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An FT-IR study of peculiarities in the polarized vibrational spectra of some aromatic nitro compounds
Journal of Molecular Structure, 245 (1991) 195-202 Elsevier Science Publishers B.V., Amsterdam
195
AN FT-IR STUDY OF PECULIARITIES IN THE POLARIZED VIBRATIONAL SPECTRA OF SOME AROMATIC NITRO COMPOUNDS
M. BELHAKEM and B. JORDANOV
Bulgarian Academy of Sciences, Institute of Organic Chemistry, BG-1113 Sofia (Bulgaria) (Received 21 August 1990 1
ABSTRACT FT-IR polarization spectra of oriented nematic solutions of nitrobenzene showed an anomaly consisting in the equal polarization of both symmetric and antisymmetric stretching bands of the nitro group. In order to clarify the observed phenomenon the polarization spectra of the nematic solutions of a series of p-substituted aromatic nitro compounds was studied. The contribution of the ring x-electrons to the vibrational transition moment of the antisymmetric -NO, stretching vibration seems to explain this anomaly.
INTRODUCTION
In some recent studies [l-3] we reported the use of the nematic liquid crystal ZLI-1538 (Merck’s notation for truns-4-tert butyl-4’ -cyanobicyclohexyl), as an anisotropic solvent for IR polarization studies of large organic compounds. Its main advantages consist in its reduced IR spectrum, comparable to that of nujol, and in the good solubility of many organic compounds in it. The solutions obtained possess a nematic temperature range in which a good orientation can be achieved, thus enabling the performance of IR polarization measurements. Nitrobenzene (NB ) was included as one of the ordinary examples for application of the differential IR linear dichroic (DIRLD) spectra for the study of the symmetry of the molecular vibrations, since we took into account that the IR spectra of this compound are fairly well assigned. A trivial discrimination between the longitudinal (along the molecular long axis) and transversal vibrations of NB was expected by the appearance of positive and negative features in the DIRLD spectrum. This expectation was certainly confirmed for all but one IR band, namely that of the antisymmetric stretching vibration of the nitro group. The symmetric and antisymmetric NO2 stretching bands turned out to exhibit equal longitudinal polarization. After excluding all possibilities for errors 0022-2860/91/$03.50
0 1991-
Elsevier Science Publishers B.V.
196
and artifacts, we realized that this phenomenon deserves special attention and therefore carried out the present study. Some preliminary results have already been reported [ 1,3]. EXPERIMENTAL
The nematic liquid crystal ZLI-1538 was purchased from Merck. Additional model nitro-compounds were used for the elucidation of the observed anomaly in the DIRLD spectrum of NB. The chemicals used were as follows: nitrobenzene (NB) and nitroethane (NE) (Fluka); p-nitrotoluene (p-NT) andp-nitroaniline (p-NA) (POCG, Poland); p-nitrophenol (p-NPh) (Jenapharm, FRG); m-dinitrobenzene (m-DNB), p-nitrobenzonitrile (p-NBN), and 4-dimethylaminonitrostilbene (DMANS) were synthesized for various purposes and kindly given to us. All spectra were measured with a Bruker IFS-113~ FT-IR spectrometer, using an automatically rotating grid polarizer (Al/KRS-5). A KBr cell of thickness 0.025 mm was assembled from two unidirectionally polished KBr windows which ensured orientation of the nematic solutions. The sample solution was prepared at above the clearing point and introduced into the warmed cell by capillarity. It was then kept at constant temperature by means of a water thermostat inside its nematic range. The presence of a nematic phase was always controlled by means of a polarizing microscope. The polarization measurement is performed by setting the plane of polarization once at - 45 ’ and once at + 45’ with respect to the optical bench of the instrument. The sample is rotated, say, at + 45’ and this position is kept constant during the measurement. The single channel spectra for both orientations of the polarizer are subtracted and the resulting spectrum represented in absorbance units. The declined, at 2 45’) polarization plane compensates for the instrument’s own polarization. RESULTS AND DISCUSSION
