Mechanism of formation of hydrogen trioxide (HOOOH) in the ozonation of 1,2-diphenylhydrazine and 1,2-dimethylhydrazine: An experimental and theoretical investigation

Article abstract of DOI:10.1021/ja036801u

Low-temperature (-78 °C) ozonation of 1,2- diphenylhydrazine in various oxygen bases as solvents (acetone-d₆, methyl acetate, tert-butyl methyl ether) produced hydrogen trioxide (HOOOH), 1,2-diphenyldiazene, 1,2-diphenyldiazene-N-oxide, and hydrogen peroxide. Ozonation of 1,2-dimethylhydrazine produced besides HOOOH, 1,2-dimethyldiazene, 1,2-dimethyldiazene-N-oxide and hydrogen peroxide, also formic acid and nitromethane. Kinetic and activation parameters for the decomposition of the HOOOH produced in this way, and identified by ¹H, ²H, and ¹⁷O NMR spectroscopy, are in agreement with our previous proposal that water participates in this reaction as a bifunctional catalyst in a polar decomposition process to produce water and singlet oxygen (O₂, ¹Δ_g). The possibility that hydrogen peroxide is, besides water, also involved in the decomposition of hydrogen trioxide is also considered. The half-life of HOOOH at room temperature (20 °C) is 16 ± 1 min in all solvents investigated. Using a variety of DFT methods (restricted, broken-symmetry unrestricted, self-interaction corrected) in connection with the B3LYP functional, a stepwise mechanism involving the hydrotrioxyl (HOOO^.) radical is proposed for the ozonation of hydrazines (RNHNHR, R = H, Ph, Me) that involves the abstraction of the N-hydrogen atom by ozone to form a radical pair, RNNHR^..OOOH. The hydrotrioxyl radical can then either abstract the remaining N(H) hydrogen atom from the RNNHR^. radical to form the corresponding diazene (RN=NR), or recombines with RNNHR^. in a solvent cage to form the hydrotrioxide, RN(OOOH)NHR. The decomposition of these very labile hydrotrioxides involves the homolytic scission of the RO-OOH bond with subsequent in cage formation of the diazene-N-oxide and hydrogen peroxide. Although 1,2-diphenyldiazene is unreactive toward ozone under conditions investigated, 1,2-dimethyldiazene reacts with relative ease to yield 1,2-dimethyldiazene-N-oxide and singlet oxygen (O₂, ¹Δ_g). The subsequent reaction sequence between these two components to yield nitromethane as the final product is discussed. The formation of formic acid and nitromethane in the ozonolysis of 1,2-dimethylhydrazine is explained as being due to the abstraction of a methyl H atom of the CH₃NNHCH₃^. radical by HOOO ^. in the solvent cage. The possible mechanism of the reaction of the initially formed formaldehyde methylhydrazone (and HOOOH) with ozone/oxygen mixtures to produce formic acid and nitromethane is also discussed.

Full text of DOI:10.1021/ja036801u

Published on Web 08/26/2003
Mechanism of Formation of Hydrogen Trioxide (HOOOH) in
the Ozonation of 1,2-Diphenylhydrazine and
1,2-Dimethylhydrazine: An Experimental and Theoretical
Investigation
Bozˇo Plesnicˇar,*,† Tell Tuttle,‡ Janez Cerkovnik,† Jozˇe Koller,† and Dieter Cremer*,‡
Contribution from the Department of Chemistry, Faculty of Chemistry and Chemical
Technology, UniVersity of Ljubljana, P.O. Box 537, 1000 Ljubljana, SloVenia, and the
Department of Theoretical Chemistry, UniVersity of Go¨teborg, Reutersgatan 2, S-41320,
Go¨teborg, Sweden
Received June 20, 2003; E-mail: bozo.plesnicar@uni-lj.si; cremer@theoc.gu.se
Abstract: Low-temperature (-78 °C) ozonation of 1,2-diphenylhydrazine in various oxygen bases as
solvents (acetone-d6, methyl acetate, tert-butyl methyl ether) produced hydrogen trioxide (HOOOH), 1,2-
diphenyldiazene, 1,2-diphenyldiazene-N-oxide, and hydrogen peroxide. Ozonation of 1,2-dimethylhydrazine
produced besides HOOOH, 1,2-dimethyldiazene, 1,2-dimethyldiazene-N-oxide and hydrogen peroxide, also
formic acid and nitromethane. Kinetic and activation parameters for the decomposition of the HOOOH
produced in this way, and identified by 1H, 2H, and 17O NMR spectroscopy, are in agreement with our
previous proposal that water participates in this reaction as a bifunctional catalyst in a polar decomposition
process to produce water and singlet oxygen (O2, 1∆g). The possibility that hydrogen peroxide is, besides
water, also involved in the decomposition of hydrogen trioxide is also considered. The half-life of HOOOH
at room temperature (20 °C) is 16 ( 1 min in all solvents investigated. Using a variety of DFT methods
(restricted, broken-symmetry unrestricted, self-interaction corrected) in connection with the B3LYP functional,
a stepwise mechanism involving the hydrotrioxyl (HOOO•) radical is proposed for the ozonation of hydrazines
(RNHNHR, R ) H, Ph, Me) that involves the abstraction of the N-hydrogen atom by ozone to form a radical
pair, RNNHR• •OOOH. The hydrotrioxyl radical can then either abstract the remaining N(H) hydrogen atom
from the RNNHR• radical to form the corresponding diazene (RNdNR), or recombines with RNNHR• in a
solvent cage to form the hydrotrioxide, RN(OOOH)NHR. The decomposition of these very labile
hydrotrioxides involves the homolytic scission of the RO-OOH bond with subsequent “in cage” formation
of the diazene-N-oxide and hydrogen peroxide. Although 1,2-diphenyldiazene is unreactive toward ozone
under conditions investigated, 1,2-dimethyldiazene reacts with relative ease to yield 1,2-dimethyldiazene-
N-oxide and singlet oxygen (O2, 1∆g). The subsequent reaction sequence between these two components
to yield nitromethane as the final product is discussed. The formation of formic acid and nitromethane in
the ozonolysis of 1,2-dimethylhydrazine is explained as being due to the abstraction of a methyl H atom of
•
the CH3NNHCH3 radical by HOOO• in the solvent cage. The possible mechanism of the reaction of the
initially formed formaldehyde methylhydrazone (and HOOOH) with ozone/oxygen mixtures to produce formic
acid and nitromethane is also discussed.
Introduction
ozonation of these compounds, which date back to the early
20th century. Molinari4a as well as Strecker and Baltes4b studied
the ozonation of 1,2-diphenylhydrazine (hydrazobenzene) and
reported 1,2-diphenyldiazene (azobenzene) to be the only
reaction product (for a summary of these studies, see ref 2).
Hydrazines occur in the environment,5,5c and it is by now
well established that humans exposed to these compounds by
drinking contaminated water and by inhaling contaminated air
may develop adverse systemic health effects or cancer.5 It is,
Reactions of saturated organic compounds with ozone are at
the focus of current research.1 Among the various less explored
topics of research in this field are the reactions of substituted
hydrazines with ozone. Although the oxidation of both aliphatic
and aromatic hydrazines with various oxidants has been
extensively studied,2,3 there are only a few reports on the
† Department of Chemistry, Faculty of Chemistry and Chemical Technol-
ogy, University of Ljubljana.
‡ Department of Theoretical Chemistry, University of Go¨teborg.
(1) (a) Plesnicˇar, B.; Cerkovnik, J.; Tuttle, T.; Kraka, E.; Cremer, D. J. Am.
Chem. Soc. 2002, 124, 11 260, and references therein. (b) Munoz, F.; Mvula,
E.; Braslavsky, S. E.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 2,
2001, 1109.
(3) (a) Nanni, E. J.; Sawyer, D. T. J. Am. Chem. Soc. 1980, 102, 7591. (b)
Calderwood, T. S.; Johlman, C. L.; Roberts, J. L.; Wilkins, C. L.; Sawyer,
D. T. J. Am. Chem. Soc. 1984, 106, 4683.
(4) (a) Molinari, E. Chem. Ber. 1907, 40, 4154. (b) Strecker, W.; Baltes, M.
Chem. Ber. 1921, 54, 2693. (c) Bailey, P. S. Ozonation in Organic
Chemistry; Academic Press: New York, 1982; Vol II, p 198.
(2) Newbold, B. T. In The Chemistry of the Hydrazo, Azo and Azoxy Groups;
Patai, S., Ed.; Wiley: New York, 1975; pp 541-597.
9
10.1021/ja036801u CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 11553-11564
11553

