Crystal structure as the basis for the lack of enantiotropic mesomorphism in 3-hydroxy-4-propionylphenyl-4′-n-alkyloxybenzoates

Article abstract of DOI:10.1134/S0018143916060114

Features of the molecular structure and crystal packing of the mesogenic compounds 3-hydroxy-4-propionylphenyl esters of 4-n-amyloxy (1), 4-n-hexyloxy (2), 4-n-heptyloxy (3), and 4-n-octyloxybenzoic acid (4) have been analyzed on the basis of X-ray diffraction (XRD) data. Comparison of the results of XRD and DSC studies of these compounds has shown that the crystalline modification studied for each of the compounds is not a precursor to the mesophase. The possibility of mesophase formation from the melt is discussed.

Full text of DOI:10.1134/S0018143916060114

ISSN 0018-1439, High Energy Chemistry, 2016, Vol. 50, No. 6, pp. 453–459. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © L.G. Kuz’mina, I.I. Konstantinov, S.I. Bezzubov, 2016, published in Khimiya Vysokikh Energii, 2016, Vol. 50, No. 6, pp. 478–484.
NANOSTRUCTURED SYSTEMS AND MATERIALS
Crystal Structure as the Basis for the Lack of Enantiotropic
Mesomorphism in 3-Hydroxy-4-Propionylphenyl-4'-n-
Alkyloxybenzoates
L. G. Kuz’minaa, *, I. I. Konstantinovb, and S. I. Bezzubova
aKurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
bTopchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 119991 Russia
*e-mail: kuzmina@igic.ras.ru
Received May 26, 2016
Abstract⎯Features of the molecular structure and crystal packing of the mesogenic compounds 3-hydroxy-
4-propionylphenyl esters of 4-n-amyloxy (1), 4-n-hexyloxy (2), 4-n-heptyloxy (3), and 4-n-octyloxybenzoic
acid (4) have been analyzed on the basis of X-ray diffraction (XRD) data. Comparison of the results of XRD
and DSC studies of these compounds has shown that the crystalline modification studied for each of the
compounds is not a precursor to the mesophase. The possibility of mesophase formation from the melt is dis-
cussed.
DOI: 10.1134/S0018143916060114
Phenyl benzoates having an alkyl or alkyloxy chain
In order to determine the relation of the crystal
as a substituent in the para-position on the benzene structure of compounds to their LC properties, we
ring exhibit liquid crystal (LC) properties [1]. Being a performed X-ray diffraction and thermographic inves-
nanostructured system, the mesophase is built from tigation of compounds 1–4.
molecular assemblies in which the balance between
O H
van der Waals and directional weak interactions (sec-
ondary bonds) is maintained [2–6]. Organic com-
pounds most frequently contain the following types of
secondary bonds characterized by different energies:
the ordinary hydrogen bonds, π…π interaction, and
weak hydrogen bonds C–H…π and X…H–C (X is a
heteroatom). Since there are no direct experimental
methods for studying the detailed structure of complex
assemblies of the mesophase, we use an approach
based on the assumption that the crystal structure of
the compound is the precursor of the mesophase and,
as such, can reflect the key structural features of the
latter. If this assumption is true, an investigation of the
crystal packing of liquid crystal compounds may shed
light not only on the structure of the mesophase, but
also on the mechanisms of phase transitions in the
crystal–mesophase–isotrope system. Previously, we
found that the number of phase transitions in the
mesophase generally coincides with the number of
O
O
O
H2n +1Cn
O
C2H5
n = 5 (1), 6 (2), 7 (3), 8 (4)
According to published data, the introduction of a
side OH group in the 3-position of the phenol moiety
of 4-propionylphenyl-4'-alkyloxybenzoates [19]
reduces the isotropization temperature (Ti) and weak-
ens the smectogenicity of these compounds.
EXPERIMENTAL
Synthesis of Compounds
3-Hydroxy-4-propionyl-4'-alkyloxybenzoates were
types of secondary bonds in the crystal structure. This prepared by reacting propionylresorcinol [20] with 4-alk-
finding give grounds to suppose that as the tempera- ylbenzoic acids in a methylene chloride solution in the
ture of the system increases, the successive degrada- presence of N,N'-dicyclohexylcarbodiimine [21] at room
tion of different types of secondary bonds (i.e., the temperature. The resulting products were purified by col-
“support contacts” of the mesophase) resulting in a umn chromatography on silica gel followed by recrystal-
decrease in the level of its structuring causes phase lization from cyclohexane. The purity of the com-
transitions in the mesophase [7–18].
pounds was checked by thin layer chromatography.
