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Spectral and acidity constants of some heterocyclic nitrogen Schiff bases
Specfrochimica Acro, Vol. 44A, No. 12, pp 1251 1254, 1988. Printed in Great Bntain.
0
S3.M)tO.OO 05%8539jSS 1988 Pergamon Press plc
Spectral and acidity constants of some heterocyclic nitrogen Schiff bases F. A. ADAM Chemistry Department, Faculty of Science, Aswan, Egypt (Received 4 June 1988; accepted 5 July 1988) Astract-The spectral behaviour of some arylidene-2-aminopurines in pure and mixed organic solvents as well as in buffer solutions of varying pH have been studied. It is recognized that the hydroxy derivatives form an intermolecular hydrogen bond solvated molecular complex with DMF solution. The pK values of the purine compound and the aminopurine Schiff bases of some aromatic aldehydes have been determined and discussed.
sodium acetate with the appropriate volume of 1.0 M HCI. The pH values were checked by a Digital MV pH meter accurate to *0.05 units.
INTRODUCTION
Scanning the literature reveals that considerable work has been done on the electronic spectral characteristics of aromatic Schiff bases [l&6]. On the other hand, less work has been reported on the spectral properties and the acidity constants of the aromatic heterocyclic nitrogen Schiff bases, in spite of their importance in the biological field [7-91. Accordingly, we published the electronic spectral behaviour of some arylidene-2-amino-5-phenyl-1,3,4_thiadiazoles [lo]. In continuation of this work the spectral properties of some heterocyclic Schiff bases derived from 2-aminopurine and aromatic aldehydes have been investigated. Moreover, the acidity constants of these derivatives were determined and are discussed in terms of their molecular structure. The compounds under investigation are:
x
= P--N(CH3)2(l).p-OH(ll).p
Electronic spectra in pure and mixed organic solvents The recorded electronic spectra of compounds I-V in ethanol exhibit three main bands (Fig. 1). The first band is located at 23G-253 nm, and can be ascribed to the Z-Z* transition within the aldehydic benzene ring [12]. The second band, lying within 27G344 nm, is assigned to a transition between n-orbitals mainly localized in the C=N bonds [13]. The third band, observed at 36&W nm, can be ascribed .to a transition within the whole molecule, due essentially to an intramolecular charge transfer. Such a charge transfer (CT) seems to originate mainly from the aryl moiety to
-OH,tn--OCH,(III);p-OCH,(IV):o-OH(V)
EXPERIMENTAL
The chemicals used in the present work were A.R. products from BDH. The SchiIf bases under investigation were prepared by condensation of the 2-aminopurine with the corresponding aromatic aldehyde in the requisite amount in an ethanolic medium as described previously [IO]. The structure and purity of the compounds were checked by i.r. spectra and microanalysis. The solvents used (EtOH, CHCI,, CCI,, Et,0 and DMF) were spectral grade BDH or E. Merck products. The electronit spectra were recorded on a Shimadzu recording 240 spectrophotometer at 25°C using 1 cm matched silica cells. The modified universal buffer series of BRITON and ROBINSON[l l] was prepared by mixing 150 ml of the 0.4 M acid mixture (phosphoric, acetic and boric) with the appropriate volume of 1.0 M NaOH solution. The acetate buffer series (pH 0.65-5.20) were prepared by mixing 50 ml of 1.0 M SAA44:12-k,
RESULTS AND DISCUSSION
the purine rings which are characterized by a high electron accepting character. Convincing evidence for this assignment is achieved from the position and molar extinction coefficient of this band which is quite sensitive to the nature of the substituent X. Generally, the excitation energy of this band decreases as the electron releasing power of the substituent X is increased. Moreover, the CT nature of the third band is substantiated by considering the spectral behaviour of the hydroxy derivative compounds (II, III and V) in buffer solution (Figs 3 and 4). The band becomes ill defined in solutions with pH < 2.1 of the acetate buffer solution. By increasing the pH of the medium within the range 2.14.7 the intensity of this band increases (Fig. 3) and a gradual red shift in its I,,,as the pH of
1251
F. A. ADAM
1252
0.8 -
0.8
IV r\
t
0.7 0.6 0.6 -
300
325
350 A,
400 nm
Electronic absorption spectra of 7.7 x lo-’ M compound V in acetate buffer solutions. A,
nm
Fig. 1. Spectra of 3 x 10m5M compounds I-V in ethanol medium. 0.8 a-O.OMofDMF b-1.3MofDMF c- 2.6MofDUF d-3.9UofDMF e-5.2MofDMF f-6SMofDUF g-7.8 MofDMF h- 9.1 M of DMF j - 10.1 Mof DMF k-11.7MofDMF 1.13.0MofDMF
0.8
0.6
pH = 11.2
0.6 -
A,
nm
Fig. 4. Electronic absorption spectra of 7.7 x lo-’ M compound V in universal buffer solutions.
