Hydrogen bonding and thermodynamic properties of (R,S)- and (R)-alanine-based selector

Article abstract of DOI:10.1016/j.molstruc.2010.06.036

We report on structural investigation of homochiral and heterochiral alanine-based selector 4-butylamino-N-[1-(3,5-dimethyl-phenylcarbamoyl)-ethyl]- 3,5-dinitrobenzamide (I) preformed by X-ray structure analysis and thermo-optical analysis. The topologies of hydrogen bonding of racemate crystals and the homochiral crystals are significantly different and affect their thermodynamic stabilities. The heterochiral rac-I crystal is characterized by high temperature and entropy fusion due to the compact packing of the molecules of opposite chirality (hydrogen bonded RS pairs). On the contrary, packing of homochiral molecules is less compact and crystals reveal low thermodynamic stability related to conformational flexibility.

Full text of DOI:10.1016/j.molstruc.2010.06.036

Journal of Molecular Structure 980 (2010) 51–58
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Hydrogen bonding and thermodynamic properties of (R,S)-
and (R)-alanine-based selector
*
´
´
ˇ
´
´
´
Ivana Gazic Smilovic, Krešimir Molcanov, Vladimir Vinkovic, Biserka Kojic-Prodic, Andreja Lesac
-
ˇ
Ruder Boškovi´c Institute, P.O. Box 180, Bijenicka cesta 54, HR-10002 Zagreb, Croatia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2010
Received in revised form 29 June 2010
Accepted 29 June 2010
Available online 6 July 2010
We report on structural investigation of homochiral and heterochiral alanine-based selector 4-butylami-
no-N-[1-(3,5-dimethyl-phenylcarbamoyl)-ethyl]-3,5-dinitrobenzamide (I) preformed by X-ray structure
analysis and thermo-optical analysis. The topologies of hydrogen bonding of racemate crystals and the
homochiral crystals are significantly different and affect their thermodynamic stabilities. The heterochi-
ral rac-I crystal is characterized by high temperature and entropy fusion due to the compact packing of
the molecules of opposite chirality (hydrogen bonded RS pairs). On the contrary, packing of homochiral
molecules is less compact and crystals reveal low thermodynamic stability related to conformational
flexibility.
Keywords:
(R,S)-, (R)- and (S)-alanine-based
compounds
Ó 2010 Elsevier B.V. All rights reserved.
X-ray structure
Thermal behavior
Hydrogen bonding
Conformational disorder
1. Introduction
The differences inmelting entropy and enthalpy betweenthe racemic
compound and its enantiomer provide the thermodynamic back-
Substituted dihydropyrimidinones (DHPMs) are known to exhi-
bit a wide range of biological activities such as antiviral, antitumor,
antibacterial, and anti-inflammatory properties, including Ca2+
channel openers [1,2]. As for many other drugs, the biological
activity of most of the DHPMs strongly depends on their absolute
stereochemistry. Pharmacologic studies with resolved enantiomers
have demonstrated multiple differences in potency or even that
the desired effect resides in one enantiomer, solely [2,3]. Therefore,
homochiral precursors and final products are of outmost impor-
tance in pharmaceutical industry [4].
Inthe frameofourresearch onbrush-type chiral stationary phases
(CSPs) we developed a group of chiral selectors based on (S)-alanine
and 4-chloro-3,5-dinitrobenzoic acid which proved to be efficient
for the liquid chromatography resolution of several racemic dihydro-
pyrimidonic compounds [5,6]. Molecular recognition of DHPM by a
chiral selector depends on the molecular geometry and accessible
conformations of the chiral selector itself. Investigations of the crystal
structures of an enantiomer and its corresponding racemate offer a
unique opportunity to study interactions of the same molecule in dif-
ferent crystalline environments. Thus, homochiral interactions are
those between molecules of the same chirality found in crystals of
an enantiomer, whereas in the racemic compound interactions be-
tween molecules of the same and opposite chirality are heterochiral.
ground that combinedwithan analysis of intermolecular interactions
in the crystals, can give valuable insight into the crystal stability.
