Molecular structure of unsubstituted oxadiazolic analog of ortho-POPOP and peculiarities of conformational structure of this class of sterically hindered organic compounds

Article abstract of DOI:10.1016/S0022-2860(01)00889-4

X-ray molecular structure of unsubstituted oxadiazolic ortho-analog of 1,4-bis-(5-phenyl-oxazolyl-2)-benzene (POPOP), synthesized via the 2,5-diphenyl-1,3,4-oxadiazole ortho-carbonic acid was revealed. Its conformation was found to be the same as for the compound obtained earlier from the 2,5-diphenyl-oxazole ortho-carbonic acid. Quantum-chemical modeling was conducted to understand the peculiarities of conformational structure of the investigated class of sterically hindered compounds.

Full text of DOI:10.1016/S0022-2860(01)00889-4

Journal of Molecular Structure 609 .2002) 29±37
www.elsevier.com/locate/molstruc
Molecular structure of unsubstituted oxadiazolic analog of
ortho-POPOP and peculiarities of conformational structure
of this class of sterically hindered organic compounds
a,
b
A.O. Doroshenko , V.N. Baumer , A.A. Verezubova , L.M. Ptyagina
a
a
*
aInstitute for Chemistry at Kharkov V.N. Karazin National University, 4 Svobody sq., Kharkov 61077, Ukraine
bUkrainian Academy of Science Institute for Single Crystals, 60 Lenin Avenue, Kharkov 61001, Ukraine
Received 20 April 2001; revised 7 September 2001; accepted 17 September 2001
Abstract
X-ray molecular structure of unsubstituted oxadiazolic ortho-analog of 1,4-bis-.5-phenyl-oxazolyl-2)-benzene .POPOP),
synthesized via the 2,5-diphenyl-1,3,4-oxadiazole ortho-carbonic acid was revealed. Its conformation was found to be the same
as for the compound obtained earlier from the 2,5-diphenyl-oxazole ortho-carbonic acid. Quantum-chemical modeling was
conducted to understand the peculiarities of conformational structure of the investigated class of sterically hindered
compounds. q 2002 Elsevier Science B.V. All rights reserved.
Keywords: X-ray structural analysis; Ortho-POPOP; Molecular modeling
1. Introduction
the usage in several technical applications, in which
reabsorption of ¯uorescence, emitted by organic lumi-
nophore, considerably decreases light output: plastic
and liquid scintillators of large dimensions, solar
energy collectors, ¯uorescence probes, etc.
Ortho-analogs of 1,4-bis-.5-phenyl-oxazolyl-2)-
benzene .POPOP) [1±3] are known as effective
abnormally high Stokes shift organic luminophores
[4±6]. Unique spectral properties of this class of
compounds are determined by the non-planar and
asymmetric conformation of their molecules in the
ground state and noticeable ¯attening, typical to
them in S1-state. Due to the excited state conformation
changes of these molecules, they are characterized by
the abnormally high Stokes shifted ¯uorescence
Dn , 8000±10 000 cm21) [3±6]. Taking their high
quantum yields also into account, this class of steri-
cally hindered compounds appear very prospective for
Owing to the sterical hindrance, which is the result
of introduction of two bulky fragments into the ortho-
positions of the central benzene ring of the ortho-
analogs of POPOP, two quasi-¯attened fragments
* Corresponding author. Tel.: 1380-572-457335; fax: 1380-572-
457130.
E-mail address: andrey.o.doroshenko@univer.kharkov.ua
.A.O. Doroshenko).
