The electronic absorption spectra of some acyl azides. Molecular orbital treatment

Article abstract of DOI:10.1016/j.saa.2007.05.066

The electronic absorption spectra of benzoyl azide and its derivatives: p-methyl, p-methoxy, p-chloro and p-nitrobenzoyl azide were investigated in different solvents. The observed spectra differ basically from the electronic spectra of aryl azides or alkyl azides. Four intense π-π^* transitions were observed in the accessible UV region of the spectrum of each of the studied compounds. The contribution of charge transfer configurations to the observed transitions is rather weak. Shift of band maximum with solvent polarity is minute. On the other hand, band intensity is highly dependent on the solvent used. The observed transitions are delocalized rather than localized ones as in the case with aryl and alkyl azides. The attachment of the C{double bond, long}O group to the azide group in acyl azides has a significant effect on the electronic structure of the molecule. The arrangements as well as energies of the molecular orbitals are different in acyl azides from those in aryl azides. The first electronic transition in phenyl azide is at 276 nm, whereas that of bezoyle azide is at 251 nm. Ab initio molecular orbital calculations using both RHF/6-311G* and B3LYP/6-31+G^* levels were carried out on the ground states of the studied compounds. The wave functions of the excited states were calculated using the CIS and the AM1-CI procedures.

Full text of DOI:10.1016/j.saa.2007.05.066

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 177–186
The electronic absorption spectra of some acyl azides
Molecular orbital treatment
Rafie H. Abu-Eittah∗, Adel A. Mohamed, A.M. Farag, Ahmed M. Al Omar
Department of Chemistry, Faculty of Science, University of Cairo, Giza, Egypt
Received 7 June 2005; received in revised form 22 March 2007; accepted 18 May 2007
Abstract
The electronic absorption spectra of benzoyl azide and its derivatives: p-methyl, p-methoxy, p-chloro and p-nitrobenzoyl azide were investigated
in different solvents. The observed spectra differ basically from the electronic spectra of aryl azides or alkyl azides. Four intense ␲–␲* transitions
were observed in the accessible UV region of the spectrum of each of the studied compounds. The contribution of charge transfer configurations
to the observed transitions is rather weak. Shift of band maximum with solvent polarity is minute. On the other hand, band intensity is highly
dependent on the solvent used. The observed transitions are delocalized rather than localized ones as in the case with aryl and alkyl azides. The
attachment of the C O group to the azide group in acyl azides has a significant effect on the electronic structure of the molecule. The arrangements
as well as energies of the molecular orbitals are different in acyl azides from those in aryl azides. The first electronic transition in phenyl azide is
at 276 nm, whereas that of bezoyle azide is at 251 nm.
Ab initio molecular orbital calculations using both RHF/6-311G* and B3LYP/6-31+G* levels were carried out on the ground states of the studied
compounds. The wave functions of the excited states were calculated using the CIS and the AM1-CI procedures.
© 2007 Published by Elsevier B.V.
Keywords: Acyl azides; Spectra of acyl azides; Molecular orbital calculations of acyl azides; Specrta and molecular orbital calculations of acyl azides
1. Introduction
nature of other substituent [7]. The values of band maxima are
quite sensitive to the substituent effects and vary systematically
with substituent constant [5].
The electronic structures of p-benzoyl azide derivatives,
RC6H4CON3, R = H, F, Cl, Br and I, were calculated using the
PPP-method and CI among all singly excited configurations.
Absorption bands in the near UV region were assigned to three
electronic transitions: two of which are due to local excitations
in the phenyl group and the other is a local excitation in the azide
group [8].
The biological activities of aroyl azides and nicotinic azides
are wide and are the subject of extensive investigations. For
instance, p-chloroando-chloroazidesareusedinfungalmetabo-
lites and have antimicrobial activities [1,2] whereas nicotinic
and isonicotinic azides have some pharmacological effects [3].
Benzoyl azide and nicotinic azide caused powerfully effective
hypotension, hematuriaandcardiacirregularitiesinrabbitswith-
out visible toxicity at doses of 10 mg/Kg by mouth [4].
The effect of substituent on the electronic absorption spec-
tra of a related series of benzyl azide derivatives, RC6H4CON3
(R = p-OCH3, p = Br, H, m-NO2 and p-NO2) were studied [5,6].
The values of λmax increase substantially when R is an electron
donating group in the para position. However, with R = NO2,
the values of λmax are at about the same values of nitrobenzene
and p-nitro benzoic acid, that is, the azido group acts as an elec-
tron withdrawing group. It has been found that halogens can act
as electron donating or withdrawing groups depending on the
While the absorption spectrum of alkyl azides is that of the
isolated azide group, the spectra of aromatic azides are essen-
tially those of the parent hydrocarbons, only a weak additional
band due to azido group appears as a shoulder on the long wave-
length side of the hydrocarbon spectrum [9]. The coupling effect
of the azido group with the aromatic system corresponds to a
charge flow from nitrogen towards the ring (the azido group is an
electron donating with a Hammett constant σp+ = −0.54) [10].
This lowers the energy of the non-bonding πyn − orbital below
the level of σ(sp2)-orbital; consequently, the lowest energy tran-
sition, namely σ(sp2) → πy∗, where the πy∗ orbital extends now
over the whole aromatic system.
∗
Corresponding author.
E-mail address: abueittah@chem-sci.cu.edu.eg (R.H. Abu-Eittah).
1386-1425/$ – see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.saa.2007.05.066

178
R.H. Abu-Eittah et al. / Spectrochimica Acta Part A 70 (2008) 177–186
In this work, the electronic absorption spectra of: benzoyl
3. Method of calculation
azide, p-methyl, p-methoxy, p-chloro, and p-nitrobenzoyl azides
have been investigated using different solvents. Gaussian anal-
ysis was used to analyze the observed spectra.