453

454
KUZ’MINA et al.
Table 1. Phase transition parameters as measured by DSC
Sample
CrI
CrII
SmA
N
Iso
1a/b
• 76/1.2
• 88.6/36.7
• 89.3/49.9
• 74.4/35.8
•
• (77.5)/0.88
• (84.6)/1.02
• 83/0.53
•
•
•
•
2a/b
3a/b
4a/b
• 81.1/3.9
• 66.1/0.1
• 56.3/37.15
• (52.8)/0.48
• (66.1)/0.94
• 77.1/1.18
• 66/2.6
• 87.7/1.05
−1
а and b refer to the temperature (°C) and ΔH (kJ mol ), respectively.
Table 2. Unit cell parameters of 1–4 according to X-ray structure analysis (XSA) and X-ray phase analysis (XRD) data
Parameters
Substance
Method
V, Å3
a, Å
b, Å
c, Å
β, °
XSA
12.5674(11)
12.55(3)
9.0363(8)
9.02(2)
16.6531(14)
16.67(4)
93.309(2)
93.2(2)
92.618(1)
92.6(1)
90
1888.0(3)
1884(1)
1968.5(2)
1976(1)
2111.8(5)
2104(1)
2207.1(4)
2203(1)
3684(1)
1
2
3
4
XRD
XSA
XRD
XSA
XRD
XSA
XRD
12.7287(7)
12.75(3)
9.2224(5)
9.23(2)
16.7869(9)
16.81(3)
18.069(2)
18.06(2)
18.085(2)
18.07(2)
7.926(1)
7.92(1)
14.745(2)
14.72(2)
90
7.956(1)
7.95(1)
15.339(2)
15.33(2)
90
90
19.00(2)
12.45(1)
15.60(2)
92.7(1)
Calorimetric Studies
1–3 to the melting point did not result in mechanical
destruction. The crystals of 4 undergo cracking at tem-
peratures above ~58°C.
The parameters of phase transitions were deter-
mined calorimetrically with a Mettler DSC 832e
instrument at a scan rate of 5 K/min. Table 1 shows
the results of measurements on polycrystalline sam-
ples in the heating mode and their subsequent cooling
for detecting the monotropic phases.
X-ray Structure Analysis
Slow crystallization of compounds 1–4 from dif-
ferent mixtures of solvents resulted in only one crystal-
line modification. A single crystal of each of the com-
pounds 1–4 suitable for X-ray structure analysis was
placed on a CCD diffractometer (SMART APEX-II)
under a flow of cooled nitrogen, and unit cell param-
eters and reflection intensities were measured using
Mo-Kα radiation and a graphite monochromator. The
primary processing of the experimental data was car-
ried out using the program SAINT [22].
X-ray Diffraction Phase Analysis
X-ray diffraction powder patterns of 1–4 were
obtained using a Bruker D8 Advance diffractometer
(CuKα 1.5418 Å, Ni filter, LYNXEYE detector, ref lec-
tion geometry). Measurements were made in the
angular range of 2θ = 5°–40°. The diffraction patterns
were processed (profile analysis, indexing, refining of
the lattice parameters) using the software package
STOE WinXPOW (Version 1.04, 07.01.1999). The
theoretical X-ray diffraction patterns of 1–4 were cal-
culated from the X-ray structure analysis data using
the program Mercury (Version 3.1). The unit cell
parameters of 1–4 are given in Table 2.
All the structures were solved by direct methods
and refined by the F2 least squares in the anisotropic
approximation for non-hydrogen atoms. The posi-
tions of hydrogen atoms were calculated geometri-
cally, excluding the OH hydrogen atom localized in
Fourier difference synthesis. All the hydrogen atoms
were refined using a riding model.
Investigation of Thermal Stability of Single Crystals
The calculations were performed using the pro-
The thermal stability of 1–4 single crystals was grams SHELXTL-Plus [23] and Olex-2 [24]. The
studied by visual inspection with a microscope at tem- main characteristics of X-ray diffraction experiments
peratures of 23–85°C. The heating of single crystals are shown in Tables 3 and 4.
