A Combined Experimental and Computational Investigation of Hydrogen-Bonded 2,7-Diazaindole-(Water)1,2 Complexes Isolated in the Gas Phase

We have presented a detailed experimental and computational analysis on the 1:1 and 1:2 complexes of 2,7-diazaindole (27DAI) with water in the gas phase. The complexes were characterized using two-color-resonant two-photon ionization (R2PI), laser induced fluorescence (LIF), single vibronic level fluorescence (SVLF), and photoionization efficiency spectroscopic methods. The 0_0^0 band of the S1←S0 electronic transition of 27DAI-H2O complex was observed at 33074 cm-1, largely red shifted by 836 cm-1 compared to that of the bare 27DAI. From the R2PI spectrum, the detected modes at 141 (v’Tx), 169 (v’Ty) and 194 (v’Ry) cm-1 were identified as the internal motions of H2O molecule in the complex. However, these modes were detected at 115 (v’’Tx), 152 (v’’Ty) and 190 (v’’Ry) cm-1 in the ground state which suggested a stronger hydrogen bonding interaction in the photo-excited state. The structural determination was aided by the detection of νNH and νOH values in the ground and excited state complex using the FDIR and IDIR spectroscopies. The detection of νNH at 3414 and νOH at 3447 cm-1 in 27DAI-H2O have shown an excellent correlation with the most stable structure consisting of N(1)-H‧‧‧O and OH‧‧‧N(7) hydrogen bonded bridging water molecule in the ground state. The structure of the complex in the electronic excited state (S1) were confirmed by corresponding bands at 3210 (νNH) and 3265 cm-1 (νOH). The IR-UV hole burning spectroscopy confirmed the presence of only one isomer in the molecular beam. The ionization energy (IE) of the 27DAI-H2O complex was obtained as 8.7890.001 eV, which was significantly higher than the 7AI-H2O complex. The 1:2 complex 27DAI-(H2O)2 was identified by a strong transition at 32565 cm-1, which was red shifted by 1345 cm-1. The corresponding FDIR spectrum resulted three bands at 3207, 3261 and 3385 cm-1, which are assigned as the hydrogen bonded νNH of 27DAI and two νOH vibrations from solvent-bridge connecting N(1)H and N(7) groups. The obtained structures of 27DAI-H2O and 27DAI-(H2O)2 have explicitly shown the formation of cyclic one- and two-solvent bridges incorporating N(1)-H‧‧‧O and O-H‧‧‧N(7) hydrogen bonds upon micro solvation. The lower excitation and higher ionization energies of the 27DAI-H2O complex compared to 7AI-H2O established higher stabilization of N-rich molecules. The solvent clusters forming linear bridge between the hydrogen/proton acceptor and donor sites in the complex can be considered a stepping stone to investigate the photoinduced tautomerization of N-bearing biologically relevant molecules.