Canadian Journal of Chemistry

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1 Canadian Journal of Chemistry Published by THE NATIONAL RESEARCH COUNCIL OF CANADA VOLUME 47 JULY 15, 1969 NUMBER 14 Raman and far infrared spectra of strinitrobenzene and strinitrobenzened3 H. F. SHURVELL AND A. R. NORRIS Department of Chemistry, Queen's University, Kingston, Ontario AND D. E. IRISH Department of Chemistry, University of Waterloo, Waterloo, Ontario Received January 30, 1969 Raman spectra have been recorded of strinitrobenzene and strinitrobenzened3 both in solution and in the solid phase. Eighteen of the 22 Raman active fundamentals in D3, have been observed for both molecules. Two fundamentals have been observed in the far infrared spectrum of each molecule. Nous avons enregistrt les spectres Raman du strinitrobenzkne et strinitrobenzkned3 en solution et aussi l'8tat solide. Dixhuit des 22 vibrations fondamentales ont ttt observtes pour les deux moltcules. Deux vibrations fondamentales ont ttt observtes dans les spectres infrarouge lointain pour chaque moltcule. Canadian Journal of Chemistry (1969) Introduction The infrared (i.r.) spectra of strinitrobenzene and strinitrobenzened3 have been discussed previously (1). Although assignments were compatible with a molecule of D,, symmetry, Raman spectra were desired to substantiate that conclusion and give a more complete vibrational assignment. The far i.r. spectrum was also needed, since two of the i.r. active fundamentals were not observed in the previous work (1). Experimental Materials were prepared in the same way as for the i.r. work (1). Raman spectra were recorded from chloroform and acetone solutions and from the polycrystalline solid. Raman spectra of solid strinitrobenzene and strinitrobenzened3 are shown in Figs. 1 and 2. Spectra were recorded on three different spectrometers; one a Cary model 81 instrument with mercury arc excitation, and two different experimental assemblies employing argon ion laser excitation and Spex 1400 double monochromators with photoelectric detection systems. One of these experimental assemblies is in the laboratory of Dr. H. J. Bemstein at the National Research Council of Canada, Ottawa; the other is in the Department of Physics at the University of Waterloo. In recording spectra of solid strinitrobenzene and strinitrobenzened3, both the 4880 and the 5145 A argon ion laser lines FIG. 1. Raman spectra of solid strinitrobenzene and strinitrobenzened3 in the region cm', excited by the 5145 A line of an argon ion laser.

2 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969 TABLE I Raman and infrared frequencies (in cm') for strinitrobenzene and strinitrobenzened3 strinitrobenzene strinitrobenzened3 Identification Raman* Infraredt Raman* Infraredt Assignment 3100 w$ 3100 w$ 1623 m, dp 1547 vs, dp 1520 vw, sh 1440 w 1365 vvs, p 1345 s, dp 1296 vw 1184 s, p 2310 vw$ 2310 vwf 1608 m, dp 1548 s, dp 1420 vw 1384 w, dp 1363 vs, p 1348 m 1298 vw 1185 s, p %9:,: p > 960 s, P 930 m, dp 830 s, p 754 vw 704 w 727 vw 520 vw 368 w, dp 334 m, p 204 s 192 sh 160 w 132 sh Fermi resonance 926 vch (vcd) vch (vcd) vcc NO, asym. stretching C vcc NO, sym. stretching NO, stretching Ring mode v CN NO2 sym. def. C YCH (YCD) NO, inplane def. accc NO2 rock Ring def. K C NO, outofplane def. BCN NO, outofplane de f. *v = very, s = strong. m = medium, w = weak, p = polarized, dp = depolarized, c = combination or overtone frequency, sh = shoulder. tlnfrared frequencies taken from ref. (I), except the lowest frequency for each molecule. $Frequency used twice. FIG. 2. Raman spectra of solid strinitrobenzene and strinitrobenzened3 in the region 1W800 cm', excited by the 4880 A line of an argon ion laser. were used to excite the R m spectra. Most of the frequencies listed in Table I are the averages of several runs on solution and solid phase samples. The frequencies are believed to be correct to within + 2 cmi in most cases. However, for the CH and CD stretching frequencies and for frequencies below 400 cm', the errors are somewhat larger. The last 4 frequencies for each molecule in Table I were taken from spectra of the solids excited by the 4880 A argon ion line. The polarization data were obtained from acetone solutions, using the Cary 81 spectrometer. Far i.r. spectra of Nujol mulls of strinitrobenzene and strinitrobenzened3 