Configurational isomerization of push-pull thiazolidinone derivatives controlled by intermolecular and intramolecular RAHB: 1H NMR dynamic investigation of concentration and temperature effects

Article abstract of DOI:10.1002/poc.700

¹H NMR spectroscopy was used to investigate hydrogen bonding in the structurally related (Z)- and (E)-5-substituted-2-alkylidene-4-oxothiazolidines in polar and apolar solvents. The equilibrated mixtures of these typical push-pull alkenes consi

Full text of DOI:10.1002/poc.700

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2004; 17: 118–123
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.700
Configurational isomerization of push–pull
thiazolidinone derivatives controlled by intermolecular
and intramolecular RAHB: 1H NMR dynamic investigation
of concentration and temperature effects
1,2
3
2
1,2
4
´
ˇ
´
Rade Markovic, * Ata Shirazi, Zdravko Dzambaski, Marija Baranac and Dragica Minic
1Faculty of Chemistry, University of Belgrade, Studentski trg 16, 11001 Belgrade, Yugoslavia
2Center for Chemistry ICTM, 11000 Belgrade, Yugoslavia
3Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106, USA
4Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12–16, 11001 Belgrade, Yugoslavia
Received 10 December 2002; revised 4 July 2003; accepted 18 July 2003
ABSTRACT: 1H NMR spectroscopy was used to investigate hydrogen bonding in the structurally related (Z)- and (E)-
epoc
5-substituted-2-alkylidene-4-oxothiazolidines in polar and apolar solvents. The equilibrated mixtures of these typical
push–pull alkenes consist of the intramolecularly H-bonded E-isomer and intermolecularly H-bonded Z-isomer in
varying proportions which depend on the solvent polarity. For a representative of the series, (E)-(5-ethoxycarbo-
nylmethyl-4-oxothiazolidin-2-ylidene)-1-phenylethanone (1), the lack of a concentration and temperature depen-
dence of the large chemical NH shift (ꢀ 12.06 ppm) in CDCl3 indicates strong intramolecular resonance-assisted
hydrogen-bond formation (RAHB). The upfield chemical shifts of the NH prꢀot1on of the (Z)-1 isomer as a function
of temperature increase and the large 1H NMR Áꢀ/ÁT value (ꢀ11.82 ppb ꢁC , Z/E ¼ 60 : 40, or ꢀ10.33 ppb ꢁCꢀ1
,
Z/E ¼ 20 : 80) in CDCl3 are explained in terms of a decrease in intermolecular H-bonding resulting in a greater
amount of free or unassociated Z-isomer. Copyright # 2004 John Wiley & Sons, Ltd.
Additional material for this paper is available in Wiley Interscience
1
KEYWORDS: push–pull alkenes; Z/E isomerization; hydrogen bonding; temperature effect; H NMR spectroscopy
INTRODUCTION
process in apolar CDCl3, to discriminate and determine
the degree of intra- and intermolecular hydrogen bonding
interactions.
There is ample evidence in the literature regarding the
ability of hydrogen bonding to influence structure or
intermolecular association of organic compounds in the
solid state and in solution.1–7 Stereodefined (Z)-5-sub-
stituted thiazolidinones 1–4 (Scheme 1) and new deriva-
tives thereof,8–10 which have attracted our attention
owing to their possible biological activity11,12 and utility
as organic intermediates, represent an excellent model to
study specific hydrogen bonding interactions in solution.
