Why don't adenine and cytosine hydrogen bond with each other?

Adenine and cytosine do not hydrogen bond with each other because their hydrogen bonding patterns are incompatible. Adenine, a purine, has specific hydrogen bond donors and acceptors that pair with thymine or uracil, while cytosine, a pyrimidine, pairs with guanine. The structural arrangement of hydrogen bond sites in adenine and cytosine does not allow for stable pairing, which is essential for the complementary base pairing required in DNA and RNA. Therefore, they do not form the necessary hydrogen bonds to stabilize a base pair.