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860.01)00889-4

30
A.O. Doroshenko et al. / Journal of Molecular Structure 609 12002) 29±37
are formed in their molecules. First of them .a)
consists of three aromatic or hetero-cycles, while the
second one .b) consists of two cycles only. Conjuga-
tion between these fragments is disrupted due to the
sterical reasons.
determined by the con®guration of the starting
compounds in their synthesis: so far as we use in
most cases 2-.20-COOH-phenyl)-5-phenyl-oxazole,
in which diphenyloxazolic fragment is initially ¯at,
it may remain practically ¯attened in all the inter-
mediate compounds and also in the ®nal molecule
of the ortho-analog of POPOP. I0n Ref. [7], we also
assumed that the usage of 2-.2 -COOH-phenyl)-5-
phenyl-1,3,4-oxadiazole as starting compound in the
synthesis of oxadiazolic ortho-POPOPs might result
in obtaining the ®nal product with the oxadiazole
cycle present in the more extended three-cycled
quasi-¯attened fragment a. In the present paper, we
describe the results of such synthesis, X-ray investi-
gation of the molecular structure of the obtained
product, and quantum-chemical modeling of mole-
cular conformation of oxadiazolic ortho-analogs of
POPOP.
It is interesting that in molecules of oxadiazolic
ortho-analogs of POPOP, in which one oxazole
cycle is substituted for the oxadiazole one, the latter
hetero-cycle always belongs to the minor, two-cycled
structural fragment b [1±7]. Our analysis of their
spectral properties allowed us to conclude that
owing to the signi®cant disruption of conjugation
between their a and b parts, caused by sterical reasons,
these compounds behave as typical bichromophoric
molecules with reduced interaction between chromo-
phoric fragments [3±5,7]: substituents, which were
introduced into the oxadiazolic part of these mole-
cules have no effect onto the absorption bands for
which the diphenyloxazolic quasi-¯attened fragment
is responsible [3±7]. Theoretically, two main confor-
mers of I, different from one another by the presence
of oxazole cycle in two- or three-cycled fragment,
must be in equilibrium, which requires fast exchange
between them. The above-presented facts also
evidence the absence of such an exchange or the
signi®cant shift of conformational equilibrium
towards the conformer with oxazolic cycle in frag-
ment a.
2. Experimental
Oxadiazolic ortho-analog of POPOP .1-.5-phenyl-
oxazolyl-2)-2-.5-phenyl-1,3,-4-oxadiazolyl-2)-benz-
ene, compound I) was synthesized by the following
scheme, quite traditional to diaryloxazoles, starting
from the diphenyloxadiazole ortho-carbonic acid, 2-
.20-COOH-phenyl)-5-phenyl-1,3,4-oxadiazole, the
synthesis of which was described in Ref. [8]. Obtained
in such a way, compound I was puri®ed by multiple
recrystallizations from octane, mp 87±888C. Other
chemicals were of commercially available grade and
were used without special puri®cation.
First, we tried to describe this feature as a result of
more favorable conjugation between oxazole cycle
and benzene rings in its 2 .and/or 5) position in
comparison with that of oxadiazole cycle [1,2].
Though calculated difference in the correspondent
conjugation energy then appeared to be very low,
we later tried to clarify such a behavior of oxadiazolic
ortho-POPOPs as a result of highly .or even comple-
tely) hindered intermolecular rotation in their mole-
cules [7]. According to that assumption, conformation
of oxadiazolic ortho-POPOPs might be completely
Crystals of compound I suitable for X-ray analysis
were obtained by slow evaporation of its concentrated
solution in propanol-2. X-ray structural experiment
was performed on automatic diffractometer Siemens
Ê
P3/PC.Mo Ka, l ˆ 0.71073 A; graphite monochro-
mator). Intensities were measured by 2u/u-scan

A.O. Doroshenko et al. / Journal of Molecular Structure 609 12002) 29±37
31
Fig. 1. X-ray molecular structure of several previously studied oxadiazolic ortho-analogs of POPOP: .1) 1-.5-phenyl-oxazolyl-2)-2-.5-.furyl-
2)-1,3,4-oxadiazolyl-2)-benzene [2], .2) 1-.5-phenyl-oxazolyl-2)-2-.5-.20-OCH3-phenyl)-1,3,4-oxadiazolyl-2)-benzene [3], .3) 1-.5-phenyl-
oxazolyl-2)-2-.5-.40-N.CH3)2-phenyl)-1,3,4-oxadiazolyl-2)-benzene [7].
