Axial atoms

An interesting question is the effect of unshared pairs of electrons on an axial atom. Pauling states that they appear to contribute little to three fold barriers, basing this on the observed fact that methyl amine with two N—H bonds and one pair has about 2/3 the barrier of ethane while methyl alcohol with one O—H bond and two pairs has approximately one third the ethane value. He explains this by pointing out that the unshared pairs, not forming bonds, have not the same reason to acquire / character, without which they will not contribute to a threefold barrier. 

Perhaps more subtle is a molecule such as PF3 (Fig. 6.le), which has symmetry. The three equatorial fiuonne atoms can be interchanged by reflection or by rotation about the C3 axis. Similarly, the two axial atoms can be reflected or rotated into each other. However, no operation allows interchange of an axial and an equatorial fluorine atom. Thus we have two sets of symmetry (and chemically) equivalent fluorine atoms. As a consequence, we would not expect P—F,v bond lengths to be the same as P—Teq bond lengths (and they are not), nor would we expect the flve fiuonne... 

All three F nuclei (/ = i) are split by the P nucleus (/ = i) with a coupling constant of Jp p 1048 Hz. At —143 °C this produces a downfield doublet (5 —67.4) for the axial atoms and an upfield doublet (S41.5) for the equatorial fluorine atom. The single equatorial fluorine atom splits each component of the doublet of the two axial fluorine atoms into another T l /i= l)withyp p= 124 Hz. In the same... 

Interestingly, despite the greater electron affinity of carbon than boron, the calculated bond orders/indices of the cage bonds from the equatorial or tropical boron atoms to the axial atoms in systems [B H ] , [CB iH ], and C2B 2H change little as axial carbon atoms replace axial boron atoms. However, axial carbon atoms reduce the electron density and so the bond order/index between equatorial or tropical boron atoms, ... 

Expressions for more complex species can be defined from the above expressions, because each atom makes an independent contribution to the moments of inertia. For example a bicapped antiprism has lx and Iy identical to a square antiprism, as the addition of atoms on the z axis cannot influence lx or Iy, so Iz = Iz (square antiprism) + 2c2, where c is the distance of the capping atoms from the origin of the polyhedron. The term 2c2 is identical to the contribution made by the axial atoms in a trigonal bipyramid. 

The hydrogen atoms of cyclohexane are described according to their position relative to the ring. Those that lie above or below the ring are said to be axial atoms. Those that lie roughly in the plane of the ring are called equatorial atoms. 

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