The NO dimer: ¹⁵N isotopic infrared spectra, line-widths, and force field

Abstract

The v ₁ fundamental bands of (¹⁵N¹⁶O)₂ and ¹⁴N¹⁶O-¹⁵N¹⁶O isotopes of the NO dimer have been studied using a long-path (200 m) low temperature (84 K) gas cell and Fourier transform infrared spectrometer. The spectra, obtained with a resolution of 0·005 cm^-1, were analysed to give accurate rotational and quartic centrifugal distortion parameters for the ground and the v ₁ = 1 vibrational states. The predissociation-limited v ₁ band line width for (¹⁴N¹⁶O)₂ was measured to be 0·0056(3) cm^-1, which is slightly less than previous determinations. The (¹⁵N¹⁶O)₂ width was found to be 0·0044(2) cm^-1, and the significant isotope dependence will help to constrain models of the predissociation process. A much larger width of 0·014(2) cm^-1 was measured for the mixed isotope, ¹⁴N¹⁶O-¹⁵N¹⁶O. This, and the mixed isotope vibrational frequency, may be understood on the basis of a simple coupled-oscillator model. The present results were used to derive a refined structure for the NO dimer: r(NO) = 1·1515(3) Å, r(NN) = 2·2630(12) Å, and ϑ(NNO) = 97·17(5)°. Considerations of the centrifugal distortion constants and harmonic force field for (NO)₂ indicate that it may be necessary to revise the previous assignment of some of the low frequency intermolecular modes, raising that of v ₆ and interchanging the two lowest frequencies, v ₃ and v ₄.