Comparative study of the structure of rhodanine, isorhodanine, thiazolidine-2,4-dione, and thiorhodanine

Article abstract of DOI:10.1023/A:1021217617954

Ab initio (HF and MP2 level) and semiempirical (AM1, PM3, MNDO) calculations on the relative stabilities and structures of the potential tautomeric forms of rhodanine, isorhodanine, thiazolidine-2,4-dione, and thiorhodanine are reported. Ab initio calculations predict that the thiooxo, oxothio, dioxo, and dithio tautomers are the most stable. These results correspond to the known experimental data. Infrared spectra of the investigated compounds were recorded for the region 4000-150 cm^-1, and the characteristic bands were compared with ab initio calculated frequencies at the HF/3-21G^(*)* level.

Full text of DOI:10.1023/A:1021217617954

Chemistry of Heterocyclic Compounds, Vol. 38, No. 9, 2002
COMPARATIVE STUDY OF THE STRUCTURE
OF RHODANINE, ISORHODANINE,
THIAZOLIDINE-2,4-DIONE, AND THIORHODANINE
1
2
1
V. Enchev , S. Chorbadjiev , and B. Jordanov
Ab initio (HF and MP2 level) and semiempirical (AM1, PM3, MNDO) calculations on the relative
stabilities and structures of the potential tautomeric forms of rhodanine, isorhodanine, thiazolidine-2,4-
dione, and thiorhodanine are reported. Ab initio calculations predict that the thiooxo, oxothio, dioxo,
and dithio tautomers are the most stable. These results correspond to the known experimental data.
-1
Infrared spectra of the investigated compounds were recorded for the region 4000-150 cm , and the
(
)
characteristic bands were compared with ab initio calculated frequencies at the HF/3-21G * * level.
Keywords: ab initio calculations, IR spectra, structure, tautomerism.
Thiazolidin-4-one-2-thione (rhodanine 1), thiazolidin-2-one-4-thione (isorhodanine 2), thiazolidine-2,4-
dithione (thiorhodanine 4), and thiazolidine-2,4-dione (3) as well as some of their derivatives are of industrial
importance. The rhodanines find application as vulcanizing agent for different kinds of rubber. Isorhodanine,
thiorhodanine, and thiazolidine-2,4-dione have a weaker influence on vulcanizing rate [1]. Addition of
rhodanine, thiazolidine-2,4-dione, and some of their derivatives to photosensitizing compositions leads to
improvement of the qualities of these compositions [2, 3]. Different thiazolidine-2,4-dione derivatives have
antimiotic [4], hypoglycemic [5], hypocholesterolemic [5], anti-inflammatory [6], antihyperlipemic [7], and
anti-diabetic [8] action. Thiazolidine-2,4-dione is an inhibitor of the corrosion of mild steels in acidic solution.
A possible explanation of its ability to inhibit metallic corrosion is its coordination at the metallic surface.
Rhodanine, thiorhodanine, and their azo derivatives are suitable as ligands in coordination compounds [9].
These compounds are also used in analytical chemistry as highly sensitive reagents for heavy metals [10, 11].
Compounds 1-4 may exist theoretically in five tautomeric forms as shown in Fig. 1. Their tautomerism
was studied experimentally by different authors (see the citations in. [12]). The structures of rhodanine [13] and
thiazolidine-2,4-dione [14] determined by X-ray studies have been reported. Experimental (UV-vis
spectroscopy) as well as theoretical quantum-chemical investigations of the electronic structure and tautomeric
equilibrium of rhodanine, isorhodanine, thiazolidine-2,4-dione, and thiorhodanine were reported by Hilal et al.
[
15]. However, semiempirical quantum-chemical calculations in this work have been performed in the
π-electron approximation by means of the Pariser–Parr–Pople method. Photoelectron spectroscopy and CNDO/2
theoretical investigations of the tautomers A of rhodanine and thiazolidine-2,4-dione (Fig. 1) were carried out in
[
16]. Detailed tautomeric study of rhodanine, isorhodanine, and thiazolidine-2,4-dione has been carried out by
1 13
IR, H and C NMR, and mass spectrometry [17]. It has been found that these compounds exist in thiooxo,
oxothio, and dioxo tautomeric forms. We have reported [18] ab initio and semiempirical (MNDO, AM1,
MNDO-PM3) investigations on the tautomerism of rhodanine.
_
_________________________________________________________________________________________
1
Institute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria; e-mail:
2
venelin@orgchm.bas.bg. Faculty of Chemistry, Sofia University, 1126 Sofia, Bulgaria. Translated from Khimiya
Geterotsiklicheskikh Soedinenii, No. 9, 1268-1278, September, 2002. Original article submitted July 14, 2001.
1
110
0009-3122/02/3809-1110$27.00©2002 Plenum Publishing Corporation

