The infrared spectrum of NO2 has been reinvestigated in the 1.4-3.4µ region under high dispersion. The nine bands observed show resolvable rotational structure and the moments of inertia obtained, in conjunction with the large rotational constant from measurements of ν2 by Wilson and Brown, yield the ground state parameters rN-o=1.1884±0.004A and ∢O—N—O=134° 4″±15″. The set of overtones and combinations observed together with ν2= 750.9 cm-1 from Wilson and Brown allow one to determine a complete set of quadratic vibrational interaction constants. The fundamental frequencies of NO2 are ν1= 1322.5 cm-1, ν2= 750.9 cm-1, and ν3= 1616.0 cm-1. An intensity alternation is observed in type A bands as is predicted by theory but which, as far as the authors are aware, has not previously been observed in a similar case. An apparently abnormal interaction, which is not adequately explained, occurs between the vibrational levels 3ν3 and 2ν1+3ν2 and likewise between ν1+3ν3 and 3ν1+3ν2.
GORDON E. MOORE, "The Spectrum of Nitrogen Dioxide in the 1.4-3.4ų, Region and the Vibrational and Rotational Constants of the NO2 Molecule," J. Opt. Soc. Am. 43, 1045-1050 (1953)