Ground state multiplicity of acylnitrenes: Computational and experimental studies

Article abstract of DOI:10.1007/s11172-005-0287-3

The singlet-triplet energy splitting (ΔE _ST = E _S - E _T ) was calculated for formylnitrene (5) and for the syn- and anti-rotamers of carboxynitrene HOC(O)N (6) by the CCSD(T) method. Extrapolation of ΔE _ST to a complete basis set was calculated to be negative for 5 and strongly positive for 6. Similar results were obtained by the G2 procedure. The reason for the dramatic stabilization of the singlet state appeared to be a special bonding interaction between the nitrogen and oxygen atoms, which results in the structure intermediate between those of nitrene and oxazirene. It was found that the B3LYP/6-31G(d) method overestimates ΔE _ST by ～9 kcal mol^-1 for 5 and by ～7 kcal mol^-1 for 6. Taking into account this overestimation and the results of DFT calculations, it was concluded that benzoylnitrene has a singlet ground state. It was proved experimentally using photolysis of benzoyl azide in an argon matrix at 12 K that benzoylnitrene has a singlet ground state and its structure is similar to that of oxazirene. Nevertheless, these singlet intermediates have low barrier to the aziridine formation, which is traditionally considered to be a typical singlet nitrene reaction.

Full text of DOI:10.1007/s11172-005-0287-3

Russian Chemical Bulletin, International Edition, Vol. 54, No. 3, pp. 525—532, March, 2005
525
Full Articles
Groundꢀstate multiplicity of acylnitrenes:
computational and experimental studies
E. A. Pritchina,a,b N. P. Gritsan,a,bꢀ and T. Ballyc
aInstitute of Chemical Kinetics and Combustion,
Siberian Branch of the Russian Academy of Sciences,
3 Institutskaya st., 630090 Novosibirsk, Russian Federation.
Fax: +7 (383) 330 7350. Eꢀmail: gritsan@kinetics.nsc.ru
bNovosibirsk State University,
2 Pirogova st., 630090 Novosibirsk, Russian Federation.
cDepartment of Chemistry, University of Fribourg,
Fribourg, Pérolles, CHꢀ1700, Switzerland
The singlet—triplet energy splitting (∆EST = ES – ET) was calculated for formylnitrene (5)
and for the synꢀ and antiꢀrotamers of carboxynitrene HOC(O)N (6) by the CCSD(T) method.
Extrapolation of ∆EST to a complete basis set was calculated to be negative for 5 and strongly
positive for 6. Similar results were obtained by the G2 procedure. The reason for the dramatic
stabilization of the singlet state appeared to be a special bonding interaction between the
nitrogen and oxygen atoms, which results in the structure intermediate between those of
nitrene and oxazirene. It was found that the B3LYP/6ꢀ31G(d) method overestimates ∆EST
by ~9 kcal mol–1 for 5 and by ~7 kcal mol–1 for 6. Taking into account this overestimation and
the results of DFT calculations, it was concluded that benzoylnitrene has a singlet ground state.
It was proved experimentally using photolysis of benzoyl azide in an argon matrix at 12 K that
benzoylnitrene has a singlet ground state and its structure is similar to that of oxazirene.
Nevertheless, these singlet intermediates have low barrier to the aziridine formation, which is
traditionally considered to be a typical singlet nitrene reaction.
Key words: acyl azides, acylnitrenes, matrix isolation, IR spectroscopy, UVꢀVis spectroꢀ
scopy, quantum chemical calculations, ab initio quantum chemical calculations, transition
states.
Photolysis of carbonyl azides RC(O)N3 gives rise to
two types of products: (i) isocyanates RNCO (the prodꢀ
ucts of the photoꢀCurtius rearrangement1) and (ii) bimoꢀ
lecular trapping products typical of the reactions of sinꢀ
glet carbonylnitrenes RC(O)N.2 It has, however, been
shown that the yield of isocyanates upon photolysis of a
series of carbonyl azides (R = tertꢀbutyl, aryl) remains
constant (~40ꢀ50%) irrespective of the presence or abꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 519—526, March, 2005.