Ab initio molecular orbital calculations were carried out
using the RHF level with 6-31+G* basis set. The molecules were
fully optimized without any constrains. Effect of electron corre-
lation on the ground state properties was considered by applying
the DFT method as given by the B3LYP/6-31+G* procedures.
The wave functions of the excited states and their energies were
calculated using both ab-initio CIS and the semiemperical AM1-
CI methods.
All calculations were carried out using the free distributed
GAMESS 6.4 program [17] and a MOPAC package [18].
Gaussian analysis of the spectra was performed using a
MATHCAD PLUS package [19].
2. Experimental
2.1. Solvents
Ethanol, methanol, acetonitrile, dichloromethane, heptane,
hexane cyclohexane and diethyl ether are all Merck AR-reagents
and used without further purification.
2.2. Compounds
4. Results and discussion
All compounds were prepared by the methods reported in
the literature [11]. The benzoyl azides were prepared by the
action of sodium nitrite on the corresponding hydrazide in
aqueous hydrochloric acid with stirring at −5 ◦C using sodium
chloride-ice bath. Stirring continued for 30 min at the same tem-
perature, and then ether is added. The ethereal layer is separated,
washed several times with cold dilute solution of sodium car-
bonate, then dried over anhydrous sodium sulphate. Ether was
removed under reduced pressure and the product was purified
by crystallization from dry ether. IR-spectra were performed to
confirm the purity of the product. The compounds prepared:
benzoyl azide, m.p. = 27 ◦C (reported value [12] is 26 ◦C), p-
methylbenzoyl azide: m.p. = 35.5 ◦C (reported value [13] is
35.0 ◦C), p-methoxybenzoyl azide: m.p. = 69–70 ◦C (reported is
70–71 ◦C [14]), p-chlorobenzoyl azide: m.p. = 42 ◦C (reported
[15] is 43 ◦C)(81), p-nitrobenzoyl azide: m.p. = 64 ◦C (reported
[16] is 65 ◦C).
4.1. Electronic absorption spectra
4.1.1. Benzoyl, p-methylbenzoyl and p-methoxybenzoyl
azides
Fig. 1 shows the absorption spectra of benzoyl azide in hep-
tane as a solvent. Gaussian analysis of the spectrum in heptane,
using a MATHCAD PLUS package [19] indicates the existence
of four electronic transitions in the accessible UV region. Band
maximum and molar extinction coefficient are given in Table 1.
The spectra of benzoyl azide have been investigated in different
solvents. There is a small shift in band maximum with solvent
polarity a result which means a minor contribution of charge
transfer configurations to the excited states of the molecule and
that there is no detectable change in the polarity of the molecule
on electronic excitation. On the other hand, band intensity varies
significantly with the type of the solvent. This is due to the fact
that acyl azides are stronger dipoles than the corresponding aryl
azides. Benzoyl azide has a dipole moment μ = 2.60D whereas
that of phenyl azide is 1.44 [20]. The values of the molar extinc-
tion coefficients of the observed transitions of benzoyl azide,
using heptane as a solvent, vary between 1150 (λmax = 284)
to 19,330 (λmax = 241) a result which indicates that all of the
observed bands are due to ␲ → ␲* localized transitions.
The spectra of the studied acyl azides are of interest because
they identify the excited states which arise on light absorption
and are potentially involved in photolytic reaction. In the acyl
azides, conjugation with the carbonyl group causes a small blue
shift of the low energy band and a slight enhancement of its
intensity as compared to aryl azides. The flow of charge from
azide group to aryl group in aryl azides is inhibited in acyl azides
as a result of the presence of the carbonyl group and this leads
to the inhibition of the perturbation effect of the azide group on
the spectra of the aryl ring in acyl azides. The contribution of
charge transfer configurations to the excited states is small. The
perturbation effect of the CON3 on the spectra of the aryl ring
has to be considered on examining the spectra of acyl azides.
The spectra of acyl azides are different from those of aryl azides
and cannot be assigned to those of isolated azide or aryl groups
as in the case of aryl azides. Molecular orbital calculations will
confirm this conclusion.
2.3. Apparatus
The spectral measurements were performed on a Perkin-
Elmer Lambda 4B UV–vis spectrophotometer using 1.0 cc fused
silica cells.
Fig. 1. Electronic absorption spectra of benzoyl azide in heptane and its Gaus-
sian analysis.

R.H. Abu-Eittah et al. / Spectrochimica Acta Part A 70 (2008) 177–186
179
Table 1
Band maxima (λmax, nm), molar extinction coefficients(ε, mol−1 L cm−1), and oscillator strengths of benzoyl azide, p-methyl azide and p-methoxybenzoyl azide in
different solvents
Solvent
Benzoyl azide
p-Methylbenzoyl azide
p-Methoxybenzoyl azide
λmax
ε
f
λmax
ε
f
λmax
ε
f
241
252
269
284
19370
12940
2160
0.219
0.097
0.028
0.007
250
260
275
286
17120
14390
3220
0.164
0.129
0.022
0.014
212
221
254
271
284
15770
14200
6500
22000
12970
0.149
0.141
0.119
0.312
0.079
Heptane
1150
1980
242
253
270
285
16140
11580
2000
0.163
0.128
0.023
0.011
251
262
276
285
17200
15900
2610
0.200
0.158
0.026
0.015
–
255
272
286
–
7500
17990
17450
–
0.132
0.248
0.187
Diethyl ether
Acetonitrile
1660
1480
243
254
271
285
14340
12580
2100
0.189
0.137
0.021
0.011
253
263
277
286
19000
16500
3210
0.221
0.177
0.032
0.016
215
223
255
273
289
13460
39320
5900
16890
17360
0.227
0.072
0.104
0.246
0.209
1350
1580
244
255
271
285
13240
11880
2000
0.163
0.128
0.023
0.011
253
264
277
286
19200
17000
3410
0.223
0.189
0.034
0.022
215
224
256
274
290
13570
5000
5700
16900
18450
0.229
0.088
0.100
0.246
0.222
Ethanol
1350
2180
245
256
272
285
13350
11800
2050
0.173
0.126
0.025
0.011
254
265
277
286
17000
15600
3410
0.198
0.162
0.034
0.022
–
256
274
290
–
5740
14330
16000
–
0.101
0.219
0.210
Dichloro-methane
1380
2180
Figs. 2 and 3 show the electronic absorption spectra of
p- and o-methylbenzoyl azide. Methyl substitution facilitates
charge transfer transitions and one observes a red shift of
band maximum in polar solvents, Table 1. The intensity of the
Methoxy substitution causes a detectable perturbation effect,
as a result five intense electronic transitions are obtained,
Figs. 4 and 5; the spectrum of p-methoxybenzoyl azide, Fig. 4
and Table 1. Variation of band maximum with solvent polarity
is minor, hence a minor change in polarity is experienced on
excitation. Bhaskar [5] studied the effect of substituent on the
electronic absorption spectra of thiocarbanilide, benzyl azide,
diphenylcarbodiimide and acetophenone. The values of band
maximum, in the case of benzyl azide, increases appreciably
when an electron donating substituent as OCH3 is present.