H
Y
7
H
Y
Y
8
N
N
N
N
3
2
4
5
H
H
H
H
H
6
X
1
9
H
H
S
X
S
X
X
S
H
A
10
B
C
H
Y
H
Y
H
Y
H
H
N
N
H
X
S
X
S
H
S
H
D
E1
E2
Fig. 1. Tautomeric forms of rhodanine (X = S, Y = O) 1, isorhodanine (X = O, Y = S) 2,
thiazolidine-2,4-dione (X = O, Y = O) 3, and thiorhodanine (X = S, Y = S) 4.
The aim of the present study was to compare the relative stabilities of tautomers of rhodanine,
isorhodanine, thiorhodanine, and thiazoline-2,4-dione as well as the structures and IR spectra of the most stable
tautomers. The quantum-chemical calculations were done using ab initio and semiempirical methods. In this
work we present theoretical and experimental investigation on the IR spectra of the above-mentioned
compounds with ab initio methods.
In order to estimate the reliability of the methods used for predicting the properties of sulfur-containing
compounds, the relative stabilities of different tautomeric forms, the geometric and electronic aspects, and the
IR spectra of the most stable tautomers were studied. The bond lengths and bond angles were considered as
geometric description of the molecules while the dipole moments were used for description of the electronic
distribution.
In agreement with the experiment [12, 17] ab initio calculations show that tautomer A is the most stable
for all compounds studied (Table 1). The difference between relative stabilities of tautomer A and the next low-
lying tautomer is more than 10 kcal/mol. The relative stabilities of A and B, C and D tautomeric forms were
(
)
( )
evaluated from calculations at the MP2(fc)/6-31G**//HF/3-21G * * and HF/6-31G**//HF/3-21G * * levels.
Those obtained from the latter turn out to be higher by 0.1-0.9 kcal/mol. Only for rotamers of tautomer E is the
difference in the range 2.5-4.0 kcal/mol. Semiempirical AM1, MNDO-PM3, and MNDO calculations do not
agree even qualitatively, neither with the experimental data [12] nor with the ab initio results for compounds
with exocyclic sulfur (rhodanine, isorhodanine, and thiorhodanine). An exception is the AM1 calculation for
rhodanine where only a qualitative agreement between experiment and quantum-chemical calculations (ab initio
and semiempirical) for thiazolidine-2,4-dione exists.
Structural data for rhodanine [13] and thiazolidine-2,4-dione [14] in the solid state are available.
However, there are no published experimental data on the structures of isorhodanine and thiorhodanine. The
(
)
HF/3-21G * * and HF/6-31G** optimized structural parameters of the most stable tautomers A for the above-
mentioned investigated compounds with the available X-ray data are given in Table 2. Geometry optimization of
(
)
tautomer A for rhodanine and thiazolidine-2,4-dione at the HF/3-21G * * level led to a minimum energy
structure that is close in bond lengths and angles to the geometry found in the crystal as well as to those
obtained at the HF/6-31G** level. For example, the differences in the C–S and C=S bond lengths calculated
(
)
with 6-31G** and 3-21G * * basis sets do not exceed 0.003 and 0.006 Å respectively. The difference is
substantial for the carbon–carbon bond lengths where calculations with the 6-31G** basis set predict shorter
(
0.011-0.016 Å) distances in comparison with the experiment.
1
111

