Thermal unimolecular dissociation

On the other hand, at high temperatures, wiUand may merge. This is illustrated in figure A3.13.3 for the classic example of thermal unimolecular dissociation [48, 49, and M ] ... 

A. Thermal Unimolecular Dissociation of the Proton-Bound Methoxide Dimer... 

Experimental studies have had an enormous impact on the development of unimolecular rate theory. A set of classical thermal unimolecular dissociation reactions by Rabinovitch and co-workers [6-10], and chemical activation experiments by Rabinovitch and others [11-14], illustrated that the separability and symmetry of normal modes assumed by Slater theory is inconsistent with experiments. Eor many molecules and experimental conditions, RRKM theory is a substantially more accurate model for the unimolecular rate constant. Chemical activation experiments at high pressures [15,16] also provided information regarding the rate of vibrational energy flow within molecules. Experiments [17,18] for which molecules are vibrationally excited by overtone excitation of a local mode (e.g. C-H or O-H bond) gave results consistent with the chemical activation experiments and in overall good agreement with RRKM theory [19]. 

Dunbar, R.C. (1994) Kinetics of thermal Unimolecular Dissociation by Ambient Infrared Radiation. J. Phys. Chem. 98 8705-8712. 

Dunbar, R. C. Kinetics of thermal unimolecular dissociation by ambient infrared radiation. J. Phys. Chem 1994, 98, 8705-8712. 

Figure 1. Fall-off curve of a thermal unimolecular dissociation or recombination reaction from Eq. (2.3).

Current studies of unimolecular reactions can be broadly divided into three categories, based on different methods of activation of the decomposing species. The first, most classical, method is that of thermal activation of the type first envisioned by Lindemann to explain unimolecular dissociation phenomena brought about by heat energy. The second method involves chemical activation, ... 

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... 

The relatively long timescales of the ionization, isolation, thermalization, reaction, and detection sequences associated with low-pressure FTICR experiments are generally thought to preclude the use of this technique as a means of examining the unimolecular dissociation of conventional metastable ions occurring on the microsecond to millisecond timescale. Nonetheless, as just demonstrated (Section IIIC), intermediates with this order of magnitude of lifetime are routinely formed in the bimolecular reactions of gaseous ions with neutral molecules at low pressures in the FTICR cell, as in Equation (13). 

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.

Conventional wisdom concerning thermal unimolecular reactions would seem to dictate that this must then be a Lindemann-type collisionally activated dissociation reaction scheme such as is in Equation (17). Application of the steady-state... 

A further feature expected of thermal, collisionally activated unimolecular dissociations is that the rate constants should exhibit an Arrhenius-type dependence on the temperature.For the tmimolecular dissociation of the (H20)5H cluster,... 

Photoisomerization of c/.y-stilbene 191 Ionic fragmentation reaction 191 Cyclopropyl radical ring-opening 192 Ionic molecular rearrangement 193 Ene reaction 196 Thermal denitrogenation 198 Unimolecular dissociation 199 Sn2 reaction 200... 

In principle, one can induce and control unimolecular reactions directly in the electronic ground state via intense IR fields. Note that this resembles traditional thermal unimolecular reactions, in the sense that the dynamics is confined to the electronic ground state. High intensities are typically required in order to climb up the vibrational ladder and induce bond breaking (or isomerization). The dissociation probability is substantially enhanced when the frequency of the field is time dependent, i.e., the frequency must decrease as a function of time in order to accommodate the anharmonicity of the potential. Selective bond breaking in polyatomic molecules is, in addition, complicated by the fact that the dynamics in various bond-stretching coordinates is coupled due to anharmonic terms in the potential. 

Although the theory was initially developed in 1952 and had been partly prompted by my prior experimental work, there were very few experimental data to which it could be applied. Around 1959 and subsequent years, B.S. Rabinovitch and coworkers used this theory to interpret their data on chemical activation [62, 69]. It may be recalled that chemical activation produced a narrower energy distribution of dissociating molecules than that in thermal unimolecular reactions and, hence, is better for testing the theory. 

J. Troe, Theory of multichannel thermal unimolecular reactions. 2. Application to the thermal dissociation of formaldehyde, /. Phys. Chem. A109 (37) (2(X)5) 8320-8328. 

Several conclusions can be drawn from Eqs. (76) and (77). First, the influence of fluctuations is the largest when the number of open channels u is of the order of unity, because then the distribution Q k) is the broadest. Second, the effect of a broad distribution of widths is to decrease the observed pressure dependent rate constant as compared to the delta function-like distribution, assumed by statistical theories [288]. The reason is that broad distributions favor small decay rates and the overall dissociation slows down. This trend, pronounced in the fall-of region, was clearly seen in a recent study of thermal rate constants in the unimolecular dissociation of HOCl [399]. The extremely broad distribution of resonances in HOCl caused a decrease by a factor of two in the pressure-dependent rate, as compared to the RRKM predictions. The best chances to see the influence of the quantum mechanical fluctuations on unimolecular rate constants certainly have studies performed close to the dissociation threshold, i.e. at low collision temperatures, because there the distribution of rates is the broadest. 

Dissociative electron capture is observed with hyperthermal electrons in NIMS electron impact experiments. In order for dissociative electron capture to take place with thermal electrons, there must be a dissociative pathway that is accessible by the thermal activation of the neutral molecule or a low-lying negative-ion state. The quantity D(R — Le) — Ea(Le) must be less than about 1.0 eV. This limit has been established empirically. Two types of dissociative thermal electron attachment have been observed in NIMS and ECD. The first occurs by unimolecular dissociation in which there is only one temperature region for many compounds. In the original work a low-temperature low-slope region was observed but unexplained. We now believe this could represent the formation of a molecular ion with an electron affinity of about 0.1 eV. The exact nature of this ion is not known, but it could represent stabilization to an excited state. In Figure 4.8 ECD data are plotted for several... 

Figure 4.9 Morse potential energy curves for chloromethane and its ions. The curves are calculated using the activation energy determined from data in Figure 4.8. The high-temperature data is for unimolecular dissociation via the curve crossing on the approach side of the molecule. Only the VEa is negative and dissociation occurs in the Franck Condon transition. The thermal energy dissociation occurs through the thermal activation of the molecule, as is the case for all DEC(l) molecules.

As with N20 the shapes of yield vs. concentration curves show little or no dependence on dose rate. As stated this is to be expected if these reactions occur in or near spurs. Again the difference between the cyclohexene yield and the bicyclohexyl yields is greater than that caused by unimolecular dissociation, implying that SF6 like N20 does not affect the yield of thermal hydrogen atoms. 

Recombination reactions are the inverse of unimolecular dissociation processes, and the associated rate coefficients correspondingly exhibit also a pressure dependence. Like thermal decomposition processes, recombination reactions require an energy transfer by collision. The pressure dependence results from the change in efficiency with which the excess energy is removed from the incipient product molecule by the third body M. The situation can be made clearer by writing the reaction as a sequence of two steps... 

The dissociation of molecules is one of the basic processes in chemistry the study of the kinetics of these reactions is therefore of considerable theoretical and practical interest, A simple method of obtaining information about dissociation reactions is to heat the gas to a sufficiently high temperature and then look for thermal decomposition. However for rich mixtures bimolecular reactions may well contribute to the reaction their influence must be separated out so that the unimolecular dissociation can be isolated. The rate of the primary dissociation is determined by elementary physical processes including both energy transfer between particles and internal energy flow. Dissociation reactions, isomerisation processes, photolytic reactions, dissociation of ions (e.g. in a mass spectrometer) and chemical activation experiments are closely related processes. 

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