The FT-IR spectra (parallel (A), perpendicular (B) and DIRLD (C) ) of the pure nematic solvent ZLI-1538 were shown in refs. 1 and 2. The poor DIRLD spectrum of ZLI-1538 offers a broad transparent region where solute bands can be reliably detected. The dichroic ratio, defined as R =A,, /A I is calculated from the parallel (A ,,) and the perpendicular (A I ) spectra. It serves for evaluating the orientation parameters of the partially oriented ensemble. In terms of the Thulstrup-Eggers-Michl (TEM ) model [ 41 and for uniaxially oriented samples the orientation parameters I& are obtained from the relation KL=Ri/(Ri+2)
(1)
where i=r, y or z (x, y, z being the molecular axes). The orientation parame-
197
ters Ki describe the average orientation of the molecular axis i with respect to the sample 2 axis. In the sample studied (ZLI-1538+NB) the nematic liquid crystal aligns with the long axes parallel to the rubbing direction to an extent comparable to that of the nematic solutions of other guests [ 21. Indeed, we measure from the spectra in Fig. 1A K(CN) =0.56. This value is evidence for a pretty good orientation. Remember that the sign of a DIRLD band is determined by the direction of the vibrational transition moment with respect to the molecular long axis [ 51. If the angle between them lies in the interval O-54.7” then a positive sign is conventionally ascribed. Transition moments lying in the interval 54.7-90” correspond to bands of opposite signs, assumed to be negative. If accidentally a transition moment turns out to build up an angle of 54.7’ with the molecular long axis (the so called “magic angle”) then no DIRLD band is present in the spectrum. A dichroic ratio R = 1 corresponds to such a band. SPECTRA OF NB ORIENTED IN ZLI-1538
The free NB molecule is planar [6] and possesses CzVsymmetry. The symmetry distribution of the 36 fundamentals into classes is 13 Al + 4 A2 + 7 B, + 12 &. All species except for A2 are IR active. For the following analysis a Cartesian frame is chosen with the z axis coinciding with the molecular long axis, the x axis perpendicular to the molecular plane and the y axis lying in the same plane. The DIRLD spectrum of NB is shown in Fig. 1A. Only the IR region 400-2500 cm-’ was considered since the C-H stretching region turned out to be strongly overlapped by the C-H stretching bands of ZLI-1538. Assuming parallel alignment of the NB z-axis, leads to an agreement of our results listed in Table 1 with the assignment of Laposa [ 6 ] and Kuwae [ 71 in the liquid and vapor states. An exception was found for the band at 1162 cm-’ which exhibits positive polarization in the DIRLD spectrum with K, = 0.49. We, therefore, assign it to A, and not to Bz as was done in refs. 6-8. Furthermore, a comparison with Ovaska’s [ 81 polarization measurements of NB embedded and oriented in stretched polyethylene (PE) shows an essential discrepancy. Ovaska recorded only the parallel and perpendicular spectrum and compared them by evaluating the dichroic ratio of each band. For the symmetric and antisymmetric stretching bands of the nitro-group he obtained dichroic ratios greater and less than unity, respectively, thus giving experimental evidence for distinguishing between both bands. The analogous measurement in ZLI-1538 led to the surprising result that the symmetric and antisymmetric stretching bands of NB nitro group had the same sign as their DIRLD bands. This phenomenon was exactly reproducible and gave no reason for doubt. No orientation model of NB in ZLI-1538 could explain the observed anomaly.
198
1346
J(sym NO*)
C6H5N02 in ZLI-1538 I
t
2500
2000
1500
1000
cm-l
Fig. 1. DIRLD spectra of NB in the 2500-400 cm-’ region: (top) oriented in the nematic phase ZLI-1538; (bottom) oriented in low density PE. The shaded bands refer to the anisotropic solvents.