One of the most often employed methods to investigate
this type of weak non-covalent interaction, present not
only in small molecules but also in nucleic acids, peptides
and proteins as well, is NMR spectroscopy.1,13–18
We report here the first 1H NMR dynamic study of the
concentration and temperature dependence of the lactam
proton chemical shift for a representative of the series, (Z)-
(5-ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene)-1-
phenylethanone (1),8 undergoing the well-defined Z/E
EXPERIMENTAL
The NMR spectra for characterization were obtained
using a Varian Gemini 2000 instrument (1H at
200 MHz, 13C at 50.3 MHz). Chemical shifts are reported
in parts per million (ppm) on the ꢀ scale from TMS as an
internal standard in the solvents specified. Variable-tem-
perature 1H NMR measurements in the temperature range
273–333 K were carried out on a Bruker AC-300 spectro-
meter using CDCl3 as a solvent, which was dried over
˚
activated molecular sieves (4 A) for 1 day. The concen-
trations of CDCl3 solutions were 0.011 or 0.016 M unless
indicated otherwise. The variable temperature was com-
puter controlled employing a BVT 2000 unit. The inter-
nal temperature was calibrated with methanol and
ethylene glycol using the Bruker Batman program. Cau-
tion was taken to increase the temperature slowly, espe-
cially when approaching 333 K, to avoid solvent
evaporation. The sample was equilibrated at the given
´
*Correspondence to: R. Markovic, Faculty of Chemistry, University of
Belgrade, Studentski trg 16, 11001 Belgrade, Yugoslavia.
E-mail: markovic@helix.chem.bg.ac.yu
Contract/grant sponsor: Ministry of Science, Republic of Serbia.
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 118–123

CONFIGURATIONAL ISOMERIZATION OF PUSH–PULL THIAZOLIDINONES
119
Table 1. Synthesis of (Z)-5-substituted-4-oxothiazolidines
1–4
temperature and a 128-scan spectrum was recorded with
0.5 Hz per point digital resolution. All chemical shifts
were referenced to the signal for residual CHCl3. A one-
dimensional NOE experiment was run on a Varian Inova
500 MHz spectrometer. Typical parameters were acquisi-
tion time 1.892 s, spectral width 7997.6 Hz with 256
repetitions and 32 K data points. Melting-points were
determined on a Micro-Heiztisch Boetius PHMK appa-
ratus and Bu¨chi apparatus and are uncorrected. The IR
spectra were recorded on a Perkin-Elmer FT-IR 1725X
spectrophotometer and are reported as wavenumbers
(cmꢀ1). Samples for IR spectral measurements were
prepared as KBr disks. Low-resolution mass spectra
were recorded using a Finnigan MAT 8230 BE spectro-
meter. Isobutane was used as the ionizing gas for the
chemical ionization (CI) mass spectra. The UV spectra
were measured on a Beckman DU-50 spectrophotometer.
Analytical thin-layer chromatography (TLC) was carried
out on Kieselgel G nach Stahl, and the spots were
visualized with iodine. Column chromatography was
Entry Product
R
M.p. ( ꢁC) Yield (%)a
1
2
3
4
(Z)-1
(Z)-2
(Z)-3
(Z)-4
Ph
NHPh
NHCH2CH2Ph 152–153
OEt 106–108
126–127
183–185
52
24
29
54
a Yields of isolated pure compounds obtained by crystallization; the use of a
large molar excess of diethyl mercaptosuccinate relative to the ꢁ-oxonitrile
derivative (molar ratio ꢁ-oxonitrile : mercapto derivative ¼ 1.73 : 1.00)
greatly improved the yields of cyclization products to 60–85%.
was stirred for 3–7.5 h. The reaction mixture was cooled
to room temperature and the separated solid was filtered,
washed with ethanol and recrystallized from 96% ethanol
to provide the final product (Z)-1–4 (Table 1). The
structural assignments of all isolated products were
1
made on the basis of spectroscopic data (IR, H and
13C NMR, MS, UV) and elemental analysis.8
For configurational isomers of the sample 1 used in the
variable-temperature (VT) 1H NMR experiment, the
following 1H NMR data are pertinent.
˚
carried out on SiO2 (silica gel 60 A, 12–26 mm, ICN
Biomedicals). Elemental analyses were performed at
the microanalysis laboratory at the Department of Chem-
istry, University of Belgrade.
