the most positively charged B atom.

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