For the purpose of this study the compound of general formulae I
(Fig. 1) was selected. The structure of the compound I comprises dini-
trobenzoyl (DNB) and dimethylphenylamide (DMA) units that are
connected via alanine moiety. The DNB unit is primarily involved in
complexation with analyte by hydrogen bonding whereas DMA
defines the wall of the chiral cavity and strengthen the rigidity of
the chiral selector, allowing uniform approach of analyte [7]. In addi-
tion, presence of aminobutyl moiety at DMB unit that replaces the
spacer towards the silica, might contribute to conformational flexibil-
ity and facilitate preorganization of the chiral selector in such a way
that interaction points are moved away from the silica surface [6].
In the present work, selectors rac-I and both enantiomers
denoted as S-I and R-I, have been prepared from corresponding
racemic, (S)-alanine and non-natural (R)-alanine, respectively and
their conformational features studied by X-ray diffraction and
thermo-optical methods; hot-stage polarizing optical microscopy
(HS-POM) and differential scanning calorimetry (DSC).
2. Experimental
2.1. General – synthesis of the selectors
Thin layer chromatography was performed on DC-Alufolien
Kieselgel 60 F254 (Merck) and the compounds were detected using
* Corresponding author. Tel.: +385 1 4571 300; fax: +385 1 4680 108.
E-mail address: andreja.lesac@irb.hr (A. Lesac).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.06.036

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I. Gazic Smilovic et al. / Journal of Molecular Structure 980 (2010) 51–58
2.2. X-ray structure analysis
O2N
O2N
O
The crystallographic analysis was carried out for rac-I and R-I,
CH3
HN
while all attempts to obtain a good crystal of S-I failed. Crystals
for X-ray diffraction were obtained by slow evaporation of dimeth-
ylformamide solutions of rac-I and from dichloromethane solu-
tions of R-I. Data collections were performed on an Enraf–Nonius
*
HN
H3C
NH
O
CAD4 diffractometer, using a graphite monochromated Cu K
a
CH3
I
(1.54179 Å) radiation at room temperature (RT, 293 K). The WinGX
standard procedure was applied for data reduction [8]. Three stan-
dard reflections were measured every 120 min as intensity control.
H3C
Absorption correction based on eight
W-scan reflections was per-
formed [9] for structures rac-I and R-I (293 K). No absorption cor-
rection was performed for R-I (100 K). The structures were solved
with SHELXS97 and refined with SHELXL97 [10,11]. The models
were refined using the full matrix least squares refinement. Hydro-
gen atoms were located from a difference Fourier map and were re-
fined as mixed free and riding entities. The atomic scattering
factors were those included in SHELXL97. Molecular geometry cal-
culations were performed with PLATON [12], and molecular graph-
ics were prepared using ORTEP-3 [13], and CCDC-Mercury [14].
Reduced crystal quality, probably originated from conforma-
tional flexibility, of the molecules affecting the intensity data. In
rac-I and molecule B of R-I (293 K) the displacement parameters
of the atoms of butyl and nitro groups were significantly larger
than those of the rest of the molecule. In molecule A of R-I
(293 K), however, thermal motion produced unrealistic geometry
of the C–C bonds and elongated, stick-shaped, ellipsoids of n-butyl
group. Therefore, in the molecule A of R-I (293 K) n-butyl group
was restrained (C–C bond length of 1.5 Å). The structure involves
dynamic disorder, which could not be modeled and the high resid-
ual electron density occurred. Cooling of a crystal of R-I to 100 K
reduced thermal motion of the atoms, but did not improve the
quality of the data due to the increased mosaic spread. The refine-
ment resulted in a relatively high R value and residual electron
density (Table 1).
O2N
O2N
O
silica
gel
Si
H3C
HN
(H2C)3HN
O
H
NH
O
CH3
CSP-1
H3C
Fig. 1. General chemical diagram of the alanine-based chiral selector
schematic presentation of the chiral stationary phase CSP-1 and its soluble
analogue S-I.
I and
Spectroline UV lamp at 254 nm. Column chromatography was per-
formed by using silica gel, particle size 0.063–0.200 mm (J.T. Ba-
ker). 1H NMR and 13C NMR spectra were recorded on Bruker AV
300 spectrometer. The melting points were determined using an
Olympus BX51 polarizing microscope equipped with a Linkam
TH600 hot stage and PR600 temperature controller. Optical rota-
tions were measured using the Optical Activity AA-10 automatic
polarimeter.