technique in the angle range from 3.04 to 35.088. 8696
total intensities in the hkl range of 0 # h # 19; 0 #
k # 14; 228 # l # 28 were measured, of which 8385
were unique ꢀRint , 6:31%†: The following main
crystallographic data were obtained:
of molecular geometry were performed using semi-
empirical method AM1 [10] by mopac 6.0 program
.with EF Ð `eigenvector following' keyword). When
constructing potential energy surface for intramole-
cular rotation in I, two intercycle torsional angles a1
and a2) were being ®xed on the de®nite values .0±
1808, step 108), while the other molecular parameters
were optimized without any restrictions.
Compound I Ð C23H15N3O2, Mr ˆ 365:38; a ˆ
ꢀ
12:0913; b ˆ 8:8452; c ˆ 17:9935 A; b ˆ 98:058;
ꢀ 3
V ˆ 1905:38 A ; monoclinic; space group P21/c; Z ˆ
4; dcalc ˆ 1:274 g cm23; T ˆ 293 K; mꢀMo Ka† ˆ
0:083 mm21; Fꢀ000† ˆ 760:
The structure was solved by the direct method and
re®ned using full-matrix least squares on .Fhkl)2 with
the weighting scheme w21 ˆ s2ꢀF† by the shelxtl
plus program [9] .goodness-of-®t on F2 ˆ 0.703),
R1 ˆ 0:0438; wR2 ˆ 0:0764 for unique re¯ections,
R1 ˆ 0:1136; wR2 ˆ 0:0889 for all independent
re¯ections with I . 2sI: The anisotropic thermal
corrections were made for all non-hydrogen atoms.
Quantum-chemical calculations with the optimization
3. Results and discussion
First, let us note that no X-ray investigation of the
molecular structure of unsubstituted compound I was
made before. All our previous conclusions concerning
the conformation of oxadiazolic ortho-POPOPs were
related to several substituted derivatives of I, mole-
cular geometries of which are presented in Fig. 1 for
informational purpose. However, strong correlation

32
A.O. Doroshenko et al. / Journal of Molecular Structure 609 12002) 29±37
Table 1
4
Ê
Ê 2
Atomic coordinates .A £ 10 ) and equivalent isotropic displace-
3
ment parameters .A £ 10 ) for 1-.5-phenyl-oxazolyl-2)-2-.5-
phenyl-1,3,4-oxadiazolyl-2)-benzene .I). Ueq is de®ned as one
third of the trace of the orthogonalized Uijtensor
Atom
X/a
4097.1)
Y/b
Z/c
Ueq
O.1)
4782.1)
5261.1)
2707.1)
5864.1)
7144.1)
2550.1)
3228.1)
2428.1)
960.2)
4088.1)
3615.1)
4309.1)
4762.1)
4288.1)
4576.1)
4600.1)
4917.1)
5195.1)
5152.1)
4853.1)
4325.1)
4042.1)
3902.1)
3598.1)
3619.1)
3315.1)
2997.1)
2967.1)
3267.1)
4348.1)
3628.1)
2937.1)
2989.1)
2337.1)
1642.1)
1583.1)
2225.1)
4940.1)
5415.3)
5338.2)
4833.1)
3964.1)
3841.3)
3326.1)
2790.3)
2753.4)
3246.1)
3466.5)
2374.1)
1176.7)
1089.7)
2180.1)
53.1)
O.2)
N.1)
1835.1)
5164.1)
1491.1)
1246.1)
3198.1)
2142.1)
1314.1)
1506.1)
2536.1)
3367.1)
4168.1)
5781.1)
5149.1)
5327.1)
4561.1)
4763.1)
5725.1)
6482.1)
6297.1)
1841.1)
1472.1)
1375.1)
1199.1)
1073.1)
1111.1)
1283.1)
1416.1)
653.8)
51.1)
72.1)
61.1)
64.1)
51.1)
49.1)
61.1)
70.1)
71.1)
63.1)
52.1)
69.1)
53.1)
53.1)
64.1)
77.1)
83.1)
85.1)
75.1)
47.1)
48.1)
51.1)
63.1)
73.1)
72.1)
69.1)
60.1)
65.3)
85.4)
81.4)
70.3)
81.4)
71.3)
70.4)
85.4)
107.5)
101.5)
57.3)
70.4)
105.5)
95.4)
95.4)
N.2)
N.3)
C.1)
C.2)
C.3)
C.4)
C.5)
273.2)
Fig. 2. X-ray molecular structure, scheme of atom numbering, and
thermal vibrations ellipsoids for 1-.5-phenyl-oxazolyl-2)-2-.5-
phenyl-1,3,4-oxadiazolyl-2)-benzene .com-pound I).
