Electronic and vibrational spectral studies of substituted mercaptopyrimidines

0584-853918653.00+0.00 Q Pergamon Journals Ltd.

Spectrochimica Acfo. Vol. 42A. No. 10. pp. 1171-l 179, 1986. Printed in Great Britain.

Electronic and vibrational spectral studies of substituted mercaptopyrimidines S. P. GUPTA,*$ SANGEETASHARMA* and R. K. C&EL? *Chemical Laboratory, D. N. College, Meerut 250002,India and tMole&ar Spectroscopyand Biophysics Laboratories, D. N. College, Meerut 250002,India (Received 15 April 1986;accepted 1 May 1986)

Abstract-The vibrational spectra (i.r., far i.r. and Rarnan) of 4,6dimethyl-2-mercaptopyrimiame and 4,6diiydroxy-2-methyhnercaptopyrimidine have been reported along with their assignments. Hydrogen bonding and tautomeric behaviour are discussed.Electronic spectra in various solvents at different pH values are recorded. The effectof a change of solvent on the electronic transitions of both compounds is explained along with the bathochromic and hypsochromic shifts observed when the neutral form of the compound is changed to the anionic or cationic form.

INTRODUCHON

Infrared and Raman spectroscopic studies are widely used in biophysical research on nucleic acids and related compounds. A full understanding of the vibrational spectra of the bases of nucleic acids is of great importance.. The i.r. and Raman spectra of simple pyrimidine bases and their derivatives have been extensively studied both experimentally and theoretically [l-6]. Not much work seems to have been reported on mercaptopyrimidines except on methylthiopyrimidines [7, S] and on substituted mercaptopyrimidines [9, lo]. Numerous sulphur substituted pyrimidines have found applications as clinically useful drugs. In many of these it was noted that the position in which sulphur was introduced was crucial to biological activity; for instance, 2-thiouracil has useful antithyroid activity [ll] and 4-thiouracil is inactive. 2-Thiopyrimidine participates in hydrogen bonds formed by SRNA [12] and hence information about the hydrogen bonding ability of the C-S bond in substituted thiopyrimidines would have considerable biological relevance. The profound interest in the molecular mechanism of gene expression places great importance on structural probes of nucleic acid bases and related compounds. MILES et al. [ 131 and CLARK and TINOCOO [14] have found evidence of the presence of electronic transitions other than n-n* and n--z* in the spectra of uracil and cytosine and their nucleosides. KLEINWACHTER et al. [7]. BRAHMS et al. [8] and STEWART and JENSEN [lS] were unable to detect the presence of any n--n* transition in the near U.V. spectral region of uracil derivatives. Thus detailed studies of the electronic transitions of substituted pyrimidines are of great importance in order to check the presence of n-n*, R-Z* and n-o* transitions. In view of the above the present paper reports the vibrational spectra of 4,6-dimethyl-2-mercaptopy-

$Author to whom correspondence should be addressed.

rimidine and 4,6-dihydroxy-2-methylmercaptopyrimidine. The near U.V. absorption spectra of both the compounds in different solvents and in alkaline and acidic media are reported. EXPERIMENTAL

Spec-pure grade samples of 4$-dimethyl-2-merfaptopyrimidine and 4.6dihydroxy-2-methyhnercaptonvrimidine (hereafter refer&l as 4,6,2-CMMP and 4,6,%dGMP, resDectivelv1were obtained from EGA-Chemie. F.R.G. Their &rity w& confirmed by elemental analysis and melting point determination (214°C for 4,6,2-DMMP and 305°C for 4,6,2DHMP). Infrared spectra of both the compounds were recorded on a Perkin-Elmer 580B spectrophotometer in Nujol mull in the region 2OO-4000cn- I, far i.r. spectra were recorded on a Polytech FIR-30 Fourier far i.r. spectrophotometer in the range 100-6OOcm-‘. Laser Raman spectra of 4,6,2-DHMP was recorded on a Spex-Ramalab spectrophotometer, using a 52 MG argon-krypton laser of wavelength 488 nm. Raman spectra of 4,6,2-DHMP could not be recorded because of its fluorescent nature. Ultraviolet spectra of both molecules were recorded on a Beckman M-35 spectrophotometer and the pHs of the solutions were measured with a Systronics 335 digital pH meter. This system was standardized at pH 4 & 0.01,7 + 0.01 and 9 k 0.01 at room temperature, with the help of buffer solutions. All the solvents used were of GR grade and were dried over calcium oxide and repeatedly distilled prior to use. The concentration of the solution in all cases was kept constant at 8 x low3 g/l. The ratio of pure solvent and acid or alkali was maintained at 9 : 1 by volume. The U.V.spectra of both compounds in the vapour phase were tried on a medium quartz spectrograph, but even under the best experimental conditions, reportable bands were not observed. RESULTS AND DISCUSSION

