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Vibrational and electronic spectral studies of substituted 1,3-dihydro-1,3-diphenyl-2-thioxo-2H,5H-pyrimidine-4,6-diones
Spectrochimica Acta, Vol. 45A, No. 9, pp. 917427, 1989.
0584-8539/89 $3.00+0.00 © 1989 Pergamon Press pie
Printed in Great Britain.
Vibrational and electronic spectral studies of substituted 1,3-dihydro1,3-diphenyl-2-thioxo-2H,5H-pyrimidine-4,6-diones V. K. AHLUWALIA*, SANGEETA SHARMA and MOHINDER KAUR Department of Chemistry, University of Delhi, Delhi 110007, India
(Received 21 December 1988; in final form 28 February 1989; accepted 1 March 1989) Abstract--The vibrational spectra of 1,3-diphenyl-; 1,3-di (4'-chlorophenyl)-; 1,3-di(3'-methoxyphenyl)-l,3dihydro-2-thioxo-2H,5H-pyrimidine-4,6-diones in solid state (KBr pellet) and in solution (CHCI3 and CC14) have been studied and assignments made. Tautomeric and hydrogen bonding behaviour are discussed. Electronic spectra in various solvents at different pH values are recorded. The effect of substituents, change of solvent on the n-n* and n-n* transitions of all the compounds is explained. The bathochromic and hypsochromic shifts observed when the neutral form changes to the cationic or anionic form depending on the pH of solution, are also discussed.
INTRODUCTION Various simple or N-substituted pyrimidinediones possess hypnotic, anaesthetic, intravenous, narcotic and anti-bacterial activity [1-4]. The widespread interest in thiobarbiturates stems in large part from their pharmacological activity, due mainly to their enhanced lipid solubility as compared to the corresponding parent barbiturates [5], resulting from the replacement of the oxygen at position 2 by the less electronegative sulphur. Thiobarbiturates with an unsubstituted 5 position are also known to form polymethine dyes with lipids and pyrimidine, the analytical applications of which are quite extensive I-6]. Vibrational modes of 2-thiobarbituric acids have been calculated previously [7,8] by normal coordinate analysis. KAZIMIERCZUK et al. [9] have also studied the u.v. and i.r. spectra of 2-thiobarbituric acid and its 1methyl-l,3-dimethyl derivatives, but no work has been done on vibrational and electronic spectra of N, Ndiphenyl 2-thiobarbituric acid (1, 3-dihydro-1, 3diphenyl-2-thioxo-2H, 5H-pyrimidine-4,6-dione). The present paper reports the vibrational (far i.r. and i.r.) and electronic spectra in polar and non-polar solvents and at various pHs of 1,3-diphenyl. (I); 1,3-di(4'chlorophenyl)- (II); 1,3-di(3'-methoxyphenyl)- (III) 1,3-dihydro-2-thioxo-2H,5H-pyrimidine-4,6-diones.
R 4.¢~"~
2,
O
pounds, indicating that the H atoms present at position 5 are involved in tautomerism. The effect of the phenyl rings on the vibrational and electronic spectrum along with effect of the presence of chloro and methoxy group is discussed. EXPERIMENTAL
The title compounds were synthesized by previously known methods, [10] and their purity was checked by element detection, melting point determination (m.pt of compound I = 245 °C, H = 203 °C, HI = 240 °C) and thin layer chromatography, Infrared spectra in solid and solution have been recorded on a Perkin-Elmer 1710 i.r. Fourier transform spectrometer,* in the 400-4000 cm -1 region. Solvents-chloroform and carbon tetrachloride--were of spectroscopic grade, and were redistilled and dried over calcium chloride prior to use. The cell used was of a NaCI prism and the path length was 0.05 ram. The concentration of the solution in chloroform was 0.1% for all three compounds. Compound II was insoluble in CCi4 and I and IH were partly soluble, hence their concentration could not be measured. No reportable bands were observed between 600-800 c m - t and above 2000 cm -~ in CCi4. Far i.r. spectra were recorded on a Polytech FIR-30 Fourier far i.r. spectrophotometer using polyethylene support, in the region 100-400cm -t. The Raman spectra of these compounds could not be recorded because of their fluorescent nature. Ultraviolet spectra were recorded on Shimadzu UV visible spectrometer-260. Path length of the cells was 10 mm. These barbiturates are insoluble in water and pure acidic solutions. Hence, to see the effect of pH on electronic transitions we have recorded the spectra of the neutral cationic and anionic species in pure methanol, methanol + HCI and methanol + NaOH, respectively. The volume of alcohol +acid or alkali was kept constant at 9:1 by volume. GR grade HCI and 1N NaOH were used. The concentration of the solutions in pure alcoholic, acidic and basic solutions was kept constant at 0.002*/. for all the compounds. Due to the poor solubility of the compounds in non-polar solvents, their concentration could not be measured.
4'
R = H , C o m p o u n d I; R = 4 - c h l o r o , C o m p o u n d II; R = 3 ' - m e t h o x y , C o m p o u n d Ill. Although both the hydrogen atoms of 2-thiobarbituric acid present at NI and N 3 have been replaced by phenyl groups, ketoenol tautomerism has been observed in all the three corn-
* Infrared spectra were recorded on a FT-i.r. rather than on dispersive i.r. spectrophotometer, to get clearly resolved spectra of the compounds (specifically in the region 2000-4000 cm- 1). 917
918
V.K. AHLUWALIAet al. RESULTS AND DISCUSSION
The i.r. spectra of compounds I, II and III are shown in Figs 1-7 and summarized in Table 1. Figures 8-13 and Table 2 depict the u.v. spectra of these compounds. Vibrations of the pyrimidine nucleus
v (C=S) mode. There has been wide disagreement with regard to the assignment of the C=S stretching frequency in nitrogen containing compounds. In compounds where the thiocarbonyi group is linked to one or two nitrogen atoms, strong vibrational coupling effects are possible and the --C=S vibration is not localized. However, three bands consistently appear in the regions 1395-1570, 1260-1420 and 940-1140 c m - t and were assigned as "-N~:~-I, II and l l l " bands. The three strong bands observed in this region (Table 1) in all the three compounds under study have tentatively been taken to represent - N - C --S l, II and IH bands. According to POUPAERT and BOUCHE [12] in 2thiobarbiturates a band at 1500 cm-~ is certainly a "thiourede" band. A strong band observed at 1510 c m - 1 in each compound may be assigned to the thiourede band. Antisymmetric and symmetric stretching modes o f - N C = S group have been observed as strong bands around 1380 and 1260cm - t respectively, which are in accordance with literature values [12,1. However, SMORYGO and IVIN [7,1 have calculated the vibrations of 2-thiobarbituric acid by normal coordinate analysis and have assigned the bands at 1533, 1135 and 889 cm-~ as the mixed vibrations of the C=S stretching and ring deformation mode. Thus, very strong bands at 1500 and l l 0 0 c m -1 in compounds I, II and I | I and at 820 c m - ~ in I and III are taken to represent the mixed vibration of the ring deformation and C=S stretching mode. SMORVOO and IVIN [7] had also calculated a frequency at 686 c m - 1 but could not observe it in 2-thiobarbituric acid. During the present investigation a medium weak band at 660 cm-~ has been assigned to v (C=S) + 6 ring. This band becomes intense in a CHC1 a solution and is shifted towards higher frequency, but is absent in a CCI 4 solution. fl (C=S). SMORYGO and IVIN [7, 8"1 had observed a band at 468 cm-~ in 2-thiobarbituric acid (2-TBA) and had assigned it to a v (C=S) vibration, but recently it has been reported [13,14] that a band between 440-490 c m - 1 in thiopyrimidine derivatives is due to the in-plane bending mode of the C=8 group. Hence, strong bands observed at 440, 490 and 460 cm-~ in compound I, II and I l l respectively have been properly assigned to the ]/(C=S) mode. ),(C=S). ARUNA and SHUNMUGAM [15,1 and LAUTIE et al. [16] have assigned the bands observed at 278 and 252 cm-~ in 2-thio- and 2,4-dithiouracil respectively to the out-of-plane bending vibration of the
C=S group. The very strong band at 272 cm-~ in compound I is assigned to ~ (C=S). This band is very weak in compound II and is absent in compound III which may be due to the presence of a chloro group at the 4' and a methoxy group at the 3' position in II and III respectively, which will sterically hinder the out-ofplane bending vibration of the thione group. v (C-O). The form of the normal mode of the C=O stretching vibration is quite sensitive to the nature of the groups attached to the carbonyl carbon atom. SMORGYO and IVIN [71 have calculated the symmetric and asymmetric C = O stretching bands for 2-TBA at 1678 and 1708 c m - 1, but during present investigation these modes have been observed at 1700 and 1720 cm -1 in all the three compounds. A possible explanation for these frequency shifts is that they are caused by a resonance competition between the n-electrons of the phenyl rings present at Nx and N 3 and the electrons of carbonyl carbon atoms, due to an overlap with the non-bonding electron pair of nitrogen atom. An increase in electron overlap between the phenyl n-electrons and the non-bonding electrons of N atoms takes place at the expense of an overlap between the carbonyl carbon n-electrons and the lone pair of the N atom. This results in an increased force constant and high stretching frequency of the C = O bond [171. The carbonyl stretching bands are sensitive to the state of the compounds. In general, a high v (C=O) is observed in the vapour state and solution phase; the highest frequency is found in low polar solvents. In crystalline solids the C=O frequencies are further
1-°I
0.5 (b)
0
L
400O
2O00
i
1200
400
Wavenumber ( cm -l )
Fig. 1. (a) Infrared spectra of compound I in KBr pellet, and (b) Infrared spectra of compound II in KBr pellet.
Spectra of N-substituted
Table
1. V i b r a t i o n a l
assignments
of 1,3-diphenyl-; 1,3-di(4'-chlorophenyl)-; and thioxo-2H,SH-pyrimidine 4,6-diones
I
KBr
. -----.
