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17 O NMR Spectroscopic Characterization and the Mechanism of Formation of Alkyl Hydrotrioxides (ROOOH) and Hydrogen Trioxide (HOOOH) in the Low‐Temperature Ozonation of Isopropyl Alcohol and Isopropyl Methyl Ether: Water‐Assisted Decomposition
Author(s) -
Plesničar Božo,
Cerkovnik Janez,
Tekavec Tomaž,
Koller Jože
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/(sici)1521-3765(20000303)6:5<809::aid-chem809>3.0.co;2-8
Subject(s) - homolysis , chemistry , isopropyl , ether , bond cleavage , photochemistry , medicinal chemistry , bond dissociation energy , ether cleavage , reaction mechanism , dissociation (chemistry) , organic chemistry , radical , catalysis
Low‐temperature ozonation of isopropyl alcohol ( 1 a ) and isopropyl methyl ether ( 1 b ) in [D 6 ]acetone, methyl acetate, and tert ‐butyl methyl ether at −78 °C produced the corresponding hydrotrioxides, Me 2 C(OH)(OOOH) ( 2 a ) and Me 2 C(OMe)(OOOH) ( 2 b ), along with hydrogen trioxide (HOOOH). All the polyoxides investigated were characterized for the first time by 17 O NMR spectroscopy of highly 17 O‐enriched species. The assignment was confirmed by GIAO/GIAO2/6‐31++G* calculations of 17 O NMR chemical shifts, which were in excellent agreement with the experimental values. Ab initio density functional (DFT) calculations at the B3LYP/6‐31G*+ZPE level have clarified the transition structure ( TS 1 , Δ E ≠ =7.4 and 10.6 kcal mol −1 , relative to isolated reactants and the complex 1 a ‐ozone, respectively) for the ozonation of 1 a ; this, together with the formation of HOOOH and some other products, indicates the involvement of radical intermediates (R . , . OOOH) in the reaction. The activation parameters for the decomposition of the hydrotrioxides 2 a and 2 b ( E a =23.5±1.5 kcal mol −1 , log A =16±1.8) were typical for a homolytic process in which cleavage of the ROOOH molecule occurs to yield a radical pair [RO . . OOH] and represents the lowest available energy pathway. Significantly the lower activation parameters for the decomposition of HOOOH ( E a =16.5±2.2 kcal mol −1 , log A =9.5±2.0) relative to those expected for the homolytic scission of the HO−OOH bond [bond dissociation energy (BDE)=29.8 kcal mol −1 , CCSD(T)/6‐311++G**] are in accord with the proposal that water behaves as a bifunctional catalyst and therefore participates in a “polar” (non‐radical) decomposition process of this polyoxide. A relatively large acceleration of the decomposition of the hydrotrioxide 2 a in [D 6 ]acetone, accompanied by a significant lowering of the activation energies, was observed in the presence of a large excess of water. Thus intramolecular 1,3‐proton transfer probably also involves the participation of water and is similar to the mechanism proposed for the decomposition of HOOOH. This hypothesis was further substantiated by the B3LYP/6‐31++G*+ZPE calculations for the participation of water in the decomposition of CH 3 OOOH, which revealed two stationary points on the potential energy surface corresponding to a CH 3 OOOH−HOH complex and a six‐membered cyclic transition state TS 2 . The energy barriers were comparable with those calculated for HOOOH, that is, Δ E ≠ =15.0 and 21.5 kcal mol −1 relative to isolated reactants and the CH 3 OOOH−HOH complex, respectively.