2.3. Thermal behavior
Suspension of 4-chloro-N-[1-(3,5-dimethyl-phenylcarbamoyl)-
ethyl]-3,5-dinitrobenzamide [5] of appropriate configuration
(0.5 mmol), n-butylamine (1.5 mmol), previously dried overnight
under solid KOH, and anhydrous tetrahydrofurane (10 mL) was
stirred overnight at room temperature. Soluble chiral selectors R-
I and S-I were isolated upon evaporation of the reaction mixture
and chromatography on silica gel, using a mixture of solvents
CH2Cl2/CH3OH (10/0.5, Rf = 0.8) as eluent while precipitated rac-I
selector was separated on G-4 filter and washed with THF. After
drying in vacuum desiccator for 5 h, the pure products were ob-
tained as the orange powders. The NMR data are identical for all
three samples.
Thermal behavior was studied by two complementary tech-
niques; hot-stage polarizing optical microscopy (HS-POM) using
an Olympus BX51 polarizing microscope equipped with a Linkam
TH600 hot stage and PR600 temperature controller and differential
scanning calorimetry. Thermograms were recorded on Perkin-El-
mer Diamond DSC, operated at scanning rates of 2–10 K minꢁ1
.
3. Results and discussion
The pure enantiomers R- and S- of a compound react differently
with other chiral molecules but not with other achiral molecules
[15]. Enantiomers show different reaction rates with other chiral
molecules (e.g. enantioselectivity of enzymes on different enantio-
mers) [16] and different solution behavior in pure chiral solvents
[15]. As expected, for R-I and S-I enantiomers identical solubility
in the achiral solvents was observed. On the other hand, whereas
R-I and S-I are readily soluble in different organic solvents, e.g. eth-
anol, methanol, acetone, chloroform etc., rac-I is only slightly sol-
uble in dimethylsulfoxide and hot dimethylformamide. The
solubility of racemate is usually lower than that of pure enantio-
mers [17]. This could be an indication that the crystal structure
of rac-I is much more stable than that of the optically pure I. Gen-
erally, the crystal packing interactions are better optimized in het-
erochiral (racemates) than in homochiral crystals (enantiomers)
leading to higher density of racemate than of enantiomeric crystals
[17,18] although the exceptions were encountered [19,20]. As a
1H NMR (DMSO-d6) d/ppm: 0.86 (3H, t, J = 7.5 Hz), 1.29 (2H, dt,
J = 7.5 and 6.4 Hz), 1.41 (3H, d, J = 7.0 Hz), 1.58–1.63 (2H, m), 2.22
(6H, s), 2.92–3.07 (2H, m), 4.57 (1H, q, J = 7.0 Hz), 6.69 (1H, s), 7.23
(2H, s), 8.41 (1H, s), 8.83 (2H, s), 8.96 (1H, d, J = 7.0 Hz), 9.88 (1H, s).
13C NMR (DMSO-d6) d/ppm: 13.43, 17.64, 19.26, 21.05, 30.98,
45.87, 50.13, 117.09, 119.18, 124.78, 131.04, 137.00, 137.59,
138.80, 140.51, 162.56, 171.03.
Selector rac-I; m.p.: 517 K; (R,S); Anal. Calcd. for C22H27 N5O6
(457.486): C, 57.76; N, 15.31; H, 5.95%. Found: C, 57.80; N,
15.19; H, 6.08%.
Selector R-I; m.p.: 471 K; ½a D20
(R) = ꢁ103.0 (10 mg/mL, DMF).
ꢀ
Anal. Calcd. for C22H27 N5O6 (457.486): C, 57.76; N, 15.31; H,
5.95%. Found: C, 57.93; N, 15.33; H, 6.07%.
Selector S-I; m.p.: 471 K; ½a D20
ꢀ
(S) = +113.0 (10 mg/mL, DMF).
Anal. Calcd. for C22H27 N5O6 (457.486): C, 57.76; N, 15.31; H,
5.95%. Found: C, 58.02; N, 15.37; H, 6.09%.

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I. Gazic Smilovic et al. / Journal of Molecular Structure 980 (2010) 51–58
Table 1
X-ray crystallographic data and structure refinement data for the compounds rac-I, R-I (100 K) and R-I (RT).