. ------
- -
- -
--
930 m - -
1020 vw --
ll00vvs
--
.
.
1380s .
. 1175 m s 1200 s
-1600 ms 1720vvs 1740 vs -. ---3030 ms
. .
----
----
--
--
. . .
. --
500 s 530ms 600m 640 m 660 mw
500 s 530ms 600mw 650 mw 670 vs
- -
620 m, b 670 vvs .
.
730 m - -
--
.
760 vvs 780 s 820 mw 830
m
ll00w
ll00vvw,
ll00vvs
ll00s
.
459 vvs 475 vs 478 vs 500 s b 534s - -
---
.
720 vs
--
- -
- -
850 ms 8 8 0
m s
940 ms 960 vvw 1020 v s
--
-m
8 7 0
-- -
1020 vs 1070 m --
l160s 1180 v w 1200 s
l160s l170s 1200 v v s
l130s ---.
1350vvs
1240 v s 1280 v v s 1300 v v s 1310s 1350vvs
1240 v s 1280 v v s 1300 v s -1350vvs
----1350vvs
1380vvs
1380s
1380vvs
1380w
---
l180m 1200 s
l180s --
---
1260 v s 1300 v v w
1260 vvs --
1350vvs 1380vs
.
CCI 4
460 ms
920 s 950 m 1020 vvs .
.
CHCi 3
. .
-540ms
b
1,3-di(Y-methoxyphenyi)-l,3-dihydro-2-
.
920 s 980 m 1020 v s .
1380m
1580w -1700vvs 1720 vvs 2840 vvw . . 2920 vs 2960 w 3000 vw --
. --
--
.
940 ms 960 vvw 1020 vvw . .
1380vs
1510 s
.
.
m
. 1350s
1510 vvs
.
-980 vs 1020 v s 1070 m
1260 vs -. 1350vvs
. 1450 ms
.
7 0 0
.
880
110" ms 127" vs 135" vs . . . . . . 225* s
--. . .
- -
.
KBr
.
- -
--
--
CHCI a
- -
919
III
- -
-690 s
.
KBr
. 127" m 135" vs 142" vs 150" m 2,12" s . . . 270* w . . . -490 m . . . . . . 505 m s 500 m 517 m , b 540ms 600w -620 vvw -660 mw ms -720 s 805 m
- -
940 s 960 mw 1020 vvs --
. 1450 w
.
. 462 vs . . -530vs
- -
1260 vvs 1300 ms . 1350vvs
CCI 4
- -
740 s 800 vvs 820 s
. ll80ms 1210 s
II
CHCI 3
110" s 127" vs 135" mw 142" vs 150" vs 212" ms 225* vvs 272* vs 387* vs 440 vvs 472 s 477 vvs 500 vw 540vs 600s 640 w 660 mw
pyrimidinediones
.
--
. 1450 m
--
1440 w 1450 m s
1420 s 1460 v s
---
--
1500 s
1490 m
1500 v v s
1510 v v s
--
-1600 m 1720vvs 1740 v v s --
1580s -1700vvs 1720 vs 2840 mw 2880 m 2920 vw 2960 3w 3000 vvw --
-1600 vs 1720vs 1740 s h 2840 vw -2940 m 2960 w 3000 vw 3000 vs
1560 vvs 1580 vvw --1725ms 1700vvs 1745 m s 1720 vvs -2840 vvw . . -2920 vvw -2960 vw -3040 w -3040 w
--3000 ms 3040 ms
1555 v v s -1725s 1745 s -------
Assignments lattice vibration v(OH. • - O) ring o.p.b/OCH3torsion C6H 5 torsion C6Hs torsion ?(C-OH) ?(C-OH) ~(c=s) ~(c=o), ~(c-o) (c=s)
- O C H 3 i.p.b. ~ring v(C=S), 6 ring ~(C=S), 6 (C,, 6 = O) v(C=S), 6 ring v(C-Cl) C H o.p.b. C H o.p.b. v(C=S), v ring C H o.p.b. vring, 6CH2 C H i.p.b. ring breathing C H i.p.b. -N-C
band Ill, vring
II S + v (C=S) CH 3 rocking C H i.p.b, 3 O H vasym. (-N--C=O), C H i.p.b. v(C-O~) vsym, (N-C=S),vring 3CH2; v(C-N) C H 3 s y m . def. v(C6-O ) + vring; CH 2 twisting CH 2 wagging, vasym. (N-C=S) C H 3 a s y m . def. CH 2 scissoring, - N - C = S II v(C=S) + vring; -N-C=S I v(C~) v(C---C) v s y m . (C4,6 = O ) v a s y m . (C4, 6 - - 0 ) vsym. CH2 vsym. CHtmethyl) vasym. CH 2 v a s y m . CH(methyl) ) v O H vCH ring v(O-H)
v = s t r e t c h i n g ; d e f o r J = d e f o r m a t i o n ; fl o r i.p.b = i n - p l a n e b e n d i n g ; 7 o r o . p . b = o u t - o f - p l a n e b e n d i n g ; s y m . = s y m m e t r i c a l ; asym. =asymmetric; s=strong; vs=very strong; vvs=very very strong; m=medium; mw=medium weak; w=weak; vw = very weak; vvw = very very weak; b = broad.
V.K. AHLUWALIAet al.
920 1.9991
literature values [7]. The y (C-OH) vibration has also been observed at 225 and 212 cm-1 in the far i.r. spectra of these compounds. CH 2 group. The CH 2 symmetric and asymmetric vibrations have been observed at 2840 and 2920 cm- 1 respectively in all three barbiturates. Very strong bands observed at 1260cm -1 in I and II and at 1280 cm-1 in III, observed in both solid state and solution are assigned to deformation modes of the > C H 2 group. The CH 2 scissoring vibration is observed at 1450 cm- 1. Other vibrations of the pyrimidine nucleus. GUPTA et al. [14] have assigned the band at 130 cm-~ to the lowest lying out-of-plane ring deformation of the pyrimidine nucleus. Hence, the very strong band observed at 135 cm -~ during the present studies is taken to represent this mode (Table 1, Fig. 7). Other vibrations of the pyrimidine nucleus of these barbiturates, such as v ring, v(C=S) + 6 ring, (C=O) etc. are well assigned in Table 1 and coincide well with literature values [7, 8].
1.7992 1.5993 1.3994 1.1995 0.9996 0.7997 0.5998 0.3999 0.2000 0
I 3200
I 2000
12tO0
400
Vibrations of the benzene ring
Wavenumber( cm-1) Fig. 2. Infrared spectra of compound Ill in KBr pellet.
lowered due to the lattice field and possible associations [11]. Also during the present investigations the (C=O) vibrations are shifted: 1700, 1720--,1720, 1740--* 1725, 1745 as we move from the solid state to a solution in CHC13 and CCI 4 (Table 1, Figs 1 and 2). Vibration of the C-OH unit. Very strong bands have been observed at 1350 crn- 1 in all the compounds in both solid state and solution, which has been taken to represent the v (C-OH) mode, in accordance with
Aromatic ring stretching region. Vibrational spectra of benzene and benzene derivatives with various numbers and arrangements have been reviewed by RANDLE and WHIFFEN [18]. Strong ring stretching modes (vCC) occur from about 1300 to 1650 cm- 1. In benzene itself, two strong degenerate bands at 1595 and 1400 cm- 1 and a weak vibration at 1310 crn- l are observed. Mono, meta or para-substituted benzenes, being however of lower symmetry, have as many as four or five ring stretching modes. Replacement of one or more carbon atoms in the benzene ring leads to surprisingly little change in the stretching mode of the ring skeletal system. In compounds I, II
104.0 96.6 88.0 81.8 ,_J
74.4
8 67.0 59.6 F-.
52.2 44.8 37.4 30.0
I
40O0
3200
I
I
2400 2000
I
/
I
1600
1200
800
Wavenumber( cm-I ) Fig. 3. Infrared spectra of compound 1 in chloroform.
400
Spectra of N-substituted pyrimidinediones
921
100
1
90 80 70
90
80I
60
6O
50
50
40
4O
30
30
20
20
10
10
0
I
t
3200
2OOO
...............
0
I
12OO
400
1800
1600
Wavenumber( cm -t) Fig. 4. Infrared spectra of compound II in chloroform.
,0
1200
800'
400
Wavenumber( c m -l ) Fig. 6. (a) Infrared spectra of compound I in carbon tetrachloride, and (b) Infrared spectra of compound Ill in carbon tetrachloride.
'
8O ,II,,
70 ~"
(I
60 SO 40
3oi 20 10 ~:x~
\
I
0
3200
2000
12100
400
Wavenumber( cm-l ) Fig. 5. Infrared spectra of compound II! in chloroform.
and IIL aromatic ring stretchings have been observed at 1350, 1380, 1500 and 1580cm - t (Table 1). A band of medium intensity is also observed at 1450 cm-1. The highest frequency vibration is a relatively weak band at 1580cm - I , the frequency assigned by RANDLE and WHIFFEN [18] to the Bt mode of
100
200
' 3OO
Wavenumber ( cm-l )
Fig. 7. Far i.r. spectra of compound I (--), 1 I ( - - - ) and III (-x-x-).
V.K. AHLUWALIAet al.
