From this exploration of the photorelaxation mechanisms of 2,6-dAP, a few conclusions can be drawn about the relationship between purine structure and excited state dynamics. The increased photostability of the 2,6-dAP 9H tautomer compared to the 7H tautomer is in line with the photostability of the adenine tautomers both in the gas phase and in solution. The 9H tautomer origin transition ( 1 ππ*) in the gas phase tends to be to the blue of the 7H tautomer, for the handful of examples that are available (see table ). The origin position is a direct measurement of the adiabatic energy of the ππ* state. The relative energy levels of the ππ* state versus the nearest nπ* states affect the excited state dynamics, as shown with 2AP. With a few exceptions (guanine , isoguanine , and hypoxanthine ), it appears most purines (adenine, (26-31) 2AP( ), 2-oxopurine , xanthine(15, 16), purine ) and pyrimidines (cytosine , thymine , and uracil( )) relax from their initially excited ππ* to an nπ* to the ground state. Instead, 2,6-dAP follows the 1 ππ*/S 0 CI path exclusively for 9H and both 1 ππ*/S 0 internal conversion and 1 ππ*/ 3 ππ* ISC for 7H. The only difference between the two forms is the placement of the hydrogen on N 7 vs N 9 and this minor difference in structure has a significant effect on the excited state dynamics.