The reason stems from two aspects: (1) excessively high temperatures cause C-C linkage deviating from their ideal positions through bending of C-C o bonds ; (2) unimolecular reaction
is associated with a notable activation barrier which is larger than the bimolecular reaction, hence becoming favored at higher temperatures .
Works on the thermal gas phase kinetics of unimolecular reaction
in the low temperature region 500-700 K have been of great interest over the last three decades.
The assumption of Romsted was that in cases, where nucleophile resides predominantly in Stern layer of the micelle, Menger and Portnoy treatment for unimolecular reaction
should hold good for bimolecular reaction also.
It was noted that an understanding of gas phase unimolecular reaction
(e.g., rearrangements) was critical in interpreting the mass spectra.
However, the advantages of the system, such as the speed of signaling of the unimolecular reaction
and the reported increased specificity caused by the close proximity of the probe to the target, may outweigh the design constraints.
These unimolecular reactions
(dissociation or isomerization) are often considered first order reactions that follow the general reactions shown in equation.
Robertson, Unimolecular Reactions
, Wiley, Chichester, UK, 1996.