Unimolecular reaction vibrational

As discussed in section A3.12.2. intrinsic non-RRKM behaviour occurs when there is at least one bottleneck for transitions between the reactant molecule s vibrational states, so drat IVR is slow and a microcanonical ensemble over the reactant s phase space is not maintained during the unimolecular reaction. The above discussion of mode-specific decomposition illustrates that there are unimolecular reactions which are intrinsically non-RRKM. Many van der Waals molecules behave in this maimer [4,82]. For example, in an initial microcanonical ensemble for the ( 211 )2 van der Waals molecule both the C2H4—C2H4 intennolecular modes and C2H4 intramolecular modes are excited with equal probabilities. However, this microcanonical ensemble is not maintained as the dimer dissociates. States with energy in the intermolecular modes react more rapidly than do those with the C2H4 intramolecular modes excited [85]. 

Beil A, Luckhaus D, Quack M and Stohner J 1997 Intramolecular vibrational redistribution and unimolecular reactions concepts and new results on the femtosecond dynamics and statistics in CHBrCIF Ber. Bunsenges. Phys. Chem. 101 311-28... 

Quack M 1991 Mode selective vibrational redistribution and unimolecular reactions during and after IR-laser excitation Mode Selective Chemistry ed J Jortner, R D Levine and B Pullman (Dordrecht Kluwer) pp 47-65... 

A) During the luultiphoton excitation of molecular vibrations witli IR lasers, many (typically 10-50) photons are absorbed in a quasi-resonant stepwise process until the absorbed energy is suflFicient to initiate a unimolecular reaction, dissociation, or isomerization, usually in the electronic ground state. 

In a unimolecular reaction, a singie moiecule fragments into two pieces or rearranges to a new isomer. A simple example of a unimolecular reaction is the decomposition of N2 O4. An N2 O4 molecule continually vibrates, and these vibrations cause its N—N bond to stretch. If the molecule has sufficient energy, the bond breaks and the moiecuie separates into two moiecuies of NO2, much like a spring that breaks if it is stretched too far. 

One point of interest deriving from the equations of TST (and Arrhenius theory) is that the upper limit for the 298 K rate constant of a unimolecular reaction that takes place with zero activation energy (of whatever sort) is roughly 10 sec . This is, in some sense, a conceptually obvious result since that is on the order of a molecular vibrational frequency, which is thought of as the mechanism by which a transition state goes to its products. 

For this QRRK analysis we will define the zero of energy as the ground-state energy of the stabilized C molecule. As in QRRK the analysis of unimolecular reactions, assume that the excited C molecule consists of s identical oscillators, each with vibrational frequency v. When we write C ( ), this indicates that the excited intermediate species has been formed with n quanta of vibrational energy thus, its total energy is E = nhv above the ground-state energy of C (which we have arbitrarily set to zero). 

G.M. Wieder and R.A. Marcus. Dissociation and Isomerization of Vibrationally Excited Species. II. Unimolecular Reaction Rate Theory and Its Application. J. Chem. Phys., 37 1835-1852,1962. 

This equation tells us what happens to an individual 03 molecule. It is an example of a unimolecular reaction, because only one reactant molecule is involved. We can picture an ozone molecule acquiring energy from sunlight and vibrating so vigorously that it shakes itself apart. If we believe that an O atom produced by the dissociation of 03 goes on to attack another 03 molecule, we write for that step... 

Nevertheless, predissociation in polyatomic molecules can be visualized as qualitatively similar to a unimolecular reaction in the Rice-Ramsperger-Kassel sense. With diatomic molecules possessing only one degree of vibrational freedom,... 

Question. Why is rotational energy not considered to be as important as vibrational energy in a unimolecular reaction In what circumstances might it be important ... 

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