The vibrational spectra (i.r., far i.r. and Raman) of 4,6,2-DHMP and 4,6,2-DMMP are shown in Figs l-3. The observed fundamental frequencies and their proposed assignments are given in Table 1. Electronic transitions observed in various solvents, at different pHs, for both compounds are given in Table 2. Near U.V. spectra of 4,6,2-DMMP and 4,6,2-DHMP are shown in Figs 4 and 5, respectively. Tautomeric forms of 4,6,2-DMMP and 4,6,2-DHMP are presented in Figs 6 and 7, respectively.

1171

S.

P.

GIJPT,~et

al.

%-;-dk 64

(b)

/

--_

,

300 3300

I

I

1

I

I

I

2700

2000

I600

I200

800

400

2

Wavenumber (cm”)

Fig. 1. (a) Infrared and (b) far i.r. spectra of 4,6-dimethyl-2-mercaptopyrimidine.

I

IO----J

350 3300

2700

I

I

2000

/

I

1400

1000

I

600

21

Wavenumber (cm-‘) Fig. 2. (a) Infrared and (b) far i.r. spectra of 4,6-dihydroxy-2-methylmercaptopyrimidine.

200

I --- 1 2800 1500

I

,

1400

1000

500

200

Wavenumber (cm-‘) Fig. 3. Laser Raman spectra of 4,6-dimethyl-2-mercaptopyrimidine.

Vibrational

spectra

The molecules under investigation are trisubstituted pyrimidines and therefore only one C-H valence oscillation is expected. NISHIMURA et al. [6] have

calculated the C,H stretching value for uracil at 3068 cm-‘. In view of this, the strong Raman band at 3040 cm-’ with an i.r. counterpart at 3070cm-’ in 4,6,2-DMMP has been assigned to the K,H vibration which is also in accordance with the literature value

1173

Substituted mercaptopyrimidines Table 1. Vibrational assignments of 4,6,2-DMMP and 4,6,2-DHMP (all values in cm-‘) 4,6,2-DMMP Raman Infrared 112s 152 VW -

110* w -

-

1301 ms 150; m 165* s 175* s 200; w 229* vs 268* vs 285* vs 462 vvs 538 ws 568 m 591 s 674 mw 725 mw 738 ms 852 vs 952 s 980 vs 1032 vs 1070 mw 1188 ms 1224 s 1320 s 1360 ms 1380 s 144OmS 1468 m 1490 mw 1568 vs 1625 vvs 1652 sh

-

1728 s

232 vs 240 vs 456 vs 532ms 560 ws 860m 952 ms 984 ws 1032 ms 1184m 1192 ms 1220 ms 1318 ms 1387 m 1432 mw 1440 mw 1476 w 1492 w 1568 ms 1588 VW -

2960 w 3000 mw 3040s -

2250 w 2835 m 2880 vs 2920 vvs 2950s 2980 ms 3030 w 3070 w 3140vw 3185 ms

4,6,2-DHMP Infrared

113* VW 121* w 130* ms 143* ms 200’ vs 210; ws 236* s 313* vs 348 mw 359 w 380 s 472 ws 519 vvs 544s 588 vvs 630 vvs 725 m 738 ms 810 vs 978 ms 995 s 1018 s 1068 m 1190s 1266~s 1342 s, b 1380 s 1420 m 1460 vs, b WOW 1618 m 1645 s,b 3160 VW (3820) s

Assignment Lattice vibration SCHJ torsion v(OH . . . 0) Ring out-of-plane bending SH torsion Y(&

-3

CH, &rsion

SCCr-s)

