The Far Infrared Spectrum of the NO Dimer

Area of Science:

• Molecular Spectroscopy
• Gas-Phase Chemistry
• Intermolecular Forces

Background:

• The structure and vibrational frequencies of weakly bound molecular complexes like the NO dimer are crucial for understanding intermolecular interactions.
• Previous studies of the NO dimer relied on condensed phase spectra, leading to uncertainties in its fundamental vibrational frequencies.

Purpose of the Study:

• To experimentally determine the far-infrared spectrum of the NO dimer in the gas phase.
• To assign and analyze the intermolecular vibrational bands of the cis-planar ON-NO complex.
• To resolve discrepancies in previously reported intermolecular vibrational frequencies.

Main Methods:

• Utilized a long-path (180 m) cooled (99 K) absorption cell.
• Employed a Bomem Fourier transform spectrometer for high-resolution spectral acquisition in the 120-450 cm-1 region.
• Analyzed four newly detected vibration-rotation bands attributed to intermolecular vibrations.

Main Results:

• Observed and assigned four weak vibration-rotation bands corresponding to intermolecular vibrations of the cis-planar NO dimer.
• Identified the nu2 (symmetric bending) fundamental at 239.36 cm-1 and the nu3 (intermolecular stretch) fundamental at 134.50 cm-1.
• Reported detailed rotational and centrifugal distortion parameters for the analyzed bands, including the nu2 + nu3 combination band and the nu6 (antisymmetric bending) fundamental.

Conclusions:

• The gas-phase far-infrared spectrum provides the first definitive measurement of the NO dimer's intermolecular vibrational frequencies.
• The determined frequencies differ significantly from those previously inferred from condensed phase spectra.
• This work resolves long-standing questions regarding the fundamental vibrational modes of the NO dimer.