Structures of (Pyrazine) 2 and (Pyrazine)(Benzene) Dimers Investigated with Infrared-Vacuum Ultraviolet Spectroscopy and Quantum-Chemical Calculations: Competition among π-π, CH···π, and CH···N Interactions.

The structures of a pyrazine dimer (pyrazine) 2 and (pyrazine)(benzene) hetero-dimer cooled in a supersonic beam were investigated by the measurement of the infrared spectra in the C-H stretching region with infrared-vacuum ultraviolet (IR-VUV) spectroscopy and quantum-chemical calculations. The stabilization energy calculation at the CCSD(T)/aug-cc-pVTZ level of theory predicted three isomers for (pyrazine) 2 and three for (pyrazine)(benzene) with energy within 6 kJ/mol. Among them, the cross-displaced π-π stacked structure is the most stable in both dimers. In the observed IR spectra, both dimers exhibited two intense bands near 3065 cm -1 , with intervals of 8 cm -1 in (pyrazine) 2 and 11 cm -1 in (pyrazine)(benzene), while only one band appeared in the monomer. For (pyrazine)(benzene), we also measured the IR spectrum of (pyrazine)(benzene- d 6 ), where the interval of the two bands was unchanged. The analysis of the observed IR spectra with anharmonic calculations suggested the coexistence of three isomers of (pyrazine) 2 and (pyrazine)(benzene) in a supersonic jet. For (pyrazine) 2 , the two isomers which were previously assigned to the H-bonded planar and the π-π stacked structures respectively were reassigned to the cross-displaced π-π stacked and T-shaped structures, respectively. In addition, the quantum chemical calculation and IR-VUV spectral measurement suggested the coexistence of the H-bonded planar isomer in the jet. For (pyrazine)(benzene), the IR spectrum of the (pyrazine) site showed a similar spectral pattern to that of (pyrazine) 2 , especially the split at ∼3065 cm -1 . However, the anharmonic analysis suggested that they are assigned to the different vibrational motions of (pyrazine). The anharmonic vibrational analysis is essential to associate the observed IR spectra with the correct structures of the dimer.