CHs torsion Y(C-CH,) Y(C-OH) CHa torsion Y(CCHo). Y(C=S) B(C-S) l3GS-C) OH torsion LJ(C=o) Ring out-of-plane bending, v(M), /?(C=S) Ring out-of-plane bending, /?(C-OH) Ring in-plane bending ;;;:z)) Ring in-plane bending v(Criq-S), Y(C,H) Ring out-of-plane bending, v(C=S) Y(N-H), Y(c--o) Ring breathing Y(Nr-H) CHs rocking Ring in-plane bending Ring stretching, &N-H) CHs rocking CHp rocking v(C=s), B(C-H) v(C-CH,) W-H) v(C-CHJ v(C-CH) Ring stretching CHa sym. def. CHs sym. def. CH, asym. def. Ring stretching, /3(N-H) CH3 asym. def. Ring stretching B(N-H) Ring stretching Ring stretching v(G=o) -r_C=S band 852 x 2, -y<=S

band

v(S-H) V

s&H

vsym.CH

v(N-H)

v, stretching; a, in-plane&ending; y, out-of-plane-bending def., deformation; sym., symmetric; asym., asymmetric; s, strong; vs, very strong; ws, very very strong; ms, medium strong; m, medium; w, weak; mw, medium weak; VW,very we& b, broad; sh, shoulder. * Far i.r. values. In parentheses: value in KBr pellet.

S. P. GuPT.4 et al.

1174 Table 2. Effects of solvents Refractive index

Solvent Ethanol Ethanol + HCI Ethanol + NaOH Methanol Methanol + HCl

1.3773

1.3362

Methanol + NaOH Water Water + HCI Water + NaOH Benzene Chloroform

1.3380

-

and pH on electronic

transitions

of 4,6,2-DMMP

and 4,6,2-DHMP

4,6,2-DMMP PH

I n-r?

7.35 0.44 12.62 8.17 0.36 13.05 7.32 0.86 12.87

3550 3740 3500 3700 3350 3570

1.5236 1.4580

3780 3610

II n--E* 2880 2990 2710 2865 2890 2770 2778 2865 2710 2860 2900

x-x*

n-0*

PH

nplI*

2160 2260 2120 2200 2265 2185 2200 2250 2190

2030 2020

5.28 0.92 12.50 7.05 0.26 13.08 5.65 0.89 12.78

2765 2770 2600 2760 2764 2600 2680 2770 2600

2080 2060 2090 2010 2000 2060

1.0

(all values in A)

4,6,2-DHMP I R-7? II n-z* 2420 2445 2420 2440

2460

2220 2200 2260 2200 2170 2100 2260 -2200

n-a* 2030 2010 2040 2050 2040 2040 2108 2100

0.50

I

,’ 0.75

I I

8 2 0.50 ,A z

8

9 D

z

0.25

/

2

0.25

1

00

2600

1

3200

i

3800

Wavelength (A)

Fig. 4. Ultraviolet absorption spectra of 4,6-dimethyl-2mercaptopyrimidine in (-) water, (---) ethanol, (--) methanol, (. .) chlorotorm and (---) benzene.

2!O(

I

I

2400

2600

3000

Wavelength (A) Fig. 5. Ultraviolet 2-mercaptopyrimidine

[16]. HARSANYI and CSAZAR[17] calculated the C,H out-of-plane bending vibration at 684 cm- I. Bands observed at 674 and 630 cm ’ in 4,6,2-DMMP and 4,6,2-DHMP, respectively, have been taken to represent this mode. BARNESet al. [ 161 have identified the ring breathing mode at 782 cm- ’ in the ix. spectra of uracil while NISHIMURA et al. [6] have calculated this mode for uracil at 740 cm- ‘. We have assigned the i.r. band at 738 cm-’ in both compounds to the ring breathing mode. HARSANYIand CSAZAR [ 171 have calculated the ring out-of-plane deformation for uracil at 119 cm I. The medium-strong bands at 130cm-’ in both 4,6,2DHMP and 4,6,2-DMMP have been assigned to the lowest lying out-of-plane ring deformation mode.

absorption spectra of 4,6-dihydroxyin (-) water, (---) ethanol and (--) methanol.

