A R T I C L E S
Cerkovnik et al.
A concentration dependence study of 1H NMR chemical shifts
of alkyl hydrotrioxide 2a and HOOOH revealed their strong
dependence on the initial concentration of cumene (1a). Namely,
chemical shifts of both polyoxides obtained during the ozonation
of 0.1 M solutions of 1a were different from chemical shifts of
these species generated by ozonizing more concentrated solu-
tions of cumene (3.6 M), with subsequent dilution to 0.05 M
solution. These phenomena most probably reflect the difference
in both the extent of the intermolecular association of the
polyoxides under the investigation, i.e., intermolecularly hydrogen-
bonded dimers and/or oligomers, and their solvation.
Scheme 1
Ozonation of triphenylmethane (1b) (0.1-0.2 M) in various
solvents at -60 °C, on the other hand, produced only the
corresponding triphenylmethyl hydrotrioxide (2b) in yields of
30-40%. The hydrotrioxide was characterized by the OOOH
1H NMR absorption at δ ) 13.38 and 13C NMR absorption at
δ ) 101.7 (C), downfield from TMS (-50 °C, acetone-d6).
All attempts to obtain 17O NMR spectra of highly 17O
enriched samples of alkyl hydrotrioxides 2a and 2b failed, most
likely because of the line broadening caused by the internal
asymmetry of the molecules.15 However, the presence of
HOOOH in the ozonized solutions of cumene was confirmed
by 17O NMR spectroscopy (HO1O2O3H: δ ) 304 (O1 and O3,
ν1/2 ) 490 Hz), 420 (O2, ν1/2 ) 490 Hz); acetone-d6, -10 °C,
H217O).5,16
HOOO Radicals in the Formation of ROOOH and
HOOOH. We have already reported theoretical and experi-
mental evidence for the “radical” mechanism of the ozonation
of isopropyl alcohol and some benzylic alcohols and ethers.5
The formation of HOOOH in the ozonation of cumene and its
complete absence in the case of triphenylmethane seem to rule
out a concerted 1,3-dipolar insertion mechanism, believed
previously to be operative in these reactions.4a Still another
mechanistic proposal involving hydride ion transfer to ozone
to form the ion pair, R+ -OOOH,17 also appears less likely.
Namely, our recent theoretical investigation of the HOOO anion
at the CCSD(T)/6-311++G(d,p) level of theory revealed an
extraordinarily long HO-OO bond in this species (1.88 Å),
indicating a predominantly noncovalent (or a very weak
covalent) interaction between HO anion and singlet oxygen.5,18
Therefore, it appears unlikely that such an assembly could
survive as a discrete molecular entity19 in organic solvents long
enough to regenerate HOOOH after protonation.
to form HOOOH (Scheme 1).21,22 Namely, it is obvious that
HOOO• cannot abstract the hydrogen atom from the triphenyl-
methyl radical inside the solvent cage. It is especially noteworthy
to point out, however, that the HOOO radical is not stable
enough to abstract the hydrogen atom from triphenylmethane
outside the solvent cage under the conditions investigated.
We have long been impressed by the ease with which
ozonations of hydrocarbons (and other saturated organic sub-
strates, i.e., ethers,23 acetals,23b and alcohols5 as well) proceed
in organic oxygen bases as solvents. We now wish to report
the results of B3LYP/6-311++G(d,p)(ZPE) theoretical studies
of the interaction between the HOOO radical and two repre-
sentative oxygen bases, i.e., acetone and dimethyl ether, which
appear to explain these phenomena. The fully optimized
structures of the complexes with minimum energies were
confirmed to be stable minima by vibrational frequency analysis.
Only the global minima energy structures were reported here
(Figure 2).