A,
hm
2. Spectra of 8 x lo-’ M compound V in CHCl,-DMF
the medium is increased within the pH range 6.6-l 1.2 by universal buffer solution (Fig. 4). This behaviour can be interpreted on the principle that the nitrogen atoms of the purine rings become protonated in a solution of low pH and therefore the CT interaction within the protonated form is expected to be reduced, i.e. the protonated form does not absorb energy in the visible region. On the other hand, as the pH of the
medium increases, the nitrogen atom becomes deprotonated and therefore its mesomeric interaction with the rest of the Schiff base molecule becomes high, consequently the CT interaction within the free base is facilitated, i.e. the free base absorbs energy in the visible region. It is worth mentioning that the increase in intensity of the CT band as a result of the deprotonation of the purine rings in the high pH range, can be considered as evidence for the coplanarity of the hetero rings and the rest of the Schiff base molecule. The intracharge
Arylidene-2-aminopurines
in pure and mixed organic solvents
1253
transfer within the whole molecule can be represented as follows:
the substituent X (pK,) (p-N(CH,),; p-OH; p-OH, m-OCH,; o-OH for compounds I, II, III and V
On using DMF as a solvent the recorded visible spectra of compounds III and V comprise a new shoulder at 1,,,= 462 and 450 nm respectively. This exceeds by far the usual solvent shifts. Thus, this behaviour can presumably be ascribed to the possible formation of a solvated molecular complex between the solute and DMF molecules through intermolecular hydrogen bonding. Since the charge transfer plays an important role in H-bonding [14], the new shoulder observed at the longer wavelength in the spectra of compounds III and V in the DMF solvent can be ascribed to an intermolecular CT transition. This involves an electron transfer from the DMF oxygen atom to the antibonding orbital of the substituent OH group. That the location of this band is at longer wavelengths than the usual n-a* transition is presumably due to the high stabilization of the polar excited state of this transition (-0-H 9) by the highly polar solvent DMF. Good evidence for the intermolecular CT nature of this shoulder is obtained from the non-linear relationship between the absorbance of this band and the molar concentration of the compound [lS]. The possibility of formation of a hydrogen bonded solvated molecular complex with DMF molecules rather than the other solvent molecules used, viz EtOH, CHCI,, Ccl, and ether, can be attributed to the low ionization potential of DMF (9.12 eV) relative to those of the latter-10.49, 11.42, 11.47 and 9.60 eV respectively [16]. The possibility of formation of hydrogen bonded solvated molecular complexes between molecules III or V and DMF molecules is further substantiated by studying the spectral behaviour ofcompounds III or V in EtOH, CHCl, and CCI, by adding increasing amounts of DMF. It is found that by increasing the DMF concentration in the medium a new band is developed at a longer wavelength, where it assumes a more or less constant absorbance value in presence of a high ratio of DMF (Fig. 2). Generally the spectra recorded display a clear isosbestic point indicating the formation of 1: 1 hydrogen bonded molecular complexes.