922
benzene; this relatively weak band is observed when substituent "X" is conjugated with the ring [19]. CH stretching region. All the three compounds under present investigation proved to have surprisingly weak absorption in the 3000-3100 cm-1 region. A very weak band observed at 3000 cm- t in I and II and at 3040 c m - t in IIl are assigned to the v CH mode. CH in-plane bending vibration. CH in-plane bending vibrations are very well assigned in their respective regions in Table 1, as suggested by RAO [11] for mono (compound I), 1, 4 (compound II) and 1,3 (compound III) disubstituted benzencs. The strong band observed at 1020 cm- t in the solid state and solution spectra is taken to represent the ring breathing vibration. CH out-of-plane bending modes. Intense absorption bands appear in the region 950-650 cm- 1, due to the CH out-of-plane bending vibrations, which are highly characteristic of the substitution type. The band positions are affected to some extent by the inductive effect of substituent, the spatial environment of the ring [20], and even the mass of substituents other than hydrogen that are attached directly to the ring 1"21]. Among molecules in the present investigation, strong interactions are possible between the ortho hydrogens of the benzene nucleus with the N atoms of the pyrimidine nucleus (A) and sulphur and oxygen atoms present at 2, 4 and 6 positions (B) as shown below:
8 o
0
200O
400O
Wavelength ( ,~ ) Fig. 8. Ultraviolet spectra of compound ! in ethanol (--), methanol (---), chloroform ( - x - x-) and CCI, (-0-(3-).
H-..O
H
J|
eA'l
H
H
-
{A)
H
C 'l
(B)
It is suggested that quasi-hydrogen bonding, as shown in the above figure, interferes with the normal pattern of CH o.p.b, vibrations ['22]. This may lead to spectral similarities of phenyl substituted 2-thiobarbituric acid derivatives with 1,2-disubstituted (compound I), 1,2,4-trisubstituted (compound If) and 1,2,3 or 1, 3, 5 trisubstituted (compound III) benzenes. An examination of Table 1 and Figs 1 and 2 makes it evident that in compound la strong band is present at 740cm -1 which is present in mono and 1,2disubstituted benzenes (770-730 cm- 1); in compound If, a medium band at 805cm -1 is present [1,4disubstituted benzene 833-810 cm- t; 1, 2, 4trisubstituted bcnzenes 822-805 cm-1 (11) ] and in compound Ill a very strong band at 760 cm-~ (1, 3disubstituted benzene 810-750 cm-t), strong band at 780 cm- 1 (1, 2, 3-trisubstituted benzene 780-760 cm- ~ and medium band at 830 cm- ~ (1, 3, 5trisubstituted benzene 865-810 cm-t), have been well assigned to CH out-of-plane deformation modes. From Table 1 and Fig. 8 it is evident that in the far i.r. region some strong or medium bands have been
observed which remain unassigned. We presume that the phenyl rings will also move along the C-N linkage and should give rise to torsional modes of the phenyl ring. Hence the very strong band observed at 142 and 150cm -1 in compounds I and II respectively, have been taken to represent this mode. It is interesting that this band is absent in compound IIl in which the - O C H 3 group is present at position 3', which will hinder the rotation of the benzene nucleus. v (C-CI). The C-CI stretching vibration generally appears in the 800-700 cm- 1 region. In compounds containing only one chloro atom this absorption is found in the narrower 750-700 cm- ~ range. Thus in compound II, the medium strong band observed at 700 cm- 1 is taken to represent the v (C-C1) vibration. Methoxy group vibrations. The strong band observed at l l 6 0 c m - t in the solid state spectra of compound III has been assigned to v(C,ryi-O). Oxygen containing compounds show a CH 3 asymmetric vibration in the 2972-2960 crn-1 region and a symmetric CH stretching mode between 2880-2861 cm- t
Spectra of N-substituted pyrimidinediones [11,1. Bands observed in this region are assigned to vs and vu CH modes in compound I l l (Table 1). Similarly, the CH a symmetric and asymmetric deformation and the CHa rocking vibrations are well assigned in Table 1. The in-plane bending vibration of the - O C H a group has been observed at 500 c m - t. All these assignments are in accordance with literature values [23,1. Very strong band observed at 135 c m - t in compound I l l has been taken to represent the torsional mode of the methoxy group.
Keto-enol tautomerism It is interesting to note that a strong band is observed at 1600 cm - t in the chloroform solution spectra of these compounds (Table 1). Emergence of this new C=C stretching vibration in the solution state makes it evident that these compounds, in chloroform solution, favour the formation of tautomer B: More-
R
H
R
0
S
(A)
H
(B)
over, strong bands have appeared in the CHC1 a solution spectra at 3020, 3040 and 300 c m - t in compounds I, II and III respectively, which have been assigned to an O - H stretching mode. Very strong bands at 1350 c m - t in the KBr and C H e l a spectra have been taken to represent the v(C6-O) mode, in accordance with literature values [7]. Deformation modes (6OH) are also well assigned in Table 1, in both the solid state and in CHCI a solution. According to SMORYGO and IVlN [7,1 in solutions of aprotic solvents barbituric acid its S and N-, alkyland 2-thio- derivatives exist in three carbonyl forms. During the present investigations v(C=C), v(O-H), v(C6-O ) and 6 OH modes are absent in solution spectra of these barbiturates in CC14, which clearly indicate that they exist in trioxo forms in CC14 solution.
Hydrogen bonding Due to the presence of phenyl groups at Nt, and Na of the pyrimidine nucleus in these compounds, the possibility of intermolecular hydrogen bonding involving atoms of atoms, is excluded and cyclic dimers or open chain multipers which can be formed in 2thiobarbituric acid will not be formed in these compounds. However, there is the possibility of intermolecular hydrogen bonding as shown below:
923
O X~N~
OH ~N--X
X
X X=Ph-R
GUPTA et al. [141 had observed a band at 121 c m - t in 4, 6-dihydroxy-2-mercapto pyrimidine and had assigned it to an intermolecularly hydrogen bonded stretching frequency of the OH • • • O group. Strong bands observed at 127 c m - t in all the three compounds have been well assigned to the v(CH • • • O) vibration.
Electronic spectra In the visible and u.v. region the wavelength intensity and general appearance of the electronic bands, along with the effect of environmental conditions such as solvent and pH of the solution are the important features for the characterization of compounds. MAUNTER and CLAYTON [24,1 and KAZIMIERCZUK et al. [9,1 have studied the ultraviolet spectra of 2thiobarbituric acid and its N, N-dimethyl derivative. They had observed two transition systems around 235 and 285 nm, but they have not assigned these transitions. JANSEEN [25,1 had examined the u.v. spectra of various thioamides and had found four band systems to be present in all of them. These compounds I, II and III are similar to the thioamides studied by JANSEEN, except that - N - C - group forms part of the pyrimidine
II nucleus. The baSd system observed around 2900 A is comparable to "Band I" observed by JANSEEN for compounds containing the "thioamide" group. This band is blue shifted as we move to solutions in ethanol or methanol. This is a characteristic feature of the n-Tt* transition that they are shifted hypsochromically with increasing solvent polarity [26,1. Hence we have assigned this transition system to n-~* transition in compounds I, II and Ill. In the region of about 200-220 nm, JANSEEN [25,1 found a band, type (IV) which appeared in the spectra of thioureas, thioamide, etc. Absorption in this region is a common feature of sulphur containing compounds. Also during the present investigations, a strong band is present in case of all the three compounds around 2200 A in ethanol and 2100 A in methanol solutions. We have assigned it to a x-~z* transition (Table 2). In pyrimidine a band is observed at 240 m/~ which corresponds to an A t ---~B2 transition and is derived from the Ato~B2~ benzene transition. CLARK and TtNOCOO [27] suggested that the 210m# band of pyrimidine corresponds to the Atg--cBtu transition of benzene. In the present studies the 2400 and 2200 A systems have been well assigned to the Ato~B2K and Alo---~Btutransition and these are due to mixed excitations of both pyrimidine nucleus and benzene group.
V.K. AHLUWAL1Aet al.
924
HUG and TINOCOO [28a] have calculated the electronic spectra of pyrimidine derivatives and demonstrated the presence of more than two systems as outof-plane transitions (x-n* and n-n*) and in-plane transitions (n-n* and n-a*). During the present investigation the 2900 J~ and 2400 A systems have been taken to represent out-of-plane n-n* and n-n* transitions and the 2200 J~ system represents the in-plane n-n* transition. Recently, similar results were observed in 4, 6-dihydroxy-2-mercapto pyrimidine [14]. According to RAO et al. [28 b] the 2600/~ band of benzene shows vibrational fine structure; the absence of a vibrational fine structure to the phenyl derivatives of group V elements is probably due to an interaction of the non-bonded electrons on the central atoms with the orbitals of the benzene ring. The 2600 J~ system observed in compounds I, II and III contains only one smooth curve (Figs 8-10). Hence we may conclude that the above explanation is true and that there is a strong interaction between the lone pair electrons of the N atom of the heteroaromatic nucleus with the 7z-electrons of the benzene ring. Solvent effect. The effect of solvent on electronic transitions has been studied extensively and it is now established that hydrogen bonding has much greater effect than any other type of solute-solvent interaction. The effect of hydrogen bonding on u.v. spectra is to cause a blue shift if the chromophore is the hydrogen bond accepter and a red shift if the chromophore is a hydrogen bond donor [29]. Compounds I, II and III contain two carbonyl and one C=S group and
•~
1
0 2000
4ooo
Wavelength ( J~ ) Fig. I0. Ultraviolet spectra of compound HI in ethanol (--), methanol (----), chloroform (- x - x -) and CC'I4 (-O-O-)
0
!