C-X vibration

The C-S stretching frequency generally appears in the region 72&570 cm-’ [18]. GREEN [19] has observed the C,H5-S stretching frequency at 684 cm- ’ in methylphenylsulphide. However GAUTHIER and LEBAS [20] assigned this vibration at 591 cm-’ in 2,4,6-trichloro-S-methylthiopyrimidine. In view of these considerations, the i.r. band at 674 cm- ’ in 4,6,2DMMP and the very strong band at 630cm-’ are taken to represent the v(C-S) vibration. This vibration is weak in the case of 4,6,2-DMMP because the -SH group is involved in tautomerism. BARNES et al. [16] have assigned the Cd=0 stretching band at 1673 cm- ’

Substituted mercaptopyrimidines

(II)

(I)

1175

(III)

(IV)

(cl Fig. 6. Tautomeric

forms

of

(a) neutral, (b) cationic and mercaptopyrimidine.

(II)

(I)

(c) anionic

forms‘ of 4,6dimethyl-2-

(IV)

(III)

OH

0 ON

(I)

(II)

(III)

(VI

(IV)

(4 Fig. 7. Tautomeric

forms

of (a) neutral, (b) cationic and (c) anionic methylmenzaptopyrimidine.

forms

of 4,6-dihydroxy-2-

1176

s. P.

&PTA

in uracil. This mode has been identified at 1645 cm- ’ in 4,6,2-DHMP. BELLAMY[21] has suggested that with aryl thioketones the C=S stretching frequency appears at around 1225-1205 cm-’ which may vary with the nature of the substituents. According to KATRITZKYand JONES[22] and LAGRANGEet al. [24] the vibration of the v(C=S) mode is expected to be intense in absorption between 1260 and 1100 cm-‘. LAUTIE et al. [24] have assigned the band at 1203 cm - ’ in 2,4-dithiouracil to this mode. However, recently SATHYANARYANA and KASMIR RAJA [lo] have calculated the vibrational frequencies for 2thiopyrimidine and have observed the band at 1210cm-’ arising due to coupled modes of C=S stretching and CH in-plane bending, and bands appearing at 748 and 477 cm- ’ to combined modes of ring out-of-plane deformation and C=S stretching. In view of these considerations, the medium Raman band observed at 1184 with an i.r. counterpart at 1188 cm- ’ is assigned to the coupled vibration of C=S stretching and CH in-plane bending in 4,6,2-DMMP. A very strong Raman band at 456 cm-’ with an i.r. counterpart at 462 cm-‘, and an i.r. frequency 725 cm-’ in 4,6,2-DMMP are taken to represent combined modes of C=S stretching and ring out-of-plane deformation. LAUTIE et al. [24] assigned the a(C=S) mode to 447 cm- ’ in 2-thiocytosine. BEETZand ASCARELLI[9] have taken the vibrational frequency 482 cm- * in 2thiocytosine to represent this mode. The very strong Raman band at 456 cm-’ with an i.r. counterpart at 462cm-’ in 4,6,2-DMMP may have some contribution from the /?(C=S) mode. For methylphenylsulphide the C6H,-S out-of-plane bending (X-sensitive a”) vibration has been proposed at 162cm.- ’ by GREEN [ 191. The strong i.r. band at 165 cm ’ in 4,6,2-DMMP and the medium-strong band at 143 cm-’ in 4,6,2DHMP have been taken to represent the C,,,-S outof-plane bending mode. ARUNAand SHUNMUGAM[l] have suggested the occurrence of a C=S out-of-plane bending vibration at 278 cm-’ in 6-amino-2thiouracil. LAUTIEet al. [24] have also assigned the i.r. band at 257 cm-’ in 2,4_dithiouracil to this mode. Thus, the very strong i.r. band at 285 cm- ’ in 4,6,2DMMP may have some contribution from the y(C=S) mode. Group vibrations

OH group. In 4,6,2-DHMP a strong i.r. band at 3820 cm-’ has been assigned to the voH fundamental. FANIRAN [25] has shown the occurrence of the OH torsional mode down to 318 cm-‘. During the present study the i.r. band at 359 cm- ’ in 4,6,2-DHMP has been assigned to the OH torsional mode. CH3 group. The CH stretching and deformation modes of the methyl group in 4,6,2-DMMP are well assigned in their respective regions in Table 1. OWEN and HESTER [26] assigned the CH, torsional mode between 177 and 240cm-‘. Recently, GOEL [27] identified this mode at 248 cm-’ in substituted toluene. In view of these considerations, the Raman

et cd.