1066ꢀ5285/05/5403ꢀ0525 © 2005 Springer Science+Business Media, Inc.

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Pritchina et al.
sence of nitrene traps.3–8 Therefore, it was concluded
that, even if carbonylnitrenes are formed during photolyꢀ
sis of carbonyl azides, they do not rearrange to isocyanꢀ
ates.2,4–7
RC(O)N and ROC(O)N, we performed highꢀlevel quanꢀ
tum chemical calculations of ∆EST values for two model
compounds, formylnitrene HC(O)N (5) and carboxyꢀ
nitrene HOC(O)N (6). In addition, we calculated barriꢀ
ers to the reactions of singlet acylnitrenes with ethylene.
The multiplicity and the structure of the ground state of
benzoylnitrene were also proved experimentally.27
It was demonstrated9,10 that direct and tripletꢀsensiꢀ
tized photolysis of benzoyl azide (1) in alkenes produces
the same trapping products and that these products are
characteristic of reactions of singlet nitrenes. Similar reꢀ
sults were obtained for 2ꢀnaphthoyl azide and a series of
substituted benzoyl azides.11–13 No nitreneꢀlike triplet
ESR spectrum was also detected after benzoyl azide phoꢀ
tolysis in glassy matrices.14,15 Although the authors11–13
were unable to determine the multiplicity of aroylnitrenes
unambiguously by direct spectroscopic observation, they
concluded that the sum total of their data left little doubt
that aroylnitrenes have singlet ground states.
Experimental
Spectroscopy in argon matrices. A gaseous mixture of 1 part
of benzoyl azide in 1000 parts of argon (mixed with 10% of N2 to
improve the optical properties of the matrix) was slowly deposꢀ
ited on a CsI plate at 19 K. Photolysis of benzoyl azide was
performed with a lowꢀpressure mercury lamp (254 nm) or a
mediumꢀpressure Hg/Xe lamp with a 313ꢀnm interference filter.
Electronic absorption spectra were obtained on a PerkinꢀElmer
Lambdaꢀ900 spectrometer (200ꢀ1000 nm). IR spectra were meaꢀ
sured on a Bomem DA3 interferometer (4000—500 cm–1) with
an MCT detector.
Quantum chemical calculations. The B3LYP/6ꢀ31G(d) calꢀ
culated geometries of singlet and triplet molecules 2 and 5 were
taken from our previous computational study20 and those of
other compounds discussed in this paper were computed by the
same method. The geometries of the model compounds 5 and 6
were reoptimized at the CCSD(T)/ccꢀpVTZ level. All B3LYP
calculated structures were ascertained to be minima on the poꢀ
tential energy surfaces by the second derivative calculations,
which also yielded harmonic frequencies (for comparison with
experimental spectra) and zeroꢀpoint vibrational energies, which
were taken into account in the singlet—triplet splitting calculaꢀ
tions. The stability of the SCF solutions for the nitrenes was
tested by CIS calculations. If no precautions were taken, the
wave function for triplet formylnitrene invariably converged to
that of the excited state. Reoptimization of the wave function
using the "stable=opt" procedure (GAUSSIANꢀ98 suite of proꢀ
grams) was required to obtain the geometric parameters and
energy characteristics of the lowest triplet state.
It is typical of nitrenes to have triplet ground states.16
The value of the singlet—triplet energy splitting ∆EST
=
ES – ET (ES and ET are the energies of the singlet and
triplet states, respectively) was determined experimenꢀ
tally in only three cases. It was found to be 36 kcal mol–1
for the simplest nitrene NH,17 31 kcal mol–1 for methylꢀ
nitrene,18 and about 18 kcal mol–1 for phenylnitrene.19
To understand why aroylnitrenes may have a singlet
ground state, we performed DFT calculations of the
singlet—triplet splittings for benzoylnitrene (2) and
2ꢀnaphthoylnitrene (3).20 The resulting ∆EST values were
found to be small (~5 kcal mol–1), but the triplet states
were still lower in energy.20 The reason for the marked
reduction of ∆EST on going from arylꢀ to aroylnitrenes
appeared to be a bonding interaction between the nitroꢀ
gen and oxygen atoms in the singlet state, resulting in a
singletꢀstate structure which is intermediate between those
of a nitrene and an oxazirene.20
On the other hand, upon photolysis and thermolysis
of azidoformates ROC(O)N3 (R = Alk, aryl) the major
products are derived from capture of acylnitrenes
ROC(O)N.2,21–25 The triplet multiplicity of the ground
state of the ethoxycarbonylnitrene EtOC(O)N (4) was
deduced on the basis of products analysis2,22,24,25 and
proved by ESR spectroscopy.11,15 A very similar ESR specꢀ
trum was recorded12 upon photolysis of (4ꢀacetylphenꢀ
oxy)carbonyl azide at 77 K.
