The unimolecular dissociation of HCO. V. Mixings between resonance states

Four-mode quantum calculations of resonance states in complex-forming bimolecular reactions: Cl−+CH3Br

The vibrational resonance states of the complexes formed in the nucleophilic bimolecular substitution (S(N)2) reaction Cl(-)+CH(3)Br-->ClCH(3)+Br(-) were calculated by means of the filter diagonalization method employing a coupled-cluster potential-energy surface and a Hamiltonian that incorporates an optical potential and is formulated in Radau coordinates for the carbon-halogen stretching modes. The four-dimensional model also includes the totally symmetric vibrations of the methyl group (C-H stretch and umbrella bend). The vast majority of bound states and many resonance states up to the first overtone of the symmetric stretching vibration in the exit channel complex have been calculated, analyzed, and assigned four quantum numbers. The resonances are classified into entrance channel, exit channel, and delocalized states. The resonance widths fluctuate over six orders of magnitude. In addition to a majority of Feshbach-type resonances there are also exceedingly long-lived shape resonances, which are associated with the entrance channel and can only decay by tunneling. The state-selective decay of the resonances was studied in detail. The linewidths of the resonances, and thus the coupling to the energetic continuum, increase with excitation in any mode. Due to the strong mixing of the many progressions in the intermolecular stretching modes of the intermediate complexes, this increase as a function of the corresponding quantum numbers is not monotonic, but exhibits pronounced fluctuations.

Rotational state-dependent mixings between resonance states of vibrationally highly excited DCO (X̃ 2A′)

Rotational state-dependent mixings between highly excited resonance states of DCO (X (2)A(')) were investigated by stimulated emission pumping spectroscopy via a series of intermediate rotational levels in the B (2)A(') electronic state of the radical. Two examples for such interactions, between pairs of accidentally nearly degenerate vibrational states at energies of E(v) approximately 6450 and E(v) approximately 10 060 cm(-1), respectively, were analyzed in detail. Deperturbations of the measured spectra provided the zeroth-order vibration-rotation term energies, widths, and rotational constants of the states and the absolute values of the vibrational coupling matrix elements. The coupled states turned out to have very different A rotational constants so that their mixings switch on or off as they are tuned relative to each other as function of the K(a) rotational quantum number. The respective zeroth-order states could be assigned to different interlaced vibrational polyads. Thus, when two states belonging to different polyads are accidentally nearly isoenergetic, even very weak interpolyad interactions may start to play important roles. The derived interpolyad coupling elements are small compared to the typical intrapolyad coupling terms so that their influences on the vibrational term energies are small. However, large effects on the widths (i.e., decay rates) of the states can be observed even from weak coupling terms when a narrow, long-lived state is perturbed by a broad, highly dissociative state. This influence contributes to the previously observed strong state-to-state fluctuations of the unimolecular decay rates of the DCO radical as function of vibrational excitation. Similar mechanisms are likely to promote the transition to "statistical" rates in many larger molecules.