TABLE 1. Calculated Relative Stabilities (kcal/mol) of the Tautomers A-E (Fig. 1) of Rhodanine, Isorhodanine,
Thiazolidine-2,4-dione, and Thiorhodanine at Different Computational Levels
Tautomers
C
Tautomers
C
Computational level
A
B
D
E1
E2
A
B
D
E1
E2
Rhodanine 1
Thiazolidine-2,4-dione 4
MNDO
PM3
AM1
0.57
3.83
0.00
0.00
0.00
0.00
0.00
0.00
6.47
18.91
16.22
22.59
21.51
17.71
17.58
16.84
2.73
9.32
11.22
10.90
18.23
9.99
0.00
2.02
7.68
38.45
26.68
23.96
26.00
22.07
2.76
3.48
0.00
7.45
8.45
16.42
19.34
25.05
20.68
18.66
18.69
18.86
9.36
10.04
12.73
21.18
13.31
25.35
25.37
24.25
7.21
10.02
21.66
23.31
38.07
24.70
34.75
34.69
32.00
0.00
8.52
32.36
28.97
14.19
16.40
15.83
0.00
0.00
0.00
0.00
0.00
0.00
0.00
14.07
18.47
20.78
18.79
15.66
15.36
15.44
20.07
20.23
33.92
21.62
32.02
32.00
29.49
10.77
42.85
30.05
27.05
29.03
25.08
HF/DH//HF/DH
HF/3-21G( //HF/3-21G
*)*
(*)*
**
**
HF/4-31G //HF/4-31G
HF/6-31G //HF/3-21G
MP2(fc)/6-31G //HF/3-21G
22.45
21.59
20.92
**
(*)*
**
(*)*
Isorhodanine 2
Thiorhodanine 4
MNDO
PM3
AM1
2.06
11.22
0.20
0.00
0.00
0.00
0.00
0.00
3.48
5.95
6.56
30.06
26.27
11.77
14.11
13.90
8.68
28.44
17.34
26.15
22.61
20.03
20.10
19.73
2.93
0.00
0.00
25.01
14.78
14.23
16.36
15.56
0.00
8.31
5.31
37.32
22.90
21.65
23.62
21.58
2.03
10.65
7.80
38.72
23.54
22.05
23.91
21.87
10.20
25.34
7.50
0.00
0.00
0.00
0.00
0.00
9.79
20.73
11.53
31.50
28.53
13.65
16.00
15.34
10.38
22.28
13.72
33.37
30.63
15.54
17.70
16.80
10.24
0.00
0.00
0.00
41.54
27.58
13.65
17.33
14.74
1.99
2.21
2.51
43.09
28.43
14.24
17.79
15.28
10.91
5.39
21.93
11.26
11.46
12.49
12.83
HF/DH//HF/DH
HF/3-21G( //HF/3-21G
*)*
(*)*
**
**
HF/4-31G //HF/4-31G
HF/6-31G //HF/3-21G
MP2(fc)/6-31G //HF/3-21G
**
(*)*
**
(*)*

(
)
TABLE 2. Ab initio calculated (3-21G * * and 6-31G** basis sets) structure parameters for tautomer A of thiazolidin-2,4-
one and thiorhodanine. Bond lengths are in Å and bond angles in degrees. For numbering of the atoms see Fig. 1
d, Å(
ω, deg
3-21G * *
Bond
1
Bond angle
5
exptl.a
2
3-21G * *
3
)
6-31G**
4
exptl.a
6
(
)
6-31G**
8
7
Rhodanine 1
C5-S1
S1-C2
C2-N3
N3-C4
C5-C4
C4-O7
C2-S6
N3-H8
1.82
1.74
1.37
1.38
1.51
1.23
1.64
1.814
1.756
1.364
1.386
1.527
1.201
1.627
0.992
1.078
1.814
1.756
1.356
1.380
1.516
1.184
1.630
0.998
1.081
C5-S1-C2
S1-C2-N3
C2-N3-C4
N3-C4-C5
C4-C5-S1
N3-C4-O7
N3-C2-S6
H8-N3-C4
92.7
93.1
110.2
119.9
109.7
107.2
124.3
125.7
120.3
93.1
109.9
119.9
110.3
106.8
124.1
125.4
120.2
118.8
116.8
112.3
106.3
123.3
124.8
C5-H9(H10)
C5-S1
S1-C2
1.819
1.770
1.396
1.352
1.530
1.625
1.198
0.992
1.077
92.4
109.4
120.2
110.2
107.2
125.9
124.4
120.9
92.0
109.4
120.2
110.5
107.9
125.3
124.2
121.1
C2-N3
N3-C4
C5-C4
C4-S7
C2-O6
N3-H8
C5-H9(H10)

