CONTRIBUTION TO THE INFRARED SPECTRA OF FIVE-MEMBERED N- AND N,S-HETEROCYCLIC COMPOUNDS1

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1 CONTRIBUTION TO THE INFRARED SPECTRA OF FIVE-MEMBERED N- AND N,S-HETEROCYCLIC COMPOUNDS1 C. N. R. Riio2 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-12, India AND R. VENKATARAGHAVAN~ Department of Organic Chemistry, Indian Institute of Science, Bangalore-13, India Received August 6, 1963 ABSTRACT Infrared spectra of 5-membered N- and K,S-heterocyclic derivatives such as triazoles, tetrazoles, thiazoles, thiadiazoles, thiatriazoles, and dithiazoles have been examined and empirical assignments have been made. The spectra of the two dithiazolidine derivatives, isoperthiocyanic acid and oxidation product of dithiobiuret, have been studied in detail. Assignments for benzthiazole derivatives are reported. The CH out-of-plane deformation bands of benzene rings fused to five-membered heterocyclic rings have been found to be in the region cm-l, characteristic of four adjacent benzenoid hydrogen atoms. INTRODUCTION Since the calculation of the normal vibrations of heterocyclic derivatives is rather tedious, the interpretation of the infrared spectra depends mainly on empirical correlations. Katritzky (1) has recently reviewed the infrared spectra of heteroarolnatic compounds and classified the vibrations in terms of the ring-stretching, ring-breathing, CH in-plane deformation, and CH out-of-plane deformation modes. We have now examined the infrared spectra of several five-membered nitrogen- and nitrogen,sulphur-heterocyclic compounds in- the light of Katritzky's review. EXPERIMENTAL Preparation of Compounds The 1,2,3-triazole derivatives were prepared by the methods-described by Lieber et al. (2,3). The tetrazole derivatives were commercially available. All the thiazole derivatives were commercially available. The 1,2,3- and 1,3,4-thiadiazole deiivatiyes were prepared by following the procedures described in the literature (4, 5). The thiatriazole derivatives were synthesized following the procedure described by Lieber et al. (6-8). Isoperthiocyanic acid was prepared by the decomposition of thiocyanic acid (9). The hydrochloride of 3,5-diimino-l,2,4-dithiazoline was prepared by the oxidation of dithiobiuret (10). Many of the benzthiazole derivatives were commercially available and some were synthesized in the laboratory following standard procedures. Spectra Infrared spectra were recorded employing Perkin-Elmer models 221 and 13'iB spectrometers fitted with NaCl prisms. The samples were in the form of solutions in CHC13 or solids dispersed in Nujol mulls or KBr pellets. RESULTS AND DISCUSSION Five-membered N-Heterocyclic Derivatives The assignments for the first two five-membered nitrogen heterocyclic compounds, 'Material taken from the Ph.D. thesis of R. Venkataraghavan submitted to the Indian Institute of Science under the guidance of C. N. R. Rao. 2To whom all the correspondence should be addressed. Present address: Department of Chzmistry, Indian Institute of Technology, Kanpur, India. 3University Grants Commission Junior Research Fellow in the Department of Organic Chenzistry. Presently in the Pure Chenzistry Division, National Research Council of Canada, Ottawa, Canada. Canadian Journal of Chemistry. Volume 42 (1964)

2 44 CANADIAN JOURNAL OF CHEMISTRY. VOL. 42, 1964 pyrrole (I) and imidazole (11), have been made by Katritzky (1). Making use of the infrared data of a few 1,2,3-triazole derivatives (111) reported by Hartzel and Benson (11), Katritzky (1) has assigned the ring-stretching frequencies around 1520, 1450, and 1410 cm-1. We have examined the infrared spectra of several 1,2,3-vicinal-triazole derivatives, particularly in the region cm-l and find that the following bands appear consistently in all of them: , , , , and cm-l. The bands in the regions , , and cm-i may be assigned to the CH in-plane deformation, ring-breathing, and CH out-of-plane deformation vibrations respectively. It should be mentioned, however, that assignments are difficult to make in many derivatives since they possess phenyl groups. We have examined the spectra of a few tetrazole derivatives (IV) and the following assignments may be made. The bands in the region cm-i may be due to the ring-stretching vibrations. The CH inplane deformations occur in the region cm-l. The bands in the region and cm-i may be assigned to the ring-breathing and CH out-of-plane deformation vibrations respectively. All the important characteristic vibrations of the five-membered N-heterocyclic derivatives have been summarized in Table I. It may be seen that the regions of absorption are very similar in all of them and infrared spectroscopr will not be of much use in qualitative analysis particularly in differentiating these compounds. The data only indicate what one should expect in the spectra of such five-membered X-heterocyclic derivatives. Further, it is not possible to assign C=N and N=N frequencies since these systems are aromatic in nature. Examination of the spectra of the five-membered N-heterocyclic compounds in the N-H stretching region shows that pyrrole derivatives (12) exhibit the highest stretching frequency (-3488 cm-l). The reported spectra of imidazoles (13) and triazoles (11) are all in the condensed phase and the N-H stretching absorption in both these kinds of derivatives appears around 3100 cm-i. We have examined the spectra of tetrazoles in very dilute solution and find N-H stretching absorption around 3145 cm-l. It is interesting to see that the N-H stretching frequency in tetrazoles is considerably lower than in pyrroles. This is what one would expect considering that the tetrazole ring is much more electronegative than pyrrole. Tetrazole is indeed an acid (pk, = 4.89).