Compound
rac-I (RT)
22H27N5O6
457.49
293
R-I (RT)
R-I (100 K)
Empirical formula
Formula wt. (g molꢁ1
T (K)
C
C22H27N5O6
457.49
293
C22H27N5O6
457.49
100
)
Crystal dimensions (mm)
Space group
a (Å)
b (Å)
c (Å)
0.25 ꢂ 0.20 ꢂ 0.10
0.32 ꢂ 0.26 ꢂ 0.12
0.40 ꢂ 0.30 ꢂ 0.10
P ꢁ 1
P 1
P 1
8.0181(9)
10.5113(9)
14.405(2)
75.441(10)
80.807(11)
73.472(8)
2
1121.5(2)
1.355
0.835
7.0890(6)
7.9496(5)
20.8571(13)
89.211(5)
89.838(6)
81.018(6)
2
1160.87(14)
1.309
0.807
6.8906(8)
7.8604(17)
20.768(4)
82.544(16)
82.555(13)
82.377(14)
2
1098.2(3)
1.383
0.853
a
(°)
b (°)
c
(°)
Z
V (Å3)
Dcalc (g cmꢁ3
)
l
(mmꢁ1
)
Tmin, Tmax
0.7916, 0.9183
3.18–76.16
0 < h < 10;
ꢁ12 < k < 13;
ꢁ17 < l < 18
5021
4673
3217
0.0495
0.0619
–
–
H
range (°)
2.12–76.38
0 < h < 8;
ꢁ9 < k < 10;
ꢁ26 < l < 26
5247
4842
4394
0.0500
0.0670
2.16–76.26
ꢁ8 < h < 8;
ꢁ9 < k < 0;
ꢁ26 < l < 25
4932
4932
4731
0
Range of h, k, l
Reflections collected
Independent reflections
Observed reflections (I P 2
Rint
r
)
R (F2)
0.0706
0.2009
Rw (F2)
0.1872
0.1949
Goodness of fit
No. of parameters
No. of restraints
1.022
383
0
0.32; ꢁ0.29
1.001
644
26
0.56; ꢁ0.33
1.015
620
3
0.61; ꢁ0.60
D
qmax
,
D
qmin (eÅ–3
)
consequence heterochiral crystals display greater stability and
higher melting points than homochiral crystals [18–21]. The struc-
tural studies involving investigation on various temperatures (X-
ray, microscopy, DSC) were performed in order to gather valuable
information on the molecular geometry, accessible conformations
of the compound I and their thermodynamic stability. The differ-
ence in molecular geometry and crystal packing between hetero-
chiral and homochiral crystals in conjunction with their
thermodynamic behavior will be discussed accordingly.
unit, A and B that differ in torsional angles of the alanine moiety
(Fig. 3, Table 2).
The homochiral crystals measured at room- and low-tempera-
tures exhibit two different conformations (Fig. 3a and b, Table 2).
The conformational differences are primarily induced by flexibility
of n-butyl chain but also different orientations of nitro groups. The
conformation of n-butyl chain in R-I (molecules A and B) at RT is
antiperiplanar about C2–C3 and C3–C4 bonds, whereas in the
low-temperature conformers it is ( )gauche [22]. The former con-
formation of n-butyl chain was also found in rac-I (RT).
Comparison of dihedral angles between least-squares best
planes of aromatic rings C5 ? C10 and C15 ? C20 illustrate the
conformational difference of the environment around the stereo-
genic centres (C12) in homochiral and heterochiral molecules at
RT (Fig. 4). Thus, the values for dihedral angles in homochiral mol-
ecules A and B amount 16.6° and 8.1°, respectively, while in heter-
ochiral rac-I it reaches 25.5°. The coplanarity of the N5 amide bond
with the aromatic ring C15 ? C20 contributes to more compact
packing in rac-I than in R-I in which this bond deviates (in both
3.1. Molecular structures of rac-I and R-I
Single crystal X-ray structure of R-I was measured at room and
low temperature (100 K, Table 1) whereas rac-I was measured at
room temperature, only, to avoid considerable mosaicity spread.
