Structure and hydrogen bond vibrations of the jet-cooled 1:1 complex between 7-azaindole and formamide: A laser-induced fluorescence spectroscopy study

Laser-induced fluorescence spectra of 7-azaindole ⋯ formamide complex are measured in a supersonic free jet expansion. Calculation at MP2/6-311++G∗∗ level predicts a cyclic doubly hydrogen-bonded structure for the complex is favoured most in the ground state. The complex emits only UV fluorescence from the locally excited state and tautomerization is inhibited under the jet-cooled environment. This photophysical behavior is consistent with the predictions of CIS/6-311++G∗∗ and TDDFT/6-311++G∗∗ calculations. The low-frequency bands in the fluorescence spectra are assigned to different hydrogen bond modes of the complex, and the spectra reveal signatures of mixing between inter- and intra-molecular vibrational modes in the excited state.