An additional argument for rejecting any unusual orientation is the behavior of all remaining DIRLD bands which exhibit the expected polarization. The same anomaly was observed in the DIRLD spectrum of m-DNB. Finally, in order to verify Ovaska’s observation [8] his measurements of NB embedded and oriented in low density PE were repeated. The DIRLD spectrum displayed in Fig. 1B shows the same pattern of the -NO, stretching bands as the NB nematic solution in ZLI-1538 (Fig. 1A). This fact makes one think that some inaccuracies seem to be present in ref. 8. In order to find a satisfactory explanation of the observed anomalous polarization of the antisymmetric stretching band of -NOz, a scheme was assumed according to which the n-electrons of the benzene ring contribute to a dipole moment change which significantly alters the expected direction of the vibrational transition moment of the antisymmetric stretching vibration of NO,. This claim needs a little more justification. The antisymmetric stretching vibration is highly characteristic, i.e. ring atoms participate in it insignificantly. The mobile n-electrons, however, can oscillate towards the nitro group and back with the frequency of the antisymmetric vibration as the antisymmetric motion of the -NO2 atoms proceeds. The latter changes the electron density on the nitrogen atom, inducing polarization in the ring x-electrons. A transversal motion of the rr-electrons is unlikely. The overall transition moment attributed to the antisymmetric -NO, vibration turns out to be formed from two components - one corresponding to
199 TABLE 1 Absorption bands of NB oriented in ZLI-1538, their orientation parameters and signs in the DIRLD spectrum Symmetry type
Al
4
Orientation parameters K
Positions cm-’
Sign in the DIRLD spectrum
In soln.’
In DPEb
In ZLI-1538
In ZLI-1538
In DPEb
1586 1480 1524 1346 1173 1161 1107 1021 852 680
1588 1477 1530(1531) 1345(1346) 1172 -
1589 1479 1527 1347 1173 1162 1107 1021 851 682
0.41 0.38 0.36 0.41 0.43 0.49 0.40 0.39 0.40 0.38
0.47 0.51 0.28(0.37) 0.57(0.41) -(0.42) -_(0.46) -_(0.41) 0.54 (0.43) 0.51(0.38)
+ + + + + + + + + +
794
(790) 700 (701) 673
794
0.23 0.24 -
0.13(0.31) O.lO(O.31) 0.14
-
704 675
1317 1308 1070
-_( 1108) -(1022) 848 (851) 678 (681) 792
1621’ 1607” 1317 -(1068)
707 677
-
1613 1316 1306 1069
0.33 0.29 0.33
0.29 -
NO -
-(0.35)
NO
“From ref. 6. bNB embedded in DPE [ 81. Values given in parentheses are from our measurements of NB embedded in low density PE. “Given as Fermi doublet in ref. 8.
the charge redistribution of the -NO, atoms and the other arising from the longitudinal x-electron displacement. To confirm the above assumption some model examples were chosen in which any possibility of a x-electron contribution can be excluded. The first example was to examine an aliphatic nitro compound. Since the simplest one, CH,N02, failed to produce nematic solutions, the next representative, nitroethane (NE), could be used, although it gave solutions of quite low degrees of orientation. Nevertheless, opposite polarizations of both stretching -NO, bands were clearly observed. Another instructive example was p-DNB. The out-of-phase antisymmetric -NO, band showed a polarization opposite to that of the out-ofphase symmetric band. In this case the n-electron contribution to the dipole moment change is equally distributed between the isolated antisymmetric motions of the -NO, groups in a way that its oppositely directed components cancel each other. The vector sum of all dipole moment changes remains perpendicular to the z-axis thus leading to the observed opposite polarization with
200
respect to the polarization of the symmetric -NO2 vibration. These two examples seem to support the above explanation. The DIRLD spectra of the remaining para-substituted nitrobenzenes used, taken in the -NO2 stretching region (1600-1300 cm-l), are displayed in Fig. 2. The positions of the -NO, stretching bands together with their signs in the DIRLD spectra are given in Table 2. 1339
1346
1339 I
1527
1533
1360
He 1555 I
1600
1400
