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Protolytic dissociation of cyanoanilines in the ground and excited state in water and methanol solutions
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Spectrochimica Acta Part A 71 (2008) 403–409
Protolytic dissociation of cyanoanilines in the ground and excited state in water and methanol solutions Beata Szczepanik a,∗ , Stanisław Styrcz a,∗ , Maciej G´ora b a
b
Institute of Chemistry, Swietokrzyska Academy, Checinska 5, 25-020 Kielce, Poland Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland Received 15 December 2007; accepted 2 January 2008
Abstract The effect of cyano substituents on the photoacidity of mono- and dicyanoanilines has been investigated. It was demonstrated that the cyano substitution increases significantly the acidity of aniline derivatives in the excited state in comparison to the ground state. 3,5-Dicyanoaniline is the strongest acid in the lowest excited singlet state, while 4-cyanoaniline is the weakest one. The derivatives of aniline with two cyano groups in o,o -position show different properties from those characteristic for aniline and other investigated cyanoanilines. In the methanol solution with sodium methanolate the anions of 2,6-dicyano-3,5-dimethylaniline and 2,6-dicyano-3,5-diphenylaniline appear already in the ground state. The electronic ground and excited state charge distributions and dipole moments of all investigated cyanoanilines have been evaluated by ab initio calculations using the GAMESS program. © 2008 Elsevier B.V. All rights reserved. Keywords: Fluorescence; Photoacidity; Cyanoanilines; Anions
1. Introduction Hydroxyarenes and aromatic amines (aniline and its derivatives) have been an object of investigation in the spectroscopy for many years, because they show large enhancement in acidity due to migration of charge from electronegative atom center to the ring in the excited state compared to the ground state [1–7]. Proton dissociation and acid–base equilibria in the excited state are strongly affected by substituents introduced in the aromatic ring [8–10]. Tolbert and Solntsev [11] and Agmon [12] have reported a remarkably increased photoacidity of monocyanonaphtols and dicyanonaphtols in the excited state, which was explained on the basis of the charge migration from the hydroxyl group to the C-5 and C-8 positions in the distal ring in the cyanonaphtol molecule. The pKa value of 2-naphtol drops from 9.5 in the ground state to 2.8 in its first excited singlet state. In the case of cyano-substituted 2-naphtol molecules the presence of the electron-withdrawing CN group makes these molecules more acidic than 2-naphtol. The doubly substituted CN derivatives
∗
Corresponding author. Tel.: +48 41 349 70 53; fax: +48 41 349 70 62. E-mail address: [email protected] (B. Szczepanik).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.01.003
are almost as strong as a mineral acid, pKa = −4.5 in the excited state. Schulman et al. [9] have investigated the proton dissociation processes in the lowest excited singlet state of cyanophenols by fluorimetric titrimetry. They found that in the excited state ocyanophenol was the strongest acid whereas the p-cyanophenol was the weakest one. The proton dissociation processes from the lowest excited singlet state of protonated m- and p-cyanoanilines and m- and pmethoxyanilines in an aqueous solution have been investigated using picosecond time-resolved fluorescence measurements by Shiobara et al. [13,14]. They have found that the position and the electronic character of the substituent strongly influence the rate of proton dissociation. The cyano substitution (with an electron-withdrawing group) at the meta-position influences the rate more strongly in comparison to aniline, while the methoxy substitution (an electron-donating group) reduces the rate remarkably. The pKa of protonated aniline, m- and p-cyanoaniline in the ground state is 4.6, 2.7 and 1.7, respectively. In the first excited state the acidity of these compounds strongly changes, resulting in pKa∗ = −6.5 for aniline, −10.5 for m-cyanoaniline and −3.7 for p-cyanoaniline [15].
