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Overtone bands in aniline and its chloro-derivatives—a low concentration study
Spectrochimica Acta Part A 59 (2003) 1299 /1306 www.elsevier.com/locate/saa
Overtone bands in aniline and its chloro-derivatives* a low concentration study
/
Vineet Kumar Rai, S.B. Rai *, D.K. Rai Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221 005, India Received 29 April 2002; received in revised form 21 August 2002; accepted 10 September 2002
Abstract Overtone spectra of aniline and its o -and m - chloro-derivatives mixed with carbon tetrachloride have been studied at different dilutions. Vibrational frequency and anharmonicity constants for the C /H stretch vibration and for the symmetric and asymmetric N /H stretch vibrations have been determined. The presence of intermolecular hydrogen bonding has been noted in all the three molecules. Intramolecular hydrogen bonding involving N /H Cl has also been detected in o -chloroaniline. # 2002 Published by Elsevier Science B.V. Keywords: Overtone spectroscopy; Inter and intramolecular hydrogen bonding; Thermal lensing; Symmetric and asymmetric stretching; Anharmonicity
1. Introduction In an earlier publication the overtone spectra of aniline and its chloro derivatives using overtone and thermal lensing technique were reported [1]. The overtone bands involving Dy /2,3,4,5 and 6 for the C /H stretch vibration and Dy /2,3,4 and 5 for the N /H stretch vibration were measured. However, the symmetric and asymmetric stretch vibrations of the NH bond could not be clearly
* Corresponding author. Tel.: /91-542-30-7329; fax: /91542-36-8468. E-mail address: [email protected] (S.B. Rai).
distinguished as the absorption peaks due to the two vibrations were overlapped. It was, therefore, thought worth while to reexamine the overtone bands of these molecules under better resolution. A later study [2] on OH vibrations had shown that dilution with CCl4 helps in observing free y (OH) stretching unaffected by hydrogen bonding. Further, it was felt that the presence of chlorine in the chloroanilines may even lead to an intramolecular hydrogen bonding which needed to be looked into. During the course of this work a similar study by Shaji and Rasheed [3] has appeared out. Points of similarity and differences with the results of these workers are discussed towards the end of this note.
1386-1425/02/$ - see front matter # 2002 Published by Elsevier Science B.V. doi:10.1016/S1386-1425(02)00312-8
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2. Experimental The aniline sample obtained from BDH India with 99.9% purity and the o- and m -chloroaniline obtained from Merck with 99% purity were used in the present experiments. No further purification was attempted. Since all these sample are sensitive to light, standard procedures to ensure protection from light were employed. The NIR spectra were recorded using a Lambda-19 UV /Vis/NIR double beam spectrophotometer, while to record the fundamental band a JASCO FTIR-5300 spectrophotometer was used. Analysis grade CCl4 was used to make mixtures with different relative concentrations. Several spectral records were made for each and every sample. Before repeating the scan every time the cuvette was cleaned and rinsed properly to avoid any contamination. All the measurements were made at room temperature (30 8C). The path length of the sample was 1 cm. The higher harmonics (Dy /6 of the C /H stretch and Dy /5 for the N/H stretch) in the three molecules aniline, o - and m chloroaniline (undiluted) have been measured using a double beam thermal lens technique [1] and we have taken that data as such for our calculations.
3. Results and discussion The infra red absorption spectra of these compounds were recorded in the spectral range 2500 /16600 cm 1. The infrared spectra show a large number of peaks near 3000 cm 1 in all these compounds due to the various C /H stretch vibrations. A doublet band consisting of an intense and a weak peak near 3400 cm 1, which are assigned to the symmetric and asymmetric stretch vibrations of the N/H bond with Dy /1. The absorption peaks in the region of 6000 cm 1 are due to the first harmonics of the CH and NH stretch vibrations. There also appear some weak absorption peaks near 5000/6000 cm 1, and are probably due to combination of C /H/N /H stretch vibrations and their deformations. The second harmonics of the two stretch vibrations are expected in the neighbourhood of 9000 cm 1.