The ∆EST values for formylnitrene and carboxynitrene were
also evaluated at the CCSD(T) levels28–30 using Dunning´s corꢀ
relationꢀconsistent basis sets ranging from double to quadrupleꢀ
zeta quality.31 Extrapolation to a complete basis set was perꢀ
formed using exponential Dunning—Feller formula.32—34 All
these calculations were performed with the GAUSSIANꢀ98 suite
of programs,35 except for the geometry optimizations of the two
model compounds at the CCSD(T)/ccꢀpVTZ level which were
carried out with the ACES II program.36
Recently,26 a laser flash photolysis study of EtOC(O)N
was carried out in order to determine the absolute rates of
spin relaxation and of the reaction of this intermediate
with tetramethylethylene (TME). The formation of tripꢀ
let ethoxycarbonylnitrene was registered (λ = 400 nm,
τ = 1.5 µs in CF2ClCFCl2 at 298K) and a reduction of its
yield and a shortening of its lifetime in the presence of
TME observed. It was deduced that singlet ethoxyꢀ
carbonylnitrene has a lifetime between 2 and 10 ns in
CF2ClCFCl2 at room temperature.26
In addition, the singlet and triplet states of formylnitrene
and carboxynitrene molecules were characterized in accordance
with the G2 procedure.37 As mentioned above, reoptimization
of the triplet formylnitrene wave function to the lowest triplet
state was necessary. It should be noted that geometry optimizaꢀ
tion of singlet formylnitrene at the HF/6ꢀ31G(d) level failed
due to an inevitable convergence to the isocyanate. In order to
overcome this obstacle, we calculated the zeroꢀpoint vibrational
energy correction for formylnitrene at the MP2(full)/6ꢀ31G(d)
level of theory.
Location of the transition states of the reactions of sinꢀ
glet acylnitrenes with ethylene was carried out at the
B3LYP/6ꢀ31G(d) level using the synchronous transitꢀguided
In order to understand the proposed difference in the
groundꢀstate multiplicity of two types of acylnitrenes,

Properties of acylnitrenes
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 3, March, 2005
527
quasiꢀNewton method38 as implemented in the GAUSSIANꢀ98
suite of programs.35 In addition to second derivative calculaꢀ
tions, all transitionꢀstate structures were subjected to intrinsic
reaction coordinate (IRC) calculations39 to identify the minima
they interconnect.
The excited state energies of benzoylnitrene were calculated
with the B3LYP/6ꢀ31G(d) geometry by the CASSCF/CASPT2
procedure40 using the ANOꢀS basis set.41 In order to arrive at a
satisfactory description of all excited states at the CASPT2 level
(i.e., remove intruder states), it was necessary to resort to the
levelꢀshifting technique,42 whereby it was carefully assured that
no artifacts are introduced. All CASSCF/CASPT2 calculations
were carried out with the MOLCAS 4 suite of programs.43
Recently,50,51 highꢀlevel calculations of the structure
and energy of triplet formylnitrene and "cyclic" singlet
species were performed. The MP2/6ꢀ31G(d) calcuꢀ
lated geometries of these intermediates are very similar to
those optimized using the B3LYP/6ꢀ31G(d) procedure.20
The ∆EST values calculated at the CASPT2/ccꢀpVTZ,
QCISD(T)/ccꢀpVTZ, and G2 levels of theory are 3.5,
0.6, and –2.9 kcal mol–1, respectively.50,51
To analyze the influence of the basis set and the level
of treatment of electron correlation on the calculated
∆EST values, we computed the geometries and energies of
the lowest singlet and triplet states of formylnitrene and
two (synꢀ and antiꢀ) rotamers of carboxynitrene at the
CCSD and CCSD(T) levels, increasing the size of the basis
set from doubleꢀ to quadrupleꢀzeta quality. Figure 1 shows
that the geometries optimized by the CCSD(T)/ccꢀpVTZ
procedure agree very well with those found by the
B3LYP/6ꢀ31G(d) method, which therefore appears to
provide adequate geometries of the lowest singlet and
triplet states of acylnitrenes.
