Evidence of vibrational-induced rotational axis switching for (HDCO)-C-12-O-16: New high-resolution analysis of the v(5) and v(6) bands and first analysis of the v(4) band (10-mu m region)

Using new high-resolution Fourier transform spectra recorded in Giessen in the 8-12 mu m region, a more extended analysis of the v(5) and v(6) bands a nd the first high-resolution study of the v(4) band of HDCO were performed. As pointed out previously [M. Allegrini. J. W, C. Johns, and A. ii. W. McK ellar, Can. J. Phys. 56, 859-864 (1978)], the energy levels of the 5(1) and 6(1) states are strongly coupled by A- and B-type Coriolis interactions. O n the other hand, it appeared that weaker resonances involving the energy l evels of the 4(1) state with those of the 5(1) and 6(1) states also had to be accounted for. Consequently, the calculation of the energy levels was pe rformed taking into account the Coriolis-type resonances linking the energy levels of the {6(1), 5(1), 4(1)} resonating states. Because of the unusual ly strong Coriolis interaction between v(5) and v(6), a nonclassical behavi or of the rotational levels of the 5(1) and 6(1) states was observed and it was necessary to use a new Hamiltonian matrix which possesses, as usual, b oth A- and B-type Coriolis operators in the 5(1) <-> 6(1) and 6(1) <-> 4(1) off diagonal blocks but differs from the classical reduced Hamiltonian whi ch is used commonly for planar C-s-type molecules. More precisely, it prove d necessary to include non-orthorhombic terms in the expansion of the rotat ional Hamiltonian of the 5(1) and 6(1) states. According to the considerati ons developed by Watson [J. K. G. Watson, in "Vibrational Spectra and Struc ture," (J. Durig, Ed.), Chap. 1, Elsevier, Amsterdam, 1977], these non-orth orhombic operators which are not symmetry forbidden are usually removed for semirigid C-s-type molecules by rotational contact transformations. In the present study, the occurrence of terms in {J(x), J(z)} in the expansions o f the rotational Hamiltonians for the 51 and 6(1) states indicates that the inertial system of HDCO differs fur each of the three {6(1), 5(1), 4(1)} r esonating stares. Therefore, HDCO becomes a good example of vibrational-ind uced rotational axis switching (VIRAS) which was already suggested as the m echanism responsible for the enhanced densities of coupled states observed in 2-fluoroethanol [H. Li, S. Erza, and L. A. Philips, J. Chem. Phys. 97, 5 956-5963 (1992)]. (C) 2000 Academic Press.