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CRYSTAL STRUCTURE AS THE BASIS FOR THE LACK
455
Table 3. Crystallographic data, solution and refinement parameters for 1 and 2
Compound
1
2
Formula
C21H24O5
356.40
monoclinic
P21/c
C22H26O5
370.43
monoclinic
P21/c
Molecular weight
Crystal system
Space group
a/Å
b/Å
c/Å
12.5674(11)
9.0363(8)
16.6531(14)
90, 93.309(2), 90
1888.0(3)
12.7287(7)
9.2224(5)
16.7869(9)
90, 92.6180(10), 90
1968.54(19)
α, β, γ/°
V/Å3
Z
4
4
calc/mg/m3
1.254
1.250
ρ
μ(MoKα)/mm−1
0.089
0.088
Temperature/ K
296
296
Angle range θ/°
2.57 ≤ θ ≤ 29.00
2.43 ≤ θ ≤ 28.00
Intervals of h, k and l indices
–17 ≤ h ≤ 17,
–12 ≤ k ≤ 12,
–21 ≤ l ≤ 21
–16 ≤ h ≤ 16,
–12 ≤ k ≤ 12,
–22 ≤ l ≤ 22
Reflections measured
Independent reflections
Refinement variables
17944
4976, Rint = 0.0364
19777
4746, Rint = 0.0260
235
248
Goodness of fit by F2
R-factors by I > 2σ(I)
R-factors by all reflections
0.992
1.038
0.0542, 0.1457
0.1205, 0.1675
–0.21/0.21
0.0456, 0.1186
0.0748, 0.1306
–0.21/0.17
Δρ (min/max)/e Å−3
The structures have been deposited with CCDC the phase transition in it is monotropic in character
under the numbers 1472457 (1), 14724578 (2), (n = 6 or 7, compounds 2 and 3), i.e., occurs during
1472459 (3), and 1472460 (4), where the correspond- cooling the N phase.
ing CIF files can be freely obtained on request on the
The crystal–crystal phase transition in polycrystal-
web site: www.ccdc.cam.ac.uk/data_request/cif.
line 4 has a high enthalpy (ΔH = 37.15 kJ/mol), that is,
this phase transition must be accompanied by a pro-
found rearrangement of the crystal structure. The
RESULTS AND DISCUSSION
resulting high-temperature crystalline modification is
The calorimetric data (Table 1) show that the heat-
the precursor of the SmA phase. The phase transition
ing of polycrystalline samples of all the compounds of this compound into the smectic phase and the sub-
results in a crystal–crystal phase transition in them.
sequent transition to the nematic (N) phase are enan-
tiotropic. During their heating, single crystals of 4 are
destroyed at a temperature of ~58°, which is somewhat
above the temperature of the crystal–crystal phase
transition in the polycrystalline sample.
The subsequent phase transitions in compounds 1
and 2 are monotropic. This means that the mesophase
is not formed by melting these compounds. It appears
only during cooling the isotropic melt. The transition
to the nematic (N) phase for compound 3 is enantio-
A comparison of the experimental and theoretical
tropic and that to the smectic (SmA) phase is mono- X-ray diffraction patterns of 1–3 shows that these are
tropic. That is, only the nematic phase is formed single-phase samples. The unit cell parameters
during melting this compound and both the nematic obtained for substances 1–3 (Table 2) as a result of the
and smectic phases emerge upon cooling the melt. For X-ray structure analysis coincide within the error with
the middle members of the homologous series, the those calculated from the powder diffraction data.
smectic phase is either absent (n = 5, compound 1) or Thus, the single crystals examined are exactly the
HIGH ENERGY CHEMISTRY Vol. 50 No. 6 2016

456
Table 4. Crystallographic data, solution and refinement parameters for 3 and 4
Compound
KUZ’MINA et al.
