Local versus hyperspherical modes of water and formaldehyde : effect of molecular complexity on mode-selective structures and dynamics

Effects of molecular complexity on mode-selective phenomena are studied for models of water and formaldehyde. Here, complexity is measured by the numbers of vibrational degrees of freedom which interact in the model systems, including both OH stretches and the bending motion for H2O and both the CH and the CO stretches for CH2O. Neglect (i.e., decoupling or ‘‘freezing’’) of the bending vibration in H2O, or the CO stretch in CH2O, yields simpler model systems which serve as references for the more complex original ones. The mode-selective phenomena that are compared for these systems include structural and dynamical effects of highly excited local and hyperspherical modes. The methods employed include expansions of vibrational states in terms of simple, i.e., Morse or harmonic-oscillaton basis functions for the individual stretches and bends, as well as fast-Fourier-transform propagations of the representative wave packets: The validity of these techniques is discussed in detail, depending on the properties of the selective states considered. The most important result is that increasing molecular complexity does not necessarily destroy all mode selectivity. However, the conservation of mode selectivity depends on the system, and on the property considered. Thus, for H2O, the structures of local modes are conserved, whereas very highly excited hyperspherical ones are modified when the bend is switched on. In contrast, for CH2O both local and hyperspherical structures are conserved, and the ratio of rates for fast local mode vs slow hyperspherical mode decay remains very large (≫100:1) when the CO stretch is coupled to the CH2 fragment. In addition, the lifetimes of local modes decrease as the complexity of the model system increases from CH2 to CH2O, indicating inverse intramolecular relaxation of vibrational energy. Extrapolation of these results suggests that mode selectivity may extend from small to larger systems.