HIGH-RESOLUTION INFRARED SPECTROSCOPY AND ONE-DIMENSIONAL LARGE AMPLITUDE MOTION IN ASYMMETRIC TOPS: HNO₃ AND H₂O₂

Large amplitude motions in molecules have been the subject of a large number of studies. The purpose of this paper is not to describe these motions extensively but rather to concentrate on the comparison of the effects of a one-dimensional large amplitude motion on two molecules, namely H₂O₂ and HNO₃. For H₂O₂ the motion is the torsion of the two equal halves O–H rotors around the O–O bond. During this motion the molecule passes through two barriers corresponding to the trans- and cis-planar configurations. As a consequence, one observes both a splitting (trans-barrier) and a staggering (cis-barrier) of the levels and it is necessary to use a double extension of the permutation-inversion group of the molecule to establish the symmetry properties and selection rules. For HNO₃, the motion is the torsion of the O–H bond relative to the much heavier , radical, and the molecule passes from one planar equilibrium configuration to an equivalent one. This torsion induces a splitting of the levels and the use of the permutation-inversion group of the molecule is required to understand its spectroscopic properties. This paper describes in detail the effects of the large amplitude motion both in H₂O₂ and HNO₃, discussing also the influence of the numerous vibration-torsion resonances affecting the excited states.