A R T I C L E S
Plesnicˇar et al.
therefore, obvious that detailed knowledge about the oxidation
pathways of these hazardous compounds is of the utmost
importance.
Diphenylhydrazine (Aldrich) was crystallized from methanol
before use.10a 1,2-Dideutero-1,2-diphenylhydrazine was syn-
thesized by Zn/ND4Cl reduction of 1,2-diphenyldiazene.10b
1,2-Diphenyldiazene-N-oxide was prepared by oxidation of 1,2-
diphenyldiazene with 3-chloroperoxybenzoic acid in dichlo-
romethane,11 and from reductive coupling of nitrobenzene with
Zn/glucose.10a 4-Methoxy-1,2-diphenylhydrazine,10b 4-methoxy-
1,2-diphenyldiazene-N-oxide, and 4-methoxy-1,2-diphenyldia-
zene-N′-oxide were prepared according to the literature proce-
dures.11 1,2-Dimethylhydrazine was obtained by neutralization
of its dihydrochloride with sodium hydroxide,12 and 1,2-
dimethyldiazene by HgO oxidation of 1,2-dimethylhydrazine
dihydrochloride.13
Ozonation Procedure. Ozone-oxygen mixtures were pro-
duced by flowing oxygen through a Welsbach T-816 ozonator.6
Ozone-nitrogen mixtures were obtained as already reported. The
concentration of ozone in the gas stream was measured
according to the literature procedure. 17O-enriched ozone was
generated by flowing 17O-enriched oxygen (58% 17O2, ISOTEC)
through a semimicro ozonator.6d
We have already reported in a preliminary form that the
reaction of 1,2-diphenylhydrazine with ozone is more complex
than originally believed, and that hydrogen trioxide (HOOOH)
is an important intermediate formed in these reactions.6 The
hydrazine-ozone reaction appears to be currently the most
reliable and efficient procedure for the production of relatively
highly concentrated solutions of HOOOH in various organic
solvents without the interfering presence of other organic
hydrotrioxides (ROOOH).7,8 Recent evidence even indicates that
HOOOH is formed as an intermediate in H2O2 and ozone
production from water and singlet oxygen by antibodies.9
Therefore, we report here the results of detailed experimental
and theoretical studies of the ozonation of 1,2-diphenylhydrazine
and 1,2-dimethylhydrazine.
Experimental Section
Instrumentation. Low-temperature 1H, 2H, 13C, and 17O
NMR spectra were recorded on a Bruker Avance 300 DPX (1H
WARNING: Although we have not had any accidents in
handling solutions of peroxides and polyoxides formed in the
ozonation of hydrazines, care should be exercised in handling
dried and concentrated solutions of these potentially hazardous
compounds.
2
NMR, 300.3 MHz; H NMR, 46.07 MHz; 13C NMR, 75.48
MHz; 17O NMR, 40.70 MHz) and on Varian Unity Inova-600
spectrometers (1H NMR, 600.09 MHz; 17O NMR, 81.37 MHz)
with TMS (1H and 13C NMR), acetone-d6 (2H NMR), and H217O
(17O NMR) as internal standards. GC/MS was performed on a
Hewlett-Packard 6890 chromatograph (HP-5MS column). Near-
IR spectra were recorded on Perkin-Elmer UV/VIS/NIR Lambda
19 spectrometer.
Product Analysis. Reaction products of the ozonation of 1,2-
diphenylhydrazine and 1,2-dimethylhydrazine were determined
by a combination of methods.6d All products, except hydrogen
peroxide, were determined by GC/MS, by using calibrated
internal standards and known reference materials. All products
were also collected and identified by NMR. Hydrogen peroxide
Materials. All solvents were the purest commercially avail-
able products and were (except of acetone-d6) rigorously dried
and distilled according to the literature methods. The purity was
checked by GC/MS. 1,2-Diphenyldiazene (99%, Aldrich),
nitrobenzene (99+%, Aldrich), 1,2-dimethylhydrazine dihydro-
chloride (99+%, Aldrich), nitromethane (99+%, Aldrich), 9,-
10-dimethylanthracene (99%, Aldrich), and 3-chloroperoxy-
benzoic acid (77%, Aldrich) were used as received. 1,2-
1
was determined by H NMR (and 17O NMR) and by other
analytical methods that have been described previously.6d
Kinetic Studies. Kinetics of the decomposition of hydrogen
trioxide was performed by following the decay of the OOOH
1
absorption by H NMR, using TMS as the internal standard.
Kinetic and activation parameters were obtained by standard
procedures.6
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employed the B3LYP hybrid functional15 because the latter
provides an acceptable cost-efficiency ratio. However, there
were several intermediates and transition states (TS) with
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11554 J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003