As we go towards the higher harmonics the intensity of the band decreases and its bandwidth increases rapidly (see Figs. 1/3). The position of the Dy /5 band of the C /H stretch and the Dy /4 band of the N/H stretch could be ascertained only after repeated scans. However, the peaks for Dy / 6 and Dy /5 for C /H and N /H vibrations, respectively, could be detected using the thermal lensing technique with relative ease and clarity. The measured frequencies of the different bands for C /H and N/H stretch vibrations are given in Table 1. The vibrational frequency and anharmonicity constants for the C /H and N /H stretch vibrations were calculated using the relation, Ev AvBv2 where v is the quantum number of the excited vibrational level and Ev is the transition energy for the (v 1/0) absorption band. A and B are related to the actual vibrational constants we( /A/B ) and wexe( //B ), whose values are given in Table 2 along with the estimated errors.
3.1. Overtone bands due to C /H stretch A comparison of these constants shows that the C /H bond stretching frequency in chloroaniline is larger than in aniline. This is clearly due to the electron withdrawing character of the chlorine atom which affects the charge distribution on the ring carbon atoms and hence the C /H bond frequency. The frequency change is smaller in o chloroaniline as compared with m -chloroaniline perhaps, due to the formation of an intramolecular hydrogen bond in o -chloroaniline. The vibrational frequency of C /H stretch vibration in benzene and in chlorobenzene is 3147 and 3172 cm 1, respectively [4]. The frequency shift of C /H stretch in chloroaniline from that of chlorobenzene is 32 cm 1, where as the frequency shift of CH stretch in o -chloroaniline and m -chloroaniline with respect to aniline is 50 and 58 cm 1, respectively. This clearly shows that the substitution of NH2 and Cl groups in place of hydrogens appreciably affect the C /H stretch frequency.
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Fig. 1. (a), (b), (c) and (d) refers the fundamental and overtone bands of CH and NH stretch vibrations in aniline. (e), (f) and (g) represents the shift of NH peak for different bands on dilution with CCl4.
3.2. Overtone bands due to N /H stretch Bands involving N/H stretch vibrations are observed for Dy /1 /5 transitions in all the three molecules. The intensity of the band decreases as Dy increases. Earlier reports showed only one
absorption peak for the NH stretching vibration, but clearly distinct peaks are seen in our spectra. To confirm the assignments we exammined the infrared spectrum of N -ethyl aniline and of NN diethyl aniline [5]. It was observed that in N -ethyl aniline there is only one peak due to the N /H
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Fig. 2. (a), (b), (c) and (d) refers the fundamental and overtone bands of CH and NH stretch vibrations in o -chloroaniline. (e), (f) and (g) represents the shift of NH peak for different bands on dilution with CCl4.
bond where as there is no peak in 3400 cm 1 in the case of NN -diethyl aniline, as expected. We also calculated the symmetric and asymmetric NH stretching frequencies in aniline, o - and m -chloroanilines using the AM1 computer program and found that the symmetric and asymmetric vibration differ by nearly 60 cm 1 in each of the three compounds, in agreement with experimental observations. The intensity of the asymmetric peak
(lying at higher wave numbers) is smaller than that of the symmetric one. The absorption frequencies corresponding to the different transitions for the symmetric and the asymmetric stretching vibrations are given in Table 1 and the calculated constants in the local mode model are given in Table 2. The molecular constants for the symmetric mode differ appreciably from those for the asymmetric mode in all the
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Fig. 3. (a), (b), (c) and (d) refers the fundamental and overtone bands of CH and NH stretch vibrations in m -chloroaniline. (e), (f) and (g) represents the shift of NH peak for different bands on dilution with CCl4.