Results and Discussion
Quantum chemical calculations of
the singletꢀtriplet splitting
Most of the calculations originally available in the
literature concerned the simplest acyl azide and acylꢀ
nitrene, that is, formyl azide and formylnitrene, respecꢀ
tively.44–49 Until recently,20,50–52 all theoretical studies
on acylnitrenes were based on lowꢀlevel ab initio calculaꢀ
tions44–49 which gave inconsistent results. As mentioned
above, B3LYP/6ꢀ31G(d) calculations20 of molecules 2,
3, and 5 in the lowest singlet state showed that their CON
fragments resemble that of a cyclic oxazirene, although
the calculated N—O distance (~1.76 Å) is much longer
than a normal N—O single bond.
In the case of formylnitrene B3LYP and CCSD calcuꢀ
lations provide strongly positive ∆EST values of 8.1 and
14.1 kcal mol–1, respectively if 6ꢀ31G(d) basis is used
(i.e., they predict triplet ground states). The splitting is
slightly reduced on going to larger basis sets; extrapolaꢀ
tion to a complete basis set gives 6.6 (B3LYP) and
9.4 kcal mol–1 (CCSD). However, ∆EST decreases draꢀ
matically on perturbative inclusion of triple excitations in
O
a
1.867
O
(1.836)
1.283
(1.280)
O
1.867
(1.852)
1.815
(1.786)
1.288
(1.283)
1.307
(1.305)
1.318
(1.319)
N
C
1.273
(1.274)
N
C
1.076
(1.088)
1.317
(1.317)
N
C
1.270
(1.273)
O
H
1.260
(1.260)
0.966
(0.974)
H
0.967
(0.975)
O
H
2.284
(2.258)
O
b
2.289
(2.273)
O
1.204
(1.219)
2.268
(2.265)
O
1.209
N
C
1.222
(1.239)
(1.221)
1.347
(1.348)
1.412
(1.389)
N
C
N
C
1.095
(1.107)
1.342
(1.344)
1.412
(1.395)
1.378
(1.363)
O
0.966
H
(0.975)
H
O
0.968
(0.976)
H
Fig. 1. Bond lengths (Å) in the formylnitrene 5 and in the synꢀ and antiꢀrotamers of carboxynitrene 6 obtained from CCSD(T)/ccꢀpVTZ
1
and B3LYP/6ꢀ31G(d) (in parentheses) calculations for the A' (a) and 3A″ (b) states.

528
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 3, March, 2005
Pritchina et al.
∆EST/kcal mol–1
∆EST/kcal mol–1
12
12
10
8
3
10
8
2
3
2
6.1
3.8
6
4
6.3
6.1
6
1
2
4
0
1
–0.7
2.5
2
–2
2
3
4
∞
n
0
Fig. 2. The values of the singlet—triplet splitting (∆EST) for
formylnitrene 5 (1) and for the antiꢀ (2) and synꢀrotamers (3) of
carboxynitrene 6 evaluated at the CCSD(T) level of theory with
Dunning´s correlationꢀconsistent basis sets ranging from double
(n = 2) to quadrupleꢀzeta (n = 4) quality, by extrapolation to a
complete basis set using the Dunning—Feller formula,32—34
and at the G2 level (arrowed). The CCSD(T)/ccꢀpVTZ and
B3LYP/6ꢀ31G(d) calculated geometries were used in the coupled
cluster calculations of formylnitrene and carboxynitrene, reꢀ
spectively.