Formation of Hydrogen Trioxide
A R T I C L E S
significant multireference character that are difficult to describe
with restricted or unrestricted DFT (RDFT and UDFT).
Therefore, we routinely carried out stability tests16 and used
broken-symmetry UDFT (BS-UDFT) when needed (i.e., in case
of negative eigenvalues of the stability matrix). BS-UDFT im-
plicitly provides a two-configurational description and leads to
reasonable results provided the splitting between the low-spin
state in question and the associated high-spin state is relatively
small.17,18 In this connection it has to be mentioned that standard
functionals based on the generalized gradient approximation
(GGA) account for a significant amount of long-range correla-
tion,19 but lead to a double-counting of correlation effects when
used in connection with BS-UDFT.20 This was another reason
why B3LYP rather than BLYP was used in this work.20
The fact that DFT with the exchange functionals presently
in use accounts for unspecified nondynamic electron correlation
is closely connected with the self-interaction error (SIE) of these
functionals.19,21 The SIE is often the reason for the good
performance of DFT in cases where other single determinant
methods fail.19,21 However, the SIE also results in a serious
failure of DFT when odd-electron situations occur in a chemical
reaction.19-22 For example, H abstraction by a radical (three-
electron situation) is wrongly described by any exchange
functional based on either the local density approximation or
GGA.23 The TS of the reaction is severely underestimated thus
leading often to reactions without barrier. We use in these
situations self-interaction corrected DFT (SIC-DFT) based on
the Perdew-Zunger approach24 and programmed in a pertur-
bational (PSIC) and a self-consistent version (SCF-SIC).19-21
For details, see ref 25.
were carried out with standard procedures based on analytical
energy gradients. Frequency calculations were performed to
characterize the optimized structures as minima or transition
states, where the transition states were found to each have a
single imaginary frequency. In addition, the vibrational frequencies
were used to obtain temperature corrected energies, enthalpies,
entropies, and free energies. For the purpose of modeling the
effects of solvation, the PISA continuum model27 was applied
using acetone as a solvent (dielectric constant ꢀ ) 20.728).
van der Waals complexes are difficult to calculate with DFT.29
However, if complexes are stabilized by H-bonding, which
include both electrostatic and some covalent bonding, DFT
performs better.30 For the purpose of getting reasonable complex
binding energies, we apply the counterpoise method31 to reduce
the basis set superposition error (BSSE).
In some cases, cyclic transition states were optimized using
the ring puckering coordinates of Cremer and Pople.32 These
coordinates make it possible to freeze individual puckering
parameters and to carry out constrained optimizations. For this
purpose the program PUCKER33 was used, which is part of the
quantum chemical package COLOGNE 2003.34 All calculations
were performed with the quantum chemical program packages
COLOGNE 200334 and Gaussian 98.35
Results and Discussion
In the following we will discuss first the ozonation of 1,2-
diphenylhydrazine, then that of 1,2-dimethylhydrazine. Calcula-
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TS energies were determined by SIC-DFT using the following
procedure. The geometry of the DFT calculation was used for
PSIC and SCF-SIC single point calculations. In this way an
estimate of the improved barrier was found. If a TS geometry
could not be determined, that found for a derivative was used
to obtain by single point calculation a SIC-DFT barrier.
Alternatively, increments were used determined for a homo-
logue. Calculations showed that in this way consistent reaction
barriers could be obtained. The corresponding activation en-
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the DFT calculation or estimating the latter from values obtained
for another hydrazine.
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M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.9,
Gaussian, Inc., Pittsburgh, PA, 1998.
(21) (a) Polo, V.; Kraka, E.; Cremer, D. Mol. Phys. 2002, 100, 1771. (b) Polo,
V.; Kraka, E.; Cremer, D. Theor. Chem. Acc. 2002, 107, 291.
(22) (a) Noodleman, L.; Post, D.; Baerends, E. J. Chem. Phys., 1982, 64, 159.
(b) Bally, T.; Sastry, G. N. J. Phys. Chem. A 1997, 101, 7923. (c) Sodupe,
M.; Bertran, J.; Rodriguez-Santiago, L.; Baerends, E. J. J. Phys. Chem. A
1999, 103, 166.
(23) (a) Merkle, R.; Savin, A.; Preuss, H. J. Chem. Phys. 1992, 97, 9216. (b)
Chermette, H.; Ciofini, I.; Mariotti, F.; Daul, C. J. Chem. Phys. 2001, 114,
1447.
(24) Perdew, J. P.; Zunger, A. Phys. ReV. B, 1981, 23, 5048.
(25) Gra¨fenstein, J.; Kraka, E.; Cremer, D. J. Chem. Phys, submitted.
9
J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003 11555