TABLE 2 (continued)
1
2
3
4
5
6
7
8
Thiazolidin-2,4-one 3
C5-S1
S1-C2
1.845
1.751
1.372
1.373
1.547
1.219
1.209
0.900
1.812
1.777
1.381
1.375
1.529
1.203
1.200
0.991
1.078
1.815
1.779
1.375
1.372
1.519
1.185
1.183
0.996
1.080
C5-S1-C2
S1-C2-N3
C2-N3-C4
N3-C4-C5
C4-C5-S1
N3-C4-O7
N3-C4-O6
H8-N3-C4
C2-N3-H8
S1-C2-O6
O6-C2-N3
O7-C4-C5
94.2
111.2
117.5
113.6
103.6
122.7
123.9
130.0
112.0
124.9
123.9
123.7
92.7
109.6
119.8
110.1
107.8
124.9
125.5
120.6
119.6
124.9
125.9
125.0
92.5
109.6
119.6
110.7
107.5
124.6
125.2
120.9
119.5
125.1
125.2
124.6
C2-N3
N3-C4
C5-C4
C4-O7
C2-O6
N3-H8
C5-H9(H10)
Thiorhodanine 4
C5-S1
S1-C2
C2-N3
N3-C4
C5-C4
C4-S7
C2-S6
N3-H8
C5-H9(H10)
1.823
1.750
1.378
1.362
1.527
1.622
1.623
0.993
1.077
1.818
1.748
1.367
1.353
1.511
1.628
1.626
0.999
1.081
C5-S1-C2
S1-C2-N3
C2-N3-C4
N3-C4-C5
C4-C5-S1
N3-C4-S7
N3-C4-S6
H8-N3-C4
C2-N3-H8
S1-C2-S6
S6-C2-N3
S7-C4-C5
92.8
109.8
120.4
109.8
107.1
125.4
124.8
120.4
119.2
125.4
124.8
124.8
92.6
109.5
120.6
110.2
107.1
124.8
124.6
120.3
119.1
125.9
124.6
125.0
_
______
a
Rhodanine 1 taken from [13]; thiazolidin-2,4-one 3 from [13].