1
o
2000
4000
Wavelength( ~ ) Fig. 11. Ultraviolet spectra of compound I; neutral (--), cationic ( - - - ) and anionic (- x - x -) form. C
2000
4OOO
Wavelength ( .~ ) Fig. 9. Ultraviolet spectra of compound H in ethanol (--), methanol (---), chloroform (- x - x -) and CCI4 (-O-O-).
thus will behave as powerful hydrogen bond acceptors. The n-n* transition system is blue shifted as we move from CHC13 to ethanol solutions (Table 2). The progressive increase in the magnitude of these blue shifts is consistent with the increasing strength of the hydrogen bond donating properties, expected in the
Spectra of N-substituted pyrimidinediones
order chloroform, ethanol and methanol. The blue shift in the n-n* band in the electronic spectra of these compounds, observed on change from non-polar to polar solvent, CHC13-,C2HsOH, are due to stabilization of the ground state by hydrogen bonding in alcoholic solution and an increase in transition energy being required to break or weaken the hydrogen bond. From Table 2 it is apparent that a red shift has been observed in the n-n* transition of all the compounds, with an increase in refractive index of the solvent, as suggested by MATAGAand KUBOTA [30]. Substituent effect on electronic spectra. In 2-thiobarbituric acid, two systems are observed at 2850 and 2350/~ (in ethanol) whereas in N, N-diphenyl 2-thiobarbituric acid (Compound I) these have been observed at 2700 and 2240/~,. This blue shift of both the n-n* and n-n* transitions may be related to substitution of two protons at positions N~ and N3 in 2thiobarbituric acid by phenyl groups in compound I. The lone pair electrons of the nitrogen atom will now be involved in resonance with the phenyl group:
.o ¢'4
~lll
I
:t
o
925
t'.q ¢'q ¢ q
e~ O ~D
ba ¢q f,q
O
:a
"E
t~
t2 ¢3 .m.m.
U -6+ e4
o=,~ =o.g.. r~
id -t O ae~ U
Hence, the availability of the lone pair electrons for conjugation in the pyrimidine nucleus will be diminished, and this should lead to a hypsochromic shift. This explanation explains very well the blue shift observed as we move from 2-thiobarbituric acid to compound I. If we now compare the electronic spectra of compounds I, II and III we shall observe that there is a conspicuous red shift in the n-n* transition as we go from compound I ~ I I ~ I I I . This red shift is more in compound III than in compound II. This is because of the mesomeric effect of the chloro and methoxy groups. However, it is interesting to note that the n-n* transition is blue shifted when we move from compound I to II or III. Once again the blue shift is more in case of III. Hence we may conclude that in 2thiobarbituric acid derivatives the n-n* transitions are shifted bathochromically and the n-n* transitions are shifted hypsochromically by o-p- directing groups, the amount of shift being related to the position of the substituent. pH effect. The molecule may exist in neutral, cationic or anionic form depending on the pH of solution. The wavelength of the n-n* band increases in the sequence: neutral molecule --* cation --* anion [26]. From Table 2 it is evident that in general a red shift is observed in 2200/~ (n-n*) system as we move from neutral molecule ~ cation ~ anion. The absorption spectra of compounds I, II and I l l at various pH are shown in Figs 11-13 respectively
926
V.K. AHLUWALIAet al. 1,3-dimethyl-2-thiobarbituric acid I-9], these compounds are fully stable in strongly alkaline medium. The dissociation pattern for these compounds is therefore as the following scheme:
2
il, I
O X ~ N ~
o H X--N3~ '
S~
"~N~ ~"0
I
x
]ii/,/':i o
2000
~
H I-
X X-~-Ph-R
4000
Wavelength ( .~ ) Fig. 12. Ultraviolet spectra of compound II; neutral (--); cationic (---) and anionic (- × - x -) form.
However, due to the presence of lone pair of electrons at : 0 and :~ atoms the possibility of protonation in acidic solution and formation of cations can not be ruled out 1"14]. In all the three compounds under investigation there is a marked red shift of the n - n * and n - n * transitions as the neutral form changes to the cationic form with a decrease in pH (Table 2). A conspicuous red shift is observed in the n-n* transition (2200 A) whereas the n-n* (2900 A) system remains unaltered, as the neutral form changes to the anionic form with an increase in pH of the solution.
CONCLUSIONS
/ I I
0
~k 2000
\x 4000
Wawlength ( ~. ) Fig. 13. Ultraviolet spectra of compound 1II, neutral (--), cationic ( - - - - ) and anionic (- x - x -) form.
and summarized in Table 2. It is clear from the isobestic point at 2205 A that only one dissociating function exists over the pH range 0-12; and as in the case of 1,3-dimethyl barbituric acid [31] and 1,3dimethyl 2-thiobarbituric acid I-9], this must involve a methylenic hydrogen at position 5. As in the case of
In substituted N,N-diphenyl-2-thiobarbituric acid derivatives the v (C=O) vibration has appeared at a higher frequency, as compared to 2-thiobarbituric acid, due to an interaction between the n-electrons of the phenyl ring with the nitrogen atom of the pyrimidine nucleus. This vibration has shifted bathochromitally by 20 cm-1 in the CHCI 3 solution. Quasi hydrogen bonding is present between the ortho hydrogens of the phenyl ring and the N atom of the heteroaromatic nucleus that has affected the normal pattern of the CH out-of-plane bending vibration. The presence of C--O and C - O H stretching and bending vibrations makes it evident that keto--enol tautomerism is present in both solid state and chloroform solution, and that the H atom at position 5 of the pyrimidine nucleus is involved in the tautomerism. The very close similarity between the solid state and solution i.r. spectra indicate that 2-thio barbituric acid derivatives retain the same molecular conformation that they have in the crystal, and that interactions in these compounds are basically intramolecular in character. Strong resonance interactions between the pyrimidine and benzene rings are expected, as a result of the planar configuration, and this is in fact apparent in the interaction of the pyrimidine and benzene ring stretching vibration in the 1300-1600 cm-1 region. The mono and para substituted benzene C - C stretching frequencies which normally fall near 1500 and 1600 c m - ~, shift to lower frequencies. The strong ring bands in these thiobarbiturates are found at about 1500, 1380 and 1350 c m - 1, which appear to be due to a combination of the vibrations of the benzene rings
Spectra of N-substituted pyrimidinediones and pyrimidine nucleus. In the electronic spectra of these compounds one n-~* (2900 A) and two lr-lr* (2400-2200 A) transitions are present. The 2400 and 2200 A systems have been taken to represent the Alo-~B2u and A l o ~ B l u transitions of benzene. The ortho-para-directing groups (-C1 and -OCH3) have shifted the n-~r* transition towards the red and ~r-~r* towards the blue due to a mesomeric effect. In general, both the transition systems are shifted bathochromically when the neutral form changes to the cationic or anionic form.
927
[11] C. N. R. RAO, Chemical Applications of Infrared Spectroscopy. Academic Press, New York (1963). [12] J. POUPAETand R. BOUCHE,J. pharm. Sci. 65 (8), 1258 (1976). [13] C. P. BEETZ JR and G. ASCRELLI,Spectrochim. Acta 36A, 299 (1980). [14] S. P. GUPTA,S. SHARMAand R. K. GOEL, Spectrochim. Acta 42A, 1171 (1986). [15] S. ARUNAand G. SHUNMUGAM,Spectrochim. Acta 41A, 531 (1985). [16] A. LAUTIE,J. HERVlEU and J. BELLOC, Spectrochim. Acta 39A, 367 (1983). [17] R. A. NYQUISTand W. J. POTTS,Spectrochim. Acta 679 (1961). [18] R. R. RANDLEand D. H. WHIFFEN,Molecular SpectroAcknowledgements--S. S. and M. K. are thankful to the scopy, pp. 111-128. Report of the Conference of the Council of Scientific and Industrial Research, New Delhi and Institute of Petroleum, London, 1954 (1955). the Department of Science and Technology, New Delhi, [19] L. J. BELLAMY,The Infrared Spectra of Complex Molerespectively for financial assistance, cules John Wiley & Sons, New York (1958). [20] J. E. STEWARTand M. HELLMANN,J. Res. nam. Bw. Stand. 60, 125 (1958). [21] M. MARGOS~tESand V. A. FASSEL,Spectrochim. Acta 7, 14 (1955). REFERENCES [22] R. D. SPENCER,Spectrochim. Acta 21, 1543 (1965). [1] D. L. TABERNand E. H. WOLWITER,J. Am. chem. Soc. [23] R. K. GOEL, S. SHARMAand A. GUPTA,Indian J. Phys. 60B, 375 (1986). 57, 1961 (1935). [2] E. E. SWANSONand K. K. CrlEN, Proc. Soc. exp. Biol. [24] H. MAUNTERand E. M. CLAYTON,J. Am. chem. Soc. 81, 6270 (1959). Med. 82, 212 (1953). [3] Y. K. STOELTINGand J. P. GRAPH,Anaesthesiology 15, [25] M. J. JANSEEN, Recl Tray. chim. 79, 454, 464, 1066 (1960). 4127 (1954). [4] L.G. AKOFYAN,A. S. ADZmKEKVAN,B. A. DARKINYAN [26] R. S. BECK[R, A. B. F. DUNCAN,F. A. MATSEN,D. R. and E. A. TUNASVAN,Biol. Zh., Arm. 29, 80, 72071 SCOTTand W. WEST,Chemical Applications of Spectroscopy. Wiley, New York (1968). (1976). [5] H.G. MAUNTERand E. M. CLAYTON,J. Am. chem. Soc. [27] L.B. CLARKand I. TINOCOOJR, J. Am. chem. Soc. 87, 11 (1965). 81, 6270 (1959). [6] R. G. SHEPERD,J. chem. Soc. 4410 (1964). [28] (a) W. HUG and I. TINOCOOJR, J. Am. chem. Soc. 95, 2803 (1973); (b) C.N.R. RAO, J. RAMACHANDRANand [7] N. A. SMORVGO and B. A. IvIN, Khim. Geterotiske Soedin 10, 1411 (1975). .A. BALLASUSRAMANIAN,Can J. Chem. 39, 171 (1961). [8] N. A. SMORYGO and B. A. IWN, Khim. Geterotiske [29] B. ELLISand P. J. F. GRIFVITnS,Spectrochim. Acta 22, 2005 (1966). Soedin 10, 1402 (1975). [9] Z. KAZIMIERCZUK,A. PSOOA and D. S.UOAR, Acta [30] N. MATAGAand T. KUaOTA, Molecular Interactions and Electronic Spectra. Marcel Dekker, New York biochim, pol. 20, 83 (1973). [10] (a) J. N. D. DASSand S. Du'rr, Proc. Indian Acad. Sci. (1970). 8A, 145 (1938); (b) R. TYAGI, Ph.D. Thesis, Delhi [31] J. J. Fox and D. SHUt3AR,Bull Soc. chim. Belg. 61, 44 University (1986). (1952).