three molecules. One notices that the frequencies for both the symmetric and asymmetric NH stretch modes in o -chloroaniline are nearly equal to their values in aniline. It appears that in ochloroaniline the change (increase) in the vibrational frequency due to the electron withdrawing
effect of the chlorine atom (added to aniline) is almost compensated by a decrease in this frequency due to the formation of an intramolecular hydrogen bond N/H Cl. Since such intramolecular hydrogen bonding is ruled out by the geometry of m -chloroaniline, there is no such
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Table 1 Transition frequencies for aniline, o -chloroaniline and m -chloroaniline 11/0
21/0
Aniline C/H stretch N/H sym. stretch N/H asym. Stretch
3038.2 (65) 3360.2 (190) 3420.0
5995.2 (102) 6678.2 (209) 6860.6
8762.1 (200) 9733.9 (266) 10054.8
o-Chloroaniline C /H stretch N/H sym. stretch N/H asym. stretch
3070.9 (80) 3377.7 (150) 3445.2
6004.1 (93) 6703.1 (162) 6881.8
m-Chloroaniline C /H (stretch) N/H sym. stretch N/H asym. stretch
3079.2 (80) 3379.0 (200) 3452.9
6013.6 (124) 6716.3 (225) 6899.4
31/0
41/0
5 1/0
61/0
11481.8 (299) 12683.0 (382)
14020"/ 15490"/
16475"/
8824.8 (223) 9782.0 (300) 10098.2
11517.6 (338) 12770.2 (453)
14114"/ 15523"/
16608"/
8808.5 (286) 9785.1 (605) 10105.6
11511.3 (599) 12787.15 (744)
14104.0"/ 15540.0"/
16578.0"/
"/, Denotes the values taken from reference [1]. ( ), The values in the brackets are FWHM.
compensation and a higher vibrational frequency (compared with aniline) is observed (Table 3). Nonat and coworkers [6,7] have studied the microwave spectrum of chloroanilines and have noted the presence of an intramolecular hydrogen bond in o -chloroaniline. Shaji and Rasheed [3] have observed the band which we have assigned to the asymmetric NH stretch mode in these molecules, but they seem to have interpreted it as a combination band. Combination bands at 7737 and 8296 cm 1, at 7758 and 8300 cm,1 and at 7808 and 8317 cm 1 have been assigned in aniline, o -chloroaniline and m -chloroaniline, respectively, in the present work.
The unusually small intensity observed for the asymmetric N /H stretching mode can be understood as follows. In the symmetric vibration both the N/H bonds change in the same way. The ‘s’ character of the nitrogen hybrid orbitals involved in the two bonds decreases as the N /H bond lengths increase. Any decrease in the ‘s’ character of the bonding hybrid also changes the character of the lone pair electron on the N -atom. Since the atomic dipole of the lone pair electron makes a significant contribution to the transition moment of the NH stretch vibration and this atomic dipole follows the change in the ‘s’ character of the bonding orbital. The change in the dipole moment
Table 2 Vibrational frequencies, anharmonicity constants and dissociation energies We /A/B (cm1)
De /w2e /4wexe (eV)
C/H stretch vibrations in aniline and its chloroderivatives Aniline 3088.19/12.3 55.99/3.6 o -Chloroaniline 3134.29/8.1 62.29/2.4 64.59/2.7 m -Chloroaniline 3140.29/9.4
3144.09/12.3 3196.49/10.5 3204.79/12.2
5.59/0.3 5.19/0.2 4.99/0.2
N/H (symmetric ) stretch vibrations in aniline and its chloroderivatives Aniline 3473.59/12.8 75.59/3.7 o -Chloroaniline 3480.59/22.6 72.19/8.6 74.69/4.3 m -Chloroaniline 3489.59/26.4
3549.19/16.5 3552.69/31.2 3564.09/30.7
5.29/0.2 5.49/0.5 5.39/0.2
N/H (asymmetric ) stretch vibrations in aniline and its chloroderivatives Aniline 3515.29/31.2 53.09/11.7 o -Chloroaniline 3533.09/24.6 54.49/9.3 m -Chloroaniline 3548.49/27.3 58.69/10.3
3568.19/42.7 3587.49/34.0 3607.09/37.5
7.49/1.1 7.49/1.0 6.99/0.9
Sample
A (cm1)
/B /wexe (cm1)
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Table 3 Dilution effect in aniline, o -chloroaniline and m -chloroaniline Serial number
Dilution (% of the sample)
11/0
2 1/0
31/0
100 (no CCl4)
3360.2* 3377.7$ 3379.0% 3375.7* 3389.1$ 3393.0% 3386.1* 3398.3$ 3404.2% 3390.9* 3402.0$ 3408.3% 3395.2* 3412.8$ 3419.7% 3404.7* 3420.3$ 3427.0%
6678.2* 6703.1$ 6716.3% 6690.3* 6714.5$ 6721.2% 6696.3* 6717.6$ 6728.8% 6703.1* 6722.8$ 6730.3% 6707.2* 6728.4$ 6734.8% 6716.0* 6731.5$ 6736.3%
9734.0* 9782.0$ 9785.1% 9741.2* 9789.1$ 9791.4% 9751.9* 9796.2$ 9794.3% 9757.5* 9801.3$ 9799.3% 9759.7* 9804.5$ 9803.5% 9762.8* 9807.6$ 9805.9%
2
50
3
20
4
10
5
1
6
0.1
*, N/H stretching frequency for aniline. $, N/H stretching frequency for o -chloroaniline. %, N/H stretching frequency for m chloroaniline.