band at 240 cm- ’ with an i.r. counterpart at 268 cm ’ and a strong i.r. band at 175 cm ’ in 4,6,2-DMMP has been assigned to the CH3 torsional mode. SH group. BELLAMY[21] has suggested that the S-H stretching vibration occurs in the range 2590-2550 cm- I. A weak band at 2550 cm- ’ in 4,6,2DMMP is taken to represent the vSH vibration, which finds support from the literature [28]. MALLICKet ul. [28] proposed the -SH torsional mode to be at 137 cm-’ in benzylmercaptan. Recently, GOEL et ~11. [29] have assigned this mode at 145 cm-’ in 2mercaptopyrimidine. In the present study the Raman bandat 152 cm-’ withan i.r.counterpart at 150 cm-’ has been well assigned to the -SH torsional mode. SCH, group. KRESZE et al. [30] have ascribed the band at 722 cm- ’ to the S-CH, stretching fundamental. GAUTHEIR and LEBAS[20, 313 also assigned this mode to around 700 cm-’ in substituted methylthiopyrimidines. In accordance with these considerations, the strong i.r. band at 710 cm-.’ in 4,6,2DHMP has been taken to represent the S-CH3 stretching vibration. According to GREEN [19] the torsional mode of the -SCH3 group occurs at around 100 cm- ‘. Thus the i.r. band at 113 cm-. ’ in 4,6,2DHMP is assigned to this mode. Hydrogen

bonding

and tuutomrric

behaciour

LAUTIE ef al. [24] reported NH stretching frequencies at around 2900cm ‘; in the i.r. spectra (KBr pellet) of 2-pyridinethione at 2870 cm _ ’ and in 2,4-dithiouracil at 2940cm-’ due to strong NH . S hydrogen bond formation. A band observed at2835cm- ’ in 4,6,2-DMMP is taken to represent the v(NH . . S) mode due to intermolecular hydrogen bonding. In substituted phenols, according to MALLICK and BANERJEE[32], a low frequency at 138 cm- ’ is assigned to the (OH 0) stretching mode due to intermolecular hydrogen bonding. JAKOBSEN[33] has assigned the v(OH 0) mode to 190 cm-’ in p-cresol. In view of these observations, the band at 121 cm-’ in 4,6,2-DHMP is taken to represent the v(OH . ‘0) mode. The presence of weak v(SH) and p(C-S) modes in 4,6,2-DMMP (Table 1) indicates that the -SH group is involved in tautomerism as the H atom from the mercapto group migrates to N, or Nj of the ring. The presence of strong v(C=S), /?(C=S) and y(C=S) vibrations further confirms that the thione form predominates over the thiol form in 4,6,2-DMMP. An examination of cc-and y-hydroxy derivatives of N-heterocyclic aromatic systems in NH, OH and C=O stretching regions both in solid and solution phases shows that they possess an amide structure [34]. The i.r. spectra of several di- and trihydroxypyrimidines show that substitution at the 4 position leads to ketonic structures whereas the substitution at the 6 position tends to retain the original enolic character. In 4,6,2-DHMP there are two hydroxyl groups attached at the 4 and 6 positions so that two vOH, two BOH and two yOH modes are expected, but we could only