0584-8539/89 $3.00+0.00 © 1989 Pergamon Press pie
Printed in Great Britain.
Vibrational and electronic spectral studies of substituted 1,3-dihydro1,3-diphenyl-2-thioxo-2H,5H-pyrimidine-4,6-diones V. K. AHLUWALIA*, SANGEETA SHARMA and MOHINDER KAUR Department of Chemistry, University of Delhi, Delhi 110007, India
(Received 21 December 1988; in final form 28 February 1989; accepted 1 March 1989) Abstract--The vibrational spectra of 1,3-diphenyl-; 1,3-di (4'-chlorophenyl)-; 1,3-di(3'-methoxyphenyl)-l,3dihydro-2-thioxo-2H,5H-pyrimidine-4,6-diones in solid state (KBr pellet) and in solution (CHCI3 and CC14) have been studied and assignments made. Tautomeric and hydrogen bonding behaviour are discussed. Electronic spectra in various solvents at different pH values are recorded. The effect of substituents, change of solvent on the n-n* and n-n* transitions of all the compounds is explained. The bathochromic and hypsochromic shifts observed when the neutral form changes to the cationic or anionic form depending on the pH of solution, are also discussed.
INTRODUCTION Various simple or N-substituted pyrimidinediones possess hypnotic, anaesthetic, intravenous, narcotic and anti-bacterial activity [1-4]. The widespread interest in thiobarbiturates stems in large part from their pharmacological activity, due mainly to their enhanced lipid solubility as compared to the corresponding parent barbiturates [5], resulting from the replacement of the oxygen at position 2 by the less electronegative sulphur. Thiobarbiturates with an unsubstituted 5 position are also known to form polymethine dyes with lipids and pyrimidine, the analytical applications of which are quite extensive I-6]. Vibrational modes of 2-thiobarbituric acids have been calculated previously [7,8] by normal coordinate analysis. KAZIMIERCZUK et al. [9] have also studied the u.v. and i.r. spectra of 2-thiobarbituric acid and its 1methyl-l,3-dimethyl derivatives, but no work has been done on vibrational and electronic spectra of N, Ndiphenyl 2-thiobarbituric acid (1, 3-dihydro-1, 3diphenyl-2-thioxo-2H, 5H-pyrimidine-4,6-dione). The present paper reports the vibrational (far i.r. and i.r.) and electronic spectra in polar and non-polar solvents and at various pHs of 1,3-diphenyl. (I); 1,3-di(4'chlorophenyl)- (II); 1,3-di(3'-methoxyphenyl)- (III) 1,3-dihydro-2-thioxo-2H,5H-pyrimidine-4,6-diones.
R 4.¢~"~
2,
O
pounds, indicating that the H atoms present at position 5 are involved in tautomerism. The effect of the phenyl rings on the vibrational and electronic spectrum along with effect of the presence of chloro and methoxy group is discussed. EXPERIMENTAL
The title compounds were synthesized by previously known methods, [10] and their purity was checked by element detection, melting point determination (m.pt of compound I = 245 °C, H = 203 °C, HI = 240 °C) and thin layer chromatography, Infrared spectra in solid and solution have been recorded on a Perkin-Elmer 1710 i.r. Fourier transform spectrometer,* in the 400-4000 cm -1 region. Solvents-chloroform and carbon tetrachloride--were of spectroscopic grade, and were redistilled and dried over calcium chloride prior to use. The cell used was of a NaCI prism and the path length was 0.05 ram. The concentration of the solution in chloroform was 0.1% for all three compounds. Compound II was insoluble in CCi4 and I and IH were partly soluble, hence their concentration could not be measured. No reportable bands were observed between 600-800 c m - t and above 2000 cm -~ in CCi4. Far i.r. spectra were recorded on a Polytech FIR-30 Fourier far i.r. spectrophotometer using polyethylene support, in the region 100-400cm -t. The Raman spectra of these compounds could not be recorded because of their fluorescent nature. Ultraviolet spectra were recorded on Shimadzu UV visible spectrometer-260. Path length of the cells was 10 mm. These barbiturates are insoluble in water and pure acidic solutions. Hence, to see the effect of pH on electronic transitions we have recorded the spectra of the neutral cationic and anionic species in pure methanol, methanol + HCI and methanol + NaOH, respectively. The volume of alcohol +acid or alkali was kept constant at 9:1 by volume. GR grade HCI and 1N NaOH were used. The concentration of the solutions in pure alcoholic, acidic and basic solutions was kept constant at 0.002*/. for all the compounds. Due to the poor solubility of the compounds in non-polar solvents, their concentration could not be measured.
4'
R = H , C o m p o u n d I; R = 4 - c h l o r o , C o m p o u n d II; R = 3 ' - m e t h o x y , C o m p o u n d Ill. Although both the hydrogen atoms of 2-thiobarbituric acid present at NI and N 3 have been replaced by phenyl groups, ketoenol tautomerism has been observed in all the three corn-
* Infrared spectra were recorded on a FT-i.r. rather than on dispersive i.r. spectrophotometer, to get clearly resolved spectra of the compounds (specifically in the region 2000-4000 cm- 1). 917
918
V.K. AHLUWALIAet al. RESULTS AND DISCUSSION
The i.r. spectra of compounds I, II and III are shown in Figs 1-7 and summarized in Table 1. Figures 8-13 and Table 2 depict the u.v. spectra of these compounds. Vibrations of the pyrimidine nucleus
v (C=S) mode. There has been wide disagreement with regard to the assignment of the C=S stretching frequency in nitrogen containing compounds. In compounds where the thiocarbonyi group is linked to one or two nitrogen atoms, strong vibrational coupling effects are possible and the --C=S vibration is not localized. However, three bands consistently appear in the regions 1395-1570, 1260-1420 and 940-1140 c m - t and were assigned as "-N~:~-I, II and l l l " bands. The three strong bands observed in this region (Table 1) in all the three compounds under study have tentatively been taken to represent - N - C --S l, II and IH bands. According to POUPAERT and BOUCHE [12] in 2thiobarbiturates a band at 1500 cm-~ is certainly a "thiourede" band. A strong band observed at 1510 c m - 1 in each compound may be assigned to the thiourede band. Antisymmetric and symmetric stretching modes o f - N C = S group have been observed as strong bands around 1380 and 1260cm - t respectively, which are in accordance with literature values [12,1. However, SMORYGO and IVIN [7,1 have calculated the vibrations of 2-thiobarbituric acid by normal coordinate analysis and have assigned the bands at 1533, 1135 and 889 cm-~ as the mixed vibrations of the C=S stretching and ring deformation mode. Thus, very strong bands at 1500 and l l 0 0 c m -1 in compounds I, II and I | I and at 820 c m - ~ in I and III are taken to represent the mixed vibration of the ring deformation and C=S stretching mode. SMORVOO and IVIN [7] had also calculated a frequency at 686 c m - 1 but could not observe it in 2-thiobarbituric acid. During the present investigation a medium weak band at 660 cm-~ has been assigned to v (C=S) + 6 ring. This band becomes intense in a CHC1 a solution and is shifted towards higher frequency, but is absent in a CCI 4 solution. fl (C=S). SMORYGO and IVIN [7, 8"1 had observed a band at 468 cm-~ in 2-thiobarbituric acid (2-TBA) and had assigned it to a v (C=S) vibration, but recently it has been reported [13,14] that a band between 440-490 c m - 1 in thiopyrimidine derivatives is due to the in-plane bending mode of the C=8 group. Hence, strong bands observed at 440, 490 and 460 cm-~ in compound I, II and I l l respectively have been properly assigned to the ]/(C=S) mode. ),(C=S). ARUNA and SHUNMUGAM [15,1 and LAUTIE et al. [16] have assigned the bands observed at 278 and 252 cm-~ in 2-thio- and 2,4-dithiouracil respectively to the out-of-plane bending vibration of the
C=S group. The very strong band at 272 cm-~ in compound I is assigned to ~ (C=S). This band is very weak in compound II and is absent in compound III which may be due to the presence of a chloro group at the 4' and a methoxy group at the 3' position in II and III respectively, which will sterically hinder the out-ofplane bending vibration of the thione group. v (C-O). The form of the normal mode of the C=O stretching vibration is quite sensitive to the nature of the groups attached to the carbonyl carbon atom. SMORGYO and IVIN [71 have calculated the symmetric and asymmetric C = O stretching bands for 2-TBA at 1678 and 1708 c m - 1, but during present investigation these modes have been observed at 1700 and 1720 cm -1 in all the three compounds. A possible explanation for these frequency shifts is that they are caused by a resonance competition between the n-electrons of the phenyl rings present at Nx and N 3 and the electrons of carbonyl carbon atoms, due to an overlap with the non-bonding electron pair of nitrogen atom. An increase in electron overlap between the phenyl n-electrons and the non-bonding electrons of N atoms takes place at the expense of an overlap between the carbonyl carbon n-electrons and the lone pair of the N atom. This results in an increased force constant and high stretching frequency of the C = O bond [171. The carbonyl stretching bands are sensitive to the state of the compounds. In general, a high v (C=O) is observed in the vapour state and solution phase; the highest frequency is found in low polar solvents. In crystalline solids the C=O frequencies are further
1-°I
0.5 (b)
0
L
400O
2O00
i
1200
400
Wavenumber ( cm -l )
Fig. 1. (a) Infrared spectra of compound I in KBr pellet, and (b) Infrared spectra of compound II in KBr pellet.
Spectra of N-substituted
Table
1. V i b r a t i o n a l
assignments
of 1,3-diphenyl-; 1,3-di(4'-chlorophenyl)-; and thioxo-2H,SH-pyrimidine 4,6-diones
I
KBr
. -----.
. ------
- -
- -
--
930 m - -
1020 vw --
ll00vvs
--
.
.