for the asymmetric mode is much smaller than for the symmetric mode.
4. Effect of dilution As mentioned earlier the absorption peaks due to the N/H stretching vibrations (symmetric and also the asymmetric) in the spectrum of all the three molecules are broadened due to intermolecular hydrogen bonding. The hydrogen bonding causes the formation of dimers, trimers etc. and the superposition of their slightly shifted absorption peaks gives a broad band similar to what is observed for OH absorption in many cases. In an earlier study [2] of the fundamental and overtone bands of OH recorded in the presence of CCl4 it was noted that addition of CCl4 inhibits formation of polymeric species. A considerable fraction of the sample then exists as monomer and gives a sharp absorption peak shifted to higher frequency as compared with the peak observed in the associated state. In the case of the three aniline compounds the spectra have been recorded at 50, 20, 10 0.1 and 0.1% (% sample in CCl4) dilutions
along with the undiluted samples. The spectra in the case of aniline, o- and m -chloroaniline are shown in Figs. 1/3 for Dy /1, 2, 3 1/0 transitions. One can clearly see that peaks show a blue shift with the decrease of concentration of the aniline compound. The maximum shift in the case of symmetric NH stretch for the fundamental band in aniline is 44 cm 1 (this value is much smaller than the shift /250 cm 1 observed in the case of the OH bond). It is thus clear that aniline may also have intermolecular hydrogen bonding though its strength is much smaller as compared with the case where OH group is involved. Whetsel et al. [8] have made a similar study on the fundamental band of some amines (including aniline and m chloroaniline) and have also noted similar shifts due to breaking of the intermolecular hydrogen bonds. It is to be noted that even in the monomeric situations the N/H stretch frequency of o -chloroaniline is smaller than in aniline which testifies to the existence of intramolecular N /H Cl hydrogen bond. Due to the relatively small ( /40 /50 cm 1) shift when these molecules reduce to the monomeric form the peak due to the free NH is seen to be overlapped with the peak of the
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associated NH. As the dilution is increased the associated peak decreases in intensity where as that of free NH increases which causes a shift towards higher frequency of the superposed peak. The asymmetric peak also shows a similar behaviour. When we compare the change in the absorption peak positions with increasing CCl4 for different values of Dy , we find that the shifts are smaller as Dy increases. These observations are also similar to the earlier observation [2] in the case of OH vibration and are related to the growing importance of N/H Cl bonds for the higher vibrational excitations.
Acknowledgements Authors are grateful to CSIR for financial assistance. One of us (Vineet Kumar Rai) also
likes to thank Banaras Hindu University for a B.H.U. fellowship.
References [1] J. Vipin Prasad, I.B. Singh, S.B. Rai, S.N. Thakur, Indian J. Phys. 69B (1995) 315. [2] P.K. Srivastav, D.K. Rai, S.B. Rai, Spectrochim. Acta 56A (2000) 1283. [3] S. Shaji, T.M.A. Rasheed, Spectrochim. Acta 57A (2001) 337. [4] J. Vipin Prasad, S.B. Rai, S.N. Thakur, Chem. Phys. Lett. 164 (1989) 629. [5] V.K. Rai, S.B. Rai, Unpublished. [6] A. Nonat, A. Bouchy, G. Rouss, J. Mol. Struct. 108 (1984) 230. [7] A. Nonat, A. Bouchy, G. Rouss, J. Mol. Spectrosc. 116 (1984) 227. [8] K.B. Whetsel, W.E. Roberson, M.W. Krell, Anal. Chem. 32 (1960) 1281.