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In the present work, we have studied the protolytic dissociation in the ground and first singlet excited electronic state of aniline derivatives with two cyano groups: 2,6-dicyano3,5-dimethylaniline (oDCA), 3,5-dicyanoaniline (mDCA), 2,6-dicyano-3,5-diphenylaniline (oDCPA) and, for comparison, of o-, m- and p-cyanoaniline (oCA, mCA and pCA) (see Scheme 1). We present: (i) the results of steady-state measurements of absorption and fluorescence in aqueous and methanolic solutions, (ii) ab initio calculations of the electronic structure, the dipole moments and molecular geometry of the investigated molecules in the electronic ground and excited states. 2. Experimental 2.1. Chemicals 2,6-Dicyano-3,5-dimethylaniline (oDCA) was synthetized using the procedure described in Ref. [16]. 2,6-Dicyano-3,5diphenylaniline (5 -amino-1,1 :3 ,1 -terphenyl-4 ,6 -dicarbonitrile) (o-DCPA) was synthetized using procedure described in Ref. [16a]. Spectroscopic data of oDCPA: 1 H NMR (DMSOd6 , 300 MHz): 6.79 (s, 1H); 6.81 (s, 2H, NH2 ); 7.49–7.54 (m, 6H); 7.60–7.65 (m, 4H); 13 C NMR (DMSO-d6 , 75 MHz): 93.97, 115.90, 118.44, 128.44, 128.58, 129.34, 137.34, 149.57, 154.02. Synthesis of 3,5-dicyanoaniline (mDCA) is described in Ref. [17]. Spectroscopic data of mDCA: 1 H NMR (500 MHz, DMSO-d6 ) 6.12 (s, 2H), 7.15 (d, 2H, J = 1.4 Hz), 7.39 (t, 1H, J = 1.4 Hz); 13 C NMR (125 MHz, DMSO-d6 ) 113.08, 117.72, 120.08, 121.15, 150.13; IR max/cm−1 : 435, 672, 865, 1328, 1343, 1443, 1599, 1630, 2230, 3344, 3463. o-Cyanoaniline (oCA), m-cyanoaniline (mCA) and pcyanoaniline (pCA) are commercial (Aldrich). The solvents were purchased from Merck and Aldrich and were spectroscopic or HPLC grade. The perchloric acid was purchased from Apolda (Germany), and sodium and sodium hydroxide from P.O.Ch. Gliwice (Poland).
3. Results 3.1. Protolytic dissociation of investigated anilines in aqueous solutions Figs. 1 and 2 show the steady-state absorption and fluorescence spectra of oCA, mCA, pCA, oDCA and mDCA measured in aqueous solutions under different pH conditions at 293 K. Detailed data for all amines are presented in Table 1. The absorption spectra obtained in neutral and alkaline solutions (3 M NaOH) are attributed to neutral molecules of all investigated amines (Fig. 1). Alkalization of the solutions does not lead to a change of the absorption spectrum, indicating the lack of cyanoanilines anions formation in aqueous solution in the ground state. The first absorption bands of oCA and mCA appear at similar energies (∼32,000 cm−1 ). In the case of pCA this band is shifted into the blue about 5000 cm−1 . The absorption spectra of derivatives with two cyano groups are shifted into the red in comparison to the derivatives with one cyano substituent (oCA and mCA) about 2000 cm−1 . The fluorescence spectra obtained in the neutral solution for all above mentioned amines are ascribed to the neutral form of cyanoanilines, but in the case of pCA we observe at the
2.2. Methods The absorption spectra were recorded on a SPECORD M500 UV–VIS spectrophotometer (Carl-Zeiss Jena). The steady-state fluorescence spectra were measured on the KONTRON SFM 25 spectrofluorimeter (Kontron Instruments Germany). The quantum yields were obtained using quinine bisulphate in 0.1 N H2 SO4 (Φf = 0.52) and 2-amino-pyridine in 0.05 M H2 SO4 (Φf = 0.74) as standards [18,19]. 2.3. Quantum chemical calculations The distributions of electronic densities and the dipole moments of the cyanoanilines molecules in the ground and first excited state were obtained on the basis of ab initio calculations in the base 6-31G*, using GAMESS program [20]. Additionally the CI calculations taking into consideration the one- and two-electron excitation were performed. The geometries optimization was obtained on the basis of ab initio calculations in the base STO-3G with the application of the AM1 method [21].
