Ethane conformers

A completely different structural motif has very recently been found in the red-brown phosphide CasPg, formed by direct fusion of Ca metal and red P in the correct atom ratio in a corundum crucible at 1000 C. The structure comprises Ca + cations and Pg anions, the latter adopting a staggered ethane conformation. (Note that P+ is isolobal with C and P with H so that C2H6 = [(P+)2(P"")6] = Ps - -) The internal P-P distance is 230.1pm and the terminal P-P distances 214.9-216.9 pm, while the internal PPP angles are 104.2-106.4° and the outer angles are 103.4-103.7°. 

Preliminary approximate calculations of the transition state in C2H6 + IIO reaction, also executed by the Partial Reservation of Double-Centered Differential Overlap (PRDDO) method [29, 30] indicate the interaction between masked ethane conformation (with a calculated internal rotation barrier equal 1.3kJ/mol) and the HO radical approaching it in the C—C plane. The distance between C—C and O—O bond sites was taken as the reaction coordinate. It is found that planar structure (II) corresponds to the transition state in the C2H6 + H02 reaction. 

Ethane conformations. The eclipsed conformation has a dihedral angle 0 = 0° and the staggered conformation has 0 = 60°. Any other conformation is called a skew conformation. 

Conformation and Conformational Analysis Conformation of Ethane Conformation of Propane Conformation of Butane Eclipsed and Staggered Eorms Ring Strains in Cycloalkanes Principles of Conformation 165... 

The ethane conformers are thus separated by a relatively low potential barrier, which can be overcome by thermal energy supplied by the collision of molecules. On the average, thermal energies of about 0.5RT per degree of freedom are transferred during such a collision. Because this is less than... 

Figure 16.7 Potential energy for ethane conformations as a fnnction of dihedral angle.

Thus, even a one-CW theory of ethane conformational isomerism would predict that staggered is more stable than eclipsed ethane, if overlap were included in the analysis (vide infra). 

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