Overtone bands in aniline and its chloro-derivatives* a low concentration study
/
Vineet Kumar Rai, S.B. Rai *, D.K. Rai Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221 005, India Received 29 April 2002; received in revised form 21 August 2002; accepted 10 September 2002
Abstract Overtone spectra of aniline and its o -and m - chloro-derivatives mixed with carbon tetrachloride have been studied at different dilutions. Vibrational frequency and anharmonicity constants for the C /H stretch vibration and for the symmetric and asymmetric N /H stretch vibrations have been determined. The presence of intermolecular hydrogen bonding has been noted in all the three molecules. Intramolecular hydrogen bonding involving N /H Cl has also been detected in o -chloroaniline. # 2002 Published by Elsevier Science B.V. Keywords: Overtone spectroscopy; Inter and intramolecular hydrogen bonding; Thermal lensing; Symmetric and asymmetric stretching; Anharmonicity
1. Introduction In an earlier publication the overtone spectra of aniline and its chloro derivatives using overtone and thermal lensing technique were reported [1]. The overtone bands involving Dy /2,3,4,5 and 6 for the C /H stretch vibration and Dy /2,3,4 and 5 for the N /H stretch vibration were measured. However, the symmetric and asymmetric stretch vibrations of the NH bond could not be clearly
* Corresponding author. Tel.: /91-542-30-7329; fax: /91542-36-8468. E-mail address: [email protected] (S.B. Rai).
distinguished as the absorption peaks due to the two vibrations were overlapped. It was, therefore, thought worth while to reexamine the overtone bands of these molecules under better resolution. A later study [2] on OH vibrations had shown that dilution with CCl4 helps in observing free y (OH) stretching unaffected by hydrogen bonding. Further, it was felt that the presence of chlorine in the chloroanilines may even lead to an intramolecular hydrogen bonding which needed to be looked into. During the course of this work a similar study by Shaji and Rasheed [3] has appeared out. Points of similarity and differences with the results of these workers are discussed towards the end of this note.
1386-1425/02/$ - see front matter # 2002 Published by Elsevier Science B.V. doi:10.1016/S1386-1425(02)00312-8
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2. Experimental The aniline sample obtained from BDH India with 99.9% purity and the o- and m -chloroaniline obtained from Merck with 99% purity were used in the present experiments. No further purification was attempted. Since all these sample are sensitive to light, standard procedures to ensure protection from light were employed. The NIR spectra were recorded using a Lambda-19 UV /Vis/NIR double beam spectrophotometer, while to record the fundamental band a JASCO FTIR-5300 spectrophotometer was used. Analysis grade CCl4 was used to make mixtures with different relative concentrations. Several spectral records were made for each and every sample. Before repeating the scan every time the cuvette was cleaned and rinsed properly to avoid any contamination. All the measurements were made at room temperature (30 8C). The path length of the sample was 1 cm. The higher harmonics (Dy /6 of the C /H stretch and Dy /5 for the N/H stretch) in the three molecules aniline, o - and m chloroaniline (undiluted) have been measured using a double beam thermal lens technique [1] and we have taken that data as such for our calculations.
3. Results and discussion The infra red absorption spectra of these compounds were recorded in the spectral range 2500 /16600 cm 1. The infrared spectra show a large number of peaks near 3000 cm 1 in all these compounds due to the various C /H stretch vibrations. A doublet band consisting of an intense and a weak peak near 3400 cm 1, which are assigned to the symmetric and asymmetric stretch vibrations of the N/H bond with Dy /1. The absorption peaks in the region of 6000 cm 1 are due to the first harmonics of the CH and NH stretch vibrations. There also appear some weak absorption peaks near 5000/6000 cm 1, and are probably due to combination of C /H/N /H stretch vibrations and their deformations. The second harmonics of the two stretch vibrations are expected in the neighbourhood of 9000 cm 1.