3
4
Formula
C23H28O5
384.45
Rhombic
Pna21
C24H30O5
398.48
Rhombic
Pna21
Molecular weight
Crystal system
Space group
a/Å
b/Å
c/Å
18.069(2)
7.9263(10)
14.7451(19)
90, 90, 90
2111.8(5)
18.085(2)
7.9559(17)
15.3393(7)
90, 90, 90
2207.1(4)
α, β, α/°
V/Å3
Z
4
4
calc/mg/m3
1.209
1.199
ρ
μ(MoKα)/mm−1
0.084
0.083
Temperature/K
296
296
Angle range θ/°
2.25 ≤ θ ≤ 28.0
2.25 ≤ θ ≤ 27.6
Intervals of h, k and l indices
–23 ≤ h ≤ 23
–10 ≤ k ≤ 9
–19 ≤ l ≤ 18
–23 ≤ h ≤ 23
–10 ≤ k ≤ 10
–19 ≤ l ≤ 19
Reflections measured
Independent reflections
Refinement variables
17441
2645, Rint = 0.0889
21016
2654, Rint = 0.0361
253
263
Goodness of fit by F2
R-factors by I > 2σ(I)
R-factors by all reflections
0.899
1.009
0.0481, 0.0915
0.1558, 0.1143
–0.10/0.19
0.0385, 0.0912
0.0702, 0.1019
–0.14/0.12
Δρ (min/max)/e Å−3
same substances as powdered 1–3 in their qualitative
In the structure of 4, the C21…C22 segment of the
aliphatic chain is disordered over two positions. There
is no disorder of this kind in the structure of 3, but it is
to be noted that the terminal atoms of the aliphatic
chain have higher values of the temperature parame-
ters than the other atoms of the molecules.
In all the four molecules, the OH group in the
3-position on the benzene ring C8…C13 forms an
intramolecular hydrogen bond with the propionyl car-
bonyl group. The (O4)H4…O5 distances of 1.84, 1.69,
1.85, and 1.84 Å in 1, 2, 3, and 4, respectively, are
common for the hydrogen bond that closes a six-
membered ring.
The molecules are nonplanar. The ester group lies
in the plane of the benzene ring C2… C7, and the ben-
zene ring C8…C13 is rotated around the O1–C8 bond
out of this plane. The dihedral angle between the
planes of the benzene rings is 67(1)°, 63(1)°, 59.5(4)°,
or 60.9(3)° in 1, 2, 3, or 4, respectively.
and quantitative compositions and structure.
The X-ray diffraction pattern of sample 4, in addi-
tion to lines corresponding to a single crystal, contains
additional lines that have been indexed in the mono-
clinic system with the following parameters: a =
19.00(2), b = 12.45(1), c = 15.60(2) Å, β = 92.7(1)°,
V = 3684(1) Å3. Phase transitions in powdered 4 dra-
matically differ from those in the powder of 3, whereas
the single crystals of these compounds are isostruc-
tural. Therefore, it can be assumed that the difference
between the thermograms of powdered 3 and 4 is due
to the monoclinic form of compound 3, which we
have failed to obtain via the slow crystallization of this
compound.
Single crystals of 1–3 do not show outward signs of
degradation until reaching the melting point. This
means that the transition to a high-temperature crys-
talline modification occurs without the destruction of
the single crystal, a fact that is consistent with their
low enthalpy values. The molecular structure of 1–4 is thermotropic LC compounds have two specific fea-
shown in Fig. 1.
Previously, we showed that the crystal packing of
tures. First, they are composed of alternating loose
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CRYSTAL STRUCTURE AS THE BASIS FOR THE LACK
457
o
O3
C4
C19
C3
C17
a
O4
O1
C1
C9
C20
C18
C21
C2
C8
C5
C10
O5
C14
C7
C6
C13
b
C11
c
O2
C12
C16
C15
O4
O3
C7
C6
C19
C17
O1
C10
C9
C22
C21
C2
C18
C20
C5
O5
C11
C8
C1
O2
C3
C4
C14
C15
C12
C13
C16
O4
Fig. 2. Fragment of the crystal packing of 1.
C10
C13
O5
C14
C9
C7
C11
O1
C6
C5
O3
C2
C3
C8
C1
C17
C18
3.9 Å. It is likely that the transition to the high-tem-
perature phase in the crystals of 3, which is not accom-
panied by the destruction of the single crystal with a
relatively low enthalpy, is associated with the expan-
sion of loose crystal areas, the presence of which is the
first condition for the formation of the mesophase
from the crystal. This explains the emergence of the
nematic phase during the melting of the high-tem-
perature modification of 3.
