The UV spectrum of the adenine analogue 9-methyl-2-aminopurine (9M-2AP) is investigated with one- and two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm−1 resolution in a supersonic jet. The electronic origin at 32 252 cm−1 exhibits methyl torsional subbands that originate from the |$0A_{1}^{{\prime \prime }}$|0A1 (l = 0) and 1E (l = ±1) torsional levels. These and further torsional bands that appear up to |$0_0^0+230$|000+230 cm−1 allow to fit the threefold (V3) barriers of the torsional potentials as |$|V^{\prime \prime }_3|\break = 50$||V3|=50 cm−1 in the S0 and |$|V^{\prime }_3 |=126$||V3|=126 cm−1 in the S1 state. Using the B3LYP density functional and correlated approximate second-order coupled cluster CC2 methods, the methyl orientation is calculated to be symmetric relative to the 2AP plane in both states, with barriers of |$V^{\prime \prime }_3=20$|V3=20 cm−1 and |$V^{\prime }_3\break =115$|V3=115 cm−1. The |$0_0^0$|000 rotational band contour is 75% in-plane (a/b) polarized, characteristic for a dominantly long-axis 1ππ* excitation. The residual 25% c-axis polarization may indicate coupling of the 1ππ* to the close-lying 1nπ* state, calculated at 4.00 and 4.01 eV with the CC2 method. However, the CC2 calculated 1nπ oscillator strength is only 6% of that of the 1ππ* transition. The 1ππ* vibronic spectrum is very complex, showing about 40 bands within the lowest 500 cm−1. The methyl torsion and the low-frequency out-of-plane |$\nu ^{\prime }_1$|ν1 and |$\nu ^{\prime }_2$|ν2 vibrations are strongly coupled in the 1ππ* state. This gives rise to many torsion-vibration combination bands built on out-of-plane fundamentals, which are without precedence in the 1ππ* spectrum of 9H-2-aminopurine [S. Lobsiger, R. K. Sinha, M. Trachsel, and S. Leutwyler, J. Chem. Phys.134, 114307 (2011)]. From the Lorentzian broadening needed to fit the |$0_0^0$|000 contour of 9M-2AP, the 1ππ* lifetime is τ ⩾ 120 ps, reflecting a rapid nonradiative transition.

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