supported between polyethylene discs were recorded on a PerkinElmer model 301 spectrophotometer in the laboratory of Dr. E. Whalley at the National Research Council of Canada, Ottawa, and are shown in Fig. 3. Results and Discussion Suectra obtained from chloroform and acetone solutions were essentially the same as the solid state spectra shown in Figs. 1 and 2. Some relative intensity differences were noted, but no evidence of splittings attributable to crystal effects was observed. The normal vibrations of the strinitrobenzene molecule span the irreducible representations of the point group D,, as follows Of these, only those vibrations belonging to the

3 SHURVELL ET AL.: RPLMAN AND I.R. SPECTRA OF strinitrobenzenes 2517 Z a W m Q I strinitrobenzene I wavenumber CM' FIG. 3. Far infrared spectra of Nujol mulls of s trinitrobenzene and strinitrobenzened, suspended between polyethylene plates. species E' and A," are i.r. active, while those belonging to the species A,', E', and E" are Raman active. Thus the r am an spectrum should contain 22 fundamentals of which 6 of species A,' will be polarized, 11 will coincide with i.r. active fundamentals of species E', and the remaining 5 will be the E" modes. Discussion of the assignments will be facilitated by using descriptions of normal vibrations based on symmetry coordinates. In certain cases it is probable that the symmetry coordinate will provide a good description of the normal mode; for example the symmetrical CH stretching mode of species A,'. In other cases, particularly for the modes of low frequency, considerable mixing may occur and the description will be at best approximate. By using the coordinates of the various groups of atoms as bases for reducible representations of the point group D,, (2), we may predict qualitatively the numbers and types of group vibrations to be expected in the Raman spectrum. Following this procedure, the Raman active vibrations mav be described as follows. There are 6 vibrations involving the carbon atoms of the benzene ring, 4 CH vibrations, 4 CN vibrations, and 8 NO, group vibrations. This qualitative approach, coupled with previous evidence from various nitrobenzenes (37) and strichlorobenzene (a), has lead to the assignments given in Table I. The assignments to the various symmetry species are based on polarization studies and on coincidences with i.r. absorption frequencies. The notation used to describe the various vibrations of the substituted benzene ring is due to Whiffen (9). A, ' Vibrations There are 6 Raman active, totally symmetric fundamentals of the strinitrobenzene molecule, which should give rise to polarized lines in the spectra. One of the modes of species A,' involves the hydrogen atoms. This can only be a symmetric stretching vibration and should be found near cm ' in strinitrobenzene and 2300 cm ' in the deuterated molecule. In the spectrum of the normal molecule, we have observed a weak line at about 3100 cm', and a very weak line near 2310 cm' for the deuterated sample. These frequencies were only observed for solid samples, and no polarization data are available. There is therefore no way of distinguishing the symmetry species and we have assigned these frequencies to both A,' and E' species of the CH and CD stretching modes. The NO, symmetric stretchingmode is assigned to a very strong polarized line at 1365 cm', which is virtually unshifted on deuteration. This frequency is well within the usual range observed for symmetric NO, stretching vibrations in various nitrobenzenes (37). The frequency is 20 cm' higher than that of the doubly degenerate NO, stretching mode of species E' in the same molecule. A strong, polarized line at 1184 cmi is assigned to the symmetric CN stretching mode. The frequency of this mode is also unaffected by deuteration of strinitrobenzene. The frequencies of the corresponding mode in strichlorobenzene, (normal and deuterated) are 1149 and 1146 cm ', respectively (8). A strong, polarized line at 1002 cmi in the spectrum of strinitrobenzene is assigned to the ring "breathing" mode. This vibration usually gives rise to frequencies near 1000 cm' in monosubstituted benzenes (9), and Scherer et al. have reported a frequency of 995 cm' in the Raman spectrum of strichlorobenzene (8). In the spectrum of deuterated strinitrobenzene, a doublet is observed at 976,960 cm '. Both components are polarized and must therefore be assigned to vibrations of A,' symmetry. Since only one fundamental is expected in this region, these two lines probably arise as a result of Fermi

