Spectrochimica Acta Part A 60 (2004) 53–56
Overtone spectra of aniline derivatives Vineet Kumar Rai, Amareswar Rai, D.K. Rai, S.B. Rai∗ Department of Physics, Laser and Spectroscopy Laboratory, Banaras Hindu University, Varanasi 221 005, India Received 22 November 2002; received in revised form 3 March 2003; accepted 15 March 2003
Abstract Overtone spectra of 2-ethylaniline, N-methylaniline, N-ethylaniline, N,N-dimethylaniline and N,N-diethylaniline have been studied in 2500–15 000 cm−1 region. Vibrational frequency and anharmonicity constants for aryl/alkyl C–H stretch and N–H stretch vibrations have been determined. The effect of substitution of C2 H5 group on the ortho position in the ring and CH3 /C2 H5 at the positions of the H-atom in NH2 group has been studied in these molecules. It is noted that the aryl C–H stretching frequency and the N–H stretching frequency is appreciably increased due to the replacement of H in NH2 group by CH3 /C2 H5 . These experimental observations are well supported by theoretical calculations for charge density on N-atom using molecular orbital AM1 method. © 2003 Elsevier B.V. All rights reserved. Keywords: Overtone spectroscopy; Hydrogen bonding; Symmetric and asymmetric stretching; Anharmonicity
1. Introduction Recently we [1,2] investigated the vibrational overtone spectrum of aniline and its chloroderivatives. It has been noted that the frequency of the C–H and N–H stretch vibrations are affected appreciably due to the presence of an electron withdrawing substituent (chlorine). Shaji and Rasheed [3] have also made a similar study on chloroderivatives of aniline. In a recent paper Shaji et al. [4] studied the spectrum of N and NN methyl derivatives of aniline and marked changes in the aryl CH stretching frequency in these molecules. Whetsel et al. [5] have studied the solvent and concentration effects on the near-infrared (NIR) N–H bands of primary aromatic amines. They concluded that the band shifts to longer wavelengths with an increase in concentration and becomes broader and weaker. The position, intensity and width of the band is shown to depend on the solvent. In this paper, we have recorded the overtone absorption spectrum of CH and NH stretch vibrations in liquid phase 2-ethylaniline, N-methylaniline, N-ethylaniline, N,Ndimethylaniline and N,N-diethylaniline in the infrared (IR) and NIR regions. Bands involving v = 1–4 of NH/CH ∗ Corresponding author. Tel.: +91-542-231-8990; fax: +91-542-31-7074. E-mail address:
[email protected] (S.B. Rai).
1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1386-1425(03)00179-3
stretch vibration in 2-ethylaniline, N-methylaniline and N-ethylaniline molecules and those with v = 1–5 for C–H stretch vibration for N,N-dimethylaniline and N,Ndiethylaniline molecules have been recorded. These data have been used to calculate vibrational frequency and anharmonicity constants for the CH (aryl as well as alkyl) and NH stretch vibrations using local mode model. We have also calculated the charge density on the N, C and H-atoms to determine changes due to the presence of CH3 /C2 H5 at different positions using molecular orbital AM1 computer program. The charge density on these atoms is in consonance with our measurements. 2. Experimental Samples of 2-ethylaniline (obtained from Merck) with 99% purity and N-methylaniline, N-ethylaniline, N,Ndimethylaniline and N,N-diethylaniline (obtained from BDH India) with 99.9% purity were used in the present experiments without any further purification. The NIR spectra were recorded using a Lambda-19UV-Vis-NIR double beam spectrophotometer, while for the fundamental band a JASCO FTIR-5300 model spectrophotometer was utilized. All the spectra were measured at room temperature (30 ◦ C) with 1 cm sample path length. Several scans were made for each and every sample with changed sensitivity so as to guard against any artifacts.
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V.K. Rai et al. / Spectrochimica Acta Part A 60 (2004) 53–56
Fig. 1. Overtone bands of CH and NH stretch vibrations in N-ethylaniline.
