Vibrational analysis of HOCl up to 98% of the dissociation energy with a Fermi resonance Hamiltonian

We have analyzed the vibrational energies and wave functions of HOCl obtain ed from previous ab initio calculations [J. Chem. Phys. 109, 2662 (1998); 1 09, 10273 (1998)]. Up to approximately 13 000 cm(-1), the normal modes are nearly decoupled, so that the analysis is straightforward with a Dunham mod el. In contrast, above 13 000 cm(-1) the Dunham model is no longer valid fo r the levels with no quanta in the OH stretch (upsilon(1) = 0). In addition to upsilon(1), these levels can only be assigned a so-called polyad quantu m number P = 2 upsilon(2) + upsilon(3), where 2 and 3 denote, respectively, the bending and OCl stretching normal modes. In contrast, the levels with v(1)greater than or equal to 2 remain assignable with three v(i) quantum nu mbers up to the dissociation (D-0 = 19 290 cm(-1)). The interaction between the bending and the OCl stretch (omega(2)congruent to 2 omega(3)) is well described with a simple, fitted Fermi resonance Hamiltonian. The energies a nd wave functions of this model Hamiltonian are compared with those obtaine d from ab initio calculations, which in turn enables the assignment of many additional ab initio vibrational levels. Globally, among the 809 bound lev els calculated below dissociation, 790 have been assigned, the lowest unass igned level, No. 736, being located at 18 885 cm(-1) above the (0,0,0) grou nd level, that is, at about 98% of D-0. In addition, 84 "resonances" locate d above D-0 have also been assigned. Our best Fermi resonance Hamiltonian h as 29 parameters fitted with 725 ab initio levels, the rms deviation being of 5.3 cm(-1). This set of 725 fitted levels includes the full set of level s up to No. 702 at 18 650 cm(-1). The ab initio levels, which are assigned but not included in the fit, are reasonably predicted by the model Hamilton ian, but with a typical error of the order of 20 cm(-1). The classical anal ysis of the periodic orbits of this Hamiltonian shows that two bifurcations occur at 13 135 and 14 059 cm(-1) for levels with upsilon(1) = 0. Above ea ch of these bifurcations two new families of periodic orbits are created. T he quantum counterpart of periodic orbits are wave functions with "pearls" aligned along the classical periodic orbits. The complicated sequence of ab initio wave functions observed within each polyad is nicely reproduced by the wave functions of the Fermi resonance Hamiltonian and by the correspond ing shapes of periodic orbits. We also present a comparison between calcula ted and measured energies and rotational constants for 25 levels, leading t o a secure vibrational assignment for these levels. The largest difference between experimental and calculated energies reaches 22 cm(-1) close to D-0 . (C) 1999 American Institute of Physics. [S0021-9606(99)01034-X].