1380s .
. 1175 m s 1200 s
-1600 ms 1720vvs 1740 vs -. ---3030 ms
. .
----
----
--
--
. . .
. --
500 s 530ms 600m 640 m 660 mw
500 s 530ms 600mw 650 mw 670 vs
- -
620 m, b 670 vvs .
.
730 m - -
--
.
760 vvs 780 s 820 mw 830
m
ll00w
ll00vvw,
ll00vvs
ll00s
.
459 vvs 475 vs 478 vs 500 s b 534s - -
---
.
720 vs
--
- -
- -
850 ms 8 8 0
m s
940 ms 960 vvw 1020 v s
--
-m
8 7 0
-- -
1020 vs 1070 m --
l160s 1180 v w 1200 s
l160s l170s 1200 v v s
l130s ---.
1350vvs
1240 v s 1280 v v s 1300 v v s 1310s 1350vvs
1240 v s 1280 v v s 1300 v s -1350vvs
----1350vvs
1380vvs
1380s
1380vvs
1380w
---
l180m 1200 s
l180s --
---
1260 v s 1300 v v w
1260 vvs --
1350vvs 1380vs
.
CCI 4
460 ms
920 s 950 m 1020 vvs .
.
CHCi 3
. .
-540ms
b
1,3-di(Y-methoxyphenyi)-l,3-dihydro-2-
.
920 s 980 m 1020 v s .
1380m
1580w -1700vvs 1720 vvs 2840 vvw . . 2920 vs 2960 w 3000 vw --
. --
--
.
940 ms 960 vvw 1020 vvw . .
1380vs
1510 s
.
.
m
. 1350s
1510 vvs
.
-980 vs 1020 v s 1070 m
1260 vs -. 1350vvs
. 1450 ms
.
7 0 0
.
880
110" ms 127" vs 135" vs . . . . . . 225* s
--. . .
- -
.
KBr
.
- -
--
--
CHCI a
- -
919
III
- -
-690 s
.
KBr
. 127" m 135" vs 142" vs 150" m 2,12" s . . . 270* w . . . -490 m . . . . . . 505 m s 500 m 517 m , b 540ms 600w -620 vvw -660 mw ms -720 s 805 m
- -
940 s 960 mw 1020 vvs --
. 1450 w
.
. 462 vs . . -530vs
- -
1260 vvs 1300 ms . 1350vvs
CCI 4
- -
740 s 800 vvs 820 s
. ll80ms 1210 s
II
CHCI 3
110" s 127" vs 135" mw 142" vs 150" vs 212" ms 225* vvs 272* vs 387* vs 440 vvs 472 s 477 vvs 500 vw 540vs 600s 640 w 660 mw
pyrimidinediones
.
--
. 1450 m
--
1440 w 1450 m s
1420 s 1460 v s
---
--
1500 s
1490 m
1500 v v s
1510 v v s
--
-1600 m 1720vvs 1740 v v s --
1580s -1700vvs 1720 vs 2840 mw 2880 m 2920 vw 2960 3w 3000 vvw --
-1600 vs 1720vs 1740 s h 2840 vw -2940 m 2960 w 3000 vw 3000 vs
1560 vvs 1580 vvw --1725ms 1700vvs 1745 m s 1720 vvs -2840 vvw . . -2920 vvw -2960 vw -3040 w -3040 w
--3000 ms 3040 ms
1555 v v s -1725s 1745 s -------
Assignments lattice vibration v(OH. • - O) ring o.p.b/OCH3torsion C6H 5 torsion C6Hs torsion ?(C-OH) ?(C-OH) ~(c=s) ~(c=o), ~(c-o) (c=s)
- O C H 3 i.p.b. ~ring v(C=S), 6 ring ~(C=S), 6 (C,, 6 = O) v(C=S), 6 ring v(C-Cl) C H o.p.b. C H o.p.b. v(C=S), v ring C H o.p.b. vring, 6CH2 C H i.p.b. ring breathing C H i.p.b. -N-C
band Ill, vring
II S + v (C=S) CH 3 rocking C H i.p.b, 3 O H vasym. (-N--C=O), C H i.p.b. v(C-O~) vsym, (N-C=S),vring 3CH2; v(C-N) C H 3 s y m . def. v(C6-O ) + vring; CH 2 twisting CH 2 wagging, vasym. (N-C=S) C H 3 a s y m . def. CH 2 scissoring, - N - C = S II v(C=S) + vring; -N-C=S I v(C~) v(C---C) v s y m . (C4,6 = O ) v a s y m . (C4, 6 - - 0 ) vsym. CH2 vsym. CHtmethyl) vasym. CH 2 v a s y m . CH(methyl) ) v O H vCH ring v(O-H)
v = s t r e t c h i n g ; d e f o r J = d e f o r m a t i o n ; fl o r i.p.b = i n - p l a n e b e n d i n g ; 7 o r o . p . b = o u t - o f - p l a n e b e n d i n g ; s y m . = s y m m e t r i c a l ; asym. =asymmetric; s=strong; vs=very strong; vvs=very very strong; m=medium; mw=medium weak; w=weak; vw = very weak; vvw = very very weak; b = broad.
V.K. AHLUWALIAet al.
920 1.9991
literature values [7]. The y (C-OH) vibration has also been observed at 225 and 212 cm-1 in the far i.r. spectra of these compounds. CH 2 group. The CH 2 symmetric and asymmetric vibrations have been observed at 2840 and 2920 cm- 1 respectively in all three barbiturates. Very strong bands observed at 1260cm -1 in I and II and at 1280 cm-1 in III, observed in both solid state and solution are assigned to deformation modes of the > C H 2 group. The CH 2 scissoring vibration is observed at 1450 cm- 1. Other vibrations of the pyrimidine nucleus. GUPTA et al. [14] have assigned the band at 130 cm-~ to the lowest lying out-of-plane ring deformation of the pyrimidine nucleus. Hence, the very strong band observed at 135 cm -~ during the present studies is taken to represent this mode (Table 1, Fig. 7). Other vibrations of the pyrimidine nucleus of these barbiturates, such as v ring, v(C=S) + 6 ring, (C=O) etc. are well assigned in Table 1 and coincide well with literature values [7, 8].
1.7992 1.5993 1.3994 1.1995 0.9996 0.7997 0.5998 0.3999 0.2000 0
I 3200
I 2000
12tO0
400
Vibrations of the benzene ring
Wavenumber( cm-1) Fig. 2. Infrared spectra of compound Ill in KBr pellet.
lowered due to the lattice field and possible associations [11]. Also during the present investigations the (C=O) vibrations are shifted: 1700, 1720--,1720, 1740--* 1725, 1745 as we move from the solid state to a solution in CHC13 and CCI 4 (Table 1, Figs 1 and 2). Vibration of the C-OH unit. Very strong bands have been observed at 1350 crn- 1 in all the compounds in both solid state and solution, which has been taken to represent the v (C-OH) mode, in accordance with
Aromatic ring stretching region. Vibrational spectra of benzene and benzene derivatives with various numbers and arrangements have been reviewed by RANDLE and WHIFFEN [18]. Strong ring stretching modes (vCC) occur from about 1300 to 1650 cm- 1. In benzene itself, two strong degenerate bands at 1595 and 1400 cm- 1 and a weak vibration at 1310 crn- l are observed. Mono, meta or para-substituted benzenes, being however of lower symmetry, have as many as four or five ring stretching modes. Replacement of one or more carbon atoms in the benzene ring leads to surprisingly little change in the stretching mode of the ring skeletal system. In compounds I, II
104.0 96.6 88.0 81.8 ,_J
74.4
8 67.0 59.6 F-.
52.2 44.8 37.4 30.0
I
40O0
3200
I
I
2400 2000
I
/
I
1600
1200
800
Wavenumber( cm-I ) Fig. 3. Infrared spectra of compound 1 in chloroform.
400
Spectra of N-substituted pyrimidinediones
921
100
1
90 80 70
90
80I
60
6O
50
50
40
4O
30
30
20
20
10
10
0
I
t
3200
2OOO
...............
0
I
12OO
400
1800
1600
Wavenumber( cm -t) Fig. 4. Infrared spectra of compound II in chloroform.
,0
1200
800'
400
Wavenumber( c m -l ) Fig. 6. (a) Infrared spectra of compound I in carbon tetrachloride, and (b) Infrared spectra of compound Ill in carbon tetrachloride.
'
8O ,II,,
70 ~"
(I
60 SO 40
3oi 20 10 ~:x~
\
I
0
3200
2000
12100
400
Wavenumber( cm-l ) Fig. 5. Infrared spectra of compound II! in chloroform.
and IIL aromatic ring stretchings have been observed at 1350, 1380, 1500 and 1580cm - t (Table 1). A band of medium intensity is also observed at 1450 cm-1. The highest frequency vibration is a relatively weak band at 1580cm - I , the frequency assigned by RANDLE and WHIFFEN [18] to the Bt mode of
100
200
' 3OO
Wavenumber ( cm-l )
Fig. 7. Far i.r. spectra of compound I (--), 1 I ( - - - ) and III (-x-x-).
V.K. AHLUWALIAet al.
922
benzene; this relatively weak band is observed when substituent "X" is conjugated with the ring [19]. CH stretching region. All the three compounds under present investigation proved to have surprisingly weak absorption in the 3000-3100 cm-1 region. A very weak band observed at 3000 cm- t in I and II and at 3040 c m - t in IIl are assigned to the v CH mode. CH in-plane bending vibration. CH in-plane bending vibrations are very well assigned in their respective regions in Table 1, as suggested by RAO [11] for mono (compound I), 1, 4 (compound II) and 1,3 (compound III) disubstituted benzencs. The strong band observed at 1020 cm- t in the solid state and solution spectra is taken to represent the ring breathing vibration. CH out-of-plane bending modes. Intense absorption bands appear in the region 950-650 cm- 1, due to the CH out-of-plane bending vibrations, which are highly characteristic of the substitution type. The band positions are affected to some extent by the inductive effect of substituent, the spatial environment of the ring [20], and even the mass of substituents other than hydrogen that are attached directly to the ring 1"21]. Among molecules in the present investigation, strong interactions are possible between the ortho hydrogens of the benzene nucleus with the N atoms of the pyrimidine nucleus (A) and sulphur and oxygen atoms present at 2, 4 and 6 positions (B) as shown below:
8 o
0
200O
400O
Wavelength ( ,~ ) Fig. 8. Ultraviolet spectra of compound ! in ethanol (--), methanol (---), chloroform ( - x - x-) and CCI, (-0-(3-).
