Dioxide and Related Molecules

Carbon Dioxide and Related Molecules.—It is not surprising that so unconventional a molecule as carbon monoxide should have a resonating structure but recognition of the faci, that the carbon dioxide molecule, for which the valence-bond formula 0—C---0 has been written ever since the development of valence theory, is not well represented by this structure alone and that other valence-bond structures also make important contributions must have come as a surprise to everyone. 

The carbon-oxygen distance in this molecule is known6 to be 1.159 A. If one structure 0=C=0 alone represented the molecule the distance should be 1.18 A, the double-bond length with the adjacent-bent-bond 

The observed resonance energy, relative to the ketonic type of double bond, is 33 kcal/mole. 

The carbon suboxide molecule is linear,10 as expected for the two double-bond structures A and B and also for structures C and D  

These four structures contain the same number of covalent bonds, but there is a separation of formal charge in C and D and none in A and B, so that C and D would be expected to make a somewhat smaller contribution to the normal state of the molecule than A and B. The values C—0 = 1.18 A and C—C 1.274 A are expected for resonance between A and B, and C—O — 1.16 A and C—C = 1.254 A for equal resonance among the four structures. The observed values11 C—O = 1.160 A and C—C = 1.279 A suggest that there is resonance 

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