However, the electron withdrawing nitro group leads to band
-
observed bands, 19200 > C > 1900, indicates that they corre-
spond to ␲ → ␲* transitions. Band maxima of p-methylbenzoyl
azide are slightly red shifted: λmax = 250, 260, 275 and 286 nm
compared to those of benzoyl azide: λmax = 241, 252, 269
and 284 nm as a result of the inductive effect of the CH3
group.
Fig. 2. Electronic absorption spectra of p-methylbenzoyl azide in heptane and
Fig. 3. Electronic absorption spectra of o-methylbenzoyl azide in heptane and
its Gaussian analysis.
its Gaussian analysis.

180
R.H. Abu-Eittah et al. / Spectrochimica Acta Part A 70 (2008) 177–186
Fig. 4. Electronic absorption spectra of p-methoxybenzoyl azide in heptane and
its Gaussian analysis.
Fig. 5. Electronic absorption spectra of o-methoxybcnzoyt azide in heptane and
its Gaussian analysis.
maximum at the same value as that of nitrobenzene. This result
shows that the acid azido group is an electron withdrawing
group.
Substitution by a nitro group causes the strongest perturba-
tion effect on the parent nucleus. The spectra of p-NO2 benzoyl
*
azide are shown in Fig. 5. Four intense ␲ → ␲ transitions are
observed with band maxima (Table 2) that differ significantly
from the previously studied compounds. Both the NO2 and
CON3 groups inhibit the electronic effect of each other, hence,
the contribution of the charge transfer configurations to the elec-
tronic states is minimum.
4.1.2. p-Chlorobenzoyl and p-nitrobenzoyl azides
The spectra of p- and o-chlorobenzoyl azides show four
electronic transitions in the 240–300 nm region, Figs. 6 and 7
*
corresponding to ␲ → ␲ but with band maxima that dif-
fer significantly from those of benzoyl, p-CH3 and p-OCH3
benzoyl azides (Table 2). All transitions are red shifted in p-
chlorobenzoyl azide as compared to those in benzoyl, p-CH3 or
p-OCH3 benzoyl azides. The highest energy transition appears at
252 nm in p-chlorobenzoyl azide compared to 241 nm in benzoyl
azide.
4.2. Molecular orbital calculations
4.2.1. Benzoyl azide
The rotation of the azide group around the C N single bond
results in two conformers of benzoyl azide. The most stable
Table 2
Band maxima (λmax, nm), molar extinction coefficients (ε, mol−1 L cm−1), and oscillator strengths of benzoyl azide, p-methyl azide and p-methoxybenzoyl azide in
different solvents
Solvent
p-Chlorobenzoyl azide
p-Nitrobenzoyl azide
λmax
ε
f
λmax
ε
f
252
263
274
286
21800
17400
3320
0.169
0.123
0.023
0.013
253
266
290
309
13700
11000
2100
780
0.167
0.128
0.025
0.007
Heptane
1780
254
263
274
286
20090
15300
3220
0.194
0.117
0.022
0.013
254
266
290
309
21800
18000
3410
0.267
0.211
0.040
0.010
Diethyl ether
Acetonitrile
Ethanol
1780
1010
254
264
275
286
18700
14300
2660
0.195
0.143
0.024
0.017
257
270
293
311
15400
12200
3200
0.191
0.143
0.038
0.010
1880
1080
254
264
276
286
24300
19300
3510
0.255
0.194
0.032
0.020
256
269
292
310
18000
15200
3090
0.225
0.178
0.046
0.014
2180
1480
256
265
276
286
16700
13900
3510
0.174
0.138
0.027
0.020
259
272
294
311
18500
15300
3600
0.230
0.179
0.043
0.011
Dichloro-methane
2180
1180

R.H. Abu-Eittah et al. / Spectrochimica Acta Part A 70 (2008) 177–186
181
Table 3
Structural properties of the ground state of the studied benzoyl azide and its derivatives using the RHF/6-311G* and (B3LYP/6-31+G*) methods
Property
Benzoyl azide
p-CH3 benzoyl
p-OCH3 benzoyl
p-Cl benzoyl
p-NO2 benzoyl
Total energy (a.u.)