H-..O
H
J|
eA'l
H
H
-
{A)
H
C 'l
(B)
It is suggested that quasi-hydrogen bonding, as shown in the above figure, interferes with the normal pattern of CH o.p.b, vibrations ['22]. This may lead to spectral similarities of phenyl substituted 2-thiobarbituric acid derivatives with 1,2-disubstituted (compound I), 1,2,4-trisubstituted (compound If) and 1,2,3 or 1, 3, 5 trisubstituted (compound III) benzenes. An examination of Table 1 and Figs 1 and 2 makes it evident that in compound la strong band is present at 740cm -1 which is present in mono and 1,2disubstituted benzenes (770-730 cm- 1); in compound If, a medium band at 805cm -1 is present [1,4disubstituted benzene 833-810 cm- t; 1, 2, 4trisubstituted bcnzenes 822-805 cm-1 (11) ] and in compound Ill a very strong band at 760 cm-~ (1, 3disubstituted benzene 810-750 cm-t), strong band at 780 cm- 1 (1, 2, 3-trisubstituted benzene 780-760 cm- ~ and medium band at 830 cm- ~ (1, 3, 5trisubstituted benzene 865-810 cm-t), have been well assigned to CH out-of-plane deformation modes. From Table 1 and Fig. 8 it is evident that in the far i.r. region some strong or medium bands have been
observed which remain unassigned. We presume that the phenyl rings will also move along the C-N linkage and should give rise to torsional modes of the phenyl ring. Hence the very strong band observed at 142 and 150cm -1 in compounds I and II respectively, have been taken to represent this mode. It is interesting that this band is absent in compound IIl in which the - O C H 3 group is present at position 3', which will hinder the rotation of the benzene nucleus. v (C-CI). The C-CI stretching vibration generally appears in the 800-700 cm- 1 region. In compounds containing only one chloro atom this absorption is found in the narrower 750-700 cm- ~ range. Thus in compound II, the medium strong band observed at 700 cm- 1 is taken to represent the v (C-C1) vibration. Methoxy group vibrations. The strong band observed at l l 6 0 c m - t in the solid state spectra of compound III has been assigned to v(C,ryi-O). Oxygen containing compounds show a CH 3 asymmetric vibration in the 2972-2960 crn-1 region and a symmetric CH stretching mode between 2880-2861 cm- t
Spectra of N-substituted pyrimidinediones [11,1. Bands observed in this region are assigned to vs and vu CH modes in compound I l l (Table 1). Similarly, the CH a symmetric and asymmetric deformation and the CHa rocking vibrations are well assigned in Table 1. The in-plane bending vibration of the - O C H a group has been observed at 500 c m - t. All these assignments are in accordance with literature values [23,1. Very strong band observed at 135 c m - t in compound I l l has been taken to represent the torsional mode of the methoxy group.
Keto-enol tautomerism It is interesting to note that a strong band is observed at 1600 cm - t in the chloroform solution spectra of these compounds (Table 1). Emergence of this new C=C stretching vibration in the solution state makes it evident that these compounds, in chloroform solution, favour the formation of tautomer B: More-
R
H
R
0
S
(A)
H
(B)
over, strong bands have appeared in the CHC1 a solution spectra at 3020, 3040 and 300 c m - t in compounds I, II and III respectively, which have been assigned to an O - H stretching mode. Very strong bands at 1350 c m - t in the KBr and C H e l a spectra have been taken to represent the v(C6-O) mode, in accordance with literature values [7]. Deformation modes (6OH) are also well assigned in Table 1, in both the solid state and in CHCI a solution. According to SMORYGO and IVlN [7,1 in solutions of aprotic solvents barbituric acid its S and N-, alkyland 2-thio- derivatives exist in three carbonyl forms. During the present investigations v(C=C), v(O-H), v(C6-O ) and 6 OH modes are absent in solution spectra of these barbiturates in CC14, which clearly indicate that they exist in trioxo forms in CC14 solution.
Hydrogen bonding Due to the presence of phenyl groups at Nt, and Na of the pyrimidine nucleus in these compounds, the possibility of intermolecular hydrogen bonding involving atoms of atoms, is excluded and cyclic dimers or open chain multipers which can be formed in 2thiobarbituric acid will not be formed in these compounds. However, there is the possibility of intermolecular hydrogen bonding as shown below:
923
O X~N~
OH ~N--X
X
X X=Ph-R
GUPTA et al. [141 had observed a band at 121 c m - t in 4, 6-dihydroxy-2-mercapto pyrimidine and had assigned it to an intermolecularly hydrogen bonded stretching frequency of the OH • • • O group. Strong bands observed at 127 c m - t in all the three compounds have been well assigned to the v(CH • • • O) vibration.
Electronic spectra In the visible and u.v. region the wavelength intensity and general appearance of the electronic bands, along with the effect of environmental conditions such as solvent and pH of the solution are the important features for the characterization of compounds. MAUNTER and CLAYTON [24,1 and KAZIMIERCZUK et al. [9,1 have studied the ultraviolet spectra of 2thiobarbituric acid and its N, N-dimethyl derivative. They had observed two transition systems around 235 and 285 nm, but they have not assigned these transitions. JANSEEN [25,1 had examined the u.v. spectra of various thioamides and had found four band systems to be present in all of them. These compounds I, II and III are similar to the thioamides studied by JANSEEN, except that - N - C - group forms part of the pyrimidine
II nucleus. The baSd system observed around 2900 A is comparable to "Band I" observed by JANSEEN for compounds containing the "thioamide" group. This band is blue shifted as we move to solutions in ethanol or methanol. This is a characteristic feature of the n-Tt* transition that they are shifted hypsochromically with increasing solvent polarity [26,1. Hence we have assigned this transition system to n-~* transition in compounds I, II and Ill. In the region of about 200-220 nm, JANSEEN [25,1 found a band, type (IV) which appeared in the spectra of thioureas, thioamide, etc. Absorption in this region is a common feature of sulphur containing compounds. Also during the present investigations, a strong band is present in case of all the three compounds around 2200 A in ethanol and 2100 A in methanol solutions. We have assigned it to a x-~z* transition (Table 2). In pyrimidine a band is observed at 240 m/~ which corresponds to an A t ---~B2 transition and is derived from the Ato~B2~ benzene transition. CLARK and TtNOCOO [27] suggested that the 210m# band of pyrimidine corresponds to the Atg--cBtu transition of benzene. In the present studies the 2400 and 2200 A systems have been well assigned to the Ato~B2K and Alo---~Btutransition and these are due to mixed excitations of both pyrimidine nucleus and benzene group.
V.K. AHLUWAL1Aet al.
924
HUG and TINOCOO [28a] have calculated the electronic spectra of pyrimidine derivatives and demonstrated the presence of more than two systems as outof-plane transitions (x-n* and n-n*) and in-plane transitions (n-n* and n-a*). During the present investigation the 2900 J~ and 2400 A systems have been taken to represent out-of-plane n-n* and n-n* transitions and the 2200 J~ system represents the in-plane n-n* transition. Recently, similar results were observed in 4, 6-dihydroxy-2-mercapto pyrimidine [14]. According to RAO et al. [28 b] the 2600/~ band of benzene shows vibrational fine structure; the absence of a vibrational fine structure to the phenyl derivatives of group V elements is probably due to an interaction of the non-bonded electrons on the central atoms with the orbitals of the benzene ring. The 2600 J~ system observed in compounds I, II and III contains only one smooth curve (Figs 8-10). Hence we may conclude that the above explanation is true and that there is a strong interaction between the lone pair electrons of the N atom of the heteroaromatic nucleus with the 7z-electrons of the benzene ring. Solvent effect. The effect of solvent on electronic transitions has been studied extensively and it is now established that hydrogen bonding has much greater effect than any other type of solute-solvent interaction. The effect of hydrogen bonding on u.v. spectra is to cause a blue shift if the chromophore is the hydrogen bond accepter and a red shift if the chromophore is a hydrogen bond donor [29]. Compounds I, II and III contain two carbonyl and one C=S group and
•~
1
0 2000
4ooo
Wavelength ( J~ ) Fig. I0. Ultraviolet spectra of compound HI in ethanol (--), methanol (----), chloroform (- x - x -) and CC'I4 (-O-O-)
0
!
1
o
2000
4000
Wavelength( ~ ) Fig. 11. Ultraviolet spectra of compound I; neutral (--), cationic ( - - - ) and anionic (- x - x -) form. C
2000
4OOO
Wavelength ( .~ ) Fig. 9. Ultraviolet spectra of compound H in ethanol (--), methanol (---), chloroform (- x - x -) and CCI4 (-O-O-).
thus will behave as powerful hydrogen bond acceptors. The n-n* transition system is blue shifted as we move from CHC13 to ethanol solutions (Table 2). The progressive increase in the magnitude of these blue shifts is consistent with the increasing strength of the hydrogen bond donating properties, expected in the
Spectra of N-substituted pyrimidinediones
order chloroform, ethanol and methanol. The blue shift in the n-n* band in the electronic spectra of these compounds, observed on change from non-polar to polar solvent, CHC13-,C2HsOH, are due to stabilization of the ground state by hydrogen bonding in alcoholic solution and an increase in transition energy being required to break or weaken the hydrogen bond. From Table 2 it is apparent that a red shift has been observed in the n-n* transition of all the compounds, with an increase in refractive index of the solvent, as suggested by MATAGAand KUBOTA [30]. Substituent effect on electronic spectra. In 2-thiobarbituric acid, two systems are observed at 2850 and 2350/~ (in ethanol) whereas in N, N-diphenyl 2-thiobarbituric acid (Compound I) these have been observed at 2700 and 2240/~,. This blue shift of both the n-n* and n-n* transitions may be related to substitution of two protons at positions N~ and N3 in 2thiobarbituric acid by phenyl groups in compound I. The lone pair electrons of the nitrogen atom will now be involved in resonance with the phenyl group:
.o ¢'4
~lll
I
:t
o
925
t'.q ¢'q ¢ q
e~ O ~D
ba ¢q f,q
O
:a
"E
t~
t2 ¢3 .m.m.
