Unimolecular dissociation potential-energy surfaces

In order to better understand the detailed dynamics of this system, an investigation of the unimolecular dissociation of the proton-bound methoxide dimer was undertaken. The data are readily obtained from high-pressure mass spectrometric determinations of the temperature dependence of the association equilibrium constant, coupled with measurements of the temperature dependence of the bimolecular rate constant for formation of the association adduct. These latter measurements have been shown previously to be an excellent method for elucidating the details of potential energy surfaces that have intermediate barriers near the energy of separated reactants. The interpretation of the bimolecular rate data in terms of reaction scheme (3) is most revealing. Application of the steady-state approximation to the chemically activated intermediate, [(CH30)2lT"], shows that. 

These, and similar data for other systems, demonstrate the tremendous potential that the MICR technique has for the qualitative elucidation of potential energy surfaces of relatively complex organic reactions. Once implementation of the quadrupolar excitation technique has been effected to relax ions to the cell center, the technique will become even more powerful, in that the determination of highly accurate unimolecular decomposition lifetimes of chemically activated intermediates will also become possible. No other technique offers such a powerful array of capabilities for the study of unimolecular dissociation mechanisms and rates. 

Fig. 3.1.5 A potential energy surface for a direct unimolecular reaction without a saddle point. The surface corresponds to a reaction like H2O — H + OH for dissociation along a fixed bond angle, where only two internuclear coordinates are required in order to specify the configuration. (Note that in this figure all energies above a fixed cut-off value Amax have been replaced by Fmax.)...

Various unimolecular reactions of H2Si=S, such as the 1,2-hydrogen shift to produce HSiSH and the dissociation reactions to H2 + SiS or to H + HSiS, have been studied220. The calculated potential energy surfaces of analogous reactions of H2Si=S and of H2Si=Q... 

Variational RRKM calculations, as described above, show that a unimolecular dissociation reaction may have two variational transition states [M, 33, 34, 35 and 36], i e. one that is a tight vibrator type and another that is a loose rotator type. Whether a particular reaction has both of these variational transition states, at a particular energy, depends on the properties of the reaction s potential energy surface [33, M and 35]- For many dissociation reactions there is only one variational transition state, which smoothly changes from a loose rotator type to a tight vibrator type as the energy is increased [26]. 

Since 1970, direct photolysis of molecules or ions in low-pressure, collisionless environments, has permitted molecules to be excited to well-defined energy levels, while the use of pulsed lasers or coincidence techniques has provided an accurate external time base with which to measure the dissociation rate constants over many orders of magnitude. It is often the case that more precise experimental results lead to fundamental changes in the theoretical models which describe the phenomena. This has not happened in the case of unimolecular reactions. The statistical theory has remained surprisingly robust. Most molecular systems that dissociate on a bound potential energy surface do so in a statistical fashion. What has changed in the past 25 years is our ability to apply the statistical theory. It is now possible to calculate Unimolecular rate constants with essentially no adjustable parameters and which are in quantitative agreement with experiments. 

The statistical dissociation rate constant can be calculated from the point of view of the reverse reaction, namely the recombination of the products to form a complex. This approach, commonly referred to as phase space theory (PST) (Pechukas and Light, 1965 Pechukas et al., 1966 Nikitin, 1965 Klots, 1971, 1972) is limited to reactions with no reverse activation energy, that is, reactions with very loose transition states. PST assumes the decomposition of a molecule or collision complex is governed by the phase space available to each product under strict conservation of energy and angular momentum. The loose transition state limit assumes that the reaction potential energy surface is of no importance in determining the unimolecular rate constant. 

In the previous sections it has been implicitly assumed that the unimolecular reaction is electronically adiabatic and, thus, occurs on a single potential energy surface. Electronically excited states (i.e., multiple potential energy surfaces) for unimolecular reactions was discussed in chapter 3 and it is assumed that the reader has read and is familiar with this material (Nikitin, 1974 Hirst, 1985 Steinfeld et al., 1989). Transitions between electronic states are particularly important for the unimolecular decomposition of ions. For example, the following two dissociation paths ... 

