Pairing of Isolated Nucleobases: Double Resonance Laser Spectroscopy of Adenine–Thymine

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Abstract
The vibronic spectrum of the adenine–thymine (A–T) base pair was obtained by one‐color resonant two‐photon ionization (R2PI) spectroscopy in a free jet of thermally evaporated A and T under conditions favorable for formation of small clusters. The onset of the spectrum at 35 064 cm−1 exhibits a large red shift relative to the π–π* origin of 9H‐adenine at 36 105 cm−1. The IR–UV spectrum was assigned to cluster structures with HNH⋅⋅⋅OC/N⋅⋅⋅HN hydrogen bonding by comparison with the IR spectra of A and T monomers and with ab initio calculated vibrational spectra of the most stable A–T isomers. The Watson–Crick A–T base pair is not the most stable base‐pair structure at different levels of ab initio theory, and its vibrational spectrum is not in agreement with the observed experimental spectrum. Experiments with methylated A and T were performed to further support the structural assignment.