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

Abstract

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.

Structures and Anharmonic Analyses of the O–H Stretching Vibrations of Jet-Cooled Benzoic Acid (BA), (BA)(H2O)n, and (BA)2(H2O)n (n = 1, 2) Clusters, and Their Ring-Deuterated Isotopologues Measured with IR-VUV Spectroscopy─Unraveling the Complex Anharmonic Couplings in the Cyclic Structures

“…Furthermore, the relative IR intensity in the 2800–3100 cm –1 region is greater than that reported in the IR-UV DR spectrum. These additional bands indicate the coexistence of several isomers of (BA) 2 (H 2 O) because the VUV laser light can ionize several isomers simultaneously . By comparing the calculated anharmonic spectra, the bands at 3700 and 3508 cm –1 may be assigned to the free O–H stretch band and the H-bonded O–H stretching band of water in the “Side type I” isomer.…”

“…The experimental scheme and setup, reported in our previous papers, ,− are shown in the Supporting Information (Figure S1 and Scheme 1). Briefly, jet-cooled benzoic acid (BA) and its hydrated clusters were generated by expanding a gas mixture of BA/H 2 O/He or BA- d 5 /H 2 O/He at a total pressure of 4 atm into the vacuum chamber through a pulsed Even-Lavie valve with a repetition rate of 10 Hz.…”

“…Although we are unable to selectively monitor different isomers with this method, it is possible to determine their structures by analyzing the observed IR spectra with the aid of quantum-chemical calculations. In addition, the direct VUV ionization can ionize different isomers of which electronic spectra have not been reported . Due to the nature of the carboxylic group, all of the (BA) m (H 2 O) n ( m = 1, 2; n = 1, 2) clusters are thought to form cyclic structures, and anharmonic coupling similar to that of (BA) 2 may occur.…”