A R T I C L E S
Plesnicˇar et al.
tions have also been carried out for the ozonation of hydrazine
itself to have a suitable reference for assessing substituent
effects.
1,2-Diphenylhydrazine (1c). Ozonation of 1,2-diphenylhy-
drazine with ozone-nitrogen or ozone-oxygen mixtures in
acetone-d6 or methyl acetate (0.1-0.5 M) at -78 °C produced
an oxygen-rich intermediate with the OOOH 1H NMR absorp-
tion at 13.3 ( 0.3 ppm downfield from Me4Si. Furthermore,
ozonation of 1,2-dideutero-1,2-diphenylhydrazine in acetone at
-78 °C produced a somewhat broader OOOD 2H NMR
absorption at 13 ppm (11 ppm relative to acetone-d6 absorption).
This polyoxide with exchangeable protons (fast exchange with
D2O and CH3OD at -60 °C) was assigned to HOOOH
(DOOOD) using 17O NMR spectroscopy for a highly 17O-
enriched species. This assignment was confirmed by 17O NMR
chemical shift calculations using either GIAO/MP2/6-311++G-
(d,p)36,6c or GIAO/CCSD(T)37 with a (11s7p2d/7s2p)[6s4p2d/
4s2p] basis.38 Results of both sets of calculations are in good
agreement with the experimental values (see Figure 1).39 The
yield of HOOOH was estimated to be 45 ( 10% (at -60 °C).
A detailed investigation of other products of ozonation of 1c
(at -60 °C) by GC/MS and NMR revealed the presence of 1,2-
diphenyldiazene (trans isomer, 65 ( 5%, mol %), 1,2-diphe-
nyldiazene-N-oxide (azoxybenzene)(35 ( 5%), hydrogen per-
oxide (30 ( 10%), and water. Ozonation of 1,2-dideutero-1,2-
diphenylhydrazine yielded deuterated hydrogen peroxide and
water.
Ozonation of an asymmetric hydrazine, i.e., 4-methoxy-
1,2-diphenylhydrazine produced, besides 4-methoxy-1,2-
diphenyldiazene, also 4-methoxy-1,2-diphenyldiazene-N′-oxide
(MeOC6H4NdN(O)C6H5).
Figure 1. (A) 1H and (B) 17O NMR spectra of hydrogen trioxide generated
by the low-temperature ozonation of 1,2-diphenylhydrazine in acetone-d6
at -20 °C. All absorptions of hydrogen trioxide completely disappeared at
+ 20 °C. (B) 17O NMR chemical shifts in parentheses refer to GIAO/MP2/
6-311++G** values6c those in brackets to GIAO/CCSD(T)/qz2p values.38
Similar results were also obtained in tert-butyl methyl ether
as solvent, i.e., HOOOH (30 ( 10%), 1,2-diphenyldiazene (75
( 5%), 1,2-diphenyldiazene-N-oxide (25 ( 5%), hydrogen
peroxide (10 ( 5%), and water. Control experiments showed
that in the ozonation of 1,2-diphenyldiazene in acetone-d6 or
tert-butyl methyl ether at low temperatures, no 1,2-diphenyl-
diazene-N-oxide could be detected and 1,2-diphenyldiazene was
recovered quantitatively.
pair 3 can recombine to form hydrazine hydrotrioxide 6, which
can dissociate to radicals 7 and then form diazene-N-oxide and
HOOH (8).
Calculated energies, enthalpies, and entropies for structures
1-8 are summarized in Table 1.
In Figure 2, calculated enthalpies for path A are shown for
R ) Ph, R ) Me, and R ) H, those for path B are given in
Figure 3 for the case R ) Ph. Ozone and hydrazine form a
H-bonded complex with a binding energy of 8 kcal/mol (6-
311++G(3df,3pd): ∆H(298) ) -3.2 kcal/mol, Table 1). In
the case of 1,2-dimethylhydrazine one NH and one CH bond
are involved in H-bonding thus increasing the complex stability
to 9 kcal/mol (for structural details, see Figure 4 and the
Supporting Information). Neither of these arrangements is
possible in the case of 1,2-diphenylhydrazine so that the complex
stability is reduced to 7.4 kcal/mol.
The proposed mechanisms of the ozonation of 1,2-diphenyl-
hydrazine is illustrated in Scheme 1.
The presence of both 1,2-diphenyldiazene and 1,2-diphenyl-
diazene-N-oxide suggests that two different competitive reaction
mechanisms are operative in the ozonation of 1,2-diphenylhy-
drazine (path A and B in Scheme 1). Path A implies the stepwise
H abstraction from hydrazine by ozone thus yielding via a
H-bonded hydrazine-ozone complex 2, an intermediate RNHNR•
•OOOH radical pair 3, the diazene-hydrogen trioxide complex
4, which can dissociate to the separated products diazene and
hydrogen trioxide (5).40 Alternatively, the intermediate radical
The activation enthalpy for H abstraction could only be
estimated to be close to 14 kcal/mol for the parent system, i.e.,
TS(2a-3a), using PSIC/B3LYP (just 5 kcal/mol at B3LYP/6-
311G++G(3df,3pd)). DFT severely underestimates the barrier
(36) (a) Gauss, J. Chem. Phys. Lett. 1992, 191, 614. (b) Gauss, J. J. Chem.
Phys, 1993, 99, 3629.
(37) Gauss, J.; Stanton, J. F. J. Chem. Phys. 1996, 104, 2574.
(38) Wu, A.; Cremer, D.; Gauss, J., in press.
(39) Our preliminary experiments by using low-temperature (-40 °C) FT IR
immersion probe (Mettler Toledo React IR 1000) revealed a typical O-H
stretching vibration at 3209 ( 15 cm-1, which disappeared upon warming
of the reaction mixture to room temperature. This vibration was tentatively
assigned to HOOOH (0.3-0.5 M in acetone). For comparison, the OH
stretching vibration for H2O2, also present in the reaction mixture, was
3350 ( 15 cm-1 under the same conditions. (Lendero, N.; Cerkovnik, J.;
Plesnicˇar, B. In preparation.)
(40) Pryor et al. considered in the ozonation of saturated hydrocarbons, alcohols,
and ethers the H abstraction and the hydride abstraction as two extremes
between which the initial reaction complex could choose according to
solvent, temperature, and other reaction conditions. Giamalva, D. H.;
Church, D. F.; Pryor, W. A. J. Am. Chem. Soc. 1986, 108, 7678. Giamalva,
D. H.; Church, D. F.; Pryor, W. A. J. Org. Chem. 1988, 53, 3429. See
also: Hellman, T. M.; Hamilton, G. A. J. Am. Chem. Soc. 1974, 96, 1530.
Nangia, P. S.; Benson, S. W. J. Am. Chem. Soc. 1980, 102, 3105.
9
11556 J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003

Formation of Hydrogen Trioxide
A R T I C L E S
Scheme 1
due to the three-electron situation in the TS (ozone acts as a
biradical thus leading to O• H• •N and the resulting large SIE
lowers the energy) yielding for TS(2b-3b) and TS(2c-3c) even
energies below that of 2. The methyl and the phenyl group
facilitate H abstraction considerably because the hydrazine
radical 3 formed is stabilized by either hyperconjugation or
π-conjugation with a phenyl ring. This stabilizing nature of
phenyl substituents for the radical intermediate has been
observed experimentally by Zhao and co-workers41 in the
measurement of bond dissociation energies.
The HOOO• radical formed in the first step can abstract a
second H atom of hydrazine directly in the solvent cage and
form a rather stable H-bonded complex between diazene and
HOOOH (4), which is 7 to 8 kcal/mol more stable than the
separated products (5, Figure 2). The formation of 4 from 2 is
exothermic by 35.1 (2a f 4a), 44.9 (2b f 4b), and 41.6 kcal/
mol (2c f 4c) thus reflecting the increased stabilization
possibilities of the substituents in the diazene (hyperconjugation
and π-conjugation). The kinetics of the reaction should be
determined by the first reaction step, which should be fast
considering the low barriers and the energy gained by the
formation of complex 2.42
The second H-abstraction is also a rapid reaction (estimated
activation enthalpies, see Figure 2), in particular if it takes place
in the solvent cage and if the second terminal O atom is already
oriented toward the second NH bond as in the case of 2a (Figure
4). A previous investigation by Ingemann and co-workers43
concluded that the abstraction of the H(N) atoms from the 1,2-
diphenylhydrazine requires a larger energy than the subsequent
H abstraction from the resulting radical, based on an analysis
of the relative N-H bond energies of the two species. In the
case of hydrazine, the first bond dissociation energy is 87.5 kcal/
mol, whereas the second leading to diazene requires just 42.6
kcal/mol.44
For both the methyl and the phenyl derivative a reorientation
of the HOOO• radical in the solvent cage is required to initiate
the second H abstraction (the second terminal O atom is oriented
in 2b toward a methyl hydrogen or away from the hydrazine
frame, see Figure 4). Such a reorientation can easily lead to a
radical recombination reaction and the formation of hydrotri-
(42) Although no detailed experimental studies on the ozonation of the parent
hydrazine were undertaken, it was obvious from our preliminary experi-
ments that HOOOH was formed in good yields in all the solvents
investigated.
(43) Ingemann, S.; Fokkens, R. H.; Nibbering, N. M. M. J. Org. Chem. 1991,
56, 607.
(44) Luo, Y.-R. Handbook of Bond Dissociation Energies in Organic Com-
pounds; CRC Press: Boca Raton, 2003.
(41) Zhao, Y.; Bordwell, F. G.; Cheng, J.-P.; Wang, D. J. Am. Chem. Soc. 1997,
119, 9125.
9
J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003 11557

