Infrared diode-laser spectra of the ν9 and ν11 N–O stretching bands of N2O4

The rotationally resolved jet-cooled infrared spectra of the b-type ν9 (b2u) fundamental band at 1757 cm−1 and the a-type ν11 (b3u) fundamental band at 1261 cm−1 of the N–O stretches of N2O4 have been recorded with a diode laser. The ν9 band was found to be unperturbed, and it was possible to assign nearly 100% of the observed lines with a signal to noise greater than 2. In contrast, most of the Ka states of the ν11 band were found to be strongly perturbed. A large number of strong lines (≊20%) are unassigned and presumably arise from the perturbing state as well as torsional hot band transitions. The rotational analysis yields precise spectroscopic constants for the ground vibrational state which are interpreted in terms of a planar centrosymmetric dimer with a N–N bond length of 1.756(10) Å. The observed nuclear-spin statistical weights and near-zero inertial defect are consistent with the planar centrosymmetric structure determined in earlier electron-diffraction studies.