2,4-diaminopyrimidine (2,4-dAPy) is the pyrimidine analogue of 2,6-dAP. From rotational contour fitting, we previously found it to have a lifetime between 10-1000 ps . Nachtigallova et al. and calculated the relaxation of 2,4-dAPy to be hindered due to a barrier while 4-aminopyrimidine (4-APy), the pyrimidine analog of adenine, is expected to exhibit ultrafast decay. We experimentally corroborated this finding for 2,4-dAPy, measuring a 766 ps lifetime near the origin (Fig. ). Singly-substituted 4-APy can access ring-puckering CIs at C 2 at low excess energy and has a short lifetime, like adenine. Meanwhile, the doubly-substituted 2,4-dAPy has some of these CIs hindered due to the second amino group, similar to dynamics in 2,6-dAP.