Bond angle bonding character

Cyclobutane has less angle strain than cyclopropane (only 19.5°). It is also believed to have some bent-bond character associated with the carbon-carbon bonds. The molecule exists in a nonplanar conformation in order to minimize hydrogen-hydrogen eclipsing strain. 

The effect of this partial double bond character on the chemical properties of chlorine atoms conjugated to double bonds is well known it corresponds in the main to a diminution in reactivity. The correlation with bond angles is discussed in a later section of this paper. 

The Dependence of Bond Angles on Single Bond-Double Bond Resonance.—In a molecule such as phosgene or 1,1-dichloroethylene the value 125°16 for the angle Cl-C-0 (0) is predicted by the theory of the tetrahedral carbon atom in case that the C-Cl bonds have no double bond character. If the double bond resonates equally among all three positions, giving the Cl-C bond one-third double bond character, we expect from symmetry... 

The geometry of nitromethane (1) is characterized by the equivalence of the two NO bonds, the single bond character of the CN bond, the coplanarity of the four nonhydrogen atoms, and a value of the zONO angle larger than 120°. The geometry of aci-nitromethane (4) is characterized by the nonequivalence of two NO bonds, the double bond character of the... 

Nitromethylene anion (5) and aci-nitromethylene anion (7) are planar molecules. In the nitromethylene anion, there is an equivalence of the two NO bonds and a clear trend of the various bond angles toward the trigonal value of 120°. By contrast, the aci-nitromethylene anion shows the nonequivalence of the two NO bonds, a typical double bond character for the CN bond, and a larger deviation of all bond angles from 120°. 

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