C19
C20
C12
C16
C21
C15
C23
C24
C22
C4
O2
C23
C22
C21
O3
C4
C19
C18
C17
C5
O4
C20
C21A
C3 O1
C9
C22A
C8
C13
C6
O5
C14
C10
C2
O1
C7
C11
C16
C12
O2
C15
Regarding the examined crystalline modification
of single crystals of 4, it is not a precursor of the high-
temperature modification that undergoes the phase
transitions revealed (Table 1).
Fig. 1. Structure of molecules 1–4 (numbering from top
down); ellipsoids of thermal standard deviations of the
atoms are given at the confidence level of 50%.
These structures belong pairwise to the monoclinic
(P21/c, structures 1 and 2) and orthorhombic (Pna21,
3 and 4) crystal systems. Since compounds 1 and 2, are
the closest homologues, as well as 3 and 4, the pair-
wise isostructural character is not surprising. At the
same time, the buildup of differences in the molecular
structure through the homologous series of 1–4
apparently leads to another, monoclinic crystalline
modification in the last member of the series (4),
whose structure cannot be judged on the basis of the
revealed structure of the rhombic modification of 3.
and close areas. The loose areas include aliphatic moi-
eties, and the close areas consist of conformationally
rigid aromatic moieties. Second, in closely packed
areas, is a system of secondary bonds, which are pre-
sumably responsible for the preservation of supramo-
lecular assemblies in the mesophase; i.e., they are the
“supporting contacts” of the mesophase [7–18].
In the crystal packing of 1 and 2 (Fig. 2), there are
no loose aliphatic areas; that is, the first condition of
mesophase formation is not fulfilled. On the basis of
the calorimetric data obtained, it can be concluded
that although the crystals of 1 and 2 grown from solu-
tions at room temperature do not represent the high-
temperature phase, they differ little from it, since the
crystal–crystal phase transition has a low enthalpy and
occurs without destruction of the single crystal.
Therefore, it is clear that these compounds do not
form a mesophase upon heating.
The second condition for the formation of the
mesophase during the melting of crystals is the exis-
tence of secondary bonds responsible for melt struc-
turing. Weak hydrogen bonds C–H…O, which link
molecules in infinite chains developing around the
helical axis 21, were revealed in the crystals of 3 and 4.
Figure 3 shows one unit of the infinite chain in the
structure of 3; a similar chain element was also found
in the crystals of 4, in which the O4…H6 distance is
2.58 Å. In intermolecular hydrogen bonding, the C4–
H4 segment of the benzene ring acts as a proton donor.
This segment is oriented onto the lone electron pair,
The crystal packing of isostructural compounds 3
and 4 also does not show the alternation of close aro-
matic and loose aliphatic areas. However, the disorder
of the aliphatic chain segment in 4 and the high values
of thermal parameters for the terminal atoms of the located on the sp2 orbital, of the oxygen atom O5. In
alkyl chain in 3 are indications that these moieties of addition, the hydrogen atoms of the C2…C7 benzene
the molecules occur in a quite loose crystalline envi- ring are pretty “acidic” because of the presence of two
ronment. In addition, the end atoms of the alkyl chain electron-withdrawing substituents, the ether and ester
form long intermolecular C…C contacts exceeding groups, on the ring. It is apparently these interactions
HIGH ENERGY CHEMISTRY Vol. 50 No. 6 2016

458
KUZ’MINA et al.
onto one another. The interplanar distance (3.66 Å in
1 and 3.68 Å in 2) corresponds to weak directional π–
π interaction.
The crystals of 1–4 were grown from the solutions
at room temperature when the flexibility of aliphatic
chains is less important for the self-organization of the
molecules in the liquid phase prior to crystal nucle-
ation and the subsequent crystal growth than in the
case of crystallization from the melts. During the crys-
tal growth from the melts, the flexibility of aliphatic
chains is high, as well as the amplitude of their thermal
vibrations. Therefore, they have a large effective size.
During the crystallization from the melt, the optimi-
zation of two conflicting requirements, the close
packing of the molecules and the presence of addi-
tional volume for mobility aliphatic chains, leads to
the situation that the chains tend to stay in close prox-
imity to each other. Indeed, in this case, an additional
volume of the space required for the mobile aliphatic
chain of one molecule (ΔV) is becoming common for
the two adjacent molecules providing that their ther-
mal fluctuations are synchronized. This reasoning can
be extended further to a large number of molecules.