TABLE 3. Calculated Dipole Moments (in Debyes) and Ionization
Potentials (in eV) for Tautomers A of Compounds 1-4 (Fig. 1)
Com-
pound
Computational level
exptl.
MNDO
AM1
PM3
3-21G(*)*
4-31G**
6-31G**
Dipole moment
1
2
3
4
A
A
A
A
1.84
1.42
1.78
3.20
1.85
1.56
1.88
1.50
2.63
1.12
1.61
1.48
2.79
1.72
1.95
2.19
2.16
1.75
1.87
1.97
2.73
1.65
1.86
2.28
2.20 [41]
2.03 [41]
Ionization potential
1
2
3
4
A
A
A
A
9.65
9.78
10.84
9.46
9.25
9.33
9.99
9.14
9.47
9.56
10.19
9.59
9.68
9.65
10.74
9.51
9.34
9.42
10.64
9.26
9.60
9.68
10.77
9.61
9.26 [16]
10.14 [16]
The results for dipole moments and ionization potentials calculated at different computational levels are
given in Table 3. It can be seen that available experimental values for dipole moments and ionization potentials
of rhodanine 1A and thiazolidine-2,4-dione 3A are in general in good agreement with the calculated
HF/4-31G** ones. Excellent agreement was also found for the measured ionization potentials of 1A and 3A and
the respective AM1 and PM3 calculated values (Table 3).
The vibrational spectra of the most stable tautomers A for the compounds studied were computed using
(
)
3
-21G * * ab initio basis set. Experimental data of the vibrational frequencies of A measured in CsI and CHCl
3
are also presented for comparison. All results are given in Table 4. Our assignments for the ab initio calculated
frequencies are based upon the analysis of the corresponding vibrational eigenvectors. Interpretation of the
calculated IR spectra is not easy since most of the normal modes are not characteristic. Only a few modes, such
as the NH, CH, C=O stretching and CH deformation, were found to be isolated.
2
Comparing the calculated spectra of the compounds with the exocyclic sulfur atom and the spectrum of
their dioxo analogue, thiazolidine-2,4-dione, we found that the wavenumbers of the NH stretching vibrations are
lower when oxygen is replaced by the sulfur atom. The NH stretching vibrations are calculated to be at 3482,
-1
3
476, and 3470 cm in the spectra of rhodanine, isorhodanine, and thiorhodanine, respectively. In the
-1
thiazolidine-2,4-dione the corresponding band was calculated at 3492 cm . The same tendency was observed
experimentally (Table 4).
-
1
A very intense carbonyl stretching band at 1730 cm was observed in a CHCl solution of isorhodanine.
3
-1
This band is shifted up to 1744 cm in the solid state spectrum. The carbonyl stretching band of rhodanine in
-
1
the CHCl solution appears as a doublet (1723, 1759 cm ) arising probably from resonance phenomena. The
3
existence of hydrogen bond dimers in the solutions used for measurements is improbable since their low
concentration of about 0.5% can hardly lead to formation of hydrogen bonding. The low solubility of rhodanine
in this solvent does not permit one to determine the position of the carbonyl band or the behavior of the NH
-1
stretchings. The C=O band is strongly shifted down to 1711 cm in the solid state spectrum due to the strong
-1
intermolecular hydrogen bonding. Therefore, the mean value of 1741 cm obtained from the positions of both
-1
maxima (1723 and 1759 cm ) was taken for comparison with the calculations. A similar situation is observed
-1
for the symmetric CH
2
stretching band where the mean value 2891 cm of the doublet frequencies 2915 and
-1
2
2
867 cm from the CsI spectrum was used for the same purpose. Both carbonyl bands observed in thiazolidine-
-1
-1
,4-dione at 1755 and 1704 cm in chloroform solution and 1738 and 1659 cm in the solid state (CsI pellet)
-1
respectively are easily assigned from the theoretical values 1756 and 1711 cm and their eigenvectors. The
corresponding calculated intensities 176 and 738 km/mol also confirm the experimental picture.
1
115

(
)
TABLE 4. HF/3-21G * * Calculated and Experimental IR Data for Tautomer A of Compounds 1-4. (Frequencies (ν) are
-1
in cm and Intensities (I) in km/mol)
Calculated
Experimental
CsI
Calculated
Experimental
Assignment
5
Assignment
10
ν
I
2
CHCl3
3
ν
I
7
CHCl3
CsI
9
1
4
Rhodanine 1
3091 m
2960 m
2891 m
1711 s
1480 sh
1443 s
1382 s
1305 m
1234 s
6
8
Isorhodanine 2
3
482
006
948
750
415
397
241
192
147
144
045
114
1
3379 m
*
NH str.
3476
3018
2958
1723
1435
1395
1261
1206
1146
1120
1069
871
115
3
8
3374 m
*
3173 m
NH str.
3
2
1
1
1
1
1
1
1
1
CH2 as. str.
CH2 sym. str.
C=O str.
HNC bend.
CH2 def.
N-C4 str. + skel. i.p.
skel. i.p.
N–C2 str.+ skel. i.p.
skel. o.p.
2950 w
2920 m
1744 s
1470 s
1388 m
1312 m
1247 w
CH2 as. str.
CH2 sym. str.
C=O str.
HNC bend.
CH2 def.
5
*
*
444
332
13
22
105
4
691
120
33
13
151
8
1741 s
1461 m
1412 s
1398 s
1307 m
1232 s
1163 s
1073 s
1032 m
885 ms
869 m
*
543
351
22
13
122
0
649
58
17
10
172
34
36
0
1730 vs
1445 s
1402 m
1321 mw
1213 m
HCS bend. + skel. i.p.
N-C4 str. + skel. i.p.
HCS bend.+ HCSC tors.
C=S str. + skel. i.p.
N-C2 str. + C=S str.
skel. o.p.
1188 s
1084 s
1170 s
1153 s
895 vw
864 vw
1177 vs
1105 m
C=S str. + skel. i.p.
skel. o.p.
8
8
7
7
6
5
5
5
4
3
2
1
94
02
70
54
46
52
41
21
68
96
45
45
887 s
880 s
823 s
786 s
682 s
skel. i.p.
807
793
722
645
skel. i.p.
skel. o.p.
C5-S1str.
C2-S1 str.
skel. o.p.
C5-S1 str.
C2-S1 str.
skel. o.p.
skel. o.p.
C2-N str. + skel. i.p.
skel. i.p.
789 m
752 m
*
14
4
798 s
623 vw
696 m
636 m
578 m
492 m
485 m
631
OCSC tors. + skel. i.p.
skel. i.p.
3
6
539
486
431
397
5
556 w
498 w
446 w
415 vw
566 m
545 s
514 s
425 s
263 s
182 s
176 s
10
32
3
skel. o.p.
skel. i.p.
skel. i.p.
skel. i.p.
skel. o.p.
skel. o.p.
41
7
456 s
418 s
292 s
164 m
skel. i.p.
skel. i.p.
5
5
8
259
152
10
7
skel. o.p.
skel. o.p.
9
7
85
6

