Final answer:
Pyrimidines cannot adopt the syn conformation due to the steric hindrance of their single six-membered ring structure, compared to the larger double ring structure of purines which allows for greater spatial flexibility.
Step-by-step explanation:
Pyrimidines are unable to adopt the syn conformation primarily due to steric hindrance caused by the arrangement of their atomic structure. Unlike purines, which have a larger double ring structure consisting of a six-membered ring fused to a five-membered ring, pyrimidines have a single six-membered ring structure. This smaller size means there is less spatial flexibility for rotation. Furthermore, the bulky ribosyl group at the C5 position in the pyrimidine ring can sterically prevent other donor atoms from adopting an orientation which would allow for stable bonding, such as tridentate binding in coordination complexes. In the context of DNA structure, pairing a pyrimidine with another pyrimidine, or a purine with another purine, would disrupt the uniformity of the double helix's width, which is maintained by specific purine-pyrimidine pairings and reinforced by hydrogen bonding.