C.6)
1051.1)
3300.1)
3868.1)
5136.1)
6651.1)
7830.1)
9240.2)
9496.2)
8333.2)
6914.2)
4803.1)
6735.1)
7657.1)
9227.1)
C.7)
C.8)
C.9)
C.10)
C.11)
C.12)
C.13)
C.14)
C.15)
C.16)
C.17)
C.18)
C.19)
C.20)
C.21)
C.22)
C.23)
H.3)
between the shape of the electronic absorption spectra
of ortho-POPOPs and conformation of their
molecules .mutual spatial orientation of their two
hetero-cycles), revealed by our quantum-chemical
calculations in Ref. [4], allowed us to make structural
conclusions on the qualitative level without making
more complicated X-ray experiments.
Both the specimens of 1-.5-phenyl-oxazolyl-2)-2-
.5-phenyl-1,3,4-oxadiazolyl-2)-benzene, synthesized
by the two schemes described above .`oxazolic' Ð
10109.2)
9457.2)
7921.2)
7009.2)
2867.6)
425.7)
H.4)
930.7)
H.5)
H.6)
2674.2)
4097.8)
6577.9)
3902.8)
4235.6)
5871.2)
7098.10)
6874.8)
1167.1)
954.2)
2 790.14)
549.6)
H.8)
3782.2)
7672.2)
10030.10)
10512.14)
8491.3)
6055.11)
9686.5)
11214.13)
10107.9)
7466.6)
5929.15)
H.11)
H.12)
H.13)
H.14)
H.15)
H.19)
H.20)
H.21)
H.22)
H.23)
1013.2)
1313.1)
1537.2)
starting from diphenyloxazole and `oxadiazolic' Ð
from diphenyloxadiazole ortho-carbonic acids), have
practically the same electronic spectra. This is the
evidence for the similarity of their molecular
Fig. 3. Scheme of packing of compound I molecules into the crystal-
line lattice.

A.O. Doroshenko et al. / Journal of Molecular Structure 609 12002) 29±37
33
Table 2
Bond lengths .A) for 1-.5-phenyl-oxazolyl-2)-2-.5-phenyl-1,3,4-oxadiazolyl-2)-benzene
Ê
Atoms
Bond length
Atoms
Bond length
Atoms
Bond length
O.1)±C.7)
O.1)±C.9)
O.2)±C.17)
O.2)±C.16)
N.1)±C.7)
N.1)±C.8)
N.2)±C.16)
N.2)±N.3)
N.3)±C.17)
C.1)±C.2)
C.1)±C.6)
C.1)±C.7)
C.2)±C.3)
C.2)±C.16)
C.3)±C.4)
C.3)±H.3)
1.3779.12)
1.3946.11)
1.3775.12)
1.3795.11)
1.3177.13)
1.3932.15)
1.3046.12)
1.4241.12)
1.3067.12)
1.4172.13)
1.4215.15)
1.4718.14)
1.4093.14)
1.4941.14)
1.3988.17)
0.8960
C.4)±C.5)
C.4)±H.4)
1.3974.16)
0.9715
C.14)±C.15)
C.14)±H.14)
C.15)±H.15)
C.17)±C.18)
C.18)±C.23)
C.18)±C.19)
C.19)±C.20)
C.19)±H.19)
C.20)±C.21)
C.20)±H.20)
C.21)±C.22)
C.21)±H.21)
C.22)±C.23)
C.22)±H.22)
C.23)±H.23)
1.3967.18)
0.8963
C.5)±C.6)
1.3874.15)
1.0040
1.0351
C.5)±H.5)
1.4771.14)
1.4104.14)
1.4108.16)
1.3993.15)
0.9546
C.6)±H.6)
C.8)±C.9)
0.9936
1.3604.15)
0.9949
C.8)±H.8)
C.9)±C.10)
C.10)±C.11)
C.10)±C.15)
C.11)±C.12)
C.11)±H.11)
C.12)±C.13)
C.12)±H.12)
C.13)±C.14)
C.13)±H.13)
1.4741.15)
1.3988.15)
1.4075.15)
1.3969.17)
0.9493
1.3840.18)
0.9910
1.3809.17)
1.0096
1.3860.18)
0.9483
1.3992.15)
0.9818
1.3830.19)
0.9973
0.9715
conformation, according to the conclusions of Ref.