Fig. 1. Absorption and corrected fluorescence spectra of oCA, mCA, pCA, oDCA and mDCA in water and in alkaline solution (3 M NaOH) at 298 K (the absorption spectrum of mDCA was normalized to the second band in water).
B. Szczepanik et al. / Spectrochimica Acta Part A 71 (2008) 403–409
Fig. 2. Absorption and corrected fluorescence spectra of oCA, oDCA and mDCA in water and in acidic solution (HClO4 , pH about 1) at 293 K (the absorption spectrum of mDCA was normalized to the second band in water).
long wavelength side the trace of an additional band of fluorescence. In the alkaline solution a new band appears at the long wavelength side for oCA, pCA and mDCA, peaking at 21,000, 23,200 and 18,500 cm−1 , respectively (Fig. 1). These bands can be ascribed to anionic forms of investigated compounds. The fluorescence spectra of mCA in neutral and alkaline solutions are similar, but further alkalization of the solution leads to a Table 1 Spectroscopic data of oCA, mCA, pCA, oDCA and mDCA at 293 K in water, alkaline and acidic solutions: molar extinction coefficient ε (dm3 mol−1 cm−1 ), absorption and fluorescence transition energy maxima ν˜ amax and ν˜ fmax (cm−1 ) and Stokes shifts ˜νSt
405
Scheme 1. Chemical structure of the compounds investigated.
change of the fluorescence spectrum with the trace of a new band appearing at the long wavelength side. This trace can confirm that also in the case of mCA an anion is formed in the excited state. In the case of oDCA we do not observe a fluorescence band corresponding to the neutral molecule, but at the long wavelength side a new band with maximum at 20,100 cm−1 appears, suggesting the emission only from anionic form of oDCA in the alkaline solution (Fig. 1). In highly acidic conditions (5 M HClO4 ) the absorption spectra of protonated form of oCA and mDCA is are observed at 36,000 and 34,500 cm−1 , respectively (Fig. 2). In the case of mCA and pCA investigated earlier by Shiobara et al. [14] under similar conditions the protonated forms of these amines absorb at similar energy like oCA (about 36,000 cm−1 ). The fluorescence spectra of oCA and mDCA show bands resulting from the presence of the deprotonated species of investigated amines (Fig. 2). oDCA is not protonated even in 5 M HClO4 . We can observe only the absorption and fluorescence bands of the neutral molecule (Fig. 2). The fluorescence quantum yield of oDCA is relatively large in water and acidic solutions (0.22 and 0.18, respectively). In the alkaline solution the anion fluoresces stronger that the neutral molecule, Φf = 0.41.
Solvent
ε
ν˜ amax
ν˜ fmax
˜νSt
oCA Water 5 M HClO4 3 M NaOH
3,200 1,000 3,100
32,000 36,000 31,700
26,200 25,800 26,200, 21,000
5,800 10,200 6,500
mCA Water 5 M HClO4 3 M NaOH
2,500 600 2,700
32,500 36,000 32,500
24,200
8,300
24,500
8,000
21,400
37,200
28,400
8,800
20,400
37,000
28,300, 23,200
8,700
3.2. Protolytic dissociation in methanol solutions
oDCA Water 5 M HClO4 3 M NaOH
8,100 7,800 4,000
29,400 29,300 29,100
26,100 26,100 20,100
3,300 3,200 9,000
mDCA Water 5 M HClO4 3 M NaOH
2,800 800 2,600
30,300 34,500 31,000
24,700 25,200 24,600, 18,500
5,600 9,300 6,400
Absorption and fluorescence spectra of oDCPA were measured in the methanol–water solution (60/40%, v/v) because of very weak solubility of this compound in neat water. Fig. 3 shows the steady-state absorption and fluorescence spectra of oDCPA recorded in methanol–water solutions (60/40%, v/v) with sodium hydroxide, in methanol with sodium hydroxide and in methanol with sodium at 293 K.
pCA Water 5 M HClO4 3 M NaOH
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Fig. 3. Absorption and corrected fluorescence spectra of oDPCA in methanol–water (6:4, v/v)—1 M NaOH solution, in methanol with NaOH and in methanol with sodium at 293 K.