respectively) were determined by considering the spectral shifts of the compounds in a series of acetate buffer solutions of pH 0.65-6.5 and universal buffer solutions pH 6.511.22. Generally, for all compounds studied by increasing the pH of the medium within pH 0.8-6.0, the absorbance of the CT band increases until a more or less constant value is reached (Fig. 3). Moreover, by gradually increasing the pH of the medium > 6.00 in the case of compounds II, III and V, the CT band acquires a gradual red shift in its A,,,,, until a constant value is reached in solutions of high pH (2 10; Fig. 4). This behaviour can be ascribed to the expected easier charge transfer interaction on increasing the pH of the medium as explained above. The recorded spectra of all compounds exhibit a clear isosbestic point within the low pH range revealing the existence of an equilibrium between the protonated and non-protonated forms of these compounds in such media. Also, a fine isosbestic point is observed in the spectra of these compounds within the high pH range (Fig. 4). This behaviour denotes the presence of an equilibrium between the neutral and ionic forms of each compound in such solutions. In the case ofcompound I the absorbance-pH curve at the selected wavelength within the low pH range displayed two inflections. This indicates that the deprotonation process of the nitrogen atoms of both the purine rings and the substituent N(CH,), occur in a stepwise manner within the pH range 0.8-6.0. The mean values of pK, and pK, were determined from the variation of absorbance with pH using the spectrophotometric half-height and limiting absorbance methods [17]. The limit of accuracy of the determined values was checked by making use of the least-squares method. The results obtained are listed in Table 2. Generally, the pK, of the purine rings increases as the electron donating character of the substituent X is increased. On the other hand, the pK, values of the compounds under investigation are higher than the acidity constant of the free purine compound (2.35f0.04). This is due to the easier CT towards the purine rings in the Schiff base compounds. This behaviour can be considered as evidence for the electron-withdrawing character of the purine rings. Furthermore, it is evident that pK, of compounds II and III is less than compound V (o-OH). This is presumably due to an interaction between the o-OH of
Determination
of the acidity
constants
The acidity constant of the purine moiety of the different compounds studied (pK,) as well as that of
F. A. ADAM
1254
Table 1. Spectral data of arylidene-2-aminopurines in organic solvents EtOH 1max e,,
Compound I p-H(CH, )Z
II p-OH
III p-OH, m-OCH,
IV p-OCH,
V o-OH
CHCl, I MX &IX
253 344 404 230 270 376 232 280 372
15000 8667 12500 11000 9000 4200 15000 9333 4833
231 282 358 235 274 380
16667 10000 2917 11167 15000 10 833
342 404
9000 11667
270 384
9300 4400
280 273
10083 4583
-
-
-
Compound I II III IV V
3.82 f 0.05 3.27 +0.03 2.79 kO.04 3.86kO.01 3.38 k 0.03
384 280 372
-
277 379
13333 10000
278 382
5.26 + 0.06 8.62kO.06 8.9lkO.05 9.19*0.06
the aldehydic moiety and the imine nitrogen atom of the azomethine group. which, in turn facilitates the liberation of the proton i.e. a high pK value. REFERENCES Cl1 J. J. CHARETTE,Spectrochim. Acta 23A, 208 (1967).
PI R. M. ISSA,A. A. EL-SAMAHYand S. H. ETAIW,2. Phys.
Chem. Leipzig 2555, 853 (1974). c31 J. MOSKAL and A. MOSKAL,J. Chem. Sot. Perkin Trans. 2, 1893 (1977). II41F. MUZALEWSKI and R. GAWINECKI, Pol. J. Chem. 55, 565 (1981).