(Z)-(5-Ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene)-
1
Push–pull (Z)-5-substituted-4-oxothiazolidines 1–4
listed in Table 1 were obtained according to the following
general procedure reported previously.8 To a stirred
suspension of the corresponding ꢁ-oxonitrile19 (3 mmol)
(Scheme 1) and diethyl mercaptosuccinate (ꢂ1% molar
excess) in 5–10 ml of ethanol, a catalytic amount of
K2CO3 was added. CAUTION: All reactions involving
diethyl mercaptosuccinate, owing to the unpleasant odor,
should be carried out in a well-ventilated hood. The
mixture was brought to reflux and the reaction mixture
1-phenylethanone [(Z)-1]. H NMR (CDCl3): ꢀ 1.26 (t,
3H, CH3, J 7.2 Hz), 3.00 (dd, 1H, CHAHBCHXS, JAB
Scheme 1
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 118–123

´
R. MARKOVIC ET AL.
120
17.5 Hz, JAX 8.2 Hz), 3.15 (dd, 1H, CHAHBCHXS, JAB
17.5 Hz, JBX 4.3 Hz), 4.19 (q, 2H, CH2O, J 7.2 Hz),
4.22 (dd, 1H, CHXS, JAX 8.2 Hz, JBX 4.3 Hz), 6.85 (s,
—
1H, CH), 7.39–7.53 (m, 3H, m- and p-Ph), 7.88–7.93
—
(m, 2H, o-Ph), 8.88 (s, 1H, NH). 13C NMR (DMSO-d6): ꢀ
14.5 (CH3), 36.4 (CH2COO), 42.5 (CHS), 61.2 (CH2O),
—
94.9 ( CH), 127.5 (o-Ph), 129.3 (m-Ph), 132.6 (p-Ph),
—
138.7 (C-1 Ph), 161.6 [ C(2)], 170.7 (C4), 176.3
—
—
(COester), 187.7 (COexo).
(E)-(5-Ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene)-
1
1-phenylethanone [(E)-1]. H NMR (CDCl3): ꢀ 1.29 (t,
3H, CH3, J 7.2 Hz), 2.91 (dd, 1H, CHAHBCHXS, JAB
Figure 1. 1H NMR spectra of Z/E mixture of derivative 1,
recorded in CDCl3 at room temperature, at regular 1 h
intervals
17.6 Hz, JAX 10.1 Hz), 3.28 (dd, 1H, CHAHBCHXS, JAB
17.6 Hz, JBX 3.7 Hz), 4.22 (q, 2H, CH2O, J 7.2 Hz), 4.29
—
(dd, 1H, CHXS, JAX 10.1 Hz, JBX 3.7 Hz), 6.32 (s, 1H,
—
CH), 7.41–7.59 (m, 3H, m- and p-Ph), 7.88–7.93 (m, 2H,
o-Ph), 12.06 (s, 1H, NH). 13C NMR (CDCl3): ꢀ 14.0
(CH3), 37.5 (CH2COO), 42.3 (CHS), 61.7 (CH2O), 94.5
upon its dissolution in CDCl3 (designated as zero time in
Fig. 1) contains, as expected, a nearly perfect set of
signals belonging to the single isomer.
The olefinic proton of the (Z)-1 isomer resonates at
considerably higher frequency owing to the deshielding
effect of the syn-lactam nitrogen, relative to the E-
analogue having this proton in a syn position to the less
electronegative sulfur atom. Proper configurational as-
signment, based on the consideration of this effect,
magnetic anisotropy and mesomeric effects, was possi-
ble, not only for the whole series 1–4, but also for
numerous derivatives thereof.9,10 One-dimensional nu-
clear Overhauser effect (NOE) experiments showed that
the irradiation of the singlet at ꢀ 6.85 ppm of the (Z)-1
isomer gave an enhancement of 4.4% to the aromatic
region and an enhancement of 1.7% to the lactam proton
singlet at ꢀ 8.88 ppm. This is in agreement with the Z-
configuration as the correct assignment. Subsequently,
the NOE experiment was conducted on a solution of the
Z/E mixture, containing about 85% of the (E)-1 isomer
after 24 h. Irradiation of the vinyl singlet at ꢀ 6.32 ppm
showed an NOE on the aromatic region, but not on the
singlet at ꢀ 12.06 ppm assigned to the lactam proton of
the (E)-1 isomer. The configurational isomerization of
1 is an intrinsic structural property, based on electronic
n–ꢂ interactions of the two electron-donor substituents
(—NH—,—S—) and one electron-acceptor, i.e. the
—
—
(
CH), 127.8 (o-Ph), 128.6 (m-Ph), 132.6 (p-Ph), 138.0
—
(C-1 Ph), 158.4 [ C(2)], 170.1 (C4), 174.6 (COester),
—
188.29 (COexo).