As we go towards the higher harmonics the intensity of the band decreases and its bandwidth increases rapidly (see Figs. 1/3). The position of the Dy /5 band of the C /H stretch and the Dy /4 band of the N/H stretch could be ascertained only after repeated scans. However, the peaks for Dy / 6 and Dy /5 for C /H and N /H vibrations, respectively, could be detected using the thermal lensing technique with relative ease and clarity. The measured frequencies of the different bands for C /H and N/H stretch vibrations are given in Table 1. The vibrational frequency and anharmonicity constants for the C /H and N /H stretch vibrations were calculated using the relation, Ev AvBv2 where v is the quantum number of the excited vibrational level and Ev is the transition energy for the (v 1/0) absorption band. A and B are related to the actual vibrational constants we( /A/B ) and wexe( //B ), whose values are given in Table 2 along with the estimated errors.
3.1. Overtone bands due to C /H stretch A comparison of these constants shows that the C /H bond stretching frequency in chloroaniline is larger than in aniline. This is clearly due to the electron withdrawing character of the chlorine atom which affects the charge distribution on the ring carbon atoms and hence the C /H bond frequency. The frequency change is smaller in o chloroaniline as compared with m -chloroaniline perhaps, due to the formation of an intramolecular hydrogen bond in o -chloroaniline. The vibrational frequency of C /H stretch vibration in benzene and in chlorobenzene is 3147 and 3172 cm 1, respectively [4]. The frequency shift of C /H stretch in chloroaniline from that of chlorobenzene is 32 cm 1, where as the frequency shift of CH stretch in o -chloroaniline and m -chloroaniline with respect to aniline is 50 and 58 cm 1, respectively. This clearly shows that the substitution of NH2 and Cl groups in place of hydrogens appreciably affect the C /H stretch frequency.
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Fig. 1. (a), (b), (c) and (d) refers the fundamental and overtone bands of CH and NH stretch vibrations in aniline. (e), (f) and (g) represents the shift of NH peak for different bands on dilution with CCl4.
3.2. Overtone bands due to N /H stretch Bands involving N/H stretch vibrations are observed for Dy /1 /5 transitions in all the three molecules. The intensity of the band decreases as Dy increases. Earlier reports showed only one
absorption peak for the NH stretching vibration, but clearly distinct peaks are seen in our spectra. To confirm the assignments we exammined the infrared spectrum of N -ethyl aniline and of NN diethyl aniline [5]. It was observed that in N -ethyl aniline there is only one peak due to the N /H
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Fig. 2. (a), (b), (c) and (d) refers the fundamental and overtone bands of CH and NH stretch vibrations in o -chloroaniline. (e), (f) and (g) represents the shift of NH peak for different bands on dilution with CCl4.
bond where as there is no peak in 3400 cm 1 in the case of NN -diethyl aniline, as expected. We also calculated the symmetric and asymmetric NH stretching frequencies in aniline, o - and m -chloroanilines using the AM1 computer program and found that the symmetric and asymmetric vibration differ by nearly 60 cm 1 in each of the three compounds, in agreement with experimental observations. The intensity of the asymmetric peak
(lying at higher wave numbers) is smaller than that of the symmetric one. The absorption frequencies corresponding to the different transitions for the symmetric and the asymmetric stretching vibrations are given in Table 1 and the calculated constants in the local mode model are given in Table 2. The molecular constants for the symmetric mode differ appreciably from those for the asymmetric mode in all the
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Fig. 3. (a), (b), (c) and (d) refers the fundamental and overtone bands of CH and NH stretch vibrations in m -chloroaniline. (e), (f) and (g) represents the shift of NH peak for different bands on dilution with CCl4.
three molecules. One notices that the frequencies for both the symmetric and asymmetric NH stretch modes in o -chloroaniline are nearly equal to their values in aniline. It appears that in ochloroaniline the change (increase) in the vibrational frequency due to the electron withdrawing
effect of the chlorine atom (added to aniline) is almost compensated by a decrease in this frequency due to the formation of an intramolecular hydrogen bond N/H Cl. Since such intramolecular hydrogen bonding is ruled out by the geometry of m -chloroaniline, there is no such
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Table 1 Transition frequencies for aniline, o -chloroaniline and m -chloroaniline 11/0
21/0
Aniline C/H stretch N/H sym. stretch N/H asym. Stretch
3038.2 (65) 3360.2 (190) 3420.0
5995.2 (102) 6678.2 (209) 6860.6
8762.1 (200) 9733.9 (266) 10054.8
o-Chloroaniline C /H stretch N/H sym. stretch N/H asym. stretch
3070.9 (80) 3377.7 (150) 3445.2
6004.1 (93) 6703.1 (162) 6881.8
m-Chloroaniline C /H (stretch) N/H sym. stretch N/H asym. stretch
3079.2 (80) 3379.0 (200) 3452.9
6013.6 (124) 6716.3 (225) 6899.4
31/0
41/0
5 1/0
61/0
11481.8 (299) 12683.0 (382)
14020"/ 15490"/
16475"/
8824.8 (223) 9782.0 (300) 10098.2
11517.6 (338) 12770.2 (453)
14114"/ 15523"/
16608"/
8808.5 (286) 9785.1 (605) 10105.6
11511.3 (599) 12787.15 (744)
14104.0"/ 15540.0"/
16578.0"/
"/, Denotes the values taken from reference [1]. ( ), The values in the brackets are FWHM.