A R T I C L E S
Plesnicˇar et al.
Table 1. Energies, Enthalpies, Entropies, and Dipole Moments of
Structures 1-8 with R ) H, R ) Me, and R ) Ph Calculated at
B3LYP/6-31G(d,p)a
1,2-diphenyldiazene-N-oxide (25 ( 5%) produced via path B
is larger than that of HOOH (10 ( 5%). The loss of hydrogen
trioxide can result from its decomposition into O2 (1∆g)
catalyzed by either water or hydrogen peroxide molecules
(reactions I-1 and I-2, in Scheme 2) as previously suggested by
us.6,9b The decomposition of HOOOH was investigated at higher
temperatures. The yield of singlet oxygen was determined with
the use of a typical O2 (1∆g) acceptor, i.e., 9,10-dimethylan-
thracene, to be 20 ( 10% (9,10-dimethylanthracene endoper-
oxide)45 in acetone-d6 and methyl acetate. The formation of O2
(1∆g) in the decomposition of HOOOH was also confirmed by
chemiluminescence in the infrared region with a maximum at
1272 nm (acetone-d6, -10 °C). The kinetic and activation
parameters for the decomposition of this simplest of polyoxides
(Table 2) are in accord with the mechanism shown in Scheme
2 (reactions I-1 and I-2) and our previous proposal6,9b although
the calculated activation enthalpies (18.8 and 17.3 kcal/mol,
Scheme 2) are 2-3 kcal/mol higher than the measured Arrhenius
energies (15.3-16.7 kcal/mol, Table 2). This difference is
obviously the result of the effect that in a solvent and with
additional water molecules present the catalytic effect is more
efficient. In any case, water or hydrogen peroxide can participate
in the decomposition of HOOOH as a bifunctional catalyst in
a “polar” decomposition process to yielding O2 (1∆g).
It should be pointed out that in all cases under investigation
sufficient water was present after the ozonation of 1,2-
diphenylhydrazine to complex all the HOOOH present in
solutions (see Figure 3.) Because water was found to be labeled
with 17O (by 17O NMR), it was most likely formed, at least in
large part, during the ozonation by the decomposition of
hydrogen trioxide as described in Scheme 2.46,47
Xu, Muller, and Goddard9b investigated reactions between
simple polyoxides using similar methods (B3LYP calculations
with 6-31G(d,p) and cc-pVTZ basis sets) as those employed in
this work. Their reaction and activation enthalpy for reaction
I-1 are close to what we obtain. In addition, these authors suggest
two other decomposition paths of hydrogen trioxide (reactions
II and III in Scheme 2). In the case that local concentrations
make the formation of a hydrogen trioxide dimer possible (one
HOOOH adopts the 3.5 kcal/mol less stable Cs-symmetrical cis
conformation by flip-flop rotation48), this can initiate the
decomposition to O2 (1∆g) (or O2 (3Σg-)) and two HOOH
molecules via intermediate O3 and H2O4 complexes.9b The rate
determining activation enthalpy of this process was calculated
to be 19.7 kcal/mol. Alternatively, O2 (1∆g) formed in the
solution reacts directly with hydrogen trioxide (activation
enthalpy: 12.0 kcal/mol, reaction III in Scheme 2) thus leading
to ozone and HOOH.
system
sym
∆E
ΣZPE ∆H(298)
ΣS
∆G(298)
µ
R ) H
1a
2a
C2
C2
C1
C1
0.0
38.1
38.1
39.2
39.2
36.2
36.2
0.0 113.4
0.0 2.14, 0.61
0.0 1.98, 0.65
2.0 0.01
6.8 0.27
8.3 2.3
0.0
-8.7
-4.0
-0.8
-4.6
-9.8
-8.1
0.0 113.4
-8.0
-3.2
-3.3
-7.1
79.9
79.9
74.6
74.6
TS(2a-3a) C1
C1
C1
C1
4.5 2.56
3a
4a
5a
6a
7a
8a
36.0 -11.5 119.5 -13.3 2.61, 1.22
36.0
38.5 -43.1
38.5 -39.3
36.8 -36.1 112.9 -35.9 0, 1.12
36.8 -35.6 112.9 -35.4 0, 1.15
40.4 -32.3
40.4 -28.1
-9.8 119.5 -11.6 2.54, 1.15
C1 -43.7
C1 -39.9
C2h -35.0
C2h -34.5
C1 -33.8
C1 -29.6
C1 -32.5
C1 -32.5
Cs -65.4
Cs -67.6
85.4 -34.8 1.65
85.4 -31.0 1.55
75.7 -21.1 2.38
75.7 -26.9 2.42
36.8 -33.4 119.7 -35.3 1.67, 2.26
36.8 -33.4 119.7 -35.3 1.83, 2.22
38.4 -65.0 114.7 -65.4 1.17, 1.76
38.4 -67.2 114.7 -67.6 1.22, 1.64
R ) Me
1b
2b
3b
4b
5b
6b
7b
8b
C2
C1
0.0
-9.4
73.5
74.1
0.0 128.5
-9.0 92.1
0.0 1.54, 0.61
1.8 2.66
C1 -14.3
C1 -54.4
C2h -44.5
C1 -35.0
C1 -35.3
Cs -73.9
71.3 -16.0 136.6 -18.4 2.05, 1.22
73.8 -53.9 85.4 -45.9 1.89
72.1 -45.6 136.6 -46.2 0, 1.13
75.1 -33.8 89.7 -22.3 2.62
72.3 -35.9 135.8 -38.1 1.83, 2.26
73.4 -73.7 129.9 -74.1 1.58, 1.76
R ) Ph
1c
2c
3c
4c
5c
6c
7c
8c
C2
C1
0.0 139.6
-8.4 140.4
0.0 164.6
0.0 1.91, 0.61
3.2 2.52
-7.4 129.2
C1 -19.3 138.0 -20.5 170.4 -22.2 1.68, 1.23
C1 -50.1 140.2 -49.0 134.1 -39.9 1.54
C2h -41.4 138.9 -41.9 166.5 -42.5 0, 1.13
C1 -27.3 140.9 -26.2 125.7 -14.6 2.15
C1 -32.3 138.5 -32.8 169.4 -34.3 1.85, 2.26
Cs -64.5 139.8 -65.0 164.7 -65.0 1.75, 1.76
a For a definition of structures see Scheme 1. Relative energies (∆E),
zero point energies (ZPE), relative enthalpies (∆H(298)), and relative Gibb’s
free energies (∆G(298)) in kcal/mol, dipole moments (µ) in Debye, entropies
(S) in cal mol/deg. - The symmetry in column Sym is given with regard
to the first molecule in a system of two molecules; the same holds for
the dipole moment. Data in italics has been calculated with the
6-311++G(3df,3pd) basis in conjunction with the B3LYP functional.
oxide 6. Although the formation of 6c is just exothermic by
5.7 kcal/mol (∆H(298), Figure 3), the hydrotrioxide should
possess so much excess energy to surmount directly a small
barrier (both DFT and SIC-DFT calculations could not locate
TS(6c-7c)) and to decompose to a hydrazine-N-oxyl radical and
the HOO• radical (7, Figure 3) in another exothermic reaction
(∆H(298) ) -6.6 kcal/mol, (Table 1). H-abstraction by the
HOO• radical leads to an diazene-N-oxide zwitterion and HOOH
(8).
The relative abundance of the 1,2-diphenyldiazene measured
experimentally suggests that path A is the preferred mechanism,
which seems to imply that a reorientation of the HOOO• radical
in the solvent cage for the recombination reaction is more
difficult than the H abstraction reaction. The required movement
(a head-tail exchange of the HOOO• radical) might only be
possible outside the solvent cage and therefore less frequent
than the H-abstraction reaction in mechanism A.
(45) (a) Corey, E. J.; Mehrotra, M. M.; Khan, A. U. J. Am. Chem. Soc. 1986,
108, 2472. (b) Clennan, E. L.; Foote, C. S. In Organic Peroxides; Ando,
W. Ed.; Wiley: New York, 1992; p 255.
(46) Lerner et al. have reported a surprising observation that ozone is formed
(besides hydrogen peroxide) in biological systems and that the first step of
its formation most likely involves the antibody catalyzed conversion of
molecular singlet oxygen and water to HOOOH (see ref 9).
(47) Water is most probably also formed in the reaction of ozone with hydrogen
trioxide and hydrogen peroxide (Xu, X.; Goddard, W. A., III. Proc. Natl.
Acad. Sci. USA, 2002, 99, 15 308). However, control experiments (-78
°C) showed that these processes are rather slow under experimental
conditions investigated.
(48) MP2/6-31G(d) calculations: D. Cremer, J. Chem. Phys., 1978, 69, 4456.
- The flip-flop rotation of HOOOH from its most stable C2-symmetrical
conformation into the mirror image of this conformation proceeds via the
Cs-symmetrical form and resembles the pseudorotation of a five-membered
ring.
The experimental yield of the hydrogen trioxide generated
via Path A (30 ( 10%) is ca. 45% lower than the corresponding
yield of the 1,2-diphenyldiazene (75 ( 5%). Also, the yield of
9
11558 J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003