2
3
4
∞
n
Fig. 3. Relative electronic energies of the triplet state of the
antiꢀrotamer of carboxynitrene 6 (1) and of the singlet states of
the synꢀ (2) and antiꢀrotamers (3) of molecule 6 obtained from
CCSD(T) calculations with Dunning´s correlationꢀconsistent
basis sets ranging from double (n = 2) to quadrupleꢀzeta (n = 4)
quality using the B3LYP/6ꢀ31G(d) calculated geometries, by
extrapolation to a complete basis set, and at the G2 level
(arrowed). The energy of the triplet state of the synꢀrotamer of
molecule 6 was chosen as a reference and set to zero.
the coupled cluster scheme. The ∆EST values obtained
for formylnitrene from CCSD(T) calculations with difꢀ
ferent basis sets are shown in Fig. 2. These values furꢀ
ther decrease with an increase in the size of the basis
set. Extrapolation to a complete basis set leads to
∆EST = –0.7 kcal mol–1. However, on adding the zeroꢀ
point vibrational energy difference between the two states,
∆EST goes back to nearly zero. At the G2 level of theory,
∆EST was calculated to be –2.9 kcal mol–1. Therefore, we
can conclude that the two spin states of molecule 5 must
lie very close in energy and that the B3LYP/6ꢀ31G(d)
method overestimates ∆EST for formylnitrene by about
9 kcal mol–1 compared to the most accurate calculaꢀ
tions (extrapolation of CCSD(T) results to a complete
basis set).
However, prediction of a singlet ground state should be
valid for aroylnitrenes. A ∆EST value of about 5 kcal mol–1
was calculated for benzoylnitrene and 2ꢀnaphthoylnitrene
by B3LYP/6ꢀ31G(d) method,20 but the highꢀlevel calcuꢀ
lations mentioned above show that it overestimates ∆EST
for formylnitrene by about 9 kcal mol–1. Assuming
that a similar error carries over to benzoylnitrene and
2ꢀnaphthoylnitrene, the B3LYP calculated ∆EST values
for these two nitrenes suggest that they have singlet ground
states (∆EST ≈ –4 kcal mol–1).
The ∆EST values obtained from B3LYP/6ꢀ31G(d) calꢀ
culations of the synꢀ and antiꢀrotamers of HOC(O)N
were found to be 13.1 and 11.0 kcal mol–1, respectively
(13.2 and 11.5 kcal mol–1, respectively, with inclusion of
zeroꢀpoint vibrational energy correction). The energies of
the structures were recalculated at the CCSD(T) level
with the ccꢀpVDZ, ccꢀpVTZ, and ccꢀpVQZ basis sets.
Figure 3 shows that at all levels of theory the synꢀ and
antiꢀrotamers of singlet carboxynitrene are very close in
energy and that the triplet state of the antiꢀrotamer is
higher in energy than that of the synꢀrotamer (Fig. 3).
The ∆EST values for two rotamers of HOC(O)N, derived
from the relative energies plotted in Fig. 3, are presented
in Fig. 2. Extrapolation to a complete basis set leads to
∆EST = 6.1 and 3.8 kcal mol–1, respectively, for the
synꢀ and the antiꢀrotamer, thus providing strong evidence
for a triplet ground state of carboxynitrene. We also conꢀ
clude that the B3LYP/6ꢀ31G(d) method overestimates
∆EST by about 7 kcal mol–1 in the case of HOC(O)N.
Note that the G2 theory also leads to the positive values
of ∆EST (4.8 kcal mol–1 for the synꢀ and 2.2 kcal mol–1 for
the antiꢀrotamer) of carboxynitrene (see Fig. 2).
