(The Q parameter indicates the puckering intensity, with Q = 0 being a planar ring.) In turn, 7-AIH + is planar in the ground state and puckered in the S3 excited state, also with a C5-C7a boat conformation (Q = 0.18 Å). This out-of-plane minimum is in good agreement with the experiment, which exhibits a low-frequency vibrational progression in S3 assigned to out-ofplane modes. The rings in the S1 and S2 minima are planar with the H atom attached with the six-membered ring N atom is marginally out of the plane.