Substituted mercantopyrimidines observe one in each case. A similar position was found for the v(C-0), B(C-O), y(C-O), v(C==O),fi(C=O) and y(C=O) vibrations. BARNES et al. [16] have assigned the band at 1673 cm-’ in the case of uracil to the v(C,=O) mode. Thus, the band at 1645 cm-’ in 4,6,2-DHMP corresponds to the v(C,+=O) vibration. Furthermore, only one NH out-of-plane bending mode is observed at SlOcm-‘, which is due to the NtH out-of-plane bending mode as suggested by BAIGDEKARand ZUNDEL [3]. In accordance with the above, it is therefore concluded that in 4,6,2-DHMP the hydroxyl group present at position 4 is in the ketonic form as its H atom has moved to Nt of the ring, whereas the hydroxyl group present at the 6 position has retained its original character and is present in the enolic form. Effect of solvents In pyrimidine a band is observed at 240 mu which corresponds to the Ai + Bz transition and is derived from the At, --, B2,, benzene transition on lowering the symmetry from D,, to C,,. CLARKand TINOCOO[ 143 suggested that the 210 and 200 mu bands of pyrimidine corresponds to the At, + B1, and At, +Eru transitions of benzene. In the present studies the 24OOA system of 4,6,2-DHMP corresponds to the At, + B,, transition of benzene. In both 4,6,2-DMMP and 4,6,2DHMP the 2200 and 2000 A systems are correlated to the A,, + B1, and At, + Et, transitions, respectively. HUG and TINOCOO[35] have calculated the electronic transitions for pyrimidine bases and showed the presence of out-of-plane transitions (n-s* and n-n*) and in-plane transitions (x-n* and n-u*) in pyrimidine bases. They also explained that the in-plane transitions are composed of x-x*, n--a* configurations. For lower energies (less than 210 nm) the x-x* contributions prevail so far. In pyrimidine the out-of-plane transitions occur at 295 nm (n-x*) and 255 nm (n-x*) and inplane transitions are present at 243 (Bzu) and 212 nm (B,,). In accordance with these considerations in the case of 4,6,2-DHMP the 2700 and 2400 A systems are taken to represent the out-of-plane n-x* and x-x* transitions, respectively, and the systems observed at around 2200 and 2008 A correspond to in-plane x-x* and n-v* transitions, respectively. In 4,6,2-DMMP the 3500 and 2800 A systems have been assigned to the inplane n-n* transitions and the in-plane n--n* and n-Q* transitions have been identified at around 2200 and 2OOOA, respectively. According to DYER [36], in compounds which contain nonbonding electrons containing groups such as -OH and -SH, there occurs a band below 21OOA which corresponds to the n-u* transition. Thus, the bands observed at around 2000 A in both compounds are correlated to the n-u* transition. Furthermore, BECKER[36] suggested that with an increase in solvent polarity n-u* transitions are blue shifted. It is interesting to note that during the present studies also the n-u* transition in both compounds is blue shifted as solvent changed from methanol to

1177

water due to an increase in polarity (Table 2 and Figs 4 and 5). The band of largest wavelength shows a pronounced blue shift on changing from n-hexane + ethanol + water. This feature suggests that the band is due to a transition of a nonbonding electron from the lone pair of a nitrogen atom to an orbital of the ring [37]. In the case of 4,6,2-DMMP a hypsochromic (blue) shift has been observed in both the n-n* transitions as the solvent is changed from CaHb -+ CHCl, + C2H,0H + CH,OH + Hz0 in the order of increasing polarity (Fig. 4). In 4,6,2-DHMP a similar hypsochromic shift has been observed for the n--IL*transition on going from ethanol + methanol -+ water (Fig. 5). The n-x* transitions undergo a bathochromic (red) shift with increasing solvent polarity [37]. This can be ascribed to the momentary polarization of the solvent by transition dipole of the solute [38]. From Table 2 it is evident that there is a marked bathochromic shift in the x-x* transition of both 4,6,2-DMMP and 4,6,2DHMP with increasing polarity of the solvent, but a slight deviation is observed in the case of 4,6,2-DHMP on going from ethanol to methanol. In general a conspicuous red shift has been observed in the n-n* transitions of both compounds, with increasing refractive index of the solvent, which is in accordance with the trend reported by MATAGAand KUBOTA [34]. K~STERet al. [39] have reported three absorption bands in the electronic spectra of 2-methylthiopyrimidine at 2870, 2510 and 22OOA. In 4,6,2-DHMP these three systems are observed at 2765, 2420 and 2220 A. It is obvious that the first two systems of 2methylthiopyrimidine are blue shifted in 4,6,2-DHMP which contains two more hydroxyl groups as compared to 2-methylthiopyrimidine. Hence it is confirmed that electronic transitions are blue shifted when powerful electron donor substituents are present in the aromatic nucleus. MASON [40] has suggested that 2-mercaptopurine exhibits an absorption peak at 330mu or longer wavelengths, a region in which the C=S group shows an absorption maximum, and hence 2-mercaptopurine exists in the thione form. During the present study 4,6,2-DMMP gave strong bands at 3350, 3500 and 3550 in water, methanol and ethanol, respectively (Fig. 6) which are absent in the case of 4,6,2-DHMP. The presence of this system in 4,6,2-DMMP shows that in pure solvents it is present in thione form. Eflect of pH variation on electronic transitions The tautomeric forms of neutral cationic and anionic forms of 4,6,2-DMMP and 4,6,2-DHMP are shown in Figs 6 and 7, respectively. As discussed previously 4,6,2-DMMP will exist in the thione form in neutral solutions [Fig. 6(a II) or (a III)]. It is obvious from Fig. 7(a) that in the neutral form of 4,6,2-DHMP one hydroxyl group is present in the enolic form while the other is in the ketonic form. (This is supported by the study of vibrational spectra also.) As the number of

