The Spectrum of Nitrogen Dioxide in the 1.4-3.4μ Region and the Vibrational and Rotational Constants of the NO_2 Molecule

The infrared spectrum of NO_2 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 ν_2 by Wilson and Brown, yield the ground state parameters r_(N–O) = 1.188±0.004A and ∢O—N—O = 134° 4′±15′. The set of overtones and combinations observed together with ν_2 = 750.9 cm^(−1) from Wilson and Brown allow one to determine a complete set of quadratic vibrational interaction constants. The fundamental frequencies of NO_2 are ν_1 = 1322.5 cm^(−1), ν_2 = 750.9 cm^(−1), and ν_3 = 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 3_(ν_3) and 2_(ν_1) + 3_(ν_2) and likewise between ν_1 + 3_(ν_3) and 3_(ν_1) + 3_(ν_2)