The pore rotational absorption spectrnk of nit& acid vapour has been observed in the &ion S-40 cm-' at a ' . 'Ie.wlution of 0.05 cm-l , using the technique of Fourier transform spdrometry. The structure of the spectrum is of considerabIe kterest, since it corresponds ciosely to the normally forbidden perpendicular pure rotation band of a symmetric top. In the Iimit of a planar ablate near symmetric rotor, the band simpfy shows a series of equispaced sharp absorption likes.
The nitric acid molecule is one exampfe of the spectroscopically interesting class of mobcules which are planar asymmetric rotors. The ground state rotational constants of JJNO, have been measured [f ,2] by microwave spectroscopy, and are given by Milten and Morton as A, = 0.4340 cm-l, B, = 0.4036 cm-l and C, = 0.2088 cm-I. This mokcule, therefore, approximates to an accidental symmetric top, at the oblate limit, with a value for the Wang oblate asymmetry parameter bg = -0.0724.
Since, for any planar body, the moment of inertia perpendicular to the plane is equal to the sum of the two moments of inertia about the axes which lie in the plane, it is possible to indentify two distinct types of pIanar, accidental symmetric tops. If we choose the molecular z-axis to be perpkndicular to the plane, then one possible accidental equality in the moments of inertia is Kj or fy" = 1:. Since the relation I; = 1: + 1; must be stisfied, this implies that one of the moments of inertia in the plane must be very small. zhus, this limit corresponds to a near prolate rotor, with the mblectiar a-axis lying in the plane. Examples of this are to be found among those molecules derived from linear molecules by the addition of a light offaxis atom. Usually, the main component of the dipole moment lies along the a-axis. The pure rotational siectra of several molecules in this class have been. recorded [3,4]; the Q-branches are well,separ&ed, but thi: P-+d R-branches may not always be resolved.