[4]. Though we understand that only the X-ray inves-
tigation could be considered as `the last provement'
for any hypothesis concerning the molecular struc-
ture.
two quasi-planar fragments could be considered in I :
diphenyloxazolyl fragment .P1 1 P2 1 P3 with mean
Ê
deviation of non-hydrogen atoms 0.0828 A) and
oxadiazolylphenyl one .P4 1 P5, mean deviation
Ê
0.0845 A). The angle between their least square
Results of our X-ray structural analysis for
compound I, which was synthesized by the alternative
`oxadiazolic' scheme, described in Section 2, are
presented in Fig. 2 .atom numbering and thermal
vibration ellipsoids are shown). Scheme of package
of this compound in the crystalline lattice is shown in
Fig. 3. Corresponding numerical data are collected in
Tables 1±3.
planes was 64.78. The intercycle bond lengths in the
main ¯attened fragment exceed the corresponding
Ê
values for the minor one: rC1±C7 ˆ 1.472 A and rC9±
Ê
Ê
Ê
ˆ 1.467 A, whereas rC2±C16 ˆ 1.495 A and rC1±
C10
ˆ 1.478 A. This highlights the difference in inter-
C7
cycle conjugation within these two parts of the
discussed molecule. Shortened intermolecular
contacts which evoke the non-planarity of I are
Ê
As in our previous papers [1±3,7], the aromatic and
hetero-cycles in the molecule of compound I are
denoted in the following manner: P1 Ð terminal
benzene ring in the oxazolic part of the molecule; P2
Ð oxazole cycle; P3 Ð central benzene ring; P4 Ð
oxadiazole cycle; P5 Ð terminal benzene ring in the
oxadiazolic part. All the cycles in the molecule I were
found to be planar. The mean deviations of their non-
hydrogen atoms from the least-squares ®tted planes
the following: O1´´´O2 ˆ 2.781 A and O1´´´C16 ˆ
Ê
2.839 A.
Our preliminary investigations show the absence of
optical activity of several ortho-analogs of POPOP in
solutions including compound I. Both theoretically
possible optical isomers were found in the crystalline
state of I .Fig. 3), they are clearly seen in the middle
of the elementary cell, which is presented in the
®gure. Thus, compound I exists both in crystalline
state and in solutions as the racemic mixture.
Another interesting feature of I in the crystalline
state is the existence of parallel layers formed by its
P1 1 P2 1 P3 and P4 1 P5 quasi-planar fragments.
