THE NO DIMER - N-15 ISOTOPIC INFRARED-SPECTRA, LINE-WIDTHS, AND FORCE-FIELD

The nu(1) fundamental bands of ((NO)-N-15-O-16)(2) and (NO)-N-14-O-16-(NO)-N-15-O-16 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 nu(1) = 1 vibrational states. The predissociation-limited nu(1) band line width for ((NO)-N-14-O-16)(2) was measured to be 0.0056(3)cm(-1), which is slightly less than previous determinations. The ((NO)-N-15-O-16)(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, (NO)-N-14-O-16-(NO)-N-15-O-16. 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) Angstrom, r(NN) = 2.2630(12) Angstrom, and theta(NNO) = 97.17(5)degrees. 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 nu(6) and interchanging the two lowest frequencies, nu(3) and nu(4).