Isomer- and species-selective infrared spectroscopy of jet-cooled 7H- and 9H-2-aminopurine and 2-aminopurine·H ₂O clusters

The infrared (IR) spectra of the supersonic-jet cooled 9H- and 7H-tautomers of 2-aminopurine (2AP) and of the 9H-2-aminopurine•H ₂O monohydrate clusters have been measured by mass- and species-selective IR-UV double resonance spectroscopy in the 3200-3900 cm ^-1 region, covering the N-H and O-H stretching vibrations. The spectra are complemented by density functional (B3LYP and PW91) and by second-order Møller-Plesset (MP2) calculations of the electronic energies and vibrational frequenciesof the respective 2AP tautomers and clusters. The 9H- and 7H-2-aminopurine tautomers were definitively identified by the shifts of their NH and NH ₂ symmetric and asymmetric stretching frequencies and by comparison to the B3LYP/TZVP calculated IR spectra. The H-bond topologies of the two previously observed 9H-2-aminopurine•H ₂O isomers (Sinha. R. K.; et al. J. Phys. Chem. A2011, 115, 6208) are definitively identified as the "sugar-edge" isomer A and the "trans-amino-bound" isomer B by comparing their IR spectra to the calculated frequencies and IR intensities of the cluster isomers A, B, C, and D, as well as to the IR spectrum of 9H-2AP. The sugar-edge isomer A involves N9-H•••OH ₂ and HOH•••N3 hydrogen bonds and is predicted to be the most stable form. The amino-bound isomer B involves NH ₂•••OH ₂ and HOH•••N1 hydrogen bonds and is calculated to lie 2.5 kJ/mol above isomer A. The H-bond topology of the "cis-amino- bound" isomer C is symmetrically related to isomer B, with a hydrogen bond to the N3 of the pyrimidine group. However, it is calculated to lie 7 kJ/mol above isomer A and indeed is not observed in the supersonic jet. Isomer D involves a single H-bond to the N7 position, is predicted to be 14 kJ/mol above A and is therefore not observed.