The intermolecular vibrations of the NO dimer

The far infrared spectrum of the NO dimer in the gas phase has been observed for the first time, using a low temperature (100 K) long-path (180 m) absorption cell. The spectroscopic results are used, together with mid-infrared observations and high level CCSD(T) ab initio calculations, to locate the four low frequency intermolecular vibrational modes of (NO)(2): nu(2) (symmetric bend)=239.361 cm(-1) nu(3) (intermolecular stretch)=134.503 cm(-1), nu(4) (out-of-plane torsion)approximate to 117 cm(-1) and nu(6) (antisymmetric bend)=429.140 cm(-1). These values agree poorly with previous determinations based on condensed-phase spectra, indicating that there are large shifts in going from the isolated gas-phase dimer to the liquid, solid, or matrix-isolated environments. (C) 1998 American Institute of Physics.