The difference in ∆EST between carboxynitrene and
formylnitrene can be attributed to a weaker bonding inꢀ

Properties of acylnitrenes
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 3, March, 2005
A
529
teraction between the nitrogen and oxygen atoms in the
1A′ state of the former molecule, which manifests itself in
lengthening of the N—O bond (Fig. 1). As there is no
apparent reason why the presence of the hydroxyl O atom
should interfere with the σꢀbonding between the N and O
atoms of the acylnitrene, we believe that it is the extenꢀ
sion of the πꢀsystem, which leads to a weakening of the
πꢀbond between these two atoms.
1.0
4
2
We also performed the B3LYP/6ꢀ31G(d) calculations
of ∆EST values for the synꢀ and antiꢀrotamers of phenꢀ
oxycarbonylnitrene, which were found to be 12.3 and
0
9.5 kcal mol–1, respectively (12.6 and 10.1 kcal mol–1
respectively, with inclusion of zeroꢀpoint vibrational
energy correction). Taking into account that the
B3LYP/6ꢀ31G(d) method overestimates ∆EST for
HOC(O)N by about 7 kcal mol–1, we may assume that
∆EST is about 5 kcal mol–1 for the synꢀ and 2.5 kcal mol–1
for the antiꢀrotamer of phenoxycarbonylnitrene. This is
in good agreement with the estimation12 for 4ꢀacetylꢀ
phenoxycarbonylnitrene (0 < ∆EST < 5 kcal mol–1).
,
1
3
–1.0
2100
2200
2300
ν/cm–1
Fig. 5. Changes in a characteristic region of the IR spectrum of
benzoyl azide 1 obtained upon 254 nm photolysis in an Ar maꢀ
trix for 2 min at 12 K (curve 1) and subsequent 313 nm phoꢀ
tolysis for 8 min (curve 2). Positions and relative oscillator
strengths of the N2 stretching bands in molecule 1 (3) and NCO
stretching bands in phenyl isocyanate 7 (4) obtained from
B3LYP/6ꢀ31G(d) calculations and scaled by 0.97.
Photolysis of benzoyl azide in argon matrices
In order to determine the groundꢀstate multiplicity
and structure of benzoylnitrene experimentally, the phoꢀ
tochemistry of benzoyl azide was studied in an Ar matrix
at 12 K. After two minutes of photolysis in Ar at 254 nm,
the optical spectrum of benzoyl azide was replaced by a
new band with a maximum around 300 nm (Fig. 4,
curve 1). Simultaneously, the IR bands of benzoyl azide,
in particular, the strong stretching band of the N3 group at
2145 cm–1 (Fig. 5, curve 1, negative peaks), disappeared
almost completely and gave rise to a set of new peaks,
notably a strong group around 2270 cm–1 (Fig. 5, curve 1,
positive peaks).
Subsequent irradiation at 313 nm led to the disappearꢀ
ance of the newly formed UV band (Fig. 4, curve 2).
Concomitantly, some of the newly formed IR peaks diꢀ
minished strongly (Fig. 6, curve 1, negative peaks),
whereas some other peaks, notably the intense group
A
3
A
0.10
2
1
0.1
0.05
1
0
0
2
3
–0.05
–0.1
–0.10
600
900
1200
1500
ν/cm–1
275
300
325
350
λ/nm
Fig. 4. Changes in the optical spectrum of benzoyl azide 1 on
254 nm photolysis in an Ar matrix for 2 min at 12 K (1) and
changes in the spectrum 1 upon subsequent irradiation at 313
nm for 8 min (2). Positions and relative oscillator strengths of
the absorption bands of the singlet benzoylnitrene 2 calculated
by the CASSCF/CASPT2 method are depicted as solid bars (3).
Fig. 6. Changes in IR spectrum after subsequent 313 nm phoꢀ
tolysis (performed after 2 min of 254ꢀnm irradiation) of benzoyl
azide 1 in an Ar matrix for 8 min at 12 K (curve 1). Positions and
relative oscillator strengths of phenyl isocyanate 7 (2) and sinꢀ
glet benzoylnitrene 2 (3) calculated by the B3LYP/6ꢀ31G(d)
and scaled by 0.97.

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Pritchina et al.
around 2270 cm–1, continued to grow (Fig. 5, curve 2;
Fig. 6, curve 1, positive peaks).