9500 12667 11000 9000 4200
10 333 5ooO
283 370
9750 5500
231 286 360 235 274 376
16667 9167 2833 11167 15000 10 833
-
282 360
PKZ
-4500
320 398 230 270 376
-
10667 2833
compounds
PK,
9200 11000
282 360
Table 2. The mean values of pK, and pK, for
arylidene-2-aminopurine
337 400
10667 2833 sAooo ,,10750
338 404 275 380
8667 11333 8400 4750
285 390 462 sh
8333 4667 1667
290 372
9500 2583
376
7167
450 sh
4333
c51 M. R. MAHMOUD, A. A. EL-SAMAHY and S. A. ELGYAR, Bull. Sot. Chiin. Fr. 11,424 (1981). PI M. R. MAHMOUD, A. M. AWAD and A. M. SHAKER, Spectrochim. Acta 41A, 1177 (1985). c71 A. A. SANTILLI,D. H. KIN and A. R. FIEBER,J. Pharm. Sci. 63, 449 (1974). PI R. S. GIORDANOand R. D. BENEMAN,J. Am. them. Sot. %, 1017 (1974). c91 B. DASH, M. PAIXA and P. K. MAHAPATRA,J. ind. Chem. Sot. LX, 772 (1983). Cl01 M. R. MAHMOUD,R. ABDEL-HAMIDEand F. A. ADAM, 2. Phys. Chem. Leipzig 262S, 551 (1981). r.111H. T. S. BRITTON,Hydrogen lons, 4th edn. Chapman and Hall, London (1952). Cl21 M. S. YOUSSEFand S. A. IBRAHIM,J. prakt. Chem. 6,923 (1985). Cl31 H. H. JAFFE,S. J. YEH and R. W. GARDNER,J. molec. Spectrosc. 2, 120 (1958). Cl41 N. MATAGAand T. KUBOTA,Molecular Interaction and Electronic Spectra, p. 293. Marcel Dekker inc., New York (1970). [ 151 M. R. MAHMOUD, H. S. EL-KASHEFand R. ABDELHAMIDE,5pectrochim. Acta 37A, 519 (1981). [16] C. R. WEAST,Handbook of Chemistry and Physics, 55th edn, p. 74. E. CRS. Press, Cleveland, OH (1975). [17] R. M. ISSA, H. SADEKand I. I. IZZAT,2. Phys. Chem. 74,
17 (1971).
0
S3.M)tO.OO 05%8539jSS 1988 Pergamon Press plc
Spectral and acidity constants of some heterocyclic nitrogen Schiff bases F. A. ADAM Chemistry Department, Faculty of Science, Aswan, Egypt (Received 4 June 1988; accepted 5 July 1988) Astract-The spectral behaviour of some arylidene-2-aminopurines in pure and mixed organic solvents as well as in buffer solutions of varying pH have been studied. It is recognized that the hydroxy derivatives form an intermolecular hydrogen bond solvated molecular complex with DMF solution. The pK values of the purine compound and the aminopurine Schiff bases of some aromatic aldehydes have been determined and discussed.
sodium acetate with the appropriate volume of 1.0 M HCI. The pH values were checked by a Digital MV pH meter accurate to *0.05 units.
