For the given compound, which of the indicated nitrogen atoms is most readily protonated?

Step-by-step solution

The given molecule is purine. The nitrogen atoms are identified as follows: 'a' is N9, 'b' is N7, 'c' is N1, and 'd' is N3. The lone pair of electrons on N9 ('a') is delocalized into the ring to maintain the aromatic 10 electron system, making it the least basic site. The lone pairs on N1 ('c'), N3 ('d'), and N7 ('b') reside in sp^2 hybridized orbitals and are available for protonation. When N1 is protonated, the resulting conjugate acid is highly stabilized by resonance. The positive charge can be extensively delo