Why can cytosine form 3 bonds but not thymine?

[wormboge]

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Why does cytosine typically form 3 bonds but not thymine?

 

[rdyotz]

I would say the key is to see that T bonds to A (as shown below). There is nowhere for a 3rd hydrogen bond to form, unlike between C & G. Orientation and polarity must match up for hydrogen bonds. The C-H bond (shown below as the bottom left corner of the 6C ring on adenine) on Adenine can't participate in hydrogen bonding since it isn't polar.

The C-G bonds are setup perfectly for 3 hydrogen bonds as depicted below.
At least that's my take on it.

Edit: it would not let me directly link image, so here is the url to image I used:
https://fthmb.tqn.com/6973M6TQEEvGw...webp)/Nucleotide-58e518d35f9b58ef7e62834d.jpg