Unimolecular dissociation resonances

Figure A3.12.9. Comparison of the unimolecular dissociation rates for HO2—>H+02 as obtained from the quantum mechanical resonances open circles) and from variational transition state RRKM step...

Tobiason J D, Dunlap J R and Rohifing E A 1995 The unimolecular dissociation of HCO a spectroscopic study of resonance energies and widths J. Cham. Phys. 103 1448-69... 

Stock C, Li X, Keller H-M, Schinke R and Temps F 1997 Unimolecular dissociation dynamics of highly vibrationally excited DCO x-A t- I- Investigation of dissociative resonance states by stimulated emission pumping spectroscopy J. Cham. Phys. 106 5333-58... 

Dobbyn A J, Stumpf M, Keller H-M and Schinke R 1996 Theoretical study of the unimolecular dissociation HO2—>H+02. II. Calculation of resonant states, dissociation rates, and O2 product state distributions J. Chem. Phys. 104 8357-81... 

In the present review, a new variation on an existing experimental method will be used to show how accurate unimolecular dissociation rate constants can be derived for thermal systems. For example, thermal bimolecular reactions are amenable to study by use of several, now well-known, techniques such as (Fourier transform) ion cyclotron resonance spectrometry (FTICR), flowing afterglow (FA), and high-pressure mass spectrometry (HPMS). In systems where a bimolecular reaction leads to products other than a simple association adduct, the bimolecular reaction can always be thought of as containing a unimolecular... 

More recently, Audier and McMahon have shown that the unimolecular dissociation spectrum of transient ions can be directly obtained from a simple manipulation of a series of FTICR spectra. The data arising from this approach very closely resemble those obtained from metastable dissociations in conventional sector spectrometers (MIKES), and it has been consequently dubbed metastable ion cyclotron resonance (MICR) spectrometry. Very briefly, the method functions as follows ... 

Figure 16. Metastable ion cyclotron resonance (MICR) spectra for the unimolecular dissociation of the chemically activated adduct ion derived from association of the methoxymethyl cation with pivaldehyde during a 2-s reaction delay at a pressure of pivaldehyde of 1.0 x 10 torr. The three spectra correspond to values of rf amplitude appropriate to eject transient intermediates with lifetimes longer than (a) 60 ps, (b) 80 ps, and (c) 1 70 ps. A partial pressure of CH4 of 1.0 x 10 torr was also present to thermalize ions. The peak at m/z 125 is a secondary reaction product of m/z 85.

B. Unimolecular Dissociation Rates RRKM Theory and Distribution of Resonances... 

There can be a difference between the dissociation of polyatomic molecules and delayed ionization in the nature of the initial excitation. In ZEKE spectroscopy the state that is optically accessed (typically via an intermediate resonantly excited state) is a high Rydberg state, that is a state where most of the available energy is electronic excitation. Such a state is typically directly coupled to the continuum and can promptly ionize, unlike the typical preparation process in a unimolecular dissociation where the state initially accessed does not have much of its energy already along the reaction coordinate. It is quite possible however to observe delayed ionization in molecules that have acquired their energy by other means so that the difference, while certainly important is not one of principle. 

RESONANCES IN UNIMOLECULAR DISSOCIATION FROM MODE-SPECIFIC TO STATISTICAL BEHAVIOR... 

R. Schinke, H.-M. Keller, M. Stumpf, C. Beck, D. H. Mordaunt, and A. J. Dobbyn, Adv. Chem. Phys., Vol. 101, Resonances in Unimolecular Dissociation From Mode-Specific to Statistical Behavior. 

Adv. Chem. Phys.. Vol. 101, Resonances in Unimolecular Dissociation From Mode-... 

The complete active space valence bond (CASVB) method is an approach for interpreting complete active space self-consistent field (CASSCF) wave functions by means of valence bond resonance structures built on atom-like localized orbitals. The transformation from CASSCF to CASVB wave functions does not change the variational space, and thus it is done without loss of information on the total energy and wave function. In the present article, some applications of the CASVB method to chemical reactions are reviewed following a brief introduction to this method unimolecular dissociation reaction of formaldehyde, H2CO — H2+CO, and hydrogen exchange reactions, H2+X — H+HX (X=F, Cl, Br, and I). 

Figure 6.1-4 Illustration of Eq. 6.1-19, the time-dependent approach to continuum resonance Raman scattering. Shown is a 2 > 1> vibrational Raman transition in Bra for Aq = 457.9 nm excitation. As examples, (A), (B) and (C) show the potential curves of the relevant ground (X = continuous line) and excited (B = 7o+m, dashed line, and 77 = 7T , dotted line) electronic states, together with the absolute values of the coordinate representations of the initial state It >= 1 >, final state ]f >= 2 >, and the time-dependent state i(r) > at times / = 0, 20 and 40 fs, respectively. The excitation and de-excitation processes and the related unimolecular dissociations are indicated schematically by vertical and horizontal arrows. For clarity of presentation, the energy gap between state (> and f> is expanded (Ganz et al., 1990).

Figure 1 (a) Illustration of the unimolecular dissociation of a reactant molecule ABC into products A and BC. p( ), Rts and Nts are the density of reactant states, the intermolecular distance of the transition state (TS) and the number of open channels at the transition state, respectively. The vertical axis on the left-hand side shows a spectrum dominated by sharp resonances, (b) Schematic representation of the energy dependence of the micro-canonical rate constant k E) (solid line). The dots represent the state-specific quantum mechanical rates kn-... 

The dissociation of weakly bound van der Waals complexes is a special case of unimolecular dissociation [20]. Because of the exceedingly weak coupling between the dissociation coordinate and the mode (or modes) initially excited, and the very low density of states of the energized complex, narrow resonances are the dominant features of van der Waals spectra. There are, of course, many similarities between the dynamics of physically bound and chemically bound molecules. The dissociation dynamics of these special molecules (or clusters) has been studied in great detail, both experimentally and theoretically. Exhaustive review articles are available [85-89] and therefore van der Waals molecules will not be particularly considered in this chapter. However, one must keep in mind that, as the density of states of van der Waals molecules increases, their dynamics becomes more and more comparable with the dynamics of strongly bound molecules [90,91]. 

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