3. Results and discussion A part of the absorption spectrum of N-ethylaniline in the region 2500–15 000 cm−1 is shown in Fig. 1. The spectrum in the 2800–3100 cm−1 region shows three peaks for all the compounds. The intensity of the peak corresponding to the lowest frequency is smaller than for the other two. This lower energy peak is ascribed to the alkyl C–H stretch vibration while the other two are assigned to aryl C–H stretch vibrations. In addition there appear two peaks near 3400 cm−1 in 2-ethylaniline and are due to symmetric and asymmetric N–H stretch vibrations. However, only one intense peak near 3400 cm−1 is seen for N-methylaniline and N-ethylaniline. The appearance of only one peak is due to the replacement of one hydrogen of NH2 group by the methyl/ethyl group. No bands in the 3400 cm−1 region are observed for N,N-dimethylaniline and N,N-diethylaniline molecules. The first overtone of C–H (aryl/alkyl) stretch and N–H stretch vibrations and their combination bands are seen near 6000 cm−1 for all the five molecules. Similarly, the second overtone bands of these vibrations in all these five molecules lie near 9000 cm−1 . The position of the fourth and in some cases the fifth overtones could be ascertained only after repeated scans as the bands are very weak. The C–H (aryl/alkyl) stretch vibrational frequencies are given in Table 1. The N–H stretch frequencies for 2-ethylaniline, N-methylaniline and N-ethylaniline are also listed in the same Table 1. The vibrational frequency and the anharmonicity constants for the C–H and the N–H stretch vibrations were calculated using the relation given by Henry and co-workers [6,7] Ev = Av + Bv2 , where we = A−B, we xe = −B. These constants along with the estimated errors in these are given in Table 2. We have also calculated the dissociation energy of CH bond using Morse potential and the values for the five molecules are given in Table 2. Electron charge
density on the N-atom and adjacent C-atom in aniline and its different derivatives are given in Table 3. 3.1. Overtone bands due to C–H stretch Since the alkyl groups (CH3 , C2 H5 . . . ) are electron donating substituents, the aryl C–H stretch frequency for Table 1 Fundamental and overtone transition frequencies (in cm−1 ) for CH and NH stretching vibrations in 2-ethylaniline, N-methylaniline, N-ethylaniline, N,N-dimethylaniline and N,N-diethylaniline 1←0
2←0
3←0
4←0
5←0
2-Ethylaniline C–H (stretch) (aryl) (alkyl C–H))
3038.4 2880.1
5952.3 5689.3
8754.2 8261.7
11491.5 10866.3
N–H (stretch) (sym.) N–H (asym.)
3385.4 3456.7
6673.7 6857.0
9772.5 10102.3
12785.7
N-Methylaniline CH (stretch) (aryl) 3032.1 (alkyl) 2887.2 NH (stretch) 3403.0
5945.1 5670.1 6695.1
8779.3 8268.2 9860.2
11458.4 10837.3 12910.3
N-Ethylaniline C–H (stretch) (aryl) (alkyl) N–H (stretch)
3034.0 2864.3 3406.0
5949.2 5625.8 6718.5
8795.1 8247.1 9878.7
11493.0 10726.4 12920.3
N,N-Dimethylaniline C–H (stretch) (aryl) 3030.0 (alkyl) 2870.0
5982.1 5632.5
8785.3 8254.3
11505.2 10718.3
14075.7
N,N-Diethylaniline C–H (stretch) (aryl) 3036.2 (alkyl) 2874.2
5987.5 5640.4
8779.1 8258.1
11502.1 10714.5
14068.3
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Table 2 Molecular constants and error estimates for C–H and NH stretching vibration in 2-ethylaniline, N-ethylaniline, N,N-dimethylaniline and N,N-diethylaniline. A (cm−1 )
−B = we xe (cm−1 )
we = A−B (cm−1 )
De = w2e /4 we xe (eV)
2-Ethylaniline (aryl) (alkyl)
3084.7 ± 2.8 2941.8 ± 19.5
53.0 ± 0.8 57.4 ± 2.6
3137.7 ± 3.6 2999.3 ± 22.1
5.8 ± 0.1 4.8 ± 0.1
N-Methylaniline (aryl) (alkyl)
3089.6 ± 5.7 2942.1 ± 11.8
55.9 ± 1.6 58.8 ± 3.4
3145.5 ± 7.4 3001.0 ± 15.2
5.5 ± 0.2 4.7 ± 0.2
N-Ethylaniline (Aryl) (Alkyl)
3094.2 ± 5.7 2936.4 ± 5.0
52.7 ± 1.6 63.4 ± 1.4
3146.9 ± 7.4 2999.9 ± 6.4
5.8 ± 0.15 4.4 ± 0.02
N,N-Dimethylaniline (Aryl) (Alkyl)
3099.7 ± 3.8 2947.8 ± 6.4
56.1 ± 1.1 66.7 ± 1.8
3155.8 ± 4.9 3014.5 ± 8.2
5.5 ± 0.1 4.2 ± 0.01
N,N-Diethylaniline (aryl) (alkyl)
3102.4 ± 6.0 2954.7 ± 6.8
57.1 ± 1.7 68.6 ± 1.9
3159.5 ± 7.1 3023.3 ± 8.7
5.4 ± 0.04 4.1 ± 0.01
N-Methylaniline (NH sym.)