An analytical sample was obtained by column chro-
matographic purification of the crude (Z)-1 on silica gel,
eluting with a gradient of toluene–ethyl acetate (100 : 0 to
50 : 50, v/v), followed by concentration of the fractions
containing the desired compound 1. Anal. Calcd for
C15H15NO4S: C, 59.00; H, 4.95; N, 4.59; S, 10.50.
Found: C, 58.76; H, 5.02; N, 4.68; S, 10.54%.
RESULTS AND DISCUSSION
Starting with the pure (Z)-1 isomer (0.011 M), the Z/E
process in CDCl3, at 0.011 M concentration, was mon-
itored at 25 ꢁC during a 15 h period at regular time
intervals (1 h) by 1H NMR spectroscopy (Fig. 1).
The progressive decrease in the Z/E ratio with time,
which reached 13 : 87 after 15 h, was based on the
observation of the signals assigned to the (Z)- and (E)-1
lactam protons which initially appear at ꢀ 8.88 and
12.06 ppm, respectively. The isomerization of (Z)-1 to
its counterpart was also followed by the gradual disap-
pearance of the vinylic proton at ꢀ 6.85 ppm and the
simultaneous growth of the signal at ꢀ 6.32 ppm. The 1H
NMR spectrum of (Z)-1 recorded almost immediately
—
COPh substituent, via the C C bond, as found for other
—
push-pull alkenes.20–27 The key factor controlling the Z/E
ratio is the strength of inter- and intramolecular hydrogen
bonds which depends on, among other factors, the polar-
ity of the medium28,29 (Table 2).
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 118–123

CONFIGURATIONAL ISOMERIZATION OF PUSH–PULL THIAZOLIDINONES
121
1
Table 2. Effects of medium polarity and solute concentration on selected H NMR chemical shifts of configurational isomers
1–4
—
—
Entry
Compound
Solvent
CH
NH(ring)
Z/E ratio
1
2
3
4
5
6
7
8
9
10
11
12
13
(Z)-1
(Z)-1
(Z)-1
(Z)-1
(Z)-2
(Z)-3
(E)-3
(Z)-3
(E)-3
(Z)-4
(Z)-4
(Z)-4
(E)-4
CDCl3 (0.011 M)
CDCl3 (0.050 M)
CDCl3 (0.050 M)
DMSO-d6 (0.011 M)
DMSO-d6
DMSO-d6
DMSO-d6
CDCl3
CDCl3
CDCl3 (0.050 M)
CDCl3 (0.050 M)
CDCl3 (0.050 M)
CDCl3
6.85
8.88a
9.88a
8.96a
11.93
11.57
11.30
11.49
9.44
11.43
9.35a
8.70a
8.28a
10.63
96 : 4 (after a few minutes)
97 : 3 (after a few minutes)
11 : 89 (after 10 days)
100 : 0
6.85
6.85
6.78
5.79
5.55
5.15
5.54
4.90
5.59
5.59
5.59
5.12
100 : 0
94 : 6b
22 : 78b
96 : 4 (after a few minutes)
43 : 57 (after 2 days)
10 : 90b(after 6 days)
a An enhancement of intermolecular hydrogen bonding in the (Z)-1 and (Z)-4 isomers, which depends on their initial concentration (entries 1 and 2), or in the
Z/E mixture (greater Z/E ratio; entries 10–12) moves the lactam proton downfield.
b Determined for equilibrated Z/E mixture.