The heterochiral crystals reveal the triclinic centrosymmetric space
group with a single molecule in an asymmetric unit (Fig. 2, Table 1)
whereas homochiral crystals appear in the analogous noncentro-
symmetric space group with two molecules in the asymmetric
Fig. 2. ORTEP drawing of the R-enantiomer of rac-I. Displacement ellipsoids are drawn at the 50% probability level.

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I. Gazic Smilovic et al. / Journal of Molecular Structure 980 (2010) 51–58
Fig. 3. Two conformers, A and B of R-I at: (a) room temperature, (b) at 100 K. Displacement ellipsoids are drawn at the 50% probability level.
Table 2
Selected torsion angles of heterochiral (rac-I, R-configuration) and homochiral (R-I) structures.
rac-I
R-I, A (RT)
R-I, B (RT)
R-I, A (100 K)
R-I, B (100 K)
C1–C2–C3–C4
C2–C3–C4–N1
C3–C4–N1–C5
C4–N1–C5–C6
C7–C8–C11–N4
C8–C11–N4–C12
C11–N4–C12–C13
C11–N4–C12–C14
N4–C12–C14–N5
C12–C14–N5–C15
C14–N5–C15–C16
ꢁ176.5(4)
ꢁ173.1(3)
ꢁ145.3(3)
ꢁ16.0(4)
25.1(4)
173.9(2)
ꢁ113.9(3)
125.9(2)
ꢁ146.4(2)
ꢁ172.9(2)
5.4(4)
174.5(12)
178.4(12)
159.7(7)
17.0(9)
161.7(3)
162.9(3)
ꢁ69.2(3)
171.0(3)
ꢁ170.3(3)
ꢁ165.2(4)
ꢁ149.8(4)
ꢁ178.4(12)
ꢁ179.8(11)
ꢁ172.3(6)
ꢁ11.8(7)
58.7(5)
67.3(5)
163.6(4)
27.8(7)
164.9(4)
162.8(4)
ꢁ68.4(5)
171.7(4)
ꢁ174.4(4)
ꢁ165.6(4)
ꢁ150.5(4)
ꢁ57.9(5)
ꢁ65.7(5)
ꢁ169.5(4)
ꢁ22.5(7)
ꢁ153.4(4)
ꢁ177.0(4)
ꢁ98.3(5)
142.5(4)
ꢁ151.1(4)
179.4(4)
ꢁ100.2(5)
139.5(4)
ꢁ136.6(4)
ꢁ170.4(4)
138.6(4)
ꢁ136.6(4)
ꢁ169.5(4)
138.3(4)
See Fig. 2. for atom labeling.
Fig. 4. Overlap of A (yellow) and B (blue) molecules in R-I at room temperature and R-enantiomer (red) selected from heterochiral (racemic) crystal. The overlap is made by
least-squares fit of atoms forming the central part of the molecule (N4, O5, C12, C13, C14, O6, N5; see Fig. 2 for atom labeling).
A and B molecules) from planarity about 30°. Overlap of A and B
molecules of R-I at room temperature and R-enantiomer of
heterochiral crystal, visualizes their conformational diversities
(Fig. 4).
H4NꢃꢃꢃO6 hydrogen bonds generating dimeric rings of R22ð10Þ. The
other amide group is proton donor to nitro group in hydrogen bond
N5–H5NꢃꢃꢃO1 that links the homochiral molecules (S–S, R–R) into
chain C22ð11Þ extending in the direction [0 1 0]. The amino group
N1–H acts as a donor to oxygen atom of the nitro group in intramo-
lecular hydrogen bond (S6). The hydrogen bonds C–HꢃꢃꢃO, formed
with strong acceptors such as amide oxygen and nitro group oxy-
gen atoms, contribute to the linkage of these chains into double
layers parallel to the direction [0 0 1].
3.2. Crystal packing
In rac-I (Fig. 5, Table 3) molecules of opposite chirality (RS
pairs) are linked by a pair of inversion symmetry-related N4–

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I. Gazic Smilovic et al. / Journal of Molecular Structure 980 (2010) 51–58
R22(10)
C22 (11)
R
R
R
S
R
S
S
S
C22 (11)
Fig. 5. Crystal packing of rac-I with differently colored R (red) and S (green) enantiomers. Hydrogen bonds N–HꢃꢃꢃO generate: heterochiral dimeric rings R22 (10) (violet dashed
lines), homochiral chains C22 (11) (black lines) and the homochiral S(6) (turquoise lines in packing diagram). Topology of hydrogen bonding network is schematically
presented.
