Intermolecular vibrational states far above the van der Waals minimum: Combination bands of the polar N2O dimer

[Display omitted] •Two combination bands of the polar isomer of N2O dimer were observed.•We probe vibrational levels quite far above the ground state of the dimer.•The observed intermolecular frequencies are in good agreement with theoretical predictions.•The observed spectrum puts the geared bend frequency above the out-of-plane torsional frequency. Infrared combination bands of the polar isomer of the N2O dimer are observed for the first time, using a tunable infrared laser source to probe a pulsed slit-jet supersonic expansion in the N2O ν1 region (≈2240 cm−1). One band involves the torsional (out-of-plane) intermolecular mode and yields a torsional frequency of 19.83 cm−1 if associated with the out-of-phase fundamental (N2O ν1) vibration of the N2O monomers in the dimer. The other band, which is highly perturbed, yields an intermolecular in-plane geared bend frequency of 22.74 cm−1. The results are compared with high level ab initio calculations. The less likely alternate assignment to the in-phase fundamental would give torsional and geared bend frequencies of 17.25 and 20.16 cm−1, respectively.