Matrix Isolation Study of the Ozonolysis of 1,3- and 1,4-Cyclohexadiene: Identification of Novel Reaction Pathways

Abstract

The ozonolysis reactions of 1,3- and 1,4-cyclohexadiene have been studied using a combination of matrix isolation, infrared spectroscopy, and theoretical calculations. Experimental and theoretical results demonstrate that these reactions predominantly do not follow the long-accepted Criegee mechanism. Rather, the reaction of O3 with 1,4-cyclohexadiene leads to the essentially barrierless formation of benzene, C6H6, and H2O3. These two species are then trapped in the same argon matrix cage and weakly interact to form a molecular complex. There is also evidence for the formation of a small amount of the primary ozonide as a minor product, formed through a transition state that is slightly higher in energy. The reaction of O3 with 1,3-cyclohexadiene follows two pathways, one of which is the Criegee mechanism through a low energy transition state leading to formation of the primary ozonide. In addition, with a similar barrier, ozone abstracts a single hydrogen from C5 while adding to C1, forming a hydroperoxy intermediate. This study presents two of the rare cases in which the Criegee mechanism is not the dominant pathway for the ozonolysis of an alkene as well as the first evidence for dehydrogenation of an alkene by ozone.

Ozonolysis Reactions of Monoterpenes: A Variational Transition State Investigation.

“…Minimum Energy Paths for the Ozonolysis of the Monoterpenes. According to the Criegee mechanism, 8,9,11 the monoterpenes ozonolysis occurs by the cycloaddition of ozone to the double bond forming the molozonide, an intermediate also known as 1,2,3-trioxolane. This initial step is highly exothermic and if, by hypothesis, the energy content is accumulated in the molozonide, it will be formed in a large distribution of excited vibrational states and then undergo fast unimolecular decomposition.…”

Matrix isolation study of the early intermediates in the ozonolysis of selected vinyl ethers

“…The C]O mode in different molecules was observed at 1733, 1742, 1740 and 1715-1722 cm À1 in cyclopentene, cyclopentadiene, 1,3-cyclohexadiene and ethylene, respectively. 24,29,35 Some CI bands could be indicated from the great change of the peaks at different temperature. Calculations of bands for two CIs indicated that each had one very intense mode, the O-O stretching mode in the 900-1000 cm À1 region.…”

“…28 Using matrix isolation, infrared spectroscopy, and theoretical calculations, the reaction between 1,4-cyclohexadiene and ozone was investigated and besides the widely-known Criegee mechanism, a new pathway to this reaction was found to lead to the formation of the benzene-H 2 O 3 complex through dehydrogenation. 29 In this study, matrix isolation technique combined with FTIR spectroscopy was used to identify and characterize early intermediate products in the reactions of ozone with EVE and n-BVE in argon matrices. Density functional theory (DFT) calculations were carried out to determine the structures of the reaction intermediate products, the relative energies of different reaction channels and to complement the experiments in assigning the products bands.…”

Estimation of mechanistic parameters in the gas-phase reactions of ozone with alkenes for use in automated mechanism construction

“…It has also been suggested that different pathways may play a more significant role for a small number of systems e.g. cyclohexadienes (Pinelo et al, 2013). Criegee intermediates are generally zwitterionic in nature, as shown in Figure 1, but the moiety is denoted simply as a >COO structure below (not to be confused with alkylperoxy radicals, ROO • ).…”