Formation of Hydrogen Trioxide
A R T I C L E S
Figure 2. Enthalpy profile for the ozonation of hydrazine (normal numbers), 1,2-dimethylhydrazine (numbers in parentheses), and 1,2-diphenylhydrazine
(numbers in brackets) according to path A (see Scheme 1). B3LYP/6-31G(d,p) calculations using RDFT, BS-UDFT, and SIC-DFT.
Figure 3. Enthalpy profile for the ozonation of 1,2-diphenylhydrazine according to paths A and B (see Scheme 1). B3LYP/6-31G(d,p) calculations using
RDFT, BS-UDFT, and SIC-DFT.
Hence, there are several decomposition possibilities for
HOOOH, which explains the relatively high loss (45%) of this
polyoxide in the reaction mixture. For HOOH, a low energy
decomposition path according to reaction Ia in Scheme 2 does
not exist. The most likely decomposition reaction (observed for
concentrated hydrogen peroxide solutions and also investigated
by Xu, Muller, and Goddard9b) starts from a dimer and yields
two water and a O2 (1∆g) molecule. The activation enthalpy is
26.9 kcal/mol,9b but can be somewhat lowered by the inclusion
of two water molecules in the reaction mechanism. In any case,
the decomposition of hydrogen peroxide is slower than that of
hydrogen trioxide and can proceed via essentially one mecha-
nism (in solutions free of acids and metal ions, which also can
act as catalysts). This seems to explain the smaller degree of
decomposition of HOOH as compared to that found for
HOOOH.
1,2-Dimethylhydrazine (1b). Ozonation of 1,2-dimethylhy-
drazine with ozone-oxygen or ozone-nitrogen mixtures in
acetone-d6 (0.01-0.1 M) at -78 °C yielded 1,2-dimethyldiazene
(trans isomer, 20 ( 10%, mol %), 1,2-dimethyldiazene-N-oxide
9
J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003 11559

A R T I C L E S
Plesnicˇar et al.
Figure 4. B3LYP/6-31G(d,p) geometries for selected structures. Distances in Å, angles in deg. For other geometries, see the Supporting Information.
(50 ( 10%), formic acid (15 ( 5%), nitromethane (10 ( 5%),
hydrogen trioxide (45 ( 10%), hydrogen peroxide (55 ( 10%),
and water. The increased yield of 1,2-dimethyldiazene-N-oxide
relative to the phenyl system is most likely due to subsequent
oxidation of the dimethyldiazene. Control experiments showed
that 1,2-dimethyldiazene reacted slowly with ozone (-78 °C)
to form 1,2-dimethyldiazene-N-oxide (90 mol %) and ni-
tromethane (10%) as the main reaction products. The proposed
mechanism of this oxidation is illustrated in Scheme 3.
Oxidation of the diazene with ozone was originally thought
to occur in a concerted symmetry-allowed [4π + 2π] reaction
via the generation of a primary ozonide. However, neither the
corresponding TS nor the ozonide ring could be located on the
potential energy surface. When performing constrained opti-
mizations with the help of the puckering coordinates32 (freezing,
e.g., the phase angle33) structures were found (testing both cis-
and trans-diazenes) that correspond to the breaking off of a
singlet oxygen molecule and that are already in the exit channel
of an O-transfer reaction from ozone to diazene (Scheme 3 and
Figure 5). The instability of the ozonide ring system results from
strong destabilizing lone pair-lone pair (lp-lp) repulsion (a
planar diazaozonide would be a 10π system with two occupied
antibondoing π* MOs). The nature of lp-lp repulsion implies
that such a system will not be significantly stabilized regardless
of the degree of puckering or the substituents of the diazene
system. The formation of 1,2-dimethyldiazene-N-oxide can only
occur via a stepwise process involving an in-plane attack of
the electrophilic ozone molecule at an electron lp of one of the
diazene N atoms and the O transfer from the ozone molecule
to the diazene.
This conclusion implies an important corollary: If the in-
plane attack of ozone is sterically hindered as in the case of
9
11560 J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003

