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Title: Low-lying excited states and nonradiative processes of 9-methyl-2-aminopurine

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{sup −1} resolution in a supersonic jet. The electronic origin at 32 252 cm{sup −1} exhibits methyl torsional subbands that originate from the 0A{sub 1}{sup ′′} (l = 0) and 1E{sup ″} (l = ±1) torsional levels. These and further torsional bands that appear up to 0{sub 0}{sup 0}+230 cm{sup −1} allow to fit the threefold (V{sub 3}) barriers of the torsional potentials as |V{sub 3}{sup ′′}|=50 cm{sup −1} in the S{sub 0} and |V{sub 3}{sup ′}|=126 cm{sup −1} in the S{sub 1} 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{sub 3}{sup ′′}=20 cm{sup −1} and V{sub 3}{sup ′}=115 cm{sup −1}. The 0{sub 0}{sup 0} rotational band contour is 75% in-plane (a/b) polarized, characteristic for a dominantly long-axis {sup 1}ππ{sup *} excitation. The residual 25% c-axis polarization may indicate coupling of the {sup 1}ππ{sup *} to the close-lying {sup 1}nπ{sup *} state, calculated at 4.00 and 4.01 eV with the CC2 method.more » However, the CC2 calculated {sup 1}nπ oscillator strength is only 6% of that of the {sup 1}ππ{sup *} transition. The {sup 1}ππ{sup *} vibronic spectrum is very complex, showing about 40 bands within the lowest 500 cm{sup −1}. The methyl torsion and the low-frequency out-of-plane ν{sub 1}{sup ′} and ν{sub 2}{sup ′} vibrations are strongly coupled in the {sup 1}ππ{sup *} state. This gives rise to many torsion-vibration combination bands built on out-of-plane fundamentals, which are without precedence in the {sup 1}ππ{sup *} 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{sub 0}{sup 0} contour of 9M-2AP, the {sup 1}ππ{sup *} lifetime is τ ⩾ 120 ps, reflecting a rapid nonradiative transition.« less
Authors:
; ; ;  [1]
  1. Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern (Switzerland)
Publication Date:
OSTI Identifier:
22255182
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 140; Journal Issue: 4; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ADENINES; COUPLING; DENSITY FUNCTIONAL METHOD; PHOTOIONIZATION; POLARIZATION; ROTATIONAL STATES; SPECTROSCOPY; TORSION; ULTRAVIOLET SPECTRA