cm-1
I 1600
1400
cm-’
Fig. 2. DIRLD spectra in the -NO2 stretching region of m-dinitrobenzene (A),p-nitroaniline (B), p-nitrophenol (C ) ,4-dimethylaminonitrostilbene (D ) , p-dinitrobenzene (E) , p-nitrotoluene (F) , p-nitrobenzonitrile (G) and nitroethane (H). The shaded bands refer to the nematic solvent. TABLE 2 Stretching vibrations of the nitro group of the nitro compounds studied and their signs from the DIRLD spectra
NPh NA DMANS NT NBN
o,,(NO,)
o,(NOd
1521(+) 1505(+) 1527(+) 1522(-_) 1533(-)
1340(f) 1331(+) 1339(+) 1345(+) 1345(+)
201
As seen from the DIRLD spectra in Fig. 2 and Table 2, positive polarization is observed for the -NO, stretching vibrations in p-NPh, p-NA and DMANS, while the opposite polarization is shown for these vibrations in p-NT and p-NBN. It is of great interest to note that the polarization behavior of the stretching vibrations of the nitro groups in DMNAS is a strong argument for discarding the possibility for any unusual orientation. Indeed, this molecule is fairly long and hence aligns most probably with its long axis parallel to the liquid crystal director, so opposite polarizations of both -NOa stretching bands are expected. This model compound showed, however, a DIRLD pattern exactly the same as NB. The long conjugated system and the positive polar effect of the (CH,),Ngroup predetermines a higher mobility of the ring n-electrons and leads to polarization analogous to that in the DIRLD spectrum of NB. Since the observed anomaly seems to be a more frequently appearing phenomenon in complicated conjugated molecules, we tried to find some theoretical reason for it. However, MNDO and ab initio calculations for the equilibrium configuration of NB and for the corresponding configuration distorted by the antisymmetric -NO, vibration did not show the expected description of the observed anomaly. Since these theoretical calculations do not lead to any explanation of the observed phenomenon we tried an empirical approach consisting of correlation of the sign of the antisymmetric -NO, vibration in the DIRLD spectrum with the existing data on (i) the effect of para substituents upon the n-withdrawing properties of -NO, [9] and (ii) the rr-electron transfer (dg) determined by a Mulliken population analysis [lo]. (i) According to this, the para substituents in the aromatic nitro compounds studied might be classified into two categories [9] (a) a-acceptor/7r-donor substituents: NH2, OH, and N (CH,),, and (b) o-acceptor/n-acceptor substituents: NOz, CN and CH,. The n-donating substituents increase the mobility of the x-electrons of the benzene ring and hence their contribution to a dipole moment change, while the n-withdrawing substituents decrease the mobility of n-electrons so that they contribute less to the dipole moment change. This may explain the polarization behavior of the antisymmetric -NO, stretching vibration observed in the DIRLD spectra with respect to the n-donating or withdrawing property of the para substituents. (ii) In ref. 10 the x-electron transfer (dq) is calculated for mono-substituted benzenes. We extended it to para substituted NB, taking the value for NB as a reference &(p-XCsHdNOz)
=4(CgHsNOz)
In Table 3 we list the calculated NB used, keeping the notations ( -sign) the substituents.
+&(C,H,X) n-electron indicating
(2)
transfer for the para substituted transfer to ( +sign) and from
202 TABLE 3 Deduced total x-electron transfer for para substituted NB (XC,H,NO,) X
Aq (lo3 e)
NH2
-89
OH
-71
NO, CH3
CN
0 23 53
From the results in Table 3 we noticed that in molecules with negative values for dq the -NO, stretching vibrations show equal polarizations in the DIRLD spectrum, while in molecules with positive dq the -NO2 stretching vibrations show opposite polarizations. ACKNOWLEDGMENT
This work is part of a project supported by the Bulgarian Ministry for Science and Education under Contract No. 508. The authors are indebted to Professor W.P. Neumann from the University of Dortmund, FRG for providing books and chemicals used in the present study.
