Problems with Singlet Diradicals and Model Chemistries

2 Problems with Singlet Diradicals and Model Chemistries 

Let us first do a reality check we ll test the ability of some model chemistry methods to perform geometry optimizations on singlet 1,3-propanediyl or trimethylene (CH2-CH2-CH2) and on singlet 1,4-butanediyl or tetramethylene ( CH2CH2CH2CH2 ), simple singlet diradicals. 

Two electrons (usually the highest-energy ones) are unpaired but of opposite spin 

3-propanediyl (CH2CH2CH2). All bond lengths and angles in these structures were standard, e.g. C-Cca. 1.5 A, C-Hca. 1.1 A, bond angles ca. 110° 

We now look at results of calculations on the 1,3- and 1,4-diradicals by methods more appropriate than the model chemistries just employed. 

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