TABLE 4 (continued)
1
2
3
4
5
6
7
8
9
10
Thiazolidin-2,4-one 3
Thiorhodanine 4
3086 m
3
492
009
951
756
711
400
360
252
207
150
108
116
1
5
3391m
3145 m, 3055 m NH str.
3470
3013
2954
1449
1395
1277
1178
1174
1141
1123
1008
865
808
795
713
637
548
479
443
420
101
3
3365 mw
*
NH str.
3
2
1
1
1
1
1
1
1
1
*
*
2923 m
2831 w
1738 s
1659 vs
1392 w
1343 s
1318 m
1233 m
~1100 sh
1165 s
893 m
~820 sh
808 s
CH2 as. str.
2914 sh
2894 m
1496 s
1386 s
1282 s
1189 vs
1181 s
CH2 as. str.
CH2 sym. str.
HNC bend.
CH2 def.
CH2 sym. str.
C4=O7 str.
C2=O6 str.
CH2 def.
7
*
175
738
10
22
93
183
3
169
40
3
37
202
34
5
0
3
31
6
20
8
1755 m
1704 vs
1406 w
1365 m
1326 s
1130 m
*
652
20
49
275
66
0
928
305
15
122
15
23
18
7
1437 vs
1399 m
1290 m
1255 s
1212 m
HCS bend.
HNC bend.
N–C5 str. + CNC bend.
C2=S7 str. + skel. i.p. + C4=S7
HCS bend.+ HCSC tors.
C2-N str. + ring def.
C2=S7 str. + NCS bend. + C4=S7
skel. o.p.
HCS bend. + N-C4 str.
ring def.+ HCS bend.
ring def.+ HCS bend.
C2-N str. + ring def.
skel. o.p.
1120 vs
1049 vs
*
1046 vs
865 s
8
8
7
7
6
6
5
5
4
4
3
1
99
18
61
58
50
29
83
46
88
43
53
65
894 w
*
skel. i.p.
873 vs
skel. o.p.
skel. i.p.
C5-S1 str.
C2-S1 str.
skel. o.p.
skel. o.p.
skel. i.p. + C2=S7
skel. i.p. + C4=S6
skel. i.p.
*
*
*
C5-S1 str. + ring def.
skel. o.p.
C2-S1 str. + ring def.
skel. o.p.
791 s
727 s
619 m
671 vs
667 s
612 m
545 w
485 w
454 w
612 w
skel. i.p.
skel. o.p.
skel. i.p.
skel. i.p.
skel. i.p.
skel. o.p.
1
544 w
499 m
470 w
404 w
576 vw
514 s
34
16
2
485 w
405 s
181 ms
333
203
132
5
2
skel. i.p.
skel. o.p.
9
1
7
skel. o.p.
89
7
skel. o.p.
_
*
______
CHCl
3
absorption; i.p. – in plane; o.p. – out of plane.