U -6+ e4
o=,~ =o.g.. r~
id -t O ae~ U
Hence, the availability of the lone pair electrons for conjugation in the pyrimidine nucleus will be diminished, and this should lead to a hypsochromic shift. This explanation explains very well the blue shift observed as we move from 2-thiobarbituric acid to compound I. If we now compare the electronic spectra of compounds I, II and III we shall observe that there is a conspicuous red shift in the n-n* transition as we go from compound I ~ I I ~ I I I . This red shift is more in compound III than in compound II. This is because of the mesomeric effect of the chloro and methoxy groups. However, it is interesting to note that the n-n* transition is blue shifted when we move from compound I to II or III. Once again the blue shift is more in case of III. Hence we may conclude that in 2thiobarbituric acid derivatives the n-n* transitions are shifted bathochromically and the n-n* transitions are shifted hypsochromically by o-p- directing groups, the amount of shift being related to the position of the substituent. pH effect. The molecule may exist in neutral, cationic or anionic form depending on the pH of solution. The wavelength of the n-n* band increases in the sequence: neutral molecule --* cation --* anion [26]. From Table 2 it is evident that in general a red shift is observed in 2200/~ (n-n*) system as we move from neutral molecule ~ cation ~ anion. The absorption spectra of compounds I, II and I l l at various pH are shown in Figs 11-13 respectively
926
V.K. AHLUWALIAet al. 1,3-dimethyl-2-thiobarbituric acid I-9], these compounds are fully stable in strongly alkaline medium. The dissociation pattern for these compounds is therefore as the following scheme:
2
il, I
O X ~ N ~
o H X--N3~ '
S~
"~N~ ~"0
I
x
]ii/,/':i o
2000
~
H I-
X X-~-Ph-R
4000
Wavelength ( .~ ) Fig. 12. Ultraviolet spectra of compound II; neutral (--); cationic (---) and anionic (- × - x -) form.
However, due to the presence of lone pair of electrons at : 0 and :~ atoms the possibility of protonation in acidic solution and formation of cations can not be ruled out 1"14]. In all the three compounds under investigation there is a marked red shift of the n - n * and n - n * transitions as the neutral form changes to the cationic form with a decrease in pH (Table 2). A conspicuous red shift is observed in the n-n* transition (2200 A) whereas the n-n* (2900 A) system remains unaltered, as the neutral form changes to the anionic form with an increase in pH of the solution.
CONCLUSIONS
/ I I
0
~k 2000
\x 4000
Wawlength ( ~. ) Fig. 13. Ultraviolet spectra of compound 1II, neutral (--), cationic ( - - - - ) and anionic (- x - x -) form.
and summarized in Table 2. It is clear from the isobestic point at 2205 A that only one dissociating function exists over the pH range 0-12; and as in the case of 1,3-dimethyl barbituric acid [31] and 1,3dimethyl 2-thiobarbituric acid I-9], this must involve a methylenic hydrogen at position 5. As in the case of
In substituted N,N-diphenyl-2-thiobarbituric acid derivatives the v (C=O) vibration has appeared at a higher frequency, as compared to 2-thiobarbituric acid, due to an interaction between the n-electrons of the phenyl ring with the nitrogen atom of the pyrimidine nucleus. This vibration has shifted bathochromitally by 20 cm-1 in the CHCI 3 solution. Quasi hydrogen bonding is present between the ortho hydrogens of the phenyl ring and the N atom of the heteroaromatic nucleus that has affected the normal pattern of the CH out-of-plane bending vibration. The presence of C--O and C - O H stretching and bending vibrations makes it evident that keto--enol tautomerism is present in both solid state and chloroform solution, and that the H atom at position 5 of the pyrimidine nucleus is involved in the tautomerism. The very close similarity between the solid state and solution i.r. spectra indicate that 2-thio barbituric acid derivatives retain the same molecular conformation that they have in the crystal, and that interactions in these compounds are basically intramolecular in character. Strong resonance interactions between the pyrimidine and benzene rings are expected, as a result of the planar configuration, and this is in fact apparent in the interaction of the pyrimidine and benzene ring stretching vibration in the 1300-1600 cm-1 region. The mono and para substituted benzene C - C stretching frequencies which normally fall near 1500 and 1600 c m - ~, shift to lower frequencies. The strong ring bands in these thiobarbiturates are found at about 1500, 1380 and 1350 c m - 1, which appear to be due to a combination of the vibrations of the benzene rings
Spectra of N-substituted pyrimidinediones and pyrimidine nucleus. In the electronic spectra of these compounds one n-~* (2900 A) and two lr-lr* (2400-2200 A) transitions are present. The 2400 and 2200 A systems have been taken to represent the Alo-~B2u and A l o ~ B l u transitions of benzene. The ortho-para-directing groups (-C1 and -OCH3) have shifted the n-~r* transition towards the red and ~r-~r* towards the blue due to a mesomeric effect. In general, both the transition systems are shifted bathochromically when the neutral form changes to the cationic or anionic form.
927
[11] C. N. R. RAO, Chemical Applications of Infrared Spectroscopy. Academic Press, New York (1963). [12] J. POUPAETand R. BOUCHE,J. pharm. Sci. 65 (8), 1258 (1976). [13] C. P. BEETZ JR and G. ASCRELLI,Spectrochim. Acta 36A, 299 (1980). [14] S. P. GUPTA,S. SHARMAand R. K. GOEL, Spectrochim. Acta 42A, 1171 (1986). [15] S. ARUNAand G. SHUNMUGAM,Spectrochim. Acta 41A, 531 (1985). [16] A. LAUTIE,J. HERVlEU and J. BELLOC, Spectrochim. Acta 39A, 367 (1983). [17] R. A. NYQUISTand W. J. POTTS,Spectrochim. Acta 679 (1961). [18] R. R. RANDLEand D. H. WHIFFEN,Molecular SpectroAcknowledgements--S. S. and M. K. are thankful to the scopy, pp. 111-128. Report of the Conference of the Council of Scientific and Industrial Research, New Delhi and Institute of Petroleum, London, 1954 (1955). the Department of Science and Technology, New Delhi, [19] L. J. BELLAMY,The Infrared Spectra of Complex Molerespectively for financial assistance, cules John Wiley & Sons, New York (1958). [20] J. E. STEWARTand M. HELLMANN,J. Res. nam. Bw. Stand. 60, 125 (1958). [21] M. MARGOS~tESand V. A. FASSEL,Spectrochim. Acta 7, 14 (1955). REFERENCES [22] R. D. SPENCER,Spectrochim. Acta 21, 1543 (1965). [1] D. L. TABERNand E. H. WOLWITER,J. Am. chem. Soc. [23] R. K. GOEL, S. SHARMAand A. GUPTA,Indian J. Phys. 60B, 375 (1986). 57, 1961 (1935). [2] E. E. SWANSONand K. K. CrlEN, Proc. Soc. exp. Biol. [24] H. MAUNTERand E. M. CLAYTON,J. Am. chem. Soc. 81, 6270 (1959). Med. 82, 212 (1953). [3] Y. K. STOELTINGand J. P. GRAPH,Anaesthesiology 15, [25] M. J. JANSEEN, Recl Tray. chim. 79, 454, 464, 1066 (1960). 4127 (1954). [4] L.G. AKOFYAN,A. S. ADZmKEKVAN,B. A. DARKINYAN [26] R. S. BECK[R, A. B. F. DUNCAN,F. A. MATSEN,D. R. and E. A. TUNASVAN,Biol. Zh., Arm. 29, 80, 72071 SCOTTand W. WEST,Chemical Applications of Spectroscopy. Wiley, New York (1968). (1976). [5] H.G. MAUNTERand E. M. CLAYTON,J. Am. chem. Soc. [27] L.B. CLARKand I. TINOCOOJR, J. Am. chem. Soc. 87, 11 (1965). 81, 6270 (1959). [6] R. G. SHEPERD,J. chem. Soc. 4410 (1964). [28] (a) W. HUG and I. TINOCOOJR, J. Am. chem. Soc. 95, 2803 (1973); (b) C.N.R. RAO, J. RAMACHANDRANand [7] N. A. SMORVGO and B. A. IvIN, Khim. Geterotiske Soedin 10, 1411 (1975). .A. BALLASUSRAMANIAN,Can J. Chem. 39, 171 (1961). [8] N. A. SMORYGO and B. A. IWN, Khim. Geterotiske [29] B. ELLISand P. J. F. GRIFVITnS,Spectrochim. Acta 22, 2005 (1966). Soedin 10, 1402 (1975). [9] Z. KAZIMIERCZUK,A. PSOOA and D. S.UOAR, Acta [30] N. MATAGAand T. KUaOTA, Molecular Interactions and Electronic Spectra. Marcel Dekker, New York biochim, pol. 20, 83 (1973). [10] (a) J. N. D. DASSand S. Du'rr, Proc. Indian Acad. Sci. (1970). 8A, 145 (1938); (b) R. TYAGI, Ph.D. Thesis, Delhi [31] J. J. Fox and D. SHUt3AR,Bull Soc. chim. Belg. 61, 44 University (1986). (1952).