Theory of thermal unimolecular reactions at low pressures. III. Superposition of weak and strong collisions

1992 ◽  
Vol 97 (1) ◽  
pp. 288-292 ◽  
Author(s):  
J. Troe
Keyword(s):  
Unimolecular Reactions ◽  
Low Pressures

Competitive unimolecular reactions at low pressures. The pyrolysis of cyclobutyl chloride

1979 ◽  
Vol 11 (1) ◽  
pp. 11-21 ◽  
Author(s):  
Keith D. King ◽  
Brendan J. Gaynor ◽  
Robert G. Gilbert
Keyword(s):  
Unimolecular Reactions ◽  
Low Pressures

Unimolecular Reactions of Methylcyclobutane at Low Pressures

1969 ◽  
Vol 91 (27) ◽  
pp. 7611-7616 ◽  
Author(s):  
Timothy F. Thomas ◽  
Paul J. Conn ◽  
D. F. Swinehart
Keyword(s):  
Unimolecular Reactions ◽  
Low Pressures

Photochemical studies of unimolecular processes VI. The unimolecular reactions of C 6 H 8 isomers and the interpretation of their photolyses

1974 ◽  
Vol 337 (1609) ◽  
pp. 257-274 ◽  
Keyword(s):  
Internal Conversion ◽  
Excited Electronic State ◽  
Photochemical Reactions ◽  
Quantum Yields ◽  
Unimolecular Reactions ◽  
Parent Molecule ◽  
Deactivation Model ◽  
Low Pressures ◽  
Step Quenching ◽  
Good Agreement

The thermal unimolecular isomerization of hexa-1, trans -3, 5-triene to the cis -compound is shown to obey the rate expression k 1 = (4.5 ± 1.5) x 10 12 exp( — 181.3 + 2.0 kJ mol -1 / RT ) s -1 . At low pressures of cyclohexa-1, 3-diene, it undergoes a thermal unimolecular conversion to benzene and hydrogen via cyclohexa-1, 4-diene; the Arrhenius parameters found for this process are k 2 = (4.7 ± 2.2) x 10 13 exp (— 258 ± 4 kJ mol -1 / RT ) s -1 . From these, other kinetic and thermodynamic data, the theory of unimolecular reactions was used to calculate the quantum yields of the main products in the gas phase photolyses of cis - and trans - hexa-1, 3, 5-triene and cyclohexa-1, 3-diene as a function of pressure. Good agreement with experiment was obtained assuming that all the observed photochemical reactions involved vibrationally exoited molecules formed by internal conversion of the initially populated excited electronic state. A limited amount of cis-trans isomerization may occur in the excited electronic states of the hexatrienes. A step-ladder deactivation model with ∆ E = 25 kJ mol -1 for the parent molecule gave good agreement with the experiments, in which there is clear evidence of multi-step quenching. It is deduced from their photochemistry that the trans and cis hexa-1, 3, 5 trienes differ in enthalpy by 11 kJ mol -1 .

scholarly journals The thermal decomposition of propionic aldehyde

1934 ◽  
Vol 146 (857) ◽  
pp. 345-356 ◽  
Keyword(s):  
Thermal Decomposition ◽  
Degrees Of Freedom ◽  
High Pressures ◽  
Unimolecular Decomposition ◽  
Complex Molecules ◽  
Unimolecular Reactions ◽  
Complex Behaviour ◽  
Large Numbers ◽  
Activated State ◽  
Low Pressures

The hypothesis put forward in 1926, namely, that unimolecular reactions are mainly confined to molecules with large numbers of internal degrees of freedom, has proved a great stimulus to the investigation of more complex molecules, such as those encountered in the encountered in the field of organic chemistry. The hypothesis was suggested by an investigation on the thermal decomposition of propionic aldehyde. Although this work was detailed enough for its original purpose, a re-investigation has become desirable in view of recent advances in chemical kinetics. According to the collision theory of activation and deactivation, the unimolecular velocity constant should decrease at low pressures. Reactions which behave in this way (so-called quasi-unimolecular reactions) are kinetically bimolecular at low pressures, but become unimolecular at high pressures. More complex behaviour is observed with gaseous acetaldehyde. This compound, while decomposing bimolecularly at high pressures, undergoes additional quasi-unimolecular decomposition at low pressures. The occurrence of independent quasi-unimolecular reactions in the acetaldehyde decomposition can be attributes to the existence of different types of activated state in the molecule. The mode of decomposition would be, on this assumption, a function of the manner in which the energy imparted to the molecule is distributed among the various molecular motions, and also the transformation probability characteristic of the different distributions.

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