Formation of Hydrogen Trioxide
A R T I C L E S
Table 2. Proton Chemical Shifts, Rate Constants, Frequency
Scheme 2
Factors, and Activation Energies for the Decomposition of
Hydrogen Trioxide (HOOOH) Formed in the Ozonation of
1,2-Diphenylhydrazine (1a) and 1,2-Dimethylhydrazine (1b) in
Various Solventsa
solvent
T (°C) δ (ppm) 104 k(T),s-1 E (kcal mol-1
)
log A
a
1a acetone-d6 -30 13.24
(0.16)b
0.1
-20 13.11
13.19
-10 13.02
13.10
(0.55)b
0.25
(1.58)b
[1.5]c
1.4
13.09
0
12.93
13.00
13.00
16.7 ( 1.5
9.4 ( 1.0
(5.0)b
[4.6]c
2.5
(15.9 ( 1.4)b (9.5 ( 1.0)b
[16.0 ( 1.4]c [9.5 ( 1.0]c
10 12.84
12.90 (14.5)b
12.88 [13.1]c
20 12.75
30 12.67
6.8
17.4
0.4
methyl
acetate
-10 12.47
0
12.38
1.43
10 12.29
15 12.25
20 12.21
30 12.14
-10 12.62
4.3
7.2
12.4
29.0
16.5 ( 1.5
15.3 ( 1.3
9.5 ( 1.1
8.3 ( 0.9
tert-butyl
methyl
ether
0.38
0
5
12.55
12.54
0.95
1.37
2.4
5.1
5.7
11.4
17.0
0.26
0.35
0.81
1.76
2.28
4.04
6.50
0.63
1.35
2.45
4.05
5.51
10 12.50
15 12.46
20 12.41
25 12.39
30 12.36
1b acetone-d6 -10 13.10
-5 13.05
0
5
12.99
12.95
17.0 ( 1.6
16.7 ( 1.6
9.6 ( 1.1
9.5 ( 1.0
10 12.91
15 12.86
20 12.81
-5 12.55
methyl
acetate
0
5
12.47
12.42
10 12.38
15 12.34
20 12.28
10.5
a c(HOOOH) ) 0.002-0.01 M. Standard deviations e(10%. By
following the decay of the HOOOH absorption. b c(HOOOH) ) 0.01-0.1
M. c Runs as in footnote b in the presence of the radical inhibitor 2,6-di-
tert-butyl-4-methylphenol (BMP). Molar ratio HOOOH:BMP ) 1:5.
three possible mechanistic alternatives for the formation of
nitromethane (see Scheme 3): (a) Ring closure of the nitroso
oxide to an azadioxirane, which can open to the nitromethane.
This reaction is similar to the isomerization of carbonyl oxide
to dioxirane and the opening of dioxirane to the methylene-
(bis)oxy biradical.49
1,2-diphenyldiazene (the ortho-H atoms of the phenyl rings in
the planar molecule block the entrance to the bay-region, in
which the N atoms are located; see the Supporting Information)
then ozonation products of the diazene can no longer be
observed. However, 1,2-dimethyldiazene reacts with relative
ease (activation enthalpy of 13.6 kcal/mol) to yield the 1,2-
dimethyldiazene-N-oxide and singlet oxygen (10) in a strongly
exothermic reaction (∆RH(298) ) -42.0 kcal/mol).
If the reaction takes place in the solvent cage, then molecular
singlet oxygen oxidizes the diazene-N-oxide and 11 is formed
(Figure 5). The activation enthalpy is 41.2 kcal/mol (∆RH(298)
) 29.3 kcal/mol), which in view of an excess energy of 55.6
kcal/mol from the first step is available for some of the
molecules (dissipation determines the yield of 11). Ni-
trosomethane and nitrosomethane oxide can recombine to form
nitromethane and nitrosomethane. Nitrosomethane can then
easily be oxidized by ozone to nitromethane. We investigated
(b) Cycloaddition of the nitroso oxide biradical to the NdO
double bond of nitrosomethane and formation of a 1,3-diaza-
2,4,5-trioxolane, which by cycloreversion can yield nitrosomethane
and nitromethane.
(c) O-transfer from the nitroso oxide to nitrosomethane thus
yielding nitromethane and nitrosomethane.
Possibility (a) is unlikely due to the fact that the activation
enthalpy for ring closure is 46.1 kcal/mol, which leads to an
enthalpy 33.4 kcal/mol above that of the reactants 9 (see Figure
5). For the cycloaddition reaction (b) a TS and the ozonide 14
(Scheme 3) were not found for reasons similar as in the case of
the diazene-ozone cycloaddition reaction (see above). Reaction
9
J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003 11561

A R T I C L E S
Plesnicˇar et al.
Scheme 3
A further point of note in the reaction of the 1,2-dimethyl-
hydrazine is the production of formic acid. This observation
suggests a third competing pathway for the reaction of the 1,2-
dimethylhydrazine radical (Scheme 4) to those outlined in
Scheme 1. In this mechanism the 1,2-dimethylhydrazine radical
reacts with the HOOO• radical (3) to produce formaldehyde
methylhydrazone and HOOOH (15), which can be ozonolyzed50
to yield formic acid and nitromethane via 16 and 17 (Scheme
4).
Although neither formaldehyde methylhydrazone51a nor
N-methyl-N-nitrosoamine,51b already proposed before as the
products of the ozonation of hydrazones,50 were detected in the
reaction mixture, control experiments showed that ozonation
of formaldehyde methylhydrazone (0.1 M in acetone, at -78
°C) actually produced nitromethane and formic acid as the main
products. The proposed mechanism shown in Scheme 4 is based
on the fact that both the NH and a CH hydrogen should be
prone to abstraction by the HOOO• radical (Schemes 1 and 4).
The corresponding TS(3-15) represents again a three-electron
situation with a large self-interaction error. It could not be found,
which suggests that the corresponding barrier is rather low thus
yielding at the DFT level a “negative” TS energy.
The CdN double bond of the hydrazone formed reacts with
ozone similarly as the NdN double bond of the diazene does
(Scheme 3), i.e., in an O-transfer step rather than a cycloaddition
step. The activation enthalpy is 25.2 kcal/mol (B3LYP/6-31G-
(d,p) calculations, Table 3), almost twice as large as in the
diazene reaction (13.6 kcal/mol, Figure 5). The O2 (1∆g) formed
can react with the hydrazone N-oxide to give N-methyl-N-
nitrosoamine and carbonyl oxide (17, Scheme 4) in an exother-
mic reaction (∆H(298) ) -11.7 kcal/mol, Table 4) analogue
to the corresponding reaction of the diazene N-oxide (Scheme
3). Carbonyl oxide is known49 to proceed via isomerization to
dioxirane (activation enthalpy 20.4 kcal/mol, ∆H(298) ) -23.4
kcal/mol), which rearranges easily to formic acid (activation
enthalpy 21.7 kcal/mol, ∆H(298) ) -89.4 kcal/mol) by
H-migration and OO-cleavage. N-methyl-N-nitrosoamine can
(c) requires an activation enthalpy of just 29 kcal/mol kcal/
mol. Considering that in the formation of 10 55.6 kcal/mol
excess energy are generated, of which just 29.3 kcal/mol are
reused in the decomposition of 10 to 11, there are still 26.3
kcal/mol left (in the best case considering dissipation) for the
O-transfer reaction and the formation of nitromethane and
nitrosomethane (13, Figure 5). Therefore, possibility (c) repre-
sents the most likely mechanism for nitromethane formation,
which was found in the reaction mixture.
Figure 5. Enthalpy profile for the ozonation of 1,2-dimethyldiazene (see Scheme 3). B3LYP/6-31G(d,p) calculations using RDFT, BS-UDFT, and SIC-
DFT.
9
11562 J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003