INTRODUCTION
Scanning the literature reveals that considerable work has been done on the electronic spectral characteristics of aromatic Schiff bases [l&6]. On the other hand, less work has been reported on the spectral properties and the acidity constants of the aromatic heterocyclic nitrogen Schiff bases, in spite of their importance in the biological field [7-91. Accordingly, we published the electronic spectral behaviour of some arylidene-2-amino-5-phenyl-1,3,4_thiadiazoles [lo]. In continuation of this work the spectral properties of some heterocyclic Schiff bases derived from 2-aminopurine and aromatic aldehydes have been investigated. Moreover, the acidity constants of these derivatives were determined and are discussed in terms of their molecular structure. The compounds under investigation are:
x
= P--N(CH3)2(l).p-OH(ll).p
Electronic spectra in pure and mixed organic solvents The recorded electronic spectra of compounds I-V in ethanol exhibit three main bands (Fig. 1). The first band is located at 23G-253 nm, and can be ascribed to the Z-Z* transition within the aldehydic benzene ring [12]. The second band, lying within 27G344 nm, is assigned to a transition between n-orbitals mainly localized in the C=N bonds [13]. The third band, observed at 36&W nm, can be ascribed .to a transition within the whole molecule, due essentially to an intramolecular charge transfer. Such a charge transfer (CT) seems to originate mainly from the aryl moiety to
-OH,tn--OCH,(III);p-OCH,(IV):o-OH(V)
EXPERIMENTAL
The chemicals used in the present work were A.R. products from BDH. The SchiIf bases under investigation were prepared by condensation of the 2-aminopurine with the corresponding aromatic aldehyde in the requisite amount in an ethanolic medium as described previously [IO]. The structure and purity of the compounds were checked by i.r. spectra and microanalysis. The solvents used (EtOH, CHCI,, CCI,, Et,0 and DMF) were spectral grade BDH or E. Merck products. The electronit spectra were recorded on a Shimadzu recording 240 spectrophotometer at 25°C using 1 cm matched silica cells. The modified universal buffer series of BRITON and ROBINSON[l l] was prepared by mixing 150 ml of the 0.4 M acid mixture (phosphoric, acetic and boric) with the appropriate volume of 1.0 M NaOH solution. The acetate buffer series (pH 0.65-5.20) were prepared by mixing 50 ml of 1.0 M SAA44:12-k,
RESULTS AND DISCUSSION
the purine rings which are characterized by a high electron accepting character. Convincing evidence for this assignment is achieved from the position and molar extinction coefficient of this band which is quite sensitive to the nature of the substituent X. Generally, the excitation energy of this band decreases as the electron releasing power of the substituent X is increased. Moreover, the CT nature of the third band is substantiated by considering the spectral behaviour of the hydroxy derivative compounds (II, III and V) in buffer solution (Figs 3 and 4). The band becomes ill defined in solutions with pH < 2.1 of the acetate buffer solution. By increasing the pH of the medium within the range 2.14.7 the intensity of this band increases (Fig. 3) and a gradual red shift in its I,,,as the pH of
1251
F. A. ADAM
1252
0.8 -
0.8
IV r\
t
0.7 0.6 0.6 -
300
325
350 A,
400 nm
Electronic absorption spectra of 7.7 x lo-’ M compound V in acetate buffer solutions. A,
nm
Fig. 1. Spectra of 3 x 10m5M compounds I-V in ethanol medium. 0.8 a-O.OMofDMF b-1.3MofDMF c- 2.6MofDUF d-3.9UofDMF e-5.2MofDMF f-6SMofDUF g-7.8 MofDMF h- 9.1 M of DMF j - 10.1 Mof DMF k-11.7MofDMF 1.13.0MofDMF
0.8
0.6
pH = 11.2
0.6 -
A,
nm
Fig. 4. Electronic absorption spectra of 7.7 x lo-’ M compound V in universal buffer solutions.