REFERENCES 1 2 3
M. Belhakem and B. Jordanov, J. Mol. Struct., 218 (1990) 309. M. Belhakem and B. Jordanov, J. Mol. Struct., 245 (1991) 29. B. Jordanov, Presented at the 4th Austrian-Hungarian Conference on Vibrational Spectroscopy, Veszprem 1990; to be published in Vibrational Spectroscopy. 4 J. Michl and E.W. Thulstrup, Spectroscopy with Polarized Light, VCH Publishers, New York, 1986, Chapters 3 and 4. 5 B. Jordanov and B. Tsankov, J. Mol. Struct., 79 (1982) 5. 6 J.D. Laposa, Spectrochim. Acta, Part A, 35 (1979) 65. 7 A. Kuwae and K. Machida, Spectrochim. Acta, Part A, 35 (1979) 27. 8 M. Ovaska and A. Kivinen, J. Mol. Struct., 101 (1983) 255. 9 S. Chowdhury, H. Kishi, G.W. Dillow and P. Kebarle, Can. J. Chem., 67 (1989) 603. 10 S. Marriott, A. Silvestro and R.D. Topsom, J. Chem. Sot., Perkin Trans. 2, (1988) 457.
195
AN FT-IR STUDY OF PECULIARITIES IN THE POLARIZED VIBRATIONAL SPECTRA OF SOME AROMATIC NITRO COMPOUNDS
M. BELHAKEM and B. JORDANOV
Bulgarian Academy of Sciences, Institute of Organic Chemistry, BG-1113 Sofia (Bulgaria) (Received 21 August 1990 1
ABSTRACT FT-IR polarization spectra of oriented nematic solutions of nitrobenzene showed an anomaly consisting in the equal polarization of both symmetric and antisymmetric stretching bands of the nitro group. In order to clarify the observed phenomenon the polarization spectra of the nematic solutions of a series of p-substituted aromatic nitro compounds was studied. The contribution of the ring x-electrons to the vibrational transition moment of the antisymmetric -NO, stretching vibration seems to explain this anomaly.
INTRODUCTION
In some recent studies [l-3] we reported the use of the nematic liquid crystal ZLI-1538 (Merck’s notation for truns-4-tert butyl-4’ -cyanobicyclohexyl), as an anisotropic solvent for IR polarization studies of large organic compounds. Its main advantages consist in its reduced IR spectrum, comparable to that of nujol, and in the good solubility of many organic compounds in it. The solutions obtained possess a nematic temperature range in which a good orientation can be achieved, thus enabling the performance of IR polarization measurements. Nitrobenzene (NB ) was included as one of the ordinary examples for application of the differential IR linear dichroic (DIRLD) spectra for the study of the symmetry of the molecular vibrations, since we took into account that the IR spectra of this compound are fairly well assigned. A trivial discrimination between the longitudinal (along the molecular long axis) and transversal vibrations of NB was expected by the appearance of positive and negative features in the DIRLD spectrum. This expectation was certainly confirmed for all but one IR band, namely that of the antisymmetric stretching vibration of the nitro group. The symmetric and antisymmetric NO2 stretching bands turned out to exhibit equal longitudinal polarization. After excluding all possibilities for errors 0022-2860/91/$03.50
0 1991-
Elsevier Science Publishers B.V.
196
and artifacts, we realized that this phenomenon deserves special attention and therefore carried out the present study. Some preliminary results have already been reported [ 1,3]. EXPERIMENTAL
The nematic liquid crystal ZLI-1538 was purchased from Merck. Additional model nitro-compounds were used for the elucidation of the observed anomaly in the DIRLD spectrum of NB. The chemicals used were as follows: nitrobenzene (NB) and nitroethane (NE) (Fluka); p-nitrotoluene (p-NT) andp-nitroaniline (p-NA) (POCG, Poland); p-nitrophenol (p-NPh) (Jenapharm, FRG); m-dinitrobenzene (m-DNB), p-nitrobenzonitrile (p-NBN), and 4-dimethylaminonitrostilbene (DMANS) were synthesized for various purposes and kindly given to us. All spectra were measured with a Bruker IFS-113~ FT-IR spectrometer, using an automatically rotating grid polarizer (Al/KRS-5). A KBr cell of thickness 0.025 mm was assembled from two unidirectionally polished KBr windows which ensured orientation of the nematic solutions. The sample solution was prepared at above the clearing point and introduced into the warmed cell by capillarity. It was then kept at constant temperature by means of a water thermostat inside its nematic range. The presence of a nematic phase was always controlled by means of a polarizing microscope. The polarization measurement is performed by setting the plane of polarization once at - 45 ’ and once at + 45’ with respect to the optical bench of the instrument. The sample is rotated, say, at + 45’ and this position is kept constant during the measurement. The single channel spectra for both orientations of the polarizer are subtracted and the resulting spectrum represented in absorbance units. The declined, at 2 45’) polarization plane compensates for the instrument’s own polarization. RESULTS AND DISCUSSION
