Jet-Cooled 2-Aminopyridine Dimer: Conformers and Infrared Vibrational Spectra

The 2-aminopyridine dimer, (2AP)(2), is linked by two N-H center dot center dot center dot N hydrogen bonds, providing a model for the Watson-Crick configurations of the adenine or cytosine self-dimers. Structure optimization of (2AP)(2) at the MP2 level with the aug-cc-pVQZ basis set establishes the existence of two nearly degenerate conformers with C-i and C-2 symmetry. Adding complete basis set extrapolation and Delta CCSD(T) corrections gives binding energies D-e = 10.70 and 10.72 kcal/mol, respectively. Both isomers are chiral, each giving rise to a pair of enantiomers. The potential energy surface of (2AP)(2) is calculated along the 2AP amino flip coordinates, revealing a 4-fold minimum low-energy region with a planar C-2h symmetric and four asymmetric transition structures. The mass-selective resonant two-photon ionization (R2PI) spectra of supersonically cooled (2AP)(2) were remeasured. Three different species (A-C) were separated and characterized by UV/UV depletion spectroscopy and by infrared (IR) depletion spectroscopy in the 2600-3800 cm(-1) range. The R2PI and IR spectra of species A and B are very similar, in agreement with the prediction of two conformers of (2AP)(2). The IR bands are assigned to the H-bonded N-H-b stretch, the N-H-2 bend overtone, and the free N-H-f stretch of (2AP)(2), based on the calculated IR spectra, thereby extending and correcting previous assignments. Conformer A is tentatively assigned as the C-2 conformer. The UV spectrum of species C is very different from those of A and B, its IR spectrum exhibiting additional O-H stretching bands. C is assigned to the (2AP)(2)center dot H2O Cluster, based on the agreement of its IR spectrum with calculated IR spectra. Complete dissociation into the (2AP)(2)(+) ion Occurs upon ionization.