Formation of Hydrogen Trioxide
A R T I C L E S
Scheme 4
Table 4. Comparison of Relative Energies, and Free Energies of
Structures 1a-8a Calculated for the Solvent Phase (Acetone) and
the Gas Phase at B3LYP/6-31G(d,p)a
system
sym
∆E
∆G(298)
µ
1a
2a
C2
C1
C1
C1
C1
C2h
C1
C1
Cs
0.0 (0.0)
-7.5 (-8.7)
-1.1 (-0.8)
-13.0 (-9.8)
-44.8 (-43.7)
-38.8 (-35.0)
-35.8 (-33.8)
-36.8 (-32.5)
-71.1 (-65.4)
0.0 (0.0)
-7.0 (2.0)
-0.8 (8.3)
-12.8 (-13.3)
-43.8 (-34.8)
-38.5 (-35.9)
-35.9 (-21.1)
-36.4 (-35.3)
-70.8 (-65.4)
2.38
0.12 (0.01)
3.11 (2.30)
3.03 (2.61)
1.85 (1.65)
0 (0)
2.83 (2.38)
1.98 (1.67)
1.38 (1.17)
TS(2a-3a)
3a
4a
5a
6a
7a
8a
a For a definition of structures see Scheme 1. Relative energies (∆E)
and relative Gibb’s free energies (∆G(298)) in kcal/mol, dipole moments
(µ) in Debye. The symmetry in column Sym is given with regard to the
first molecule in a system of two molecules. The values in parentheses
correspond to the gas-phase values.
steps further. We consider the mechanism shown in Scheme 4
as reasonable to explain the ozonation products of hydrazones.
The mechanism of the ozonation of the hydrazines 1a, 1b,
and 1c has been explored quantum chemically in the gas phase
while experimental investigations were performed in solution.
For the purpose of assessing possible solvent effects on the
reaction mechanism, we calculated electrostatic effects caused
by acetone in the case of the ozonation of 1a. Some selected
data are summarized in Table 4. They reveal that the solvent
acetone should favor reaction path B somewhat more (reactions
become more exothermic), although the mechanisms discussed
above will not change. However, a solvent, which can establish
H-bonds to the reaction partners can have a significant effect
on the mechanism. For example, water is generated in the
reaction, which may form H-bonded complexes with reaction
partners and intermediates thus influencing reaction barriers and
reaction enthalpies. Further studies will have to be carried out
to investigate these effects.
Table 3. Energies, Enthalpies, Entropies, and Dipole Moments of
Structures Involved in the Ozonation of 1,2-Dimethyldiazene as
Calculated at B3LYP/6-31G(d,p)a
system
sym
∆E
0.0 57.7
13.6 58.4
ZPE ∆H(298)
S
∆G(298)
µ
Conclusions
9
C2h
C1
Cs
0.0 126.5
13.6 86.9
0.0 0, 0.61
25.4
In the present investigation we have shown that several
mechanisms are involved in the oxidation of substituted
hydrazines by ozone where the preferred reaction path depends
on the nature of the substituent (alkyl or aryl). In any case, the
ozonation reaction leads to a major production of hydrogen
trioxide (HOOOH), and represents insofar the best method for
the preparation of this polyoxide in significant amounts without
the interfering presence of organic hydrotrioxides (ROOOH).
(1) Ozonation is initialized by the formation of an ozone-
hydrazine complex, followed by stepwise H-abstraction and the
generation of a diazene-hydrogen trioxide complex, which is
49 kcal/mol more stable than the reactants.
(2) Alternatively, radical recombination can lead to a hydra-
zine hydrotrioxide, (R2NOOOH), which decomposes quickly
even at low temperatures to diazene-N-oxide and HOOH. But
the yield of these products is smaller because, the radical
recombination requires either diffusion out of the solvent cage
or an overall rotation of HOOO• in the solvent cage.
TS(9-10)
5.58
1.58, 0
3.39
2.37, 2.39
2.90
4.01
2.54, 2.39
3.48, 2.39
2.05
1.82
7.16
3.21, 0
3.90, 3.96
3.90, 3.68
3.90, 2.53
3.90, 2.30
3.90, 1.44
3.48, 2.59
10
-43.4 59.3 -41.9 123.1 -40.8
TS(10-11) C1
11 Cs
TS(11-12) C1
TS(11-13) C1
-1.1 59.0 -0.8 83.9 11.9
-12.1 57.3 -12.7 127.5 -13.0
35.7 28.5
16.5 58.1
-1.4 60.1
33.4 65.3
16.3 87.7
-2.0 131.0
33.2
27.8
-2.3
12
13
3b
15
Cs
Cs
C1
C1
-76.6 61.4 -75.7 136.6 -77.6
0.0 60.1
-28.2 53.6 -27.3 69.0 -25.3
-2.8 58.2 -2.1 90.2 10.6
0.0 73.7
0.0
TS(15-16) C1
16
17
C1
Cs
-54.9 59.0 -53.0 124.1 -50.6
-65.7 58.1 -64.7 129.0 -63.6
-44.2 38.3 -44.3 118.8 -43.1
TS(17-18) C1
18
C2V -89.8 40.0 -88.1 118.5 -86.8
TS(18-19)b C2V -66.6 38.6 -66.4
19
20
Cs -180.1 40.9 -177.5 118.9 -176.3
Cs -147.3 45.1 -144.7 129.4 -144.5
a For a definition of structures see Scheme 3 and Scheme 4. Relative
energies (∆E), zero point energies (ZPE), relative enthalpies (∆H(298))
and relative Gibb’s free energies (∆G(298)) in kcal/mol, dipole moments
(µ) in Debye, entropies (S) in cal mol/deg. - The symmetry in column
Sym is given with regard to the first molecule in a system of two molecules;
where two molecules form a system the sum of their zero point energies
and the sum of their entropies are reported. b Data for the reaction of 18 to
19 via TS(18-19) from ref 49.
(49) See, for example, Cremer, D.; Kraka, E.; Szalay, P. G. Chem. Phys. Lett.,
1998, 292, 97, and references therein.
(50) For previous studies on the ozonation of carbon-nitrogen double bonds,
see: (a) Erickson, R. E.; Andrulis, P. J., Jr.; Collins, J. C. Lungle, M. L.;
Merger, G. D. J. Org. Chem. 1969, 34, 2961. (b) Enders, D.; Wortmann,
L.; Peters, R. Acc. Chem. Res. 2000, 33, 157.
(51) (a) Begtrup, M.; Nytoft, H. P. J. Chem. Soc., Perkin Trans. 1, 1985, 81.
(b) Castro, A.; Leis, J. R.; Pena, M. E. J. Chem. Soc., Perkin Trans. 2,
1989, 1861.
be oxidized to nitromethane and NO2, however, no attempt was
made at present to explore experimentally these mechanistic
9
J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003 11563

A R T I C L E S
Plesnicˇar et al.
(3) A new mechanism for the decomposition of hydrogen
trioxide involving both hydrogen peroxide and water in a
pericyclic process (via the hydrogen-bonded HOOOH-HOOH-
HOH complex) is proposed thus lowering the yield of HOOOH
in the ozonation of hydrazines.
(4) For alkyl hydrazines, H can be abstracted from the CH3
group in R-position to the N thus leading to a hydrazone. In
the case of 1,2-dimethylhydrazine, the corresponding hydrazone
is ozonolyzed further yielding nitromethane and formic acid.
Acknowledgment. At Ljubljana, this work was financially
supported by the Ministry of Education, Science, and Sport of
the Republic of Slovenia and at Go¨teborg by the Swedish
Research Council (Vetenskapsrådet). Calculations were done
on the supercomputers of the Nationellt Superdatorcentrum
(NSC), Linko¨ping, Sweden. D.C. thanks the NSC for a generous
allotment of computer time. B. P. and J. C. thank Dr. J. Plavec
(National Institute of Chemistry, Ljubljana) for running 17O
NMR spectra on the 600 MHz Varian spectrometer.
Supporting Information Available: Geometries and energy
data (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA036801U
9
11564 J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003