3469.2 ± 2.7
60.8 ± 0.
3530.0 ± 3.7
6.3 ± 0.01
N-Ethylaniline (N–H sym.)
3460.6 ± 6.1
66.2 ± 1.8
3526.8 ± 7.4
5.9
2-Ethylaniline (sym.) (asym.)
3476.01 ± 8.9 3523.8 ± 11.5
60.7 ± 3.5 51.6 ± 4.3
3536.7 ± 9.1 3575.4 ± 11.5
6.4 ± 0.3 7.7 ± 0.5
Molecule C–H stretch vibration
N–H stretch vibration
ethylaniline is expected to be smaller than in aniline. On substitution of C2 H5 group at the O-position the magnitude of the aryl CH stretch vibrational frequency becomes smaller than in aniline. On the other hand the aryl C–H stretching frequency for N-ethylaniline (where one hydrogen of the NH2 group has been replaced by C2 H5 ) is larger than in aniline and much larger than for 2-ethylaniline. This C–H stretch frequency is further increased when both the hydrogen atoms of the NH2 group are replaced by CH3 /C2 H5 . The observed (aryl) C–H stretch frequency for 2-ethylaniline, N-methylaniline, N-ethylaniline, N,N-dimethylaniline and N,N-diethylaniline are 3137.9, 3145.5, 3146.9, 3155.8 and 3159.5 cm−1 , respectively, while that for aniline is 3144.0 cm−1 [2]. This is well correlated with the charge on the N-atom as it gets reduced on replacing the hydrogen by the alkyl group. CH3 /C2 H5 and NH2 all are strong electron Table 3 Electron charge density on N-atom and adjacent C-atom in different derivatives of aniline Molecule
Charge density on N-atom (in units of electronic charge)
Charge density on adjascent C-atom (in units of electronic charge)
Aniline 2-Ethylaniline N-Methylaniline N-Ethylaniline N,N-Dimethylaniline N,N-Diethylaniline
−0.326 −0.325 −0.290 −0.279 −0.282 −0.271
+0.0541 +0.0516 +0.0635 +0.0615 +0.0744 +0.0907
donating groups. It appears that the substitution of C2 H5 or CH3 at the place of H in the NH2 group reduces the accumulation of electrons at N-atom. This reduction induces it to withdraw electrons from the adjacent C-atom of the ring. Thus the electron density on the ring carbon atom decreases and consequently there is a reduction on the C–H bond length. The mechanical frequency for CH stretching is thus increased. So due to substitution of CH3 /C2 H5 for H in NH2 the aryl CH stretching frequency shows an increase. In order to verify this expectation of a reduced charge in N-atom we made a molecular orbital calculation for these molecules using AM1 method. The electron charge density at different atoms in aniline, 2-ethylaniline, N and NN methyl and ethyl anilines are given in Table 3. From this Table it is obvious that the excess charge density at N-atom follows the trend N,N-dimethylaniline
