IR–UV double resonance spectra of pyrazine dimers: Competition between CH⋯π, π⋯π and CH⋯N interactions

We present size- and isomer-selective IR–UV double resonance spectra of two pyrazine dimer isomers. The most stable isomer has a planar structure, stabilized by two CH ��� N contacts. The other isomer has a stacked, cross-displaced structure. Our assignment is supported by B3LYP-D calculations. RI-MP2 calculations tend to overestimate the stability of the stacked and T-shaped isomers.