1178

S. P. GUPTA et al.

tautomers of cations protonated at N, and N3 is the same [(Fig. 6(b) and 7(b)], protonation may take place at both N, and N, in both compounds. In the case of 4,6,2-DHMP, on addition of alkali deprotonation may take place at N, and N, of the ring and the exocyclic oxygen atoms and a resonance hybrid [Fig. 7(C V)] is proposed for its anion. Similarly, a resonance hybrid is possible for the anion of 4,6,2-DMMP shown in Fig. 3.6(C IV). HUG and TINOCOO [35] have classified the nucleic acid bases into two types; (a) bases with cytosine (C) type spectra and (b) bases with uracil (U) type spectra. In the C-type spectra, besides cytosine, the anion and enol form of uracil, which lack a proton at NJ, are included. On examining the tautomers of neutral and anionic forms of 4,6,2-DMMP and 4,6,2-DHMP (Figs 6 and 7), it can be noticed that out of the two nitrogen atoms one carries a proton while the other is devoid of a proton. Hence the neutral and anionic forms of both compounds can be included in the C category. HUG and TINOCOO [35] have reported a distinct blue shift in the z--11* and n--R* transitions of uracil with increase of pH (uracil enol -+ uracil anion) which is a characteristic of bases with C-type spectra. From Table 2 it is obvious that in general with the increase of pH (neutral -+ anion) a hypsochromic shift is observed in n-z* and R-Z* transitions of both 4,6,2DMMP and 4,6,2-DHMP in all solvents (Table 2). ALBERT and BARLIN [41] have also reported a marked hypsochromic shift with increase in pH in the absorption spectra of 2-mercaptopyrimidine and 2methylthiopyrimidine. BORODAVKIN et al. [42] have also observed a blue shift in the absorption spectra of an aqeuous solution of cytosine with increase in pH. These results are in accordance with the trend observed during the present investigation. HUG and TINOCOO [35] reported the appearance of a shoulder around 225 nm when the pH of a solution of uridine is raised from neutral to 12. In the case of 4,6,2-DHMP shoulders are present in neutral alcoholic and aqueous solutions which become quite strong on addition of alkali. KISTER et al. [39] reported a hypsochromic shift in the electronic transitions of pyrimidine 2-thione on changing the solvent from pure ethanol to NaOH. During the present study also a band observed at 2880 8, in ethanol is shifted to 2710 A in pure NaOH solution in the case of 4,6,2-DMMP. Similarly in the case of 4,6,2-DHMP a band observed at 2765A in ethanol is blue shifted to 26OOA in pure NaOH solution. BECKER [37] has suggested that the best identification of the nonbonding electrons in a transition is a large blue shift of the suspected band upon excitation in a strong acid solution which is capable of binding the electron by protonation. It is interesting to note that the n-n* transition in all solvents in case of 4,6,2DMMP and in ethanol and methanol in 4,6,2-DHMP are blue shifted on addition of acid (neutral -+ cation). KISTER et al. [39] observed a bathochromic shift in

the electronic transitions of 2-methylthiopyrimidine on going from ethanol to HCl. Similar results have been observed in the case of 4,6,2-DHMP (Table 2). In 4,6,2-DMMP there is a conspicuous red shift in all solvents with a decrease in pH, which is supported by literature values [43]. MASON [44] has stated that the cation of 2mercapto-4-methylpyrimidine absorbs at a longer wavelength than any other charged species of this compound. In the case of 4,6,2-DMMP also in all solvents the value of absorption maxima is greatest for the cation in all systems observed (Table 2).

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