Such a construction of crystalline lattice results in
the presence of de®nite free volume in it. Thus, it
becomes clear why the excited state structural
ꢀ
ꢀ
are the following: P1 ˆ 0:0037 A; P2 ˆ 0:0032 A;
ꢀ
ꢀ
ꢀ
P3 ˆ 0:0026 A; P4 ˆ 0:0039 A; P5 ˆ 0:0039 A: The
angles between the planes passing through these
cycles are P1 and P2 ˆ 12:008; P2 and P3 ˆ 5:188;
P3 and P4 ˆ 57.888; and P4 and P5 ˆ 12.688. Thus,
in the complete analogy with other compounds of
the ortho-POPOP series, studied by us earlier [1±7],

34
A.O. Doroshenko et al. / Journal of Molecular Structure 609 12002) 29±37
Table 3
Valent angles .8) for 1-.5-phenyl-oxazolyl-2)-2-.5-phenyl-1,3,4-oxadiazolyl-2)-benzene
Atoms
Angle
Atoms
Angle
C.7)±O.1)±C.9)
C.17)±O.2)±C.16)
C.7)±N.1)±C.8)
C.16)±N.2)±N.3)
C.17)±N.3)±N.2)
C.2)±C.1)±C.6)
C.2)±C.1)±C.7)
C.6)±C.1)±C.7)
C.3)±C.2)±C.1)
C.3)±C.2)±C.16)
C.1)±C.2)±C.16)
C.4)±C.3)±C.2)
C.4)±C.3)±H.3)
C.2)±C.3)±H.3)
C.5)±C.4)±C.3)
C.5)±C.4)±H.4)
C.3)±C.4)±H.4)
C.6)±C.5)±C.4)
C.6)±C.5)±H.5)
C.4)±C.5)±H.5)
C.5)±C.6)±C.1)
C.5)±C.6)±H.6)
C.1)±C.6)±H.6)
N.1)±C.7)±O.1)
N.1)±C.7)±C.1)
O.1)±C.7)±C.1)
C.9)±C.8)±N.1)
C.9)±C.8)±H.8)
N.1)±C.8)±H.8)
C.8)±C.9)±O.1)
C.8)±C.9)±C.10)
O.1)±C.9)±C.10)
C.11)±C.10)±C.15)
C.11)±C.10)±C.9)
C.15)±C.10)±C.9)
C.12)±C.11)±C.10)
C.12)±C.11)±H.11)
105.59.8)
102.72.7)
104.42.10)
106.67.8)
105.76.9)
118.02.10)
125.37.10)
116.52.10)
119.58.10)
115.76.9)
124.56.9)
121.30.11)
119.3
C.10)±C.11)±H.11)
C.13)±C.12)±C.11)
C.13)±C.12)±H.12)
C.11)±C.12)±H.12)
C.14)±C.13)±C.12)
C.14)±C.13)±H.13)
C.12)±C.13)±H.13)
C.13)±C.14)±C.15)
C.13)±C.14)±H.14)
C.15)±C.14)±H.14)
C.14)±C.15)±C.10)
C.14)±C.15)±H.15)
C.10)±C.15)±H.15)
N.2)±C.16)±O.2)
119.9
120.71.14)
119.6
119.6
119.53.14)
120.2
120.2
120.71.14)
119.6
119.6
120.03.13)
120.0
120.0
119.3
112.13.9)
125.59.9)
122.14.8)
112.69.9)
127.59.10)
119.72.8)
119.02.11)
122.11.10)
118.83.10)
119.52.12)
120.2
119.21.11)
120.4
120.4
N.2)±C.16)±C.2)
O.2)±C.16)±C.2)
N.3)±C.17)±O.2)
120.35.12)
119.8
119.8
N.3)±C.17)±C.18)
O.2)±C.17)±C.18)
C.23)±C.18)±C.19)
C.23)±C.18)±C.17)
C.19)±C.18)±C.17)
C.20)±C.19)±C.18)
C.20)±C.19)±H.19)
C.18)±C.19)±H.19)
C.21)±C.20)±C.19)
C.21)±C.20)±H.20)
C.19)±C.20)±H.20)
C.22)±C.21)±C.20)
C.22)±C.21)±H.21)
C.20)±C.21)±H.21)
C.21)±C.22)±C.23)
C.21)±C.22)±H.22)
C.23)±C.22)±H.22)
C.22)±C.23)±C.18)
C.22)±C.23)±H.23)
C.18)±C.23)±H.23)
121.50.12)
119.3
119.3
112.89.9)
126.68.10)
120.42.9)
110.97.10)
124.5
120.2
120.80.13)
119.6
119.6
124.5
120.26.13)
119.9
106.13.9)
135.30.10)
118.55.9)
118.84.11)
122.44.10)
118.72.10)
120.16.12)
119.9
119.9
120.32.13)
119.8
119.8
120.08.12)
120.0
120.0
relaxation process which determines the abnormally
high ¯uorescence Stokes shifts of ortho-POPOPs and
requires the high-amplitude intramolecular motions is
realized not only in ¯uid solutions, but also in the rigid
crystalline state [4,5].