Table 3
Geometric parameters of hydrogen bonds present in the rac-I and R-I crystals.
Structure
Hydrogen bond
D–H (Å)
HꢃꢃꢃA (Å)
D–HꢃꢃꢃA (Å)
D–HꢃꢃꢃA (°)
Symm. operation on A
rac-I (RT)
N4–H4 NꢃꢃꢃO6
N5–H5 NꢃꢃꢃO1
N1–H1 NꢃꢃꢃO3
C12–H12ꢃꢃꢃO1
C7–H7ꢃꢃꢃO6
0.86(3)
0.83(3)
0.96(3)
0.98(3)
0.99(3)
0.98(4)
1.02(3)
0.90(6)
2.09(3)
2.63(3)
1.89(3)
2.59(3)
2.82(3)
2.59(4)
2.51(3)
2.69(5)
2.931(3)
3.269(3)
2.649(3)
3.442(3)
3.345(4)
3.379(4)
3.497(3)
3.441(5)
163(3)
166(2)
135(3)
146(3)
114(2)
138(3)
164(3)
141(5)
1 ꢁ x, 1 ꢁ y, 1 ꢁ z
ꢁx, 1 ꢁ y, 1 ꢁ z
x, y, z
x, ꢁ1 +y, z
1 ꢁ x,1 ꢁ y,1 ꢁ z
1 ꢁ x, 1 ꢁ y, 1 ꢁ z
2 ꢁ x, ꢁy, 1 ꢁ z
2 ꢁ x, ꢁy, 1 ꢁ z
C13–H13BꢃꢃꢃO2
C20–H20ꢃꢃꢃO5
C22–H22AꢃꢃꢃO4
R-I (100 K)
N4A–H4AꢃꢃꢃO6B
N4B–H4BꢃꢃꢃO6A
N5A–H5AꢃꢃꢃO5B
N5B–H5BꢃꢃꢃO5A
N1A–H1AꢃꢃꢃO4A
N1B–H1BꢃꢃꢃO3B
C9A–H9AꢃꢃꢃO6B
C9B–H9BꢃꢃꢃO6A
C12A–H12AꢃꢃꢃO5B
C13A–H13BꢃꢃꢃO5B
C21B–H21DꢃꢃꢃO1A
C22B–H22DꢃꢃꢃO5A
0.93(9)
0.86
0.86
0.87(9)
0.86
0.86
0.93
0.93
0.98
0.96
2.12(9)
2.21
2.19
2.12(9)
2.08
2.03
2.50
2.89
2.67
2.70
3.031(5)
3.008(5)
3.025(5)
2.978(5)
2.637(5)
2.626(5)
3.081(5)
3.514(8)
3.291(5)
3.373(5)
3.130(6)
3.408(5)
164(7)
155
165
170(7)
121
125
121
125
122
128
1 + x, ꢁ1 + y, z
ꢁ1 + x, 1 + y, z
x, y, z
x, y, z
x, y, z
x, y, z
1 + x, ꢁ1 + y, z
ꢁ1 + x,1 + y, z
x, y, z
x, y, z
x, y, z
0.96
0.96
2.46
2.46
127
168
x, 1 + y, z
R-I (RT)
N4A–H4AꢃꢃꢃO6B
N4B–H4FꢃꢃꢃO6A
N5A–H5AꢃꢃꢃO5B
N5B–H5BꢃꢃꢃO5A
N1A–H1GꢃꢃꢃO3A
N1B–H1HꢃꢃꢃO3B
C9A–H9AꢃꢃꢃO6B
C9B–H9BꢃꢃꢃO6A
C12A–H12AꢃꢃꢃO5B
C18B–H18BꢃꢃꢃO1A
C22B–H22FꢃꢃꢃO1A
C22A–H22CꢃꢃꢃO3B
C21B–H21FꢃꢃꢃO2A
0.86
0.86
0.86
0.86
0.87(3)
1.13(6)
0.92(4)
0.82
0.97(4)
1.09(6)
0.96
2.23
2.25
2.16
2.16
1.86(4)
1.63(6)
2.56(5)
2.94(8)
2.67(4)
2.72(7)
2.67
3.078(5)
3.062(6)
3.004(4)
2.994(6)
2.606(6)
2.604(6)
3.098(5)
3.443(5)
3.330(5)
3.397(7)
3.470(7)
3.445(7)
3.223(9)
167
158
167
165
142(5)
141(6)
118(4)
117(6)
126(3)
120(4)
141
1 + x, ꢁ1 + y, z
ꢁ1 + x, 1 + y, z
x, y, z
x, y, z
x, y, z
x, y, z
1 + x, ꢁ1 + y, z
ꢁ1 + x, 1 + y, z
x, y, z
ꢁ1 + x, 1 + y, z
ꢁ1 + x, 1 + y, z
1 + x, ꢁ1 + y, z
x, y, z
0.96
0.96
2.71
2.54
134
129
The crystal packing of R-I at both temperatures reveals the
same topology of hydrogen bond network but completely different
comparing to rac-I (Fig. 6, Table 3). In R-I crystal the molecule A vs.
