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

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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.

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