The FT-IR spectra (parallel (A), perpendicular (B) and DIRLD (C) ) of the pure nematic solvent ZLI-1538 were shown in refs. 1 and 2. The poor DIRLD spectrum of ZLI-1538 offers a broad transparent region where solute bands can be reliably detected. The dichroic ratio, defined as R =A,, /A I is calculated from the parallel (A ,,) and the perpendicular (A I ) spectra. It serves for evaluating the orientation parameters of the partially oriented ensemble. In terms of the Thulstrup-Eggers-Michl (TEM ) model [ 41 and for uniaxially oriented samples the orientation parameters I& are obtained from the relation KL=Ri/(Ri+2)
(1)
where i=r, y or z (x, y, z being the molecular axes). The orientation parame-
197
ters Ki describe the average orientation of the molecular axis i with respect to the sample 2 axis. In the sample studied (ZLI-1538+NB) the nematic liquid crystal aligns with the long axes parallel to the rubbing direction to an extent comparable to that of the nematic solutions of other guests [ 21. Indeed, we measure from the spectra in Fig. 1A K(CN) =0.56. This value is evidence for a pretty good orientation. Remember that the sign of a DIRLD band is determined by the direction of the vibrational transition moment with respect to the molecular long axis [ 51. If the angle between them lies in the interval O-54.7” then a positive sign is conventionally ascribed. Transition moments lying in the interval 54.7-90” correspond to bands of opposite signs, assumed to be negative. If accidentally a transition moment turns out to build up an angle of 54.7’ with the molecular long axis (the so called “magic angle”) then no DIRLD band is present in the spectrum. A dichroic ratio R = 1 corresponds to such a band. SPECTRA OF NB ORIENTED IN ZLI-1538
The free NB molecule is planar [6] and possesses CzVsymmetry. The symmetry distribution of the 36 fundamentals into classes is 13 Al + 4 A2 + 7 B, + 12 &. All species except for A2 are IR active. For the following analysis a Cartesian frame is chosen with the z axis coinciding with the molecular long axis, the x axis perpendicular to the molecular plane and the y axis lying in the same plane. The DIRLD spectrum of NB is shown in Fig. 1A. Only the IR region 400-2500 cm-’ was considered since the C-H stretching region turned out to be strongly overlapped by the C-H stretching bands of ZLI-1538. Assuming parallel alignment of the NB z-axis, leads to an agreement of our results listed in Table 1 with the assignment of Laposa [ 6 ] and Kuwae [ 71 in the liquid and vapor states. An exception was found for the band at 1162 cm-’ which exhibits positive polarization in the DIRLD spectrum with K, = 0.49. We, therefore, assign it to A, and not to Bz as was done in refs. 6-8. Furthermore, a comparison with Ovaska’s [ 81 polarization measurements of NB embedded and oriented in stretched polyethylene (PE) shows an essential discrepancy. Ovaska recorded only the parallel and perpendicular spectrum and compared them by evaluating the dichroic ratio of each band. For the symmetric and antisymmetric stretching bands of the nitro-group he obtained dichroic ratios greater and less than unity, respectively, thus giving experimental evidence for distinguishing between both bands. The analogous measurement in ZLI-1538 led to the surprising result that the symmetric and antisymmetric stretching bands of NB nitro group had the same sign as their DIRLD bands. This phenomenon was exactly reproducible and gave no reason for doubt. No orientation model of NB in ZLI-1538 could explain the observed anomaly.
198
1346
J(sym NO*)
C6H5N02 in ZLI-1538 I
t
2500
2000
1500
1000
cm-l
Fig. 1. DIRLD spectra of NB in the 2500-400 cm-’ region: (top) oriented in the nematic phase ZLI-1538; (bottom) oriented in low density PE. The shaded bands refer to the anisotropic solvents.