−506.2494
−506.2016a
−506.1886b
2.19 [2.60]c
−545.2942
−548.1946
−548.5081
2.625
−620.1573
−623.7509
−623.7176
3.805
−965.1749
−968.7968
−968.7833
1.550
−709.7710
−713.7083
−713.6947
4.257
Dipole moment (D)
Total bond order
C1 C6 (ring)
C1 O5
C1 N2
N2 N3
1.030
1.965
1.046
1.109
2.389
1.036
1.965
1.042
1.010
2.388
1.096
1.961
1.097
1.117
2.385
1.024
1.968
1.053
1.104
2.392
1.013
1.973
1.065
1.093
2.398
N3 N4
Charge density
C1
N2
N3
N4
O5
C6H5
+0.614
−0.487
+0.411
−0.062
−0.465
−0.011
+0.612
−0.488
+0.412
−0.066
−0.458
0.048
+0.615
−0.491
+0.412
−0.068
−0.471
+0.200
+0.617
−0.486
+0.410
−0.058
−0.462
+0.045
+0.420
−0.485
+0.410
−0.044
−0.453
+0.322
Energy
HOMO (a.u.)
LUMO (a.u.)
−0.34770
−0.33720
−0.32480
−0.3619
−0.3770
+0.07420
+0.07780
+0.08510
+0.0634
+0.0240
a
Conformer A.
Conformer B.
Experimental dipole moment.
b
c
conformer (A) is planer and is of lower energy, 0.013 a.u. than
the non planer one (B) at the B3LYP/6-31+G* level, Table 3.
agrees satisfactorily with the experimental value, 2.60D [20].
Comparing the results of both levels shows that the DFT elon-
gates the C O, C N and N N bonds with maximum er◦ror of
˚
0.03 A, whilethedifferenceincaseofbondanglesisonly1 . The
RHF results in Table 3 lead to some important predictions, the
˚
˚
C1 C6 bond length is 1.488 A compared to 1.554 A for a typical
C C single bond which indicates a weak ␲-overlap between the
benzene ring and the C O carbon. Along the same line, C1 C6
bond order is 1.030, which is slightly larger than that between
Conformer (A) will be used for all coming calculations. The
optimized geometry of benzoyl azide is calculated using the
RHF/6-311G* and B3LYP/6-31+G* methods, results are given
in Tables 3 and 4. The calculated dipole moment is 2.19 D and
˚
singly bonded C C atoms. The bond length C1 N2 is 1.402 A
compared to 1.472 A for a pure single C N bond and 1.287A for
a pure C N double bond. Thus in benzoyl azide the C N bond
˚
˚
Table 4
Parameters of the equilibrium geometry of the ground state of benzoyl azide and its derivatives using the RHF/6-311G* and (B3LYP/6-31+G*) methods
Parameters
Benzoyl azide
p-CH3
p-OCH3
p-Cl
p-NO2
Bond length
˚
r-(C1 C6) (A)
r-(C1 O5)
r-(C1 N2)
r-(N2 N3)
r-(N3 N4)
1.488 (1.485)
1.197 (1.218)
1.402 (1.435)
1.259 (1.247)
1.088 (1.133)
1.485 (1.402)
1.481 (1.475)
1.488 (1.483)
1.496 (1.491)
1.190 (1.218)
1.403 (1.436)
1.254 (1.246)
1.082 (1.132)
1.191 (1.220)
1.405 (1.439)
1.253 (1.246)
1.082 (1.133)
1.188 (1.218)
1.399 (1.433)
1.256 (1.248)
1.081 (1.131)
1.188 (1.217)
1.394 (1.427)
1.260 (1.250)
1.080 (1.130)
Bond angle
θ(O5C1N2) (◦)
θ(C6C1N2)
θ(C1N2N3)
θ(N2N3N4)
θ(C7C6C1)
θ(N4N3N2C1)0
123.64 (124.70)
113.20 (112.7)
111.23 (114.4)
175.30 (174.7)
117.60 (117.9)
180.0 (180.0)
123.02 (124.56)
113.27 (112.66)
111.30 (114.431)
175.27 (174.678)
117.92 (118.115)
180.0 (180.0)
123.80 (125.00)
113.21 (112.7)
111.31 (114.3)
175.35 (174.8)
118.01 (118.3)
180.0 (180.0)
123.40 (124.57)
113.07 (112.60)
111.24 (114.40)
175.26 (174.60)
122.86 (118.00)
180.0 (180.0)
123.94 (124.10)
112.97 (112.6)
111.22 (114.4)
175.17 (174.4)
122.24 (117.7)
180.0 (180.0)
θ(C6C1N2N3)0
180.0 (180.0)
180.0 (180.0)
180.0 (180.0)
180.0 (180.0)
180.0 (180.0)

182
R.H. Abu-Eittah et al. / Spectrochimica Acta Part A 70 (2008) 177–186
Table 5
Transition energies (eV) calculated at the ab initio, CIS/B3LYP/6-341+G* and
the semi-empirical AM1/6-311G*levels
Compound
CIS/B3LYP/6-31+G*
AM1/6-311G*
Exp.
Benzoyl azide
5.158
5.669
5.853
5.918
4.45
4.65
5.00
5.50
4.37
4.61
4.92
5.15
p-Cl
5.938
5.999
6.006
6.010
4.38
4.58
4.64
5.28
4.34
4.53
4.71
4.92
p-Me
p-MeO
5.693
5.805
5.843
4.41
4.50
4.90
5.33
4.34
4.51
4.77
4.96
Fig. 6. Electronic absorption spectra of p-chlorobenzoyl azide in heptane and
its Gaussian analysis.
5.990
6.203
4.28
4.40
5.09
5.53
5.80
4.37
4.58
4.89
5.61
5.81
is essentially a single bond, the bond order is 1.046. The bond
˚
˚
length N(2) N(3) is 1.255 A compared with 1.240 A for N N
˚
double bond and 1.400 A for pure N N single bond and 1.100
p-NO2
5.641
5.695
5.846
5.883
4.20
4.43
4.77
5.21
4.01
4.28
4.68
4.90
for N N triple bond. Hence, in benzoyl azide, N(2) N(3) bond
is essentially a double bond. On the other hand, the N(3) N(4)
˚
bond length is 1.081 A reflecting a nice triple bond character
with a bond order of 2.389. The above results lead to two res-
onating structures (c) and (d) for benzoyl azide, structure (c) is
favored for the ground state.
