59-340 Quantum Chemistry

Clarifications

1. Length of polyene - 10.08.2007

There was some confusion as to how the length of 154 pm was chosen for a butadiene molecule.  This is how it's done:

Butadiene, like all polyenes, is not a linear molecule; however, for simplicity, we assume that the pi electrons move along a line who length can be estimated as equal to two C=C bond lengths (2 x 135 pm) plus one C-C bond (154 pm) plus the distance of a carbon atom radius at each end (2 x 77pm = 154 pm), giving a total distance of 578 pm.  So the additional 154 pm does not come from an extra unaccounted for C-C bond, but from the terminal C radii themselves.
 

 

Copyright Rob Schurko, 2000-7
Updated: 13/08/2007
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