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4 46 CANADIAN JOURNAL OF CHEMISTRY. VOL Five-membered N,S-Heterocyclic Derizvztives The simplest member of this group of derivatives is thiazole (V) and the characteristic frequencies of thiazole derivatives have been assigned by Katritzky (1) and Taurins, Fenyes, and Jones (14). We have examined the spectra of>-substituted amino-thiazoles, thiazoline derivatives (VI), and thiazolidine derivatives (VII) and find that the assign- ments are very much similar to those in simple thiazole derivatives. All the assignments are summarized in Table 11. We have examined the spectra of derivatives of 1,2,3-, 1,2,4-, and 1,3,4-thiadiazoles (VIII, IX, and X respectively) and have made the relevant VIII IX X assignments (Table 11). The regions of absorption are similar in all these derivatives. Recently, the existence of 1,2,3,4-thiatriazole ring system (XI) has been established by Lieber et ul. (8, 15). We have examined the spectra of 5-(substituted)-amino-1,2,3,4- thiatriazoles (XI I), 5-(substituted)mercapto-l,2,3,4-thiatriazoles (XIII), 4-substituted- 1,2,3,4-thiatriazoline-5-thiones (XIV), salts of 1,2,3,4-thiatriazoline-5-thiones (XV), and 5-(substituted)phenyl-1,2,3,4-thiatriazoles (XVI) and find that all of then1 show similar N-N N-N R-N-N I1 fi I1 4 I I1 HRN-C RS-C S=C x \s/ 's/ \S/ XI1 XI11 XIV XV XVI

5 a Important characteristic vibratioils of some 5-inembered N,S-heterocyclic derivatives Band positions, cm-i Iiing-breathing and CH in-plane CH out-of-plane Compound n* Ring-skeletal deformations deformations Thiazole (1, 14) Methylthiazoles (14) 5-Substituted mercaptothiatriazoles Salts of thiatriazoline-5-8 thiones 5-Substituted phenylthiatriazoles *Number of compounds studied.?data were taken from the reference indicated. TMay be due to CII in-plane rleformation. 5"-N-C=S bands" (29) are not listed. llthe bands at 1742 and 1656 cm-1 are assigned to the free and hydrogen bonded carbonyl groups respectively..iidata are not available in this region.

6 48 CANADIAN JOURNAL OF CHEMISTRY. VOL characteristic vibrations of the heterocycle. The relevant assignments are shown in Table 11. The frequencies characteristic of the 1,2,3,4-thiatriazole ring system have been useful in distinguishing this structure from other alternatives proposed earlier (16 (a, b)). I~o~erthiocyanic acid, C2H2NzS3, which is prepared by the decomposition of thiocyanic acid has been assigned various structures (XVII, XVIII, XIX) in the literature (17). XVI I XVI I I XIX Recently, based on chemical and X-ray evidence (18, 19), the 1,2,4-dithiazoline structure (XIX) has been established. We have studied the infrared spectra of isoperthiocyanic acid and its silver and barium salts and the relevant assignments are given below: 3175 VN-H (not present in salts), 1631 VC=N, N-C=S I, 1372 and 1325 ring-stretching, I 1307-N-C=S 11, 1144, 1082, and 1036 ring-skeletal, 1005-NC=S 111, 630 ring- I breathing or deformation, 600 vcps, and 418 vsps. The results further confirm the 5-imino- 1,2,4-dithiazolidine-3-thione structure (XIX) for isoperthiocyanic acid. Another 1,2,4- dithiazoline derivative, whose structure has been established as XX (20), is produced by the peroxide oxidation of dithiobiuret. The tentative infrared assignments for this coinpound are shown below: 3333 and 3205 V ~ H ~ 2933, (triplet) v,-,,+, 1675 and 1639 vo and BNR2, 1515, 1464, and 1372 ring-stretching, 1311 vc+, 1046 ring-skeletal, 630 ring-breathing or deformation, 600 VC-8, and 418 vs-s. The characteristic vibrations of the five-membered N,S-heterocyclic derivatives summarized in Table I1 are all found to be similar. Just as in the case of the five-membered N-heterocyclic derivatives, infrared data may not be of great value in qualitative analysis and since most of these derivatives are aromatic in nature it is difficult to assign characteristic ring N=N or C=N frequencies. Benzene Ring Fused to a Five-membered Heterocyclic Ring OISullivan (21) has studied the infrared spectra of several heterocyclic molecules possessing a benzene ring fused to a five-membered ring and has assigned the characteristic bands of such compounds. We have examined the infrared spectra of several benzthiazole derivatives (XXI) and the assignments are shown below: , , , and cm-l, ring-stretching vibrations; cm-l, a number (-10) I