An additional argument for rejecting any unusual orientation is the behavior of all remaining DIRLD bands which exhibit the expected polarization. The same anomaly was observed in the DIRLD spectrum of m-DNB. Finally, in order to verify Ovaska’s observation [8] his measurements of NB embedded and oriented in low density PE were repeated. The DIRLD spectrum displayed in Fig. 1B shows the same pattern of the -NO, stretching bands as the NB nematic solution in ZLI-1538 (Fig. 1A). This fact makes one think that some inaccuracies seem to be present in ref. 8. In order to find a satisfactory explanation of the observed anomalous polarization of the antisymmetric stretching band of -NOz, a scheme was assumed according to which the n-electrons of the benzene ring contribute to a dipole moment change which significantly alters the expected direction of the vibrational transition moment of the antisymmetric stretching vibration of NO,. This claim needs a little more justification. The antisymmetric stretching vibration is highly characteristic, i.e. ring atoms participate in it insignificantly. The mobile n-electrons, however, can oscillate towards the nitro group and back with the frequency of the antisymmetric vibration as the antisymmetric motion of the -NO2 atoms proceeds. The latter changes the electron density on the nitrogen atom, inducing polarization in the ring x-electrons. A transversal motion of the rr-electrons is unlikely. The overall transition moment attributed to the antisymmetric -NO, vibration turns out to be formed from two components - one corresponding to
199 TABLE 1 Absorption bands of NB oriented in ZLI-1538, their orientation parameters and signs in the DIRLD spectrum Symmetry type
Al
4
Orientation parameters K
Positions cm-’
Sign in the DIRLD spectrum
In soln.’
In DPEb
In ZLI-1538
In ZLI-1538
In DPEb
1586 1480 1524 1346 1173 1161 1107 1021 852 680
1588 1477 1530(1531) 1345(1346) 1172 -
1589 1479 1527 1347 1173 1162 1107 1021 851 682
0.41 0.38 0.36 0.41 0.43 0.49 0.40 0.39 0.40 0.38
0.47 0.51 0.28(0.37) 0.57(0.41) -(0.42) -_(0.46) -_(0.41) 0.54 (0.43) 0.51(0.38)
+ + + + + + + + + +
794
(790) 700 (701) 673
794
0.23 0.24 -
0.13(0.31) O.lO(O.31) 0.14
-
704 675
1317 1308 1070
-_( 1108) -(1022) 848 (851) 678 (681) 792
1621’ 1607” 1317 -(1068)
707 677
-
1613 1316 1306 1069
0.33 0.29 0.33
0.29 -
NO -
-(0.35)
NO
“From ref. 6. bNB embedded in DPE [ 81. Values given in parentheses are from our measurements of NB embedded in low density PE. “Given as Fermi doublet in ref. 8.