Polar solvents (EtOH, DMSO, acetone) enhance sulfur
or nitrogen participation in the ground-state polarization,
favoring the resonance forms A and B (Scheme 1). In
fact, A and B increase the stability of the Z-isomers via
intermolecular H-bonding. The configurational stability
of Z-isomers 1–4 should be also attributed to the strong
Thus, the data in Table 3 show a gradual upfield shift of
the NH chemical shift of (Z)-1, from ꢀ 8.88 (Z/E ratio
96 : 4) to 8.32 ppm (Z/E ratio 13 : 87) with decreasing
concentration of (Z)-1 (see also Table 2, footnote a). This
case is typical for intermolecular H-bonds. Accordingly,
the shielding of the NH proton accompanying the con-
centration decrease of the (Z)-1 isomer reflects the
decrease in the population of the form with intermole-
cular H-bonds.13–15, 32–34
In contrast to the strong ionic-type intermolecular
hydrogen bond interactions between the solvent, such
as DMSO or ethanol, and (Z)-1, the solute–solvent inter-
actions are negligible in CDCl3 (see below). Thus, the
electrostatic oxygen–sulfur interactions (structure A).10
30
´
´
Angyan and co-workers have reported that in a large
number of sulfur-containing heterocycles, the interac-
tions of non-bonded S and O may influence the physico-
chemical properties and chemical reactivity of these
compounds. Consistent with this, the stereospecific for-
mation of the (Z)-thiazolidinone derivatives 1–4 in etha-
nol is understandable. In line with the postulated solvent
stabilization is also the fact that the 1H NMR spectrum of
the Z-isomers in DMSO-d6 does not change with time
(Table 2, entries 4 and 5). In the case of the E-isomers, the
dominant thermodynamic species in a non-polar solvent,
e.g. in CDCl3, is the neutral, intramolecularly H-bonded
structure. In other words, the intermolecular H-bonding,
present in the original Z-isomer, is suppressed in a non-
polar solvent, inducing the rearrangement around the
double bond. As depicted in Scheme 1, a fast exchange
process (Z-associated Ð Z-free) precedes the rearrange-
ment around the double bond. Worth noting is the
experimental fact that the equilibrated mixtures of thia-
zolidinone derivatives 1–4 in non-polar solvents (CHCl3
or toluene), enriched in the E-isomer, revert in the solid
state, upon solvent evaporation, almost completely to the
configurationally more stable Z-isomer. The high chemi-
cal shift of the NH proton for (E)-1 (ꢀ 12.06 ppm), the
unchanged frequency and the intensity enhancement of
its signal, being proportional to the simultaneous con-
centration increase of the E-isomer, are typical of strong
Table 3. 1H NMR chemical shifts (ppm) of the NH proton of
(Z)-1 in CDCl3 as a function of concentration at room
temperaturea
Chemical shift difference
(Áꢀ ¼ ꢀNH0 ꢀ ꢀNHx)
—
Time (h)
(Z)-1 (%)
ꢀNHx
0b
1
2
3
4
5
6
7
96
82
70
62
56
51
45
40
35
31
26
22
18
17
14
13
8.88
8.80
8.73
8.67
8.62
8.58
8.54
8.50
8.47
8.45
8.42
8.40
8.38
8.38
8.34
8.33
0.08
0.15
0.21
0.26
0.30
0.34
0.38
0.41
0.43
0.46
0.48
0.50
0.50
0.54
0.55
8
9
10
11
12
13
14
15
a ꢀNH0 value from spectrum 0 at 298 K (recorded immediately upon sample
dissolution); selected ꢀNHx values from spectra 1–15 (Fig. 1).
b Low abundance of the (E)-1 isomer (<5%) at time 0 indicates that the Z/E
isomerization of the Z-isomer in CDCl3 begins in time enough to prepare
sample and record the spectrum.
—
—
intramolecular resonance-assisted ꢃ ꢃ ꢃHN—C C—C
—
—
Oꢃ ꢃ ꢃ hydrogen bonding (RAHB).1,31 Conversely, the
effects of a change in concentration of (Z)-1 on the
chemical shift of the NH chemical shift were apparent.
Copyright # 2004 John Wiley & Sons, Ltd.
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R. MARKOVIC ET AL.
122
It is noteworthy that higher values of the NH chemical
shifts were obtained for the Z/E process of 0.016 M (Z)-1
conducted at 45 ꢁC as indicated by the points off the lines
over the whole Z/E range. The downfield shift of the NH
proton from 0.10 ppm (Z/E ratio 60 : 40) to 0.06 ppm (Z/E
ratio 30 : 70) at 45 ꢁC as a result of a concentration
increase from 0.011 to 0.016 M evidently implies the
enhancement of intermolecular interactions. The magni-
tude of these interactions is reflected by the value of the
downfield shift that is proportional to the population of
the Z-isomer in a Z/E mixture. This is reasonable since
both the (Z)-1 isomer of higher initial concentration and
Z/E mixtures enriched in this isomer will increase its
tendency to form intermolecular self-association (see also
Table 2, entries 1–3 and 10–12).