Thus, this experiment indicates very clearly the forꢀ
mation of at least two products on the 254 nm photolysis
of benzoyl azide. The first of these products has an abꢀ
sorption maximum at ~300 nm and rearranges to the other
primary product on exposure to 313 nm light.
tral data, about 75% of phenyl isocyanate is formed after
complete decomposition of benzoyl azide (2 min of phoꢀ
tolysis at 254 nm). Using the initial slope of the kinetic
curves of the formation of the products 7 and 2 on 254ꢀnm
irradiation, we can estimate the 7 : 2 yield ratio in the
primary process at 64 : 36.
A positive answer can now be given to the question
"Are aroylnitrenes groundꢀstate singlets?"11 Calculations
and experiments now agree that aroylnitrenes are indeed
groundꢀstate singlets, but calculations show that these
species have electronic structures and geometries that are
intermediate between those of nitrenes and oxazirenes.
Therefore, a new question arises — do these species unꢀ
dergo reactions that are typical of singlet nitrenes?
Recently,27 the photochemistry of benzoyl azide in
solution at ambient temperatures was studied by nanosecꢀ
ond timeꢀresolved IR spectroscopy. It was found that
photolysis of 1 produces two primary species, namely,
singlet benzoylnitrene (2) and phenyl isocyanate (7). The
singlet nitrene 2 undergoes an insertion into solvent C—H
bonds to give amides (cyclohexane, τ = 0.9 µs) and adds
to acetonitrile to give an ylide (CD3CN, τ = 0.3 µs).27
The lifetime of 2 was 6 µs in the most inert solvent
CH2Cl2.27
The formation of phenyl isocyanate (7), along with
products derived from singlet benzoylnitrene, is well
known to occur upon photolysis of benzoyl azide at room
temperature.2,6–10 Therefore, we assumed that the prodꢀ
ucts of the benzoyl azide photolysis at 12 K are phenyl
isocyanate and benzoylnitrene, the latter in turn being
converted to compound 7 on subsequent photolysis.
Next to the very intense doublet at 2266/2289 cm–1
,
the IR spectrum of the final product of photolysis (Fig. 6,
curve 1, positive peaks) exhibits many less intense peaks
in the range of 500–1800 cm–1 (570, 642, 755, 904, 1028,
1074, 1117, 1285, 1517, and 1604 cm–1). All of them
correlate well with the IR spectrum calculated for phenyl
isocyanate (Fig. 6, curve 2).
The absorption band at ~300 nm (Fig. 4) belongs preꢀ
sumably to the singlet benzoylnitrene, which has a strucꢀ
ture intermediate between that of a nitrene and an
oxazirene. Indeed, the two most intense transitions
in the spectrum of this species calculated by the
CASSCF/CASPT2 procedure are at 306 and 290 nm,
respectively (solid bars in Fig. 4). In addition, a very weak
transition to the first excited state is predicted at 513 nm.
It consists mainly of electron promotion from an allylic
type πꢀorbital to the lowꢀlying σ*ꢀorbital of the very weak
N—O bond.
The experimental IR spectrum (529, 606, 689, 770,
1071, 1145, 1238, 1325, 1341, 1459, and 1765 cm–1) of a
transient species with an absorption maximum at 300 nm
(Fig. 6, curve 1, negative peaks) is also in very good
agreement with the calculated IR spectrum of singlet
benzoylnitrene (Fig. 6, curve 3).
Hence, two products, phenyl isocyanate 7 and a sinꢀ
glet species 2 with a structure intermediate between those
of a nitrene and an oxazirene, are produced on photolysis
of benzoyl azide 1 (Scheme 1). According to the IR specꢀ
Stereospecific formation of aziridines in reactions with
olefins is traditionally considered to be a characteristic
reaction of singlet nitrenes.16 To understand how the sinꢀ
glet acylnitrenes with their unusual electronic structure
add to olefins, we modeled the reactions of singlet
acylnitrenes with ethylene.