For the studied compounds.
θ(N3N4N2C1) between the plane of the azide group and that
of C6H5CO is ∼180◦ indicating a planar configuration of the
molecule.
The optimized geometry of benzoyl azide at both levels was
used as an entry to calculate the excited states of the molecule
Table 6
The weight percent of the atomic orbitals in the wave functions of molecular
orbitals ꢀ25–ꢀ30 of the different chromophores of the studied benzoyl azides
The distribution of charge density is calculated and shown
in Table 3. The bond angle, θ(N2N3N4) is 175.30◦ in the
optimized geometry of benzoyl azide, hence a slight bent
structure is assigned to the azide group and indicating the
contribution of conformer (a). However, the dihedral angle
MO
wt.%
25, ␲
28, π1∗
␺
29, ␴
␺
30, π2∗
*
␺
␺
26, ␲
␺
27, ␲
3
␺
1
2
Chromophore benzoyl azide
N3
CO
C6H5
79
18
1
8
3
89
1
1
98
23
32
55
86
10
4
56
1
43
p-Methylbenzoyl azide
N3
CO
p-CH3C6H4
83
14
3
1
1
98
5
2
93
48
24
28
94
5
1
45
1
54
p-Methoxybenzoyl azide
N3
CO
82
17
1
1
1
98
2
3
95
26
30
44
90
8
2
4
1
95
p-CH3OC6H4
p-Chlorobenzoyl azide
N3
CO
p-ClC6H4
81
17
2
1
1
98
3
2
95
20
25
55
90
9
1
55
4
41
p-Nitrobenzoyl azide
N3
CO
p-NO2C6H4
21
13
66
8
1
91
61
4
35
4
9
87
94
3
3
70
13
17
Fig. 7. Electronic absorption spectra of o-chlorobenzoyl azide in heptane and
its Gaussian analysis.

R.H. Abu-Eittah et al. / Spectrochimica Acta Part A 70 (2008) 177–186
183
Table 7
CI wave functions of the excited states and the corresponding transition energies (ꢁE) of benzoyl azide
State functions
Assignment
ꢁE (eV)
Calc.
Obs.
4.37
π3 → π1∗: C6H5 → C6H5CON3, delocalized
π2 → π1∗: C6H5 → C6H5CON3, delocalized
π1 → π1∗: CON3 → C6H5CON3, delocalized
π1 → π2∗: CON3 → C6H5N3, delocalized
ΨEX I
=
−1
0.18φ27
0.47φ26
φ
28
4.45
4.65
5.00
−1
−1
φ
28
28
30
0.37φ25
φ
0.30φ−1φ
25
π3 → π1∗: C6H5 → C6H5CON3, delocalized
ΨEX II
=
−1
4.61
4.92
π3 → π2∗: C6H5 → C6H5N3, delocalized
0.63φ27
φ
28
−1
0.28φ27
φ
30
π2 → π1∗: C6H5 → C6H5CON3, elocalized
π2 → π2∗: C6H5 → C6H5N3, delocalized
π1 → π1∗: CON3 → C6H5CON3, delocalized
π1 → π2∗: CON3 → C6H5N3, delocalized
ΨEX III
=
−1
0.45φ26
0.18φ26
φ
28
−1
−1
φ
30
28
30
0.46φ25
φ
0.17φ−1φ
ΨEX IV
=
25
π2 → π1∗: C6H5 → C6H5CON3, delocalized
π1 → π1∗: CON3 → C6H5CON3, delocalized
π1 → π2∗: CON3 → C6H5N3, delocalized
π2 → π2∗: C6H5 → C6H5N3, delocalized
π3 → π1∗: C6H5 → C6H5CON3, delocalized
−1
0.33φ26
0.41φ25
0.34φ25
0.24φ26
φ
28
−1
−1
−1
−1
φ
28
φ
30
φ
30
φ
28
5.52
5.15
0.15φ27
using the ab initio CIS as well as the semi-empirical methods
AM1-CI methods. The results are given in Table 5. The AM1-CI
results match nicely with the experimental values. The SCF-
MO’s were calculated, the weight percent of the atomic orbitals
of the different chromophores in the wave functions of the three
highest occupied, ␸25–␸27 (␲1–␲3), and the three lowest unoc-
cupied, ␸28–␸30 (π1∗, σ∗ and π2∗), molecular orbitals are given in
Table 6. Excited configurations are those which result from a
one-electron transition between the three highest occupied MOs
and the lowest three unoccupied MOs. The CI matrix is solved,
the wave functions and corresponding energies of the first four
excited states are given in Table 7.
uration is that in which an electron is transferred from molecular
orbital ␸ (98% localized on the benzene ring) to molecular
27
orbital ␸28, a de-localized orbital over the whole molecule. The
same interpretation applies to the second configuration. The
third configuration corresponds to an electron transition from
␸25, localized on the CON3 group to ␸28, delocalized over the
whole molecule. The fourth configuration is also between de-
localized molecular orbitals. Hence, one can say that electronic
transitions in aroyl azides are not those of the aryl ring and or of
the azide group as is the case with aryl azides. The first electronic
*
transition of benzoyl azide, Table 7, is a ␲ → ␲ delocalized
transition, the calculated transition energy, ꢁE, is 4.45 eV, which
corresponds nicely with the experimental one, 4.37 eV. It is evi-
dent that the spectra of aroyl azides differ basically from the
spectra of aryl azides. The first electronic transition in phenyl
Analysis of the wave functions of the excited states is illus-
trative. The wave function of the first excited state, ψex1, is built
up from a−li1near combination of four el−ec1tronic configurations,
−1
namely, ϕ27 ϕ28, ϕ2−61ϕ28, ϕ25
ϕ
and ϕ ϕ30. The first config-
azide is a ␴ → ␲ transition (from the azide group to a de-
*
28
25
Table 8
CI wave functions of the excited states and the corresponding transition energies (ꢁE) of p-methylbenzoyl azide
State functions
Assignment
ꢁE (eV)
Calc.