For this reason, the crystal packing of LC compounds
obtained by cooling the melt, must consist of alternat-
ing loose and dense areas, that is, be different from the
one formed upon crystallization from solutions. The
cooling of the melts is accompanied by the emergence
of weak directional interactions between molecules to
provide their frameworking, i.e., the appearance of the
mesophase.
O4
2.57
H6
Fig. 3. Pair of molecules of 3 linked by the C–H…O hydro-
gen bond; the distance is given in Å.
Fig. 4. Centrosymmetric π-π dimer in the crystal packing
of 1 (in two projections).
d1
d2
Analysis of XRD data for LC compounds [7–18]
shows that there are two kinds of crystal packing of
mesogenic crystals that undergo the crystal–meso-
phase phase transition (scheme).
d2
d1
The molecules are packed in the head-to-tail mode
in the upper graph and in the head-to-head mode in
the lower graph. A greater distance of the aliphatic
parts of the molecules from each other relative to the
hard cores can be achieved in two ways, by arranging
the aliphatic chains on the opposite sides of the rigid
central core (top graph) and displacing the hard cores
of adjacent molecules with respect to each other with
a simultaneous bent of the aliphatic chains from the
long axes of the rigid cores (bottom graph). Both kinds
of arrangement of the aliphatic moieties lead to a
greater distance of the aliphatic, rather than aromatic
parts from each other; in both cases, the distances are
d1 < d2. Both of these arrangements are met in crystal
packings of LC compounds [7–18].
Fig. 5. Scheme. The graph showing two versions of the
crystal packing of LC compounds; the rectangles represent
the hard core of the molecule, and the zigzags depict ali-
phatic chains.
that are responsible for the appearance of the nematic
phase during the melting of the high-temperature
phase of crystals 3.
In contrast to the crystals of 3 and 4, there is
π-stacking interaction yielding centrosymmetric
dimers in the crystal packings of 1 and 2. The structure
of such a dimer in 1 is shown in Fig. 4; the dimer in 2
has a similar structure.
In the crystal structures of the test homologues,
there can be two types of directional weak interac-
tions, π-stacking and weak hydrogen bonding C–
H…O. Therefore, the cooling of an isotropic melt can
entail the successive appearance, first, of the nematic
In the dimer, the conjugated segments of the phase due to the formation of interactions of one type
C8…C13 benzene rings of the molecules and the clos- (in the case of compound 1) and, then, the smectic
est substituents to them, the O atoms, are projected phase due to the formation of the second type of inter-
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CRYSTAL STRUCTURE AS THE BASIS FOR THE LACK
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9. Kuz’mina, L.G., Pestov, S.M., Kochetov, A.N.,
Churakov, A.V., and Lermontova, E.Kh., Crystallogr.
Rep., 2010, vol. 55, no. 5, p. 786.
action, which is indeed the case observed in com-
pounds 2, 3, and 4.
10. Kuz’mina, L.G. and Kucherepa, N.S., Crystallogr.
CONCLUSIONS
Rep., 2011, vol. 56, no. 2, p. 242.
In the test crystals of compounds 1–4, the existing 11. Kuz’mina,
L.G.,
Kucherepa,
N.S.,
and
Churakov, A.V., Crystallogr. Rep., 2012, vol. 56, no. 2,
crystal packing is not of the type that precedes the for-
mation of the mesophase. The presence of two types of
weak directional interactions, the “supporting con-
tacts”, in the crystals opens the possibility for the for-
mation of both the nematic or smectic phases upon
cooling the isotropic melts of the compounds.
p. 213.
12. Kuz’mina,
L.G.,
Kucherepa,
N.S.,
and
Rodnikova, M.N., Crystallogr. Rep., 2008, vol. 53,
no. 6, p. 1016.
13. Kuz’mina, L.G., Kucherepa, N.S., and Rodni-
kova, M.N., Crystallogr. Rep., 2008, vol. 53, no. 6,
p. 1023.
ACKNOWLEDGMENTS
This work was supported by the Russian Science
Foundation, project no. 16-13-10273.
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Translated by S. Zatonsky
HIGH ENERGY CHEMISTRY Vol. 50 No. 6 2016