Thus, contrary to our earlier expectations [7], we
found for the compound I, synthesized by the alter-
native `oxadiazolic' scheme, the same molecular
conformation with oxazole cycle in more extended
quasi-¯attened fragment a, as for the other oxadia-
zolic ortho-analogs of POPOP .Fig. 1), all of which
were synthesized on the base of diphenyloxazole
ortho-carbonic acid [1±7]. So, now we have to return
to our ®rst description of the conformational behavior
of this group of compounds based on the thesis of
more favorable conjugation between oxazole cycle
and benzene ring in its position 2 in comparison
with that of correspondingly substituted oxadiazole
cycle, in spite of the fact that calculated energetic
difference between two main conformers of I is not
very large.
Among the experimental facts, which could support

A.O. Doroshenko et al. / Journal of Molecular Structure 609 12002) 29±37
35
Fig. 4. Molecular structure of 2-.20-COOH-phenyl)-5-phenyl-oxazole [11] and 2-.20-COOH-phenyl)-5-phenyl-1,3,4-oxadiazole [12].
this statement are the results of X-ray structural
analysis of two carbonic acids that could be used as
starting materials in two alternative schemes of synth-
esis of I, diphenyloxazole-ortho-carbonic acid .II)
[11] and diphenyl-1,3,4-oxadiazole-ortho-carbonic
acid .III) [12]. Their molecular geometries are
presented in Fig. 4.
typical to the molecule II. Owing to more substantial
conjugation between oxazole cycle and benzene ring
in position 2, the intercycle angles in II are much
lower than in the case of molecule III discussed
earlier. Thus, one could consider the remaining of
common p-conjugated system of II; however,
substantially weakened due to the distortions in
planarity caused by formation of intermolecular H-
bonds. So, we could assume that the energy of conju-
gation in this molecule is comparable with the energy
of formation of intermolecular hydrogen bonds
between neighboring II molecules in the crystalline
state.
To clarify the peculiarities of conformational struc-
ture of I not only in the crystalline state, but also in gas
phase and/or solutions, we conducted quantum-
chemical modeling with the use of semiempirical
AM1 method. It had shown satisfactory applicability
to description of the torsional effects in several
organic molecules [13±15]. Our own AM1 calcula-
tions of torsional potentials in biphenyl molecule
show that this method is no worse than several
novel high level ab initio schemes, whose application
to biphenyl was described in Ref. [16].
For the molecules of both discussed acids, strong
intermolecular hydrogen bonding between COOH
group and the nitrogen atom of the hetero-cycle of
neighboring molecules is typical in the crystalline
state. This results in the dramatic distortion of their
planarity .Fig. 4). Let us start our further considera-
tion from oxadiazolic compound III. Probably, conju-
gation between the carboxylic group of III and the
benzene ring, to which it is connected, is more favor-
able, than that of 1,3,4-oxadiazole .Odz) cycle and
benzene ring in position 2 of the latter. Thus, in the
crystalline state of III its Ph-COOH moiety is rotated
out of the plane of Odz-Ph fragment on the angle close
to 908 and conjugation between these parts of the
discussed molecule is completely disrupted. We
could also conclude that the energy of intermolecular
hydrogen bond formation is much higher than the
energy of conjugation between oxadiazole cycle and
benzene ring in its position 2. Another behavior is
The result of our modeling of conformational
structure of I is the potential energy surface for the

36
A.O. Doroshenko et al. / Journal of Molecular Structure 609 12002) 29±37
Fig. 5. Potential energy .kcal mol21) surface for the rotation of oxazole a1, degrees) and oxadiazole a2, degrees) cycles around the ordinary
bonds, which connect them to the central benzene ring of the molecule of compound I, calculated by AM1 method. Arrow 1 corresponds to the
racemization and arrow 2 to the exchange between two main probable conformers of 1-.5-phenyl-oxazolyl-2)-2-.5-phenyl-1,3,4-oxadiazolyl-
2)-benzene.