The absorption spectrum measured in methanol solutions with NaOH shown in Fig. 3 is attributed to a neutral molecule of oDCPA (maximum 27,200 cm−1 ). As a result of alkalization of methanol–water solution (60/40%, v/v) very weak absorption appears at the long wavelength side, which could be ascribed to the anion of oDCPA. Addition of sodium to the methanol solution leads to the change of the absorption spectrum with a new band appearing at the long wavelength side, with the maximum at 22,000 cm−1 . This band can be ascribed to the anionic form of oDCPA. The fluorescence spectra recorded in methanol–water and methanol solutions with NaOH show two bands: the band with maximum at 23,200 cm−1 which can be ascribed to the neutral molecule of oDCPA and the band at the long wavelength side, with the maximum at 17,400 cm−1 ascribed of the oDCPA anion. In methanol with addition of sodium only one band appears peaking at 17,400 cm−1 ). The absorption and fluorescence spectra were measured under the same conditions for other derivatives of aniline with two cyano groups—oDCA and mDCA. Fig. 4 shows the absorption and fluorescence spectra of oDCA in methanol, methanol with sodium and the alkaline water solution. Absorption spectra of oDCA in the alkaline water solution and in methanol are blue shifted about 2000 cm−1 in comparison to oDCPA and correspond to the neutral molecule. In the methanolate solution a new band appears on the long wavelength side similarly to oDCPA, and is shifted also about 2000 cm−1 compared with the anion of oDCPA (Fig. 4). The fluorescence spectrum recorded in methanol with maximum at 26,100 cm−1 was ascribed to the neutral molecule of oDCA. In methanol–water solutions with addition of sodium hydroxide and in methanol with sodium, only one band appears peaking at about 20,100 cm−1 , which can be ascribed to the anion (Fig. 4).
Fig. 4. Absorption and corrected and normalized fluorescence spectra of oDCA in methanol with sodium, alkaline water solution (1 M NaOH) and methanol at 293 K. Excitation fluorescence spectrum was measured in methanol + Na solution, λobs = 20,200 cm−1 .
In the case of mDCA we do not obtain the anion in the ground state in the methanolate solution. We observed only the absorption spectrum corresponding to the neutral molecule. 3.3. Acidity constants The change of acidity constant −pKa value in the excited state, was calculated on the basis of the F¨orster cycle according to Eq. (1) [22]: pKa =
0.625 − Nhc A− HA A HA = − ν˜ 00 ν˜ 00 − ν˜ 00 ν˜ 00 2.3RT T
(1)
where A− ν˜ 00 =
a +ν f ˜ A− ν˜ A− 2
HA ν˜ 00 =
a +ν f ˜ HA ν˜ HA 2
A− HA are the energies and ν˜ 00 T is the temperature on the Kelvin, ν˜ 00 (cm−1 ) of the 0–0 transitions for the anion A− and the neutral a and ν a are the wavenumbers of the absorption ˜ HA acid HA, ν˜ A− f and band maxima of the anion A− and the neutral acid HA, ν˜ A− f ν˜ HA are the wavenumbers of the fluorescence maxima of these species. In the case of the estimation of pKa for investigated amines in aqueous solutions the problem is the lack of absorption spectra of the anion in the alkaline solution. In this situation, we have used another approach of the calculation of pKa , which can be performed using the difference in energy between the
B. Szczepanik et al. / Spectrochimica Acta Part A 71 (2008) 403–409 Table 2 Wavenumbers of fluorescence maxima for o-, m-, pCA, oDCA and mDCA in water (pH ∼ 6) and strongly basic aqueous solution (293 K): ν˜ fmax (cm−1 ), pKa calculated from Eq. (2) Compound
ν˜ fmax (cm−1 ) pH ∼ 6
ν˜ fmax (cm−1 ) 2 M NaOH
pKa
oCA pCA oDCA mDCA
26,200 28,400 26,100 24,700
21,000 23,200 20,100 18,500
−11.1 −11.1 −12.8 −13.2
407
including alcohols can be sometimes considerably larger than that for aqueous solutions [23]. pKa estimated from the F¨orster cycle in methanolic solutions (calculated from the 0–0 transition wavenumbers) is found to be −11.7 for oDCA and oDCPA. The value obtained basing on the energy differences of the fluorescence maxima is about one unit greater and identical in aqueous and methanolic solutions for oDCA (Tables 2 and 3). We can suppose that the photoacidity enhancement in the excited state is comparable in both solutions for the investigated cyanoanilines.