Obs.
4.34
π3 → π1∗: p-CH3C6H4 → p-CH3C6H4CON3, delocalized
π1 → π1∗: CON3 → p-CH3C6H4CO N3, delocalized
π1 → π2∗: N3CO → p-CH3C6H4N3, delocalized
ΨEX I
=
−1
0.30φ30
0.53φ28
0.30φ
ΨEX II
φ
31
31
31
31
4.41
4.59
4.90
−1
φ
31
33
−1
φ
π3 → π1∗: p-CH3C6H4 → p-CH3C6H4CO N3, delocalized
π3 → π2∗: p-CH3C6H4 → p-CH3C6H4N3, delocalized
π2 → π1∗: p-CH3C6H4 → p-CH3C6H4CO N3, delocalized
=
28
−1
0.46φ30
0.39φ30
0.33φ
ΨEX III
φ
4.51
4.77
−1
φ
33
31
−1
φ
π3 → π1∗: p-CH3C6H4 → p-CH3C6H4CO N3, delocalized
π2 → π1∗: p-CH3C6H4 → p-CH3C6H4CON3, delocalized
π2 → π2∗: p-CH3C6H4 → p-CH3C6H4N3, delocalized
π3 → π1∗: p-CH3C6H4 → p-CH3C6H4CO N3, delocalized
π3 → π2∗: p-CH3C6H4 → p-CH3C6H4N3, delocalized
π1 → π1∗: CON3 → p-CH3C6H4CO N3, delocalized
π1 → π2∗: N3CO → p-CH3C6H4N3, delocalized
=
29
−1
0.16φ30
0.51φ29
0.44φ
ΨEX IV
φ
−1
φ
31
33
−1
φ
=
29
−1
0.33φ30
0.52φ30
0.30φ28
0.12φ28
φ
5.33
4.96
−1
−1
−1
φ
33
φ
31
φ
33

184
R.H. Abu-Eittah et al. / Spectrochimica Acta Part A 70 (2008) 177–186
localized orbital over the whole molecule) whereas the first
electronictransitioninbenzoylazideisa␲ → ␲* transitionstart-
ing from a molecular orbital mainly localized over the benzene
ring to a molecular orbital de[localized over the whole molecule.
A similar analysis of the wave functions of the other excite
4.2.3. p-Methoxybenzoyl azide
The optimized geometry is calculated, results are given in
Tables 3 and 4. As is the case with the previous compounds,
essential single bond exists between C1 and C6 (bond order is
1.096) which indicate a weak ␲ conjugation between the aryl
part and the rest of the molecule, a true double bond exists in
the C O group, (bond order is 1.965), pronounced single bond
characters exist between C(1) and N(2) and between N(2) and
N(3), bond orders are 1.097 and 1.117 respectively whereas
the N(3) N(4) bond has a pronounced triple bond characte◦r
states, ψEX II
−
is given in Table 7. It is observed that
EX IV
the calculated four electronic transitions are ␲ → ␲* transitions.
These results agree with the spectral observations, Fig. 1.
4.2.2. p-Methylbenzoyl azide
ˆ
(bond order is 2.385). Again, the bond angle θ(NNN) is 175.35
The optimized geometry of the molecule was calculated
using ab initio RHF and DFT procedures. Results are given in
Tables 3 and 4. As in the case with benzoyl azide, the C1 C6
bond order is 1.036 indicating an essential single bond and
the absence of ␲ conjugation between the benzene ring and
the CON3 group. The C O bond order is 1.965 indicating a
true double bond character. The triple bond character between
N3 N4 is apparent, bond order is 2.388. The bent structure ◦of
indicating a slight deviation from linearity in the azide group.
Excited states were calculated, the type and composition
of the three highest occupied molecular orbitals, ␸31–␸
(␲1–␲3), and the three lowest-vacant molecular orbitals ␸34–␸
33
36
(π1∗, σ∗, π2∗) are given in Table 6. Excitation is considered to
occur by a one electron transfer between of the three high-
est occupied and any of the three lowest unoccupied molecular
orbitals. The CI matrix is solved and the wave functions as well
as the energies of the first four excited states are given in Table 9.
It is to be noted that none of the calculated and observed transi-
ˆ
the N3 group is clear as the bond angle θ(N2N3N4) is 175.27 .
The wave functions of the SCF-MO’s were calculated, the
weight percent of the coefficients of the highest three occu-
pied molecular orbitals ␸28–␸30, designated as ␲1-␲3, and
the lowest three vacant molecular orbitals ␸31-␸33, designated
π1∗, σ∗ and π2∗ are given in Table 6.
Excitationwasconsideredforaone-electrontransferbetween
the highest three occupied and the lowest three unoccupied
molecular orbitals. The CI –matrix was solved, the state func-
tions and the corresponding energies are given in Table 8. The
correspondence between the calculated and experimental tran-
sition energies is satisfactory.
Analysis of the wave functions of the excited states indicates
clearly that none of the electronic trasitions of p-methylbenzoyl
azide is localized over any of the chromophores (aryl, aroyl
or azide) of the molecule as is the case in the spectra of
aryl azides. The wave function of the first excited state of p-
methylbenzoyl azide con−si1sts of a li−n1ear combination of the
configurations: ϕ3−01ϕ31, ϕ28 ϕ31 and ϕ28 ϕ33 which corresponds
to a ␲–␲* delocalized transition observed experimentally at
4.34 eV and calculated at 4.41 eV (Fig. 8).
*
tions is an n → ␲ transition. The correspondence between the
calculated and experimental transition energies is satisfactorily.