compensation and a higher vibrational frequency (compared with aniline) is observed (Table 3). Nonat and coworkers [6,7] have studied the microwave spectrum of chloroanilines and have noted the presence of an intramolecular hydrogen bond in o -chloroaniline. Shaji and Rasheed [3] have observed the band which we have assigned to the asymmetric NH stretch mode in these molecules, but they seem to have interpreted it as a combination band. Combination bands at 7737 and 8296 cm 1, at 7758 and 8300 cm,1 and at 7808 and 8317 cm 1 have been assigned in aniline, o -chloroaniline and m -chloroaniline, respectively, in the present work.
The unusually small intensity observed for the asymmetric N /H stretching mode can be understood as follows. In the symmetric vibration both the N/H bonds change in the same way. The ‘s’ character of the nitrogen hybrid orbitals involved in the two bonds decreases as the N /H bond lengths increase. Any decrease in the ‘s’ character of the bonding hybrid also changes the character of the lone pair electron on the N -atom. Since the atomic dipole of the lone pair electron makes a significant contribution to the transition moment of the NH stretch vibration and this atomic dipole follows the change in the ‘s’ character of the bonding orbital. The change in the dipole moment
Table 2 Vibrational frequencies, anharmonicity constants and dissociation energies We /A/B (cm1)
De /w2e /4wexe (eV)
C/H stretch vibrations in aniline and its chloroderivatives Aniline 3088.19/12.3 55.99/3.6 o -Chloroaniline 3134.29/8.1 62.29/2.4 64.59/2.7 m -Chloroaniline 3140.29/9.4
3144.09/12.3 3196.49/10.5 3204.79/12.2
5.59/0.3 5.19/0.2 4.99/0.2
N/H (symmetric ) stretch vibrations in aniline and its chloroderivatives Aniline 3473.59/12.8 75.59/3.7 o -Chloroaniline 3480.59/22.6 72.19/8.6 74.69/4.3 m -Chloroaniline 3489.59/26.4
3549.19/16.5 3552.69/31.2 3564.09/30.7
5.29/0.2 5.49/0.5 5.39/0.2
N/H (asymmetric ) stretch vibrations in aniline and its chloroderivatives Aniline 3515.29/31.2 53.09/11.7 o -Chloroaniline 3533.09/24.6 54.49/9.3 m -Chloroaniline 3548.49/27.3 58.69/10.3
3568.19/42.7 3587.49/34.0 3607.09/37.5
7.49/1.1 7.49/1.0 6.99/0.9
Sample
A (cm1)
/B /wexe (cm1)
V.K. Rai et al. / Spectrochimica Acta Part A 59 (2003) 1299 /1306
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Table 3 Dilution effect in aniline, o -chloroaniline and m -chloroaniline Serial number
Dilution (% of the sample)
11/0
2 1/0
31/0
100 (no CCl4)
3360.2* 3377.7$ 3379.0% 3375.7* 3389.1$ 3393.0% 3386.1* 3398.3$ 3404.2% 3390.9* 3402.0$ 3408.3% 3395.2* 3412.8$ 3419.7% 3404.7* 3420.3$ 3427.0%
6678.2* 6703.1$ 6716.3% 6690.3* 6714.5$ 6721.2% 6696.3* 6717.6$ 6728.8% 6703.1* 6722.8$ 6730.3% 6707.2* 6728.4$ 6734.8% 6716.0* 6731.5$ 6736.3%
9734.0* 9782.0$ 9785.1% 9741.2* 9789.1$ 9791.4% 9751.9* 9796.2$ 9794.3% 9757.5* 9801.3$ 9799.3% 9759.7* 9804.5$ 9803.5% 9762.8* 9807.6$ 9805.9%
2
50
3
20
4
10
5
1
6
0.1
*, N/H stretching frequency for aniline. $, N/H stretching frequency for o -chloroaniline. %, N/H stretching frequency for m chloroaniline.