the charge redistribution of the -NO, atoms and the other arising from the longitudinal x-electron displacement. To confirm the above assumption some model examples were chosen in which any possibility of a x-electron contribution can be excluded. The first example was to examine an aliphatic nitro compound. Since the simplest one, CH,N02, failed to produce nematic solutions, the next representative, nitroethane (NE), could be used, although it gave solutions of quite low degrees of orientation. Nevertheless, opposite polarizations of both stretching -NO, bands were clearly observed. Another instructive example was p-DNB. The out-of-phase antisymmetric -NO, band showed a polarization opposite to that of the out-ofphase symmetric band. In this case the n-electron contribution to the dipole moment change is equally distributed between the isolated antisymmetric motions of the -NO, groups in a way that its oppositely directed components cancel each other. The vector sum of all dipole moment changes remains perpendicular to the z-axis thus leading to the observed opposite polarization with
200
respect to the polarization of the symmetric -NO2 vibration. These two examples seem to support the above explanation. The DIRLD spectra of the remaining para-substituted nitrobenzenes used, taken in the -NO2 stretching region (1600-1300 cm-l), are displayed in Fig. 2. The positions of the -NO, stretching bands together with their signs in the DIRLD spectra are given in Table 2. 1339
1346
1339 I
1527
1533
1360
He 1555 I
1600
1400
cm-1
I 1600
1400
cm-’
Fig. 2. DIRLD spectra in the -NO2 stretching region of m-dinitrobenzene (A),p-nitroaniline (B), p-nitrophenol (C ) ,4-dimethylaminonitrostilbene (D ) , p-dinitrobenzene (E) , p-nitrotoluene (F) , p-nitrobenzonitrile (G) and nitroethane (H). The shaded bands refer to the nematic solvent. TABLE 2 Stretching vibrations of the nitro group of the nitro compounds studied and their signs from the DIRLD spectra
NPh NA DMANS NT NBN
o,,(NO,)
o,(NOd
1521(+) 1505(+) 1527(+) 1522(-_) 1533(-)
1340(f) 1331(+) 1339(+) 1345(+) 1345(+)
201
As seen from the DIRLD spectra in Fig. 2 and Table 2, positive polarization is observed for the -NO, stretching vibrations in p-NPh, p-NA and DMANS, while the opposite polarization is shown for these vibrations in p-NT and p-NBN. It is of great interest to note that the polarization behavior of the stretching vibrations of the nitro groups in DMNAS is a strong argument for discarding the possibility for any unusual orientation. Indeed, this molecule is fairly long and hence aligns most probably with its long axis parallel to the liquid crystal director, so opposite polarizations of both -NOa stretching bands are expected. This model compound showed, however, a DIRLD pattern exactly the same as NB. The long conjugated system and the positive polar effect of the (CH,),Ngroup predetermines a higher mobility of the ring n-electrons and leads to polarization analogous to that in the DIRLD spectrum of NB. Since the observed anomaly seems to be a more frequently appearing phenomenon in complicated conjugated molecules, we tried to find some theoretical reason for it. However, MNDO and ab initio calculations for the equilibrium configuration of NB and for the corresponding configuration distorted by the antisymmetric -NO, vibration did not show the expected description of the observed anomaly. Since these theoretical calculations do not lead to any explanation of the observed phenomenon we tried an empirical approach consisting of correlation of the sign of the antisymmetric -NO, vibration in the DIRLD spectrum with the existing data on (i) the effect of para substituents upon the n-withdrawing properties of -NO, [9] and (ii) the rr-electron transfer (dg) determined by a Mulliken population analysis [lo]. (i) According to this, the para substituents in the aromatic nitro compounds studied might be classified into two categories [9] (a) a-acceptor/7r-donor substituents: NH2, OH, and N (CH,),, and (b) o-acceptor/n-acceptor substituents: NOz, CN and CH,. The n-donating substituents increase the mobility of the x-electrons of the benzene ring and hence their contribution to a dipole moment change, while the n-withdrawing substituents decrease the mobility of n-electrons so that they contribute less to the dipole moment change. This may explain the polarization behavior of the antisymmetric -NO, stretching vibration observed in the DIRLD spectra with respect to the n-donating or withdrawing property of the para substituents. (ii) In ref. 10 the x-electron transfer (dq) is calculated for mono-substituted benzenes. We extended it to para substituted NB, taking the value for NB as a reference &(p-XCsHdNOz)
=4(CgHsNOz)
In Table 3 we list the calculated NB used, keeping the notations ( -sign) the substituents.
+&(C,H,X) n-electron indicating
(2)
transfer for the para substituted transfer to ( +sign) and from
202 TABLE 3 Deduced total x-electron transfer for para substituted NB (XC,H,NO,) X
Aq (lo3 e)
NH2
-89
OH
-71
NO, CH3
CN
0 23 53
From the results in Table 3 we noticed that in molecules with negative values for dq the -NO, stretching vibrations show equal polarizations in the DIRLD spectrum, while in molecules with positive dq the -NO2 stretching vibrations show opposite polarizations. ACKNOWLEDGMENT
This work is part of a project supported by the Bulgarian Ministry for Science and Education under Contract No. 508. The authors are indebted to Professor W.P. Neumann from the University of Dortmund, FRG for providing books and chemicals used in the present study.
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