Analysis of the form of the wave functions of the excited
states is significant. The wave function of the first excited
state consists of a linear combinatio−n1of the following excited
configurations. The configuration ϕ33 ϕ34 which represent an
electron transfer from molecular orbital ␸33, which is extending
over the CH3OC6H4–chromophore, to the molecular orbital ␸
34
which delocalized over the−w1hole molecule p-methoxybenzoyl
azide. The configuration ϕ33 ϕ36 represents an electron transi-
tion between molecular oritals localized over the CH3OC6H4
chromophore. The configuration ϕ32
−1
ϕ
represents an elec-
34
tron transition from a molecular orbital extending over the
CH3OC6H4 part of the molecule to a molecular orbital de-
localized over the whole molecule p-methoxybenzoyl azide.
In a similar way one analyzes the wave functions of the
other excited states. This analysis indicates that all the observed
transitions in the electronic absorption spectrum of para-
*
methoxybenzoyl azide are de-localized ␲–␲ transitions. This
conclusion agrees with the experimental observations: weak
effect of solvent polarity on band maximum (absence of con-
tribution of charge transfer configurations to the excited states),
high intensity of the observed transition (absence of n–␲* tran-
sition).
4.2.4. p-Chlorobenzoyl azide
The ab-initio results of the optimized geometry of the
ground state are given in Tables 3 and 4. Excited states were
calculated by AM1 procedure. The CI matrix of the elec-
tronic configurations which result from one electron transition
between any of the three highest occupied molecular orbitals,
␸28(␲1)–␸30(␲3), to any of the three lowest vacant molecular
orbitals ϕ31(π∗) − ϕ33(π∗), ␸ is a ␴ molecular orbital, is
*
32
1
2
solved. The wave functions of the excited states are given in
Table 10. The results indicate that none of the observed tran-
sitions in the electronic spectrum of p-chlorobenzoyl azide is a
Fig. 8. Electronic absorption spectra of p-nitrobenzoyl azide in heptane and its
Gaussian analysis.

R.H. Abu-Eittah et al. / Spectrochimica Acta Part A 70 (2008) 177–186
185
Table 9
CI wave functions of the excited states and the corresponding transition energies (ꢁE) of p-methoxybenzoyl azide
State functions
Assignment
ꢁE (eV)
Calc.
Obs.
4.37
π3 → π1∗: p-CH3OC6H4 → p-CH3OC6H4CON3, delocalized
π3 → π2∗: Localized on p-CH3OC6H4
ΨEX I
=
−1
0.14φ33
0.58φ33
0.36φ
ΨEX II
φ
34
34
34
4.28
4.40
−1
π2 → π1∗: p-CH3OC6H4 → p-CH3OC6H4CON3, delocalized
φ
36
34
−1
φ
π3 → π1∗: p-CH3OC6H4 → p-CH3OC6H4CON3, delocalized
=
32
−1
π2 → π2∗: Localized on p-CH3OC6H4
0.58φ33
0.36φ32
0.13φ
ΨEX III
φ
4.58
4.89
−1
π2 → π1∗: p-CH3OC6H4 → p-CH3OC6H4CON3, delocalized
φ
36
34
−1
φ
π3 → π1∗: p-CH3OC6H4 → p-CH3OC6H4CON3, delocalized
π2 → π1∗: p-CH3OC6H4 → p-CH3OC6H4CON3, delocalized
π2 → π2∗: Localized on p-CH3OC6H4
=
32
−1
0.23φ33
0.14φ32
φ
5.09
−1
−1
φ
34
π1 → π1∗: CON3 → p-CH3OC6H4CON3, delocalized
0.38φ32
φ
36
0.52φ−1φ
ΨEX IV = 34
31
π2 → σ*: CT p-CH3OC6H4 → N3
−1
π1 → π1∗: CON3 → p-CH3OC6H4CON3, delocalized
␲3 → ␴*: CT p-CH3OC6H4 → N3
+0.70φ33
φ
+
35
5.53
5.80
5.61
5.85
−1
−1
0.24φ31
φ
34
φ
35
φ
34
−
π1 → π1∗: CON3 → p-CH3OC6H4CON3, delocalized
0.60φ33
0.26φ
ΨEX V
−
−1
=
31
π3 → π2∗: Localized on p-CH3OC6H4
−1
π2 → π1∗: p-CH3OC6H4 → p-CH3OC6H4CON3, delocalized
π2 → π2∗: Localized on p-CH3OC6H4
0.35φ33
0.57φ32
0.18φ32
φ
36
−1
−1
φ
34
36
φ
Table 10
CI wave functions of the excited states and the corresponding transition energies (ꢁE) of p-chlorobenzoyl azide
State function
Assignment
ꢁE (eV)
Calc.
Obs.
π3 → π1∗: p-ClC6H4 → p-ClC6H4CON3, delocalized
π1 → π1∗: N3CO → p-ClC6H4CON3, delocalized
π1 → π2∗: N3CO → p-ClC6H4N3, delocalized
ΨEX I
=
−1
0.33φ30
0.48φ28
0.36φ
ΨEX II
φ
31
31
4.38
4.34
−1
φ
31
33
−1
φ
π3 → π1∗: p-ClC6H4 → p-ClC6H4CON3, delocalized
π3 → π2∗: p-ClC6H4 → p-ClC6H4N3, delocalized
π1 → π1∗: N3CO → p-ClC6H4CON3, delocalized
=
28
−1
0.51φ30
0.37φ30
0.28φ
ΨEX III
φ
4.58
4.64
4.53
4.71
−1
φ
33
31
−1
φ
π2 → π1∗: p-ClC6H4 → p-ClC6H4CON3, delocalized
=
28
−1
π2 → π2∗: p-ClC6H4 → p-ClC6H4N3, delocalized
0.58φ29
φ
31
−1
0.40φ29
φ
33
π3 → π1∗: p-ClC6H4 → p-ClC6H4CON3, delocalized
π3 → π2∗: p-ClC6H4 → p-ClC6H4N3, delocalized
π1 → π1∗: N3CO → p-ClC6H4CON3, delocalized
π1 → π2∗: N3CO → p-ClC6H4N3, delocalized
ΨEX IV
=
−1
0.46φ30
0.32φ30
0.34φ28
0.18φ28
φ
31
5.28
4.92
−1
−1
−1
φ
33
φ
31
φ
33
localized or corresponds to an n → ␲* transition. The correspon-
dence between calculated and experimental transition energy is
satisfactory.