B is rotated for about 180° to avoid steric hindrance between C13-
methyl groups attached to the stereogenic C12 atoms (of both
conformers) and to make approachable both amide groups for
hydrogen bonding between two neighboring conformers (A, B).
Two amide groups of each of the conformers acting as donors

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I. Gazic Smilovic et al. / Journal of Molecular Structure 980 (2010) 51–58
C22(8)
A
B
A
B
R22 (10)
R22 (14)
Fig. 6. Crystal packing of R-I with two molecules in the asymmetric unit: A (light-red) and B (dark-red). Intermolecular N–HꢃꢃꢃO hydrogen bonds between the conformers A
and B generate R22 (10) (violet dashed lines), R22 (14) (green dashed lines), C22 (8), and intramolecular S(6) bonds (turquoise).
Table 4
behavior summarized in Table 4. While rac-I simply melted at
497 K enantiomers R-I and S-I displayed two melting points that
correspond to the melting of two crystal forms.
Temperatures of fusion (Tfus/K), enthalpies (kJ molꢁ1) in italics and the dimensionless
value of
DS/R in square brackets, temperatures of glass transition (Tg/K) and
corresponding specific heat (J molꢁ1 Kꢁ1) in italics of heterochiral and homochiral
selector.
Polarizing optical microscopy investigation of homochiral selec-
tors revealed the presence of two crystal forms. Upon heating as
the plate-like crystals obtained from the solution (denoted as Crs)
melt while the needle-like crystals (denoted as Crm) grow (Fig. 7)
and then melt at higher temperature. On cooling neither heterochi-
ral nor homochiral selector recrystallize from the melt, instead for-
mation of an amorphous solid was observed.
The thermal behavior was confirmed and quantified by calori-
metric measurements. The DSC trace of the first heating run (2 K/
min) for R-I (Fig. 8a) shows that melting of Crs at 453 K is followed
by immediate crystallization of Crm at 455 K and final melting at
Selector
rac-I
Tfus (Crs)
Tfus (Crm
)
Tg
497
63.03 [15.25]
453
19.59 [5.20]
456
21.74 [5.73]
351
0.27
351
0.29
351
0.24
R-I
S-I
471
36.84 [9.4]
471
38.21 [9.74]
Crs, crystals obtained from the solution; Crm, crystals obtained from the melt.
and acceptors in the hydrogen bonds N–HꢃꢃꢃO generate the ring
motif R22ð10Þ and R22ð14Þ that are interlinked by a chain C22ð8Þ along
the direction [0 1 0]. Analogous to rac-I the amino group N1–H in
both conformers of R-I act as a donor to oxygen atom of the nitro
group establishing an intramolecular hydrogen bond.