As seen from Table 1, HOOO radical forms relatively strongly
bonded complexes with oxygen bases. For comparison, the
complex of HOOO• with water, recently reported by Aloisio
and Francisco,24,25 has also been investigated. The strength of
the intermolecular hydrogen bond (the energy difference
between the complex and the sum of energies of both compo-
nents of the complex) decreases in the following order: acetone
(AC)-HOOO• (8.55 kcal/mol) > dimethyl ether (DME)-HOOO•
(7.04 kcal/mol) > HOH-HOOO• (5.91 kcal/mol).26 A consider-
able structural perturbation of the hydrotrioxyl radical27 was
The above experimental evidence, together with the previ-
ously reported retention of configuration in the ozonation of
saturated hydrocarbons,7a as well as the log A values (7-8) for
these reactions,20 supports the involvement of tight radical pairs,
i.e., [R• •OOOH], in the ozonation of C-H bonds, as suggested
previously by Hamilton,7a Whiting,7b and Pryor,20 allowing both
the collapse of the pair to the ROOOH and the abstraction of
the hydrogen atom from the alkyl radical (R•) by HOOO radical
(21) We are currently investigating (DFT-IRC) some other possible mechanistic
pathways for the reactions of R and OOOH radicals in the cage (ozonolysis
of isopropyl alcohol,5 and some other saturated substrates).
(22) A recent theoretical study on the ozonation of hydrocarbons by using the
semiempirical AM1 method supported the involvement of a biradical
transition state leading to alkyl and hydrotrioxyl radicals. (Timerghazin,
Q. K.; Khursan, S. L.; Shereshovets, V. V. J. Mol. Struct. (Theochem) 1999,
489, 87.)
(23) (a) Stary, F. E.; Emge, D. E.; Murray, R. W. J. Am. Chem. Soc. 1976, 98,
1880. Plesnicˇar, B.; Kovac, F.; Schara, M. J. Am. Chem. Soc. 1988, 110,
214. (b) Kovac, F.; Plesnicˇar, B. J. Am. Chem. Soc. 1979, 101, 2677.
(24) Aloisio, S.; Francisco, J. S. J. Am. Chem. Soc. 1999, 121, 8592.
(25) For some theoretical studies on the HOOO radical, see: (a) Blint, R. J.;
Newton, M. D. J. Chem Phys. 1973, 59, 6220. (b) Mathisen, K. B.;
Siegbahn, P. E. M. Chem. Phys. 1984, 90, 225. (c) Dupuis, M.; Fitzgerald,
G.; Hammond, B.; Lester, W. A.; Schaefer, H. F. J. Chem. Phys. 1986,
84, 2691. (d) Vincent, M. A.; Hillier, I. H. J. Phys. Chem. 1995, 99, 3109.
(e) Jungkamp, T. P. W.; Seinfeld, J. H. Chem. Phys. Lett. 1996, 257, 15.
(f) Yu, H. G.; Varandas, A. J. C. Chem. Phys. Lett. 2001, 334, 173.
(26) Binding energies of the HOOO• -HOH complex: B3LYP/6-311++G(2df,-
(15) Amour, T. S.; Fiat, D. Bull. Magn. Reson. 1979, 1, 118. Kintzinger, J.-P.
In NMR-Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld, R.,
Eds.; Springer: Berlin, 1981; Vol. 17, pp 1-64.
(16) Our preliminary results of the ozonation of some other hydrocarbons, for
example substituted cumenes, ethylbenzenes, tetralin, and norcaran, showed
that besides the corresponding hydrotrioxides (ROOOH), hydrogen trioxide
(HOOOH) was formed in these reactions too.
(17) Nangia, P. S.; Benson, S. W. J. Am. Chem. Soc. 1980, 102, 3105.
(18) Koller, J.; Plesnicˇar, B. J. Am. Chem. Soc. 1996, 118, 2470.
(19) A detailed high-level theoretical investigation on the structure of this unusual
molecule is currently underway.
(20) Giamalva, D. H.; Church, D. F.; Pryor, W. A. J. Org. Chem. 1988, 53,
3429.
2p)(ZPE)
) 5.0 kcal/mol; CCSD(T)/6-311++G(2df, 2p)//B3LYP/6-
311++G(2df,2p) ) 6.5 kcal/mol.24
9
406 J. AM. CHEM. SOC. VOL. 124, NO. 3, 2002