Figure 2. Lactam proton NMR chemical shifts for the (Z)-1
isomer against temperature. *Initial concentration of (Z)-1 in
CDCl3: 0.011 M, except at 45 ꢁC when it was 0.016 M
1
In summary, this VT H NMR dynamic spectroscopic
study provides clear evidence of an intrinsic interplay
between the structural properties of the (Z)- and (E)-1
thiazolidinone isomers and concentration and tempera-
ture effects on the lactam proton chemical shift. In
principle, the competition between (a) the strong intra-
molecular H-bonds within the (E)-1 isomer formed in the
apolar CDCl3 solvent and (b) intermolecular hydrogen
bonds in the (Z)-1 isomer is reflected by the unchanged
downfield chemical shift of the NH proton and its con-
sistent upfield trend, respectively. The large ꢀNH values of
(E)-1–4 in a non-polar solvent, which depend on the
strength of the hydrogen bond, indicate that the intramo-
lecular RAHB contribute to the stabilization of the E-
form. The larger temperature dependence of the NH
chemical shift variation for the (Z)-1 isomer, expressed
as ÁꢀNH/ÁT, is in agreement with the formation of a
greater amount of an unassociated (Z)-1 species for which
the lactam proton is shielded in CDCl3. We hope that the
push–pull thiazolidinone derivatives 1–4, bearing struc-
tural and functional similarities to peptides, may serve as
a good system to study and mimic weak non-covalent
interactions.
NH resonance of (Z)-1 is shifted downfield by 3.05 ppm
in DMSO (ꢀ 11.93 ppm; Table 2, entry 4) relative to ꢀ
8.88 ppm in CDCl3 (entry 1). The chelate-type H-bonding
of major (E)-1 isomer is stronger in CDCl3 than inter-
molecular hydrogen bonding in (Z)-1. Following this
reasoning, an apolar solvent would weaken the intermo-
lecular H-bonds to the benefit of the formation of in-
tramolecular bonds. Short and obviously strong
intramolecular RAHB are formed as the configuration
of the molecule brings the neutral donor and acceptor
—
groups involved, i.e. N—H and O C, into close contact.
—
The signals corresponding to the lactam proton in the
(Z)-1 and (E)-1 isomers were used to follow the stereo-
dynamic of push–pull derivative 1. The temperature
dependence of the NH chemical shifts based on the VT
1H NMR technique1,4,8 permits further discrimination to
be made between the inter- and intramolecular hydrogen
bonding in model substrate 1 (Fig. 2). A lack of tem-
perature dependence of the NH chemical shift (ꢀ
12.06 ppm), assigned to intramolecularly H-bonded iso-
mer (E)-1 experiencing the same electronic environment,
was expected. In the case of the (Z)-1 isomer, NH
chemical shifts in CDCl3 were plotted against tempera-
ture over a range of Z/E ratios of 60 : 40 to 20 : 80. Large
temperature coefficients were observed (ÁꢀNH/
Acknowledgment
This work was supported in part by the Ministry of
Science, Republic of Serbia.
ÁT ¼ ꢀ11.8 ppb ꢁCꢀ1, Z/E ¼ 60 : 40, or ꢀ10.3 ppb ꢁCꢀ1
,
Z/E ¼ 20 : 80) in agreement with a solvent-free or unas-
sociated Z-isomer. The consistent upfield shift of the
lactam proton as a function of temperature increase
appears to be due to the disruption of the intermolecular
hydrogen bonds, leading to a shielding of the lactam
proton. We assume that the large value of the Áꢀ/ÁT
coefficients reflects the decrease in the solute–solute
interaction in CDCl3, rather than relatively weak so-
lute–CDCl3 interactions. This is consistent with the
conclusion of Stevens et al.13 that the NH group of linear
peptides exposed to CDCl3 has a low value of Áꢀ/ÁT ,
implying weak hydrogen bonds in CDCl3.
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