intramolecular rotation of its two hetero-cycles
around the ordinary bonds, which connect them to
the central benzene ring .Fig. 5).
conformer,
R
Ð
universal gas constant,
1.987 cal mol21821, T Ð absolute temperature .K):
expꢀ2DHi=RT†
X
The intramolecular rotation potential energy surface
of I is characterized by the presence of two minima,
which correspond to two main conformations of this
molecule: with oxazole .X) or oxadiazole cycle .Y)
included into the more extended quasi-¯attened frag-
ment, which consists of three rings .a in the scheme in
Section 1 of this paper). As it was expected, the `X-ray
like' X-conformation of I .which is shown in Fig. 2) is
energetically more favorable, approximately on 0.55±
0.6 kcal mol21. This might correspond to 73%molar
contents of the conformer X in the equilibrium mixture,
estimated according to the following formula, where
DHi is the calculated enthalpy of formation of ith
mi ˆ
expꢀ2DHi=RT†
i
Energetic barriers on the pathway of the mutual
conversion of these two main conformers of I,
which is schematically denoted by the arrow number
2 in Fig. 5, is ,2.4 kcal mol21 for forward X ! Y and
,1.8 kcal mol21 for the backward Y ! X transforma-
tions. These values exceed several times the energy of
Brownian movement for the room temperature,
RT ˆ ,0.6 kcal mol21, so thermal activation of both
X $ Y conformational transformations is hardly
probable at normal conditions. This circumstance

A.O. Doroshenko et al. / Journal of Molecular Structure 609 12002) 29±37
37
leva, A.V. Vankevich, O.P. Shilo, S.N. Yarmolenko, V.M.
Scherschukov, V.G. Mitina, O.A. Ponomaryov, Zhurn.
Obsch. Khim. 64 .1994) 646±652 in Russian.
determines practical absence of free exchange
between these two alternative conformations of I
and thus, the bichromophoric nature of the investi-
gated oxadiazolic ortho-analogs of POPOP [4,5].
The energetic barrier for racemization, which is
shown by the arrow number 1 on Fig. 5, is much
lower, near 0.05 kcal mol21. This de®nes the practi-
cally free exchange between theoretically possible
optical isomers of the main conformers of I and is
the reason for the absence of optical activity of this
compound in solutions.
[2] A.O. Doroshenko, L.D. Patsenker, V.N. Baumer, L.V. Chepe-
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V.M. Scherschukov, V.G. Mitina, O.A. Ponomaryov, Mol.
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[4] A.O. Doroshenko, A.V. Kirichenko, V.G. Mitina, O.A. Pono-
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Our X-ray investigation shows the identity of mole-
cular conformations of 1-.5-phenyl-oxazolyl-2)-2-.5-
phenyl-1,3,4-oxadiazolyl-2)-benzene, obtained from
diphenyloxazole or diphenyl-1,3,4-oxadiazole ortho-
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its position 2 in comparison with that of 1,3,4-oxadia-
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POPOP, which was made by semiempirical method
AM1, con®rms this assumption and ®nally supports
our earlier conclusion about the bichromophoric
nature of this class of sterically hindered compounds.
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[1] A.O. Doroshenko, L.D. Patsenker, V.N. Baumer, L.V. Chepe-