fluorescence maxima of the anionic and neutral forms (Eq. (2)).
3.4. Quantum chemical calculations
Nhc max − ν˜ f (A ) − ν˜ fmax (HA) 2.3RT 0.625 max − (2) = ν˜ f (A ) − ν˜ fmax (HA) T The data used for the calculation of pKa in aqueous solutions are collected in Table 2. In methanolic solutions pKa for oDCA and oDCPA have been calculated using the 0–0 energy, because we have obtained absorption spectra of anilines anions in these conditions. For comparison, pKa was determined also from the fluorescence maxima of the neutral and anionic form of investigated anilines. Results are collected in Table 3. The acidity of the investigated amines in the aqueous solution strongly changes in the excited state, resulting in pKa from −11.1 for oCA and pCA to −12.8 and −13.2 for oDCA and mDCA (Table 2). The photoacidity enhancement obtained on the basis of the F¨orster cycle from measurements in non-aqueous solvents
Ab initio calculations of the molecular geometry for the ground and lowest excited states of the mono- and disubstituted cyanoanilines were carried out by Kolek et al. [17,24–27]. The authors have compared the geometry change upon electronic excitation for aniline and 2-, 3-, 4-cyanoaniline, 2,6-, 3,5-dicyanoaniline and 2,6-dicyano-3,5-dimethyl-aniline. The aniline molecule is non-planar in the ground electronic state, with the NH2 plane at 42◦ with respect to the ring plane. In the lowest singlet excited state the aniline molecule becomes planar [27]. For mono-cyanoanilines and 3,5-dicyanoaniline the ab initio calculations predict a non-planar equilibrium geometry in the ground electronic state (all these molecules have a pyramidal amino group in ground state) like in the case of aniline. Geometric optimization of these aniline derivatives gives almost planar structure in the S1 excited state. The situation is different only in the case of 2,6-dicyanoaniline and 2,6-dicyano3,5-dimethylaniline. The molecules having two cyano groups in ortho positions reveal different properties from those character-
pKa =
Fig. 5. The distributions of the charge densities (in the area of amino- and cyanogroup) for oDCA, mDCA, o-, m- and pCA in the S0 and S1 (values in brackets).
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Table 3 Wavenumbers of absorption and fluorescence maxima for oDCA and oDCPA in methanol (MeOH) and in methanolate solution (methanol + Na) (MeONa) (293 K): abs and ν f ˜ max ν˜ max (cm−1 ); 0–0 transition wavenumbers of the neutral molecule and the anion: ν˜ 00 (cm−1 ); pKa as calculated from Eq. (1) Compound
oDCA oDCPA a b c
abs (cm−1 ) ν˜ max
f ν˜ max (cm−1 )
pKa a
MeOH
MeONa
MeOH
MeONa
29, 000 27, 200
24,000 22,000
26, 200 23, 200
20,200 17,400
−12.8 −12.4
ν˜ 00 (cm−1 )b
pKa c
MeOH
MeONa
27, 600 25, 200
22,100 19,700
−11.7 −11.7
pKa determined from the fluorescence band maxima. ν˜ 00 determined from the mean of absorption and fluorescence band maxima. pKa calculated from the 0–0 transition wavenumbers.