7 RAO AND \'ENKATARAGHA\'-4N: HETEROCYCLIC COMPOUNDS 49 of bands due to the CH in-plane deformation, ring-breathing, and ring-skeletal vibrations. It is not possible to assign the C=N frequency in these conjugated systems. O'Sullivan (22) has pointed out that the out-of-plane CH deformations of benzene rings fused to five-membered rings are found in the region cm-i characteristic of o-disubstituted benzenes (four adjacent hydrogen atoms). We have found that all the 2-substituted benzthiazole derivatives show a band around 750 cm-i due to CH out-ofplane deformation. Exarrli~lation of the data in the literature shows that fluorene (XXII) (23), carbazole (XXIII) (23), 3-substituted indoles (XXIV) (24, 25), thionaphthene (XXV) (26), and benzoxazoles (XXVI) (27) show a band in the region cm-l XXII XXIII XXIV XXV XXVI characteristic of four adjacent benzenoid hydrogen atoms. REFERENCES 1. A. R. KATRITZKY. Quart. Revs. 13, 353 (1959). 2. E. LIEBER, T. S. CHAO, and C. N. R. RAO. J. Org. Chem. 22, 654 (1957). 3. E. LIEBER, T1 S. CHAO, and C. N. R. RAO. Can. J. Chem. 37, 118 (1959). 4. E. LIEBER, N. CALVA~ICO, and C. N. R. RAO. J. Org. Chem. 28, 257 (1963). 5. K. S. SURESH and C. N. R. RAO. J. Ind. Chem. Soc. 37, 581 (1960). 6. E. LIEBEK, C. N. PILLAI, and R. D. HITES. Can. J. Chem. 35, 832 (1957). 7. E. LIEBER and J. RAMACHANDRAN. Can. J. Chem. 37, ). 8. E. LIEBER, E. OFTEDAHL, and C. N. R. RAO. J. Org. Chem. 28, 194 (1963). 9. ~YOHLER. Ann. Physik. 69, 273 (1821). 10. P. IT. PREISLER and PI. M. BATEAIAN. J. Am. Chem. Soc. 69, 2632 (1947). 11. L. IT. HARTZEL and F. R. BEKSON. J. Am. Chem. Soc. 76, 667 (1954). 12. U. EISNER and R. L. ERSKINE. J. Chem. Sac. 971 (1958). 13. D. GARFISKEL and J. T. EDSALL. J. Am. Chem. Soc. 80, 3807 (1958). 14. A. TAURISS, J. G. E. FENYES, and R. N. JONES. Can. J. Chern. 35, 423 (1957). 15. E. LIEBER, C. N. PILLAI, J. RAMACHANDRAN, and R. D. HITES. J. Org. Chem. 22, 1750 (1957). 16. (a) G. B. L. SMITH, F. WILCOXON, and A. W. BROWNE. J. Am. Chem. Soc. 45, 2604 (1923). (b) E. OLIVERI-MANDALA. Gazz. Chim. Ital. 52, 11, 139 (1922). 17. L. L. BAMBAS. In The chemistry of heterocyclic compounds: Five-membered heterocyclic compounds with nitrogen and sulfur or nitrogen, sulfur and oxygen (except thrazole). Interscience, New York A. SRINIVASAN, T. R. KASTURI, and C. N. R. RAO. Unpublished results. 19. A. HORDVIK. Acta Chem. Scand. 15, 1186 (1961) Foss and 0. TJOMSLAND. Acta Chem. Scand. 12, 1799 (1958). 21. D. G. O'SULLIVAN. J. Chem. Soc (1960). 22. D. G. O'SULLIVAN. Spectrochim. Acta, 16, 762 (1960). 23. C. G. CANNON and G. B. B. M. SUTHERLAND. Spectrochim. Acta, 4, 373 (1951). 24. J. B. BROWK, H. B. HENBEST, and E. R. H. JOPJES. J. Chem. Soc (1952) F. MILLICH and E. E. BECKER. J. Org. Chem. 23, 1096 (1958). 26. P. LAIIBERT and J. LECOMTE. Compt. rend. 208, 1148 (1939). 27. H. M. RANDALL, R. G. FOWLER, N. FUSON, and J. R. DANGL. Infrared determination of organic str~lctl~req Vnn - Nostrand.. Co. Inc.. New York G. D. THORN. Can. J. Chem. 38, 1489 (1960). 29. C. K. R. RAO and R. VENKATARAGHAVAN. Spectrochim. Acta, 18, 541 (1962).