Gas-phase vibrations of the anionic, hydrogen-bonded dimer of 9-methylguanine

The gas-phase InfraRed Multiple Photon Dissociation (IRMPD) spectra in the fingerprint region of the anion (m/z 163) and mono-deprotonated homodimer of 9-methyl-guanine (2, m/z 329) and its d₅ analogue (2-d₅) are reported here. The anionic dimer is reverse Watson-Crick paired with the hydrogen bonding pattern ADD/DAA (where A stands for acceptor and D stands for donor), in which the site of negative charge is unambiguously found to be N₁. The match between experimental vibrational spectra and DFT-computed spectra is good with the exception of the region between 2000 and 2900 cm^-1 that contains symmetrical and antisymmetrical N-H stretches of the adjacent hydrogen-bond donor-donor pair.