istic for aniline. The interactions between the amino group and the cyano groups in these molecules induce significant rehybridization at the nitrogen atom, thus in the ground S0 and excited S1 states the molecules of these compounds have an almost planar structure [27]. In this paper we have performed ab initio calculations for all the investigated cyanoanilines molecules in the ground and first excited state. The charge distribution in the ground electronic state S0 and the lowest S1 electronic excited state are presented in Fig. 5. The results presented in Fig. 5 show that the sum of charge densities on atoms of the amino group becomes more positive in the first excited state in comparison to the ground state for all investigated amines. The charge densities on the amino group decrease after excitation in the following order: oDCA > mDCA > oCA > mCA > pCA The charge densities on atoms of the cyano groups have considerably negative values. In the excited state these values become more negative in the same order in which the charge densities on the amino group become more positive. The decrease of charge density on the amino group in the excited state corresponds to the direction of the decrease of the investigated amines basicity with the pKa of the dicyano-derivatives being higher than that for the monocyano-derivatives of aniline. The dipole moments in the ground state show values from 3.72 for oDCA to 5.70 D for pCA (Table 4). The dipole moments change rather little upon excitation, μ is from 0.14 D for pCA to 1.14 D for mCA. The highest increase is observed for meta cyanoderivatives of aniline while the weakest for pCA, which stands in correlation with the pKa values for these compounds (Table 2).
Table 4 Ground state dipole moments (D) (μg ) and the changes of the dipole moment upon excitation (in the first excited singlet state) (μ (D)) Compound
μg (D)
μ (D)
oCA mCA pCA mDCA oDCA
4.43 5.07 5.70 5.31 3.72
0.84 1.14 0.14 0.96 0.57
4. Conclusions The presented study provides new information concerning the protolytic reaction of mono- and dicyanoanilines in the ground and first excited singlet state. In alkaline aqueous solutions the anions of all monocyanoanilines and mDCA were not formed in the ground state even in strong alkaline solutions, which remains in accordance with the character of aromatic amines which can form anions only in strong bases like ammonia in the presence of sodium or potassium amides [28]. In the excited state the acidity of amino group increases strongly: pKa ∼ −11 for o- and pCA and ∼−13 for derivatives with two cyano groups and the anions are formed for all amines. mDCA is a stronger acid in the S1 state than oDCA while oDCA is a stronger acid than oCA and pCA. The order of acidity in the S1 state suggests that the inductive effect of cyano substituents is more important in the case of cyanoanilines than the resonance effect. In the acidic aqueous solution the monocyanoanilines and the derivative with two cyano groups in meta position form the cations in the ground state in accordance with the alkaline character of the amino group, so we can observe the cation absorption for oCA and mDCA. In the excited state the fluorescence of the neutral molecule is observed only, like in the case of m- and p-cyanoaniline [14], suggesting the proton dissociation reaction of the protonated cyanoaniline derivatives in the excited state. The derivatives of aniline with two cyano groups in o,o position show different properties from those characteristic for mono-cyanoanilines and mDCA. oDCA and oDPCA did not form the cations in the ground state nor in the excited state. The two substituents in the proximity of amino group act as steric hindrances, making the protonation of amino group difficult. In the methanol solution with addition of sodium we obtained the anions of all studied anilines in the excited state, but only in the case of oDCA and oDCPA the anions appeared already in the ground state in these conditions. The presence of two cyano groups in ortho position in the case of oDCA and oDCPA causes considerable migration of negative charge from the nitrogen atom to the aromatic ring and as a result the basicity of the amino group can decrease more strongly in the ground state than for derivatives with one cyano group and aniline. This decrease of basicity is so considerable that the anion formation is possible in the ground state in methanol solution containing the sodium methanolate.