An analysis of the theoretical spectra of rhodanine, isorhodanine, and thiorhodanine shows that the C=S
stretching vibration is always coupled with the in-plane deformation vibrations of the five-membered ring. The
bands for which this vibration contributes to the other normal modes are found to be in the region
-
1
1
200-1000 cm (Table 4). In contrast to them the C–S stretching vibrations in these compounds are
characteristic. An exception is thiazolidine-2,4-dione where the C–S vibrations are coupled with the ring
deformation coordinates (Table 4). In compounds 1-4 the bands due to the C–S stretching vibration were found
-1
to be located in the region 823-619 cm .
Our study includes also thiorhodanine, whose IR spectra obviously indicate the presence of rhodanine,
although its melting point coinsides well with that reported by Grishchuk [19]. This means that thiorhodanine
decomposes during measurement of its IR spectrum because the rhodanine bands increase with the time. The
strongly pronounced thiorhodanine instability has also been noticed by Grishchuk [19].
Lebedev and Yakimenko [20] have reported the presence of strong bands at 455, 417, 293, 165 and
-
1
-1
1
63 cm in the spectrum of isorhodanine. We observed such strong bands in the region 500-150 cm in the IR
spectra of rhodanine, isorhodanine, and thiazolidine-2,4-dione.
EXPERIMENTAL AND CALCULATIONS
Rhodanine (thiazolidin-4-one-2-thione) was synthesized according to Nencki [21] from
monochloroacetic acid and ammonium thiocyanate. The product was recrystallized from water and dried in
vacuum; mp 168-170°C. Thiazolidine-2,4-dione was synthesized according to Volhard [22] from
monochloroacetic acid and thiourea. The product was recrystallized from water and dried in vacuum;
mp 120-122°C. Isorhodanine (4-thioxothiazolidin-2-one) was synthesized according to Grishchuk et al. [23]
from thiazolidine-2,4-dione and phosphorus pentasulfide in dioxane. The product was recrystallized from
1
,2-dichloroethane and dried in vacuum; mp 158-162°C. Thiorhodanine (2,4-thiazolidinedithione) was
synthesized according to Grishchuk [19] from rhodanine and phosphorus pentasulfide in dry dioxane. The
product was recrystallized from methanol and dried in a desiccator in vacuum; mp 98-100°C.
The IR spectra of rhodanine, isorhodanine, thiazolidine-2,4-dione, and thiorhodanine were recorded
with the Bruker IFS-113V FTIR spectrometer. Solid state spectra were taken in CsI pellet in the spectral region
-1
4
000-150 cm . The spectra of saturated solutions in CHCl were recorded in a 0.1 mm KBr cell in the spectral
3
-1
region 4000-400 cm .
Ab initio calculations were performed by means of the GAMESS [24] program. The standard
(
)
Dunning/Hay [25], 3-21G * * [26, 27], 4-31G** [28-30] and 6-31G** [31, 32] basis sets were used at the
(
)
Hartree-Fock level. The 3-21G * * basis set places d-type Gaussian functions on the second-row sulfur atom
(
)
only and on the hydrogen atoms. Additional single-point HF/6-31G**//HF/3-21G * * calculations were then
carried out. For tautomers A of compounds 1-4 geometry optimization was also carried out at the HF/6-31G**
-
4
level. Ab initio geometry optimizations were done within C symmetry and the gradient threshold was 1.0·10
1
( )
Hartree/Bohr. Subsequently, the effect of electron correlation was studied at the MP2/6-31G**//HF/3-21G * *
level. Single-point energies at the MP2 level [33] were calculated with frozen core MP2(fc).
Semiempirical MNDO [34, 35], AM1 [36, 37], MNDO-PM3 [38] methods were also used. The
calculations were carried out with the MOPAC 6.0 program package [39]. The geometries of the investigated
compounds were completely optimized using the EF [40] procedure. For all calculations the option PRECISE
was used.
The IR frequencies and intensities for the most stable tautomers of compounds studied were computed
(
)
at the HF/3-21G * * level using the numerical derivative procedure incorporated in the GAMESS program.
Calculated frequencies were scaled by a factor of 0.893 to correct for vibrational anharmonicity at the SCF
level.
1
118

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