4 2518 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969 resonance between the ring breathing fundamental of species A,', and an overtone or combination line belonging to the same irreducible representation. None of the i.r. or Raman active fundamentals would have an overtone in the region of 970 cm ', and no combination of these fundamentals belonging to species A,' lies in this region. The second component of the Fermi doublet must be either an overtone of an inactive fundamental or involve an inactive fundamental. The observed frequency of the ring breathing mode in strichlorobenzened3 was 956 cm' (8). A polarized line at 825 cm' is assigned to the NO, symmetric deformation mode. In the i.r. spectrum of nitrobenzene, Stephenson et al. (5) have assigned a peak at 850 cml to this vibration. We have previously assigned i.r. bands near 725 cm' in the spectra of normal and deuterated strinitrobenzene to a symmetric NO, deformation mode of species E' (1). At that time we stated that this provided support for the assignment made by Green et al. (4) of a peak at 677 cm' in the i.r. spectrum of strinitrobenzene to the NO, symmetric deformation mode. In view of the present Raman spectra, it would appear that the assignment of Stephenson et al. (5) is the better one, and this conclusion is in agreement with the work of Pinchas et al. (6) on ''0 labelled nitrobenzene. The last polarized line at 355 cm l is attributed to a symmetric ring deformation. In strichlorobenzene the corresponding mode gives rise to a Raman line at 379 cml. This vibration involves motion of the carbon atoms to which the NO, groups are attached (or C1 atoms in the case of strichlorobenzene). Scherer (10) has given diagrams of the planar vibrations of strichlorobenzene and strichlorobenzened,, and it is likely that the corresponding A,' modes in strinitrobenzene have similar forms, with the chlorine atoms replaced by nitro groups. E' Vibrations The 11 E' type vibrations are active in both the i.r. and Raman spectra. However, definite coincidences have been observed in only 7 cases. The agreement between i.r. and Raman frequencies is good in these cases. The doubly degenerate CH and CD stretching modes should give rise to Raman lines near cm for strinitrobenzene and 2300 cm' for the deuterated molecule. Only one line has been observed in each region. These lines have been assigned to the CH and CD stretching modes of species A,', but it is possible that the E' modes could have similar frequencies. We have therefore attributed the observed lines to both A,' and E' type fundamentals. One of the far i.r. frequencies should be due to an E' type fundamental and a coincident Raman line is expected. We have assigned the line at 160 cm ' in the spectrum of strinitrobenzene to the lowest frequency E' fundamental, an inplane CN deformation mode (PCN). A line at the same frequency was observed for the deuterated molecule. The corresponding frequencies in the far i.r. region were 156 and 155 cm' for the normal and deuterated molecules respectively. The differences between i.r. and Raman frequencies are 5 cm ', which is probably within experimental error. A second far i.r. band was observed at 130 cm ' for strinitrobenzene, and at 129 cm ' for the deuterated molecule. These appear to coincide with Raman features observed as shoulders on the exciting line at 133 and 132 cm ' for the normal and deuterated molecules respectively, and offer an alternative assignment for the lowest frequency E' mode. However, we have assigned the i.r. bands to the A," mode of lowest frequency, and the Raman lines to an E" mode in each molecule. The i.r. frequencies assigned to E' type modes but not observed in the present Raman spectra were at 1075 and 620 cml for strinitrobenzene and 806 and 518 cm' for the deuterated molecule (1). These were assigned respectively to the inplane CH deformation (PCH) and ring deformation (accc) vibrations. A very weak Raman line observed at 520 cm ' in the spectrum of solid strinitrobenzened3, probably corresponds to the 518 cm' i.r. absorption. The assignments of the other E' type vibrations are the same as for the previous i.r. work (1). E" Vibrations There should be 5 depolarized lines in the Raman spectra of both strinitrobenzene and strinitrobenzened,, having no counterparts in the i.r. spectra. These are the outofplane E" vibrations. Two lines and two shoulders have been located in the spectrum of each molecule. A weak line at 754 cm' that is displaced to 704 cm ' on deuteration is attributed to the outofplane CH bending mode (ych). The corresponding pair of lines in the Raman spectra of normal and deuterated strichlorobenzene were located