See, for example, D. L. Bunker, /. Chem. Phys., 40,1946 (1963). Monte Carlo Calculations. IV. Further Studies of Unimolecular Dissociation. D. L. Bunker and M. Pattengill,/. Chem. Phys., 48, 772 (1968). Monte Carlo Calculations. VI. A Re-evaluation erf Ae RRKM Theory of Unimolecular Reaction Rates. W. J. Hase and R. J. Wolf, /. Chem. Phys., 75,3809 (1981). Trajectory Studies of Model HCCH H -P HCC Dissociation. 11. Angular Momenta and Energy Partitioning and the Relation to Non-RRKM Dynamics. D. W. Chandler, W. E. Farneth, and R. N. Zare, J. Chem. Phys., 77, 4447 (1982). A Search for Mode-Selective Chemistry The Unimolecular Dissociation of t-Butyl Hydroperoxide Induced by Vibrational Overtone Excitation. J. A. Syage, P. M. Felker, and A. H. Zewail, /. Chem. Phys., 81, 2233 (1984). Picosecond Dynamics and Photoisomerization of Stilbene in Supersonic Beams. II. Reaction Rates and Potential Energy Surface. D. B. Borchardt and S. H. Bauer, /. Chem. Phys., 85, 4980 (1986). Intramolecular Conversions Over Low Barriers. VII. The Aziridine Inversion—Intrinsically Non-RRKM. A. H. Zewail and R. B. Bernstein,... 

The energy disposal and effective upper state lifetimes have been reproduced using classical trajectory calculations a quasi-diatomic assumption was made to determine the slope of the section through the upper potential energy surface along the N—a bond from the shape of the u.v. absorption profile. The only adjustable parameter was the assumption of a parallel transition in the quasi-diatomic molecule. In contrast, a statistical adiabatic channel model which assumed dissociation via unimolecular decomposition out of vibrationally and rotationally excited level in the ground electronic state (following internal con-... 

The title molecule plays an important role in atmospheric chemistry and combustion processes, with its ground-state potential energy surface being also frequently employed as a prototype in unimolecular dissociation calculations. In this subsection, we focus on the NO2 potential energy surfaces of A symmetry. Because they are outstandingly complicated, the interpretation of the visible spectrum of NO2 poses an extremely difficult problem both to experimentalists and theoreticians. Similarly, such surfaces are crucial to study the dynamics of the reaction... 

The major changes observed in the OH-Ar potential energy surface upon electronic excitation of OH prompted us to examine the dynamics occurring on these surfaces. The unimolecular dissociation dynamics of OH-Ar complexes were investigated by preparing the complexes with one quantum of OH vibrational excitation (vqh), which is more than sufficient energy to break the OH-Ar bond. We have found that the resultant vibrational predissociation dynamics of OH-Ar differ enormously in its ground and excited electronic states. ... 

The MCTDH method allows for a treatment of quantum dynamics of (mainly) bound states in medium size molecular systems (with number of degrees of freedom up to 24). Many applications employing accurate potential energy surfaces are successfully achieved and compared with the relevant experimental spectral data, such as vibrational absorption, vibronic spectrum and so on. The method, however, is known not necessarily to work well for the evaluation of reaction rate in case of unimolecular dissociation with chaotic motions and hard recombination reaction systems. In these systems it is quite difficult to reproduce the d5mamics accurately in terms of a small number of reference multireference orbitals. 

Chemical dynamics simulations were performed to study the unimolecular decomposition of microcanonical ensembles versus energy of the Cl — CH Br ion lipole complex.An analytic potential energy surface was used for the simulations. The complex has two unimolecular reaction paths, i.e. dissociation to C/ + CHjBr or isomerization to the CICH —Br ion iipole complex. The simulations were performed for energies of 30-80 kcal mol and the resulting non-exponential N t)IN 0) were lit by a sum of three exponentials, i.e. eqn (20.18). The resulting// and ki fitting parameters are listed in Table 20.2. 

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