for the asymmetric mode is much smaller than for the symmetric mode.
4. Effect of dilution As mentioned earlier the absorption peaks due to the N/H stretching vibrations (symmetric and also the asymmetric) in the spectrum of all the three molecules are broadened due to intermolecular hydrogen bonding. The hydrogen bonding causes the formation of dimers, trimers etc. and the superposition of their slightly shifted absorption peaks gives a broad band similar to what is observed for OH absorption in many cases. In an earlier study [2] of the fundamental and overtone bands of OH recorded in the presence of CCl4 it was noted that addition of CCl4 inhibits formation of polymeric species. A considerable fraction of the sample then exists as monomer and gives a sharp absorption peak shifted to higher frequency as compared with the peak observed in the associated state. In the case of the three aniline compounds the spectra have been recorded at 50, 20, 10 0.1 and 0.1% (% sample in CCl4) dilutions
along with the undiluted samples. The spectra in the case of aniline, o- and m -chloroaniline are shown in Figs. 1/3 for Dy /1, 2, 3 1/0 transitions. One can clearly see that peaks show a blue shift with the decrease of concentration of the aniline compound. The maximum shift in the case of symmetric NH stretch for the fundamental band in aniline is 44 cm 1 (this value is much smaller than the shift /250 cm 1 observed in the case of the OH bond). It is thus clear that aniline may also have intermolecular hydrogen bonding though its strength is much smaller as compared with the case where OH group is involved. Whetsel et al. [8] have made a similar study on the fundamental band of some amines (including aniline and m chloroaniline) and have also noted similar shifts due to breaking of the intermolecular hydrogen bonds. It is to be noted that even in the monomeric situations the N/H stretch frequency of o -chloroaniline is smaller than in aniline which testifies to the existence of intramolecular N /H Cl hydrogen bond. Due to the relatively small ( /40 /50 cm 1) shift when these molecules reduce to the monomeric form the peak due to the free NH is seen to be overlapped with the peak of the
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V.K. Rai et al. / Spectrochimica Acta Part A 59 (2003) 1299 /1306
associated NH. As the dilution is increased the associated peak decreases in intensity where as that of free NH increases which causes a shift towards higher frequency of the superposed peak. The asymmetric peak also shows a similar behaviour. When we compare the change in the absorption peak positions with increasing CCl4 for different values of Dy , we find that the shifts are smaller as Dy increases. These observations are also similar to the earlier observation [2] in the case of OH vibration and are related to the growing importance of N/H Cl bonds for the higher vibrational excitations.
Acknowledgements Authors are grateful to CSIR for financial assistance. One of us (Vineet Kumar Rai) also
likes to thank Banaras Hindu University for a B.H.U. fellowship.
References [1] J. Vipin Prasad, I.B. Singh, S.B. Rai, S.N. Thakur, Indian J. Phys. 69B (1995) 315. [2] P.K. Srivastav, D.K. Rai, S.B. Rai, Spectrochim. Acta 56A (2000) 1283. [3] S. Shaji, T.M.A. Rasheed, Spectrochim. Acta 57A (2001) 337. [4] J. Vipin Prasad, S.B. Rai, S.N. Thakur, Chem. Phys. Lett. 164 (1989) 629. [5] V.K. Rai, S.B. Rai, Unpublished. [6] A. Nonat, A. Bouchy, G. Rouss, J. Mol. Struct. 108 (1984) 230. [7] A. Nonat, A. Bouchy, G. Rouss, J. Mol. Spectrosc. 116 (1984) 227. [8] K.B. Whetsel, W.E. Roberson, M.W. Krell, Anal. Chem. 32 (1960) 1281.