5 SHURVELL ET AL.: RAMAN AND I.R. SPECTRA OF STRMITROBENZENES 2519 at 869 and 712 cm'. The smaller shift in the trinitro compounds is difficult to explain. A fairly strong line at 212 cm' in the spectrum of strinitrobenzene shifts to 204 cm' in the spectrum of the deuterated molecule. These frequencies are assigned to an outofplane ring deformation (4CC). Coupling with other E" type vibrations could account for the low frequency of this mode in strinitrobenzene as compared with other substituted benzenes (1 I). The other normal modes of species E" are two outofplane NO, deformations and an outofplane CN bending mode (ycn). We have arbitrarily assigned the two shoulders to NO, deformations. The missing line may either be too weak to be observed in the Raman spectrum, or is obscured by another fundamental. Far Infrared Spectra The far i.r. spectra of strinitrobenzene and strinitrobenzened3 are shown in Fig. 3. The observed frequencies were 156 and 130 cm ' for the normal molecule and 155 and 129 cm' for the deuterated compound. The absorption at higher frequency coincides with a Raman line for both molecules, and this is assigned to the inplane CN deformation mode of species E'. The lower frequency absorption is then assigned to the outofplane CN deformation of species A,". Overtones and Combinations Two very weak lines in the Raman spectrum of strinitrobenzene at 1520 and 1296 cm ' have been assigned to overtone or combination frequencies. In the spectrum of strinitrobenzened,, 4 very weak lines at 1420, 1298, 989, and 754 cm', in addition to one component of the Fermi doublet at 976, 960 cm' have been assigned to combinations or overtones. Conclusions We have observed 18 of the 22 Raman active fundamentals in D,, of strinitrobenzene and strinitrobenzened3 in the present work and have made qualitative assignments to these frequencies in terms of symmetry coordinates based on group vibrations. Three of the remaining Raman active fundamentals belonging to the E' species have been observed in previous i.r. spectra. The two fundamentals observed in the far i.r. spectra complete the total of 15 i.r. active modes. There remain, however, 6 fundamentals inactive in both spectra, and the frequencies of these modes can onlv be estimated from normal coordinate calculations. We hope to make these calculations in the near future. Acknowledgments We are very grateful to Dr. H. J. Bernstein and Dr. E. Whalley of the National Research Council of Canada, Ottawa, and Dr. A. Anderson of the Department of Physics, University of Waterloo, for placing the facilities of their laboratories at our disposal. We thank Mr. E. A. Symons for help with the preparation and analysis of strinitrobenzened3, and Mrs. J. Weerheim for technical assistance. The financial support of the National Research Council of Canada is gratefully acknowledged. 1. H. F. SHURVELL, J. A. FANIRAN, E. A. SYMONS, and E. BUNCEL. Can. J. Chem. 45, 117 (1967). 2. F. A. COTTON. Chemical applications of group theory. John Wiley and Sons, Inc., New York R. R. RANDLE and D. H. WHIFFEN. J. Chem. Soc (1952). 4. J. H. S. GREEN, W. KYNASTON, and A. S. LINDSEY. Spectrochim. Acta, 17, 486 (1961). 5. C. V. STEPHENSON, W. C. COBURN, JR., and W. S. WILCOX. Spectrochim. Acta, 17, 933 (1961). 6. S. PINCHAS, D. SAMUEL, and B. L. SILVER. Spectrochim. Acta, 20, 179 (1964). 7. K. C. MEDHI. Spectrochim. Acta, 20, 675 (1964). 8. J. R. SCHERER, J. C. EVANS, W. W. MUELDER, and J. OVEREND. Spectrochim. Acta, 18, 57 (1962). 9. D. H. WHIFFEN. J. Chem. Soc (1956). 10. J. R. SCHERER. Planar vibrations of chlorinated benzenes. The Dow Chemical Company, Midland, Michigan F. F. BENTLEY, L. D. SMITHSON, and A. L. ROZEK. Infrared spectra and characteristic frequencies. Interscience Publishers, Inc., New York