are evident. The calculated dipole moment is 4.26D, less than
that of nitrobenzene, which indicates that the electron withdraw-
ing effect of the NO2 group is inhibited by that of the CON3
group. The bond order between the N-atom of the NO2 group
and C of the phenyl ring is only 0.822 which indicates an
essential single bond and weak ␲-conjunction between the nitro
group and the benzene ring. The same behavior exists between
C
of the benzene ring and C of the CON 3 group. The bond
◦
ˆ
angle θ(NNN) is 176.16 which reflects an extent of linearity
of the azide group. The partial triple bond character between the
terminal adjacent nitrogen atoms and the partial double bond
character between the inner nitrogen atoms are apparent, bond
orders are 2.398 and 1.093, respectively.
4.2.5. p-Nitrobenzoyl azide
The optimized geometry of p-NO2 benzoyl azide has been
calculated using the ab initio method and the 6-311G* basis set.
The results are given in Tables 3 and 4. Some important results
Excited states were calculated using the AM1-procedures as
was done with the previously studied compounds, results are

186
R.H. Abu-Eittah et al. / Spectrochimica Acta Part A 70 (2008) 177–186
Table 11
CI wave functions of the excited states and the corresponding transition energies (ꢁE) of p-nitrobenzoyl azide
State function
Assignment
ꢁE (eV)
Calc.
Obs.
4.01
π3 → π1∗: CT N3 → p-NO2C6H4CO
ΨEX I
=
−1
π3 → π2∗: p-NO2C6H4CO N3 → p-NO2C6H4CON3, delocalized
π1 → π1∗: p-NO2C6H4CON3 → p-NO2C6H4CO, delocalized
π1 → π2∗: p-NO2C6H4N3 → p-NO2C6H4N3, delocalized
0.50φ35
0.33φ35
φ
36
4.20
−1
−1
φ
38
36
38
0.20φ33
φ
0.28φ−1φ
ΨEX II
=
33
π3 → π1∗: CT N3 → p-NO2C6H4CO
−1
4.43
4.77
4.28
4.66
π2 → π1∗: Localized on p-NO2C6H4
0.32φ35
φ
36
−1
0.63φ34
φ
36
π3 → π2∗: p-NO2C6H4CO N3 → p-NO2C6H4CON3, delocalized
π2 → π2∗: CT p-NO2C6H4 → CON3
ΨEX III
=
−1
0.33φ35
0.14φ34
0.59φ
ΨEX IV
φ
38
−1
π1 → π1∗: p-NO2C6H4CON3 → p-NO2C6H4CO, delocalized
φ
38
36
−1
φ
π3 → π1∗: CT N3 → p-NO2C6H4CO
=
33
−1
π3 → π2∗: p-NO2C6H4CON3 → p-NO2C6H4CON3, delocalized
π2 → π1∗: Localized on p-NO2C6H4
0.35φ35
φ
36
−1
034φ3−5 1
φ
5.21
4.90
38
π1 → π1∗: p-NO2C6H4CON3 → p-NO2C6H4CO, delocalized
π2 → π2∗: CT p-NO2C6H4 → CON3
0.26φ34 φ36
−1
−1
0.32φ33
0.25φ34
φ
36
38
φ
given in Table 11. The contribution of charge transfer configu-
rations to the excited states is minor. None of the observed or
calculated transitions is an n → ␲ transitions.
References
*
[1] I.M. Shaw, A. Talor, Can. J. Chem. 64 (1) (1986) 164.
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[3] H. Giono, R. Camain-Giabicani, P. Giono, Bull. Mem. Fac. Mixte Med.
Pharm. Dakar 11 (1963) 280.
5. Conclusions
The observed electronic absorption spectra of the studied acyl
azides indicate a marked perturbation effect of the CON3 group
on the energy levels of the benzene ring in spite of the fact
that calculations show a minor ␲-conjugation between the ring
and the CON3group. The observed spectra show weak sol-
vent effect and marked band-overlap. The weak solvent effect
indicates that the contribution of charge transfer configurations
to the excited states is not significant. Gaussian analysis of the
observed spectra indicate intense bands, a result which means
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[11] Y.Yu. Kawa, Y. Tsuno, J. Am. Chem. Soc. 79 (1957) 5530.
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[13] T. Curtius, H. Franzen, Ber 35 (1902) 3239.
[14] C. Naegeli, A. Tyabji, L. Conrad, Helv. Chem. Acta 21 (1938) 1127.
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[16] T. Curtius, O. Trachmann, J. Prakt. Chem. 51 (1985) 165.
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[18] K. Hermansson, L. Ojamae, University of Uppsala, Institute of Chemistry,
Report UUIC-B19-500 (1994).
*
the absence of n → ␲ transitions.
Ab initio molecular orbital calculations give the geome-
try optimized values for charge density distribution, dipole
moment, bond order, bond length and bond angles. The res-
onating forms contributing to the real structures of the molecule
were predicted and pseudo-linearity of the azide group was clear.
AM1-calculations give a good insight to the nature of the excited
states and to the identification of the observed transitions. Calcu-
lations indicated that the electronic spectra of aroyl azides result
from transitions between orbitals de-localized over the whole
molecule and not between localized orbitals as is the case with
aryl azides.
[19] Mathcad plus 6.0, from Mathsoft© 1986–1994 Mathsoft Inc.
[20] Ya.K. Syrkin, E.A. Shott-L’vova, Dokl. Akad. Nauk S.S.S.R. 87 (1952)
639.