A,
hm
2. Spectra of 8 x lo-’ M compound V in CHCl,-DMF
the medium is increased within the pH range 6.6-l 1.2 by universal buffer solution (Fig. 4). This behaviour can be interpreted on the principle that the nitrogen atoms of the purine rings become protonated in a solution of low pH and therefore the CT interaction within the protonated form is expected to be reduced, i.e. the protonated form does not absorb energy in the visible region. On the other hand, as the pH of the
medium increases, the nitrogen atom becomes deprotonated and therefore its mesomeric interaction with the rest of the Schiff base molecule becomes high, consequently the CT interaction within the free base is facilitated, i.e. the free base absorbs energy in the visible region. It is worth mentioning that the increase in intensity of the CT band as a result of the deprotonation of the purine rings in the high pH range, can be considered as evidence for the coplanarity of the hetero rings and the rest of the Schiff base molecule. The intracharge
Arylidene-2-aminopurines
in pure and mixed organic solvents
1253
transfer within the whole molecule can be represented as follows:
the substituent X (pK,) (p-N(CH,),; p-OH; p-OH, m-OCH,; o-OH for compounds I, II, III and V
On using DMF as a solvent the recorded visible spectra of compounds III and V comprise a new shoulder at 1,,,= 462 and 450 nm respectively. This exceeds by far the usual solvent shifts. Thus, this behaviour can presumably be ascribed to the possible formation of a solvated molecular complex between the solute and DMF molecules through intermolecular hydrogen bonding. Since the charge transfer plays an important role in H-bonding [14], the new shoulder observed at the longer wavelength in the spectra of compounds III and V in the DMF solvent can be ascribed to an intermolecular CT transition. This involves an electron transfer from the DMF oxygen atom to the antibonding orbital of the substituent OH group. That the location of this band is at longer wavelengths than the usual n-a* transition is presumably due to the high stabilization of the polar excited state of this transition (-0-H 9) by the highly polar solvent DMF. Good evidence for the intermolecular CT nature of this shoulder is obtained from the non-linear relationship between the absorbance of this band and the molar concentration of the compound [lS]. The possibility of formation of a hydrogen bonded solvated molecular complex with DMF molecules rather than the other solvent molecules used, viz EtOH, CHCI,, Ccl, and ether, can be attributed to the low ionization potential of DMF (9.12 eV) relative to those of the latter-10.49, 11.42, 11.47 and 9.60 eV respectively [16]. The possibility of formation of hydrogen bonded solvated molecular complexes between molecules III or V and DMF molecules is further substantiated by studying the spectral behaviour ofcompounds III or V in EtOH, CHCl, and CCI, by adding increasing amounts of DMF. It is found that by increasing the DMF concentration in the medium a new band is developed at a longer wavelength, where it assumes a more or less constant absorbance value in presence of a high ratio of DMF (Fig. 2). Generally the spectra recorded display a clear isosbestic point indicating the formation of 1: 1 hydrogen bonded molecular complexes.
respectively) were determined by considering the spectral shifts of the compounds in a series of acetate buffer solutions of pH 0.65-6.5 and universal buffer solutions pH 6.511.22. Generally, for all compounds studied by increasing the pH of the medium within pH 0.8-6.0, the absorbance of the CT band increases until a more or less constant value is reached (Fig. 3). Moreover, by gradually increasing the pH of the medium > 6.00 in the case of compounds II, III and V, the CT band acquires a gradual red shift in its A,,,,, until a constant value is reached in solutions of high pH (2 10; Fig. 4). This behaviour can be ascribed to the expected easier charge transfer interaction on increasing the pH of the medium as explained above. The recorded spectra of all compounds exhibit a clear isosbestic point within the low pH range revealing the existence of an equilibrium between the protonated and non-protonated forms of these compounds in such media. Also, a fine isosbestic point is observed in the spectra of these compounds within the high pH range (Fig. 4). This behaviour denotes the presence of an equilibrium between the neutral and ionic forms of each compound in such solutions. In the case ofcompound I the absorbance-pH curve at the selected wavelength within the low pH range displayed two inflections. This indicates that the deprotonation process of the nitrogen atoms of both the purine rings and the substituent N(CH,), occur in a stepwise manner within the pH range 0.8-6.0. The mean values of pK, and pK, were determined from the variation of absorbance with pH using the spectrophotometric half-height and limiting absorbance methods [17]. The limit of accuracy of the determined values was checked by making use of the least-squares method. The results obtained are listed in Table 2. Generally, the pK, of the purine rings increases as the electron donating character of the substituent X is increased. On the other hand, the pK, values of the compounds under investigation are higher than the acidity constant of the free purine compound (2.35f0.04). This is due to the easier CT towards the purine rings in the Schiff base compounds. This behaviour can be considered as evidence for the electron-withdrawing character of the purine rings. Furthermore, it is evident that pK, of compounds II and III is less than compound V (o-OH). This is presumably due to an interaction between the o-OH of
Determination
of the acidity
constants
The acidity constant of the purine moiety of the different compounds studied (pK,) as well as that of
F. A. ADAM
1254
Table 1. Spectral data of arylidene-2-aminopurines in organic solvents EtOH 1max e,,
Compound I p-H(CH, )Z
II p-OH
III p-OH, m-OCH,
IV p-OCH,
V o-OH
CHCl, I MX &IX
253 344 404 230 270 376 232 280 372
15000 8667 12500 11000 9000 4200 15000 9333 4833
231 282 358 235 274 380
16667 10000 2917 11167 15000 10 833
342 404
9000 11667
270 384
9300 4400
280 273
10083 4583
-
-
-
Compound I II III IV V
3.82 f 0.05 3.27 +0.03 2.79 kO.04 3.86kO.01 3.38 k 0.03
384 280 372
-
277 379
13333 10000
278 382
5.26 + 0.06 8.62kO.06 8.9lkO.05 9.19*0.06
the aldehydic moiety and the imine nitrogen atom of the azomethine group. which, in turn facilitates the liberation of the proton i.e. a high pK value. REFERENCES Cl1 J. J. CHARETTE,Spectrochim. Acta 23A, 208 (1967).