571 K. In addition, the Crm form has the enthalpy
36.84 kJ molꢁ1
and corresponding entropy of fusion
R = 9.40) almost twice higher than the Crs form
19.59 kJ molꢁ1
(
D
Hfus
Sfus
Hfus
=
/
=
)
(
D
(
D
;
D
Sfus/R = 5.20) pointing to a substantial difference
in thermodynamic stability and in ‘‘crystal ordering” between the
two forms. In the cooling cycle (Fig. 8b) the observed low intensity
3.3. Thermal behavior
exothermic transition (D
Cp = 0.29 J molꢁ1 Kꢁ1) to an amorphous
solid at 351 K was assigned to a glass transition. In the second
heating run (10 K/min) this solid undergoes the glass transition
The differences in the crystal packing of the heterochiral and
homochiral selectors are reflected in the variations of the thermal
Fig. 7. Photomicrographs of R-I obtained in the first heating run: (a) plate-like crystals of the Crs form at 423 K, (b) needle-like crystals of the Crm form at 467 K (magnification
500ꢂ).

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I. Gazic Smilovic et al. / Journal of Molecular Structure 980 (2010) 51–58
Fig. 8. DSC thermograms of R-I (a) the first heating run at rate 2 K/min and (b) the first cooling run at rate 10 K/min.
at 355 K followed by crystallization at 444 K (see ESI Fig. S1c). The
newly formed crystal form melt at 470 K, approximate the same
melting temperature observed for the Crm form in the first heating
run. Absence of the endothermic signal at 456 K in the second
heating cycle reveals that the structure of Crs is permanently trans-
formed into Crm form under thermal stress. As expected, a similar
behavior pattern was observed for the S-I enantiomer (see ESI, Figs.
S2 and S3). However, although the melting points of Crm forms and
Tg of both enantiomers are the same, surprisingly the tempera-
tures of melting of the Crs form (456 K) and crystallization
(457 K) of the Crm form of S-enantiomer are higher than those ob-
served for the R-enantiomer. Despite of a small difference in the
melting temperatures of two enantiomeric Crs forms, thermody-
namic data associated with this process (Table 4) are roughly equal
suggesting that the both enantiomeric Crs forms are of the same
thermodynamic stability and the ‘‘crystal ordering” as expected.
Calorimetric studies performed on rac-I are in agreement with
microscopy observation revealing single melting point at 497 K
tals, heterochiral and homochiral, the intermolecular hydrogen
bonding involving amide groups represents the building block of
crystal packing. Presence of aminobutyl chain and dinitrophenyl
group in the molecular structure allows additional hydrogen bond-
ing. Thus in crystal of rac-I both nitro groups are engaged: inter-
molecular bonding between H of dimethylphenyl-amide group
and oxygen of nitro group on neighboring molecule (H5NꢃꢃꢃO10)
links the homochiral molecules (S–S, R–R), and intramolecular
bonding between H of aminobutyl group and oxygen atom of the
nitro group occurs. In the homochiral crystal of R-I the nitro group
is forming only intramolecular hydrogen bonding with the adja-
cent amino group giving to the rest of molecule higher degree of
conformational freedom. Furthermore, the flexibility of n-butyl
chain is associated with large thermal motions of these atoms that
do not disappear at 100 K, leading to dynamical disorder. Thermo-
optical investigation reveals that observed difference in crystal
packing has a great impact on thermodynamic properties. Despite
of flexibility of n-butyl chain, intermolecular hydrogen bonding
that fix not only the middle alanine moiety but also the terminal
with enthalpy of fusion of
parameter Sfus/R of 15.25 and on cooling a glass transition at
351 K (
Cp = 0.27 J molꢁ1 Kꢁ1), (see ESI, Figs. S4 and S5).
D
Hfus = 63.03 kJ molꢁ1 and entropy
D
dinitrophenyl group resulted in the high entropy of fusion (DSfus/
D
R = 15.25) of rac-I pointing to good ‘‘crystal ordering”. On the
contrary, the homochiral crystal Crs, where only amide groups
form hydrogen bonding, is unstable under thermal stress and its
The results presented above show that hydrogen bonding pri-
mary control the packing mode in the crystals. The packing of het-
erochiral crystal of rac-I is 3.5% denser than the homochiral crystal
of R-I (1.355 g cmꢁ3 vs. 1.309 g cmꢁ3) that reflected on its higher
temperature of fusion and thermodynamic stability. In both crys-
entropy term
DSfus/R abate to 5.20 suggesting considerable re-
duced ‘‘crystal ordering” most likely caused by conformational dis-
order within the crystal.

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4. Conclusion
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Acknowledgements
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The authors are indebted to Dr. Irina Pucic from Ruder Boškovic
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2010.06.036.