Calculations of barriers to reactions
of singlet acylnitrenes with ethylene
It is known2 that addition of both singlet and triplet
acylnitrenes to the C=C bonds results in aziridines, but
proceeds by drastically different reaction mechanisms.
Singlet nitrenes react in a single step, and the addition is
stereospecific. Conversely, triplet nitrenes undergo a twoꢀ
step reaction via a 1,3ꢀdiradical and, as a result, the addiꢀ
tion is not stereospecific.
Here we only examine the addition of singlet acylꢀ
nitrenes RC(O)N (R = H, OH, Ph) to ethylene. The geꢀ
ometries of the aziridines and the transition states (TS)
leading there were optimized by the B3LYP/6ꢀ31G(d)
method. The TS structure for the addition of formylnitrene
to ethylene and the structure of the resulting aziridine are
shown in Figure 7. It is seen that the TS apparently occurs
very early along the reaction coordinate, as the lengths of
the forming C—N bonds are very long (~2.8 Å, Fig. 7).
The height of the barrier to this reaction was found to be
Scheme 1
only 6.7 kcal mol–1
.
We also calculated the TS structures of the reactions
of the singlet molecules 2 and 6 with ethylene (Table 1).
Once more we found very early transition states with long

Properties of acylnitrenes
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 3, March, 2005
531
∆E0/kcal mol–1
available in the literature, but this value is consistent with
the high yield of aziridines on photolysis of benzoyl azide
in the presence of olefin traps.2,8—10
H
10
TS
1.288
N
C
1.271
Analysis of the results obtained in a study26 of the
reaction of singlet ethoxycarbonylnitrene 4 with TME
allows the rate constant for this addition reaction to be
estimated at (1—5)•109 L mol–1 s–1 at ambient temperaꢀ
ture. According to our calculations, the barrier to the
reaction of singlet molecule 6 (which could serve as a
model of molecule 4) with ethylene is much lower than
the barriers to reactions of ethylene with molecules 2
and 5 (Table 1). The very rough estimation of the rate
constant for the addition of singlet molecule 6 to ethylꢀ
ene as
O
0
2.089
2.803
HC(O)N + C2H4
–10
–20
–30
–40
–50
–60
C
C
1.342
k ≈ 5•1011•exp[–∆H#/(RT )]
H
for T = 298 K leads to a k value of about 1.3•109
L mol–1 s–1, which reasonably agrees with the pubꢀ
lished data.26
Thus, although the singlet intermediates are structurꢀ
ally similar to cyclic oxazirene, their reactivities are typiꢀ
cal of singlet nitrenes.
1.213
O
C
1.389
N
1.446
1.456
–70
–80
C
C
1.504
–74.3
This work was carried out with the financial support
from the Russian Foundation for Basic Research (Project
No. 01ꢀ03ꢀ32864), the Ministry of Education and Sciꢀ
ence of the Russian Federation (grant No. E02ꢀ5.0ꢀ27),
the Division of Chemistry and Materials Science of
the Russian Academy of Sciences (Project No. 2000ꢀ
061.560ꢀ00), and the Swiss National Science Foundation
(SCOPES, grant No. 7SUPJ062336).
Fig. 7. Energy diagram for the addition of singlet formylꢀ
nitrene 5 to ethylene C2H4. The relative electronic energies
(∆E0/kcal mol–1) were calculated by the B3LYP/6ꢀ31G(d)
method with inclusion of ZPE corrections. The sum of the enerꢀ
gies of the singlet 5 and C2H4 was set to zero. Shown are the
transitionꢀstate and the reaction product (aziridine) structures
optimiized by the same method.
Table 1. Bond lengths of the forming C—N bonds (r, Å) and
imaginary frequencies (ωimag/cm–1) in the transition states of
the reactions of singlet acylnitrenes RC(O)N with ethylene;
values of the barriers (∆E0#) to reactions and the enthalpies
(∆E0reac) of these reactions calculated for 0 K; and the activaꢀ
tion enthalpies (∆H#) and enthalpies (∆H reac) of these reactions
calculated for 298 K by the B3LYP/6ꢀ31G(d) method
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Received November 26, 2003;
in revised form April 30, 2004