PI R. M. ISSA,A. A. EL-SAMAHYand S. H. ETAIW,2. Phys.
Chem. Leipzig 2555, 853 (1974). c31 J. MOSKAL and A. MOSKAL,J. Chem. Sot. Perkin Trans. 2, 1893 (1977). II41F. MUZALEWSKI and R. GAWINECKI, Pol. J. Chem. 55, 565 (1981).
9500 12667 11000 9000 4200
10 333 5ooO
283 370
9750 5500
231 286 360 235 274 376
16667 9167 2833 11167 15000 10 833
-
282 360
PKZ
-4500
320 398 230 270 376
-
10667 2833
compounds
PK,
9200 11000
282 360
Table 2. The mean values of pK, and pK, for
arylidene-2-aminopurine
337 400
10667 2833 sAooo ,,10750
338 404 275 380
8667 11333 8400 4750
285 390 462 sh
8333 4667 1667
290 372
9500 2583
376
7167
450 sh
4333
c51 M. R. MAHMOUD, A. A. EL-SAMAHY and S. A. ELGYAR, Bull. Sot. Chiin. Fr. 11,424 (1981). PI M. R. MAHMOUD, A. M. AWAD and A. M. SHAKER, Spectrochim. Acta 41A, 1177 (1985). c71 A. A. SANTILLI,D. H. KIN and A. R. FIEBER,J. Pharm. Sci. 63, 449 (1974). PI R. S. GIORDANOand R. D. BENEMAN,J. Am. them. Sot. %, 1017 (1974). c91 B. DASH, M. PAIXA and P. K. MAHAPATRA,J. ind. Chem. Sot. LX, 772 (1983). Cl01 M. R. MAHMOUD,R. ABDEL-HAMIDEand F. A. ADAM, 2. Phys. Chem. Leipzig 262S, 551 (1981). r.111H. T. S. BRITTON,Hydrogen lons, 4th edn. Chapman and Hall, London (1952). Cl21 M. S. YOUSSEFand S. A. IBRAHIM,J. prakt. Chem. 6,923 (1985). Cl31 H. H. JAFFE,S. J. YEH and R. W. GARDNER,J. molec. Spectrosc. 2, 120 (1958). Cl41 N. MATAGAand T. KUBOTA,Molecular Interaction and Electronic Spectra, p. 293. Marcel Dekker inc., New York (1970). [ 151 M. R. MAHMOUD, H. S. EL-KASHEFand R. ABDELHAMIDE,5pectrochim. Acta 37A, 519 (1981). [16] C. R. WEAST,Handbook of Chemistry and Physics, 55th edn, p. 74. E. CRS. Press, Cleveland, OH (1975). [17] R. M. ISSA, H. SADEKand I. I. IZZAT,2. Phys. Chem. 74,
17 (1971).