B. Szczepanik et al. / Spectrochimica Acta Part A 71 (2008) 403–409
The charge migration from the amino group to the benzene ring is the result of the structural change of the amino group in the S1 state. Amino nitrogen of monocyanoanilines in the ground state sp3 has the configuration with almost pyramidal structure, while upon excitation the configuration of the amino group changes to sp2 with almost planar structure [14]. The unusual behaviour of oDCA and oDCPA in protolytic reactions results not only from the strongest interactions between the amino and cyano groups in comparison to all the other aniline derivatives examined, but also from the presence of two cyano groups in o,o -position which make the recombination of the anion with the proton difficult, leading to the stabilization of the anion already in the ground state. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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[14] S. Shiobara, S. Tajima, S. Tobita, Chem. Phys. Lett. 380 (2003) 673– 680. [15] H. Shizuka, S. Tobita, Proton transfer reactions in the excited states, in: V. Ramamurthy, K.S. Schanze (Eds.), Organic Photochemistry and Photophysics, Molecular and Supramolecular Photochemistry, vol. 14, CRC Press, Boca Raton, London, New York, 2006. [16] R. Hull, N. Campbell, W.H. Stafford, J.F.K. Wilshire, L.H. Briggs, G.A. Nicholls, A.W. Johnson, N. Sheppard, A.R. Todd, J. Ratcliffe, D. BryceSmith, K.E. Howlett, L.H. Groves, G.A. Swan, J. Chem. Soc. (1951) 1136; (a) J. Sepioł, P. Milart, Tetrahedron 41 (1985) 5261. [17] P. Kolek, K. Pirowska, Ł. Chacaga, J. Najbar, Phys. Chem. Chem. Phys. 5 (2003) 4096. [18] S.R. Meech, D. Phillips, J. Photochem. 23 (1983) 193. [19] W.M. Melhuish, Nat. Bur. Stand. 378, Proc. Conf. NBS, Gaithersburg, 1970. [20] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen, S. Koseki, N. Matsunaga, K.A. Nguyen, S.J. Su, T.L. Windus, M. Dupuis, J.A. Montgomery, J. Comput. Chem. 14 (1993) 1347. [21] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [22] T. F¨orster, Z. Elektrochem. Ber. Bunsenges. Physik. Chem. 54 (1950) 42. [23] D. Pines, E. Pines, in: J.T. Hynes, J.P. Klinman, H.H. Limbach, R.L. Schowen (Eds.), Hydrogen Transfer Reactions, Wiley-VCH, Weinheim, 2007, p. 377. [24] K. Pirowska, P. Kolek, J. Goclon, J. Najbar, Chem. Phys. Lett. 387 (2004) 165. [25] P. Kolek, K. Pirowska, J. Najbar, Phys. Chem. Chem. Phys. 3 (3) (2001) 4874. [26] P. Kolek, K. Pirowska, M. G´ora, B. Kozik, J. Najbar, Chem. Phys. 285 (2002) 55. [27] P. Kolek, P. Gajdek, K. Pirowska, J. Najbar, J. Mol. Struct. 511/512 (1999) 117–131. [28] E. D¨oller, T. F¨orster, H. Renner, Z. Elektrochem., Bor. Bunsengers Physik Chem. 56 (1952) 137.
Spectrochimica Acta Part A 71 (2008) 403–409
Protolytic dissociation of cyanoanilines in the ground and excited state in water and methanol solutions Beata Szczepanik a,∗ , Stanisław Styrcz a,∗ , Maciej G´ora b a
b
Institute of Chemistry, Swietokrzyska Academy, Checinska 5, 25-020 Kielce, Poland Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland Received 15 December 2007; accepted 2 January 2008
Abstract The effect of cyano substituents on the photoacidity of mono- and dicyanoanilines has been investigated. It was demonstrated that the cyano substitution increases significantly the acidity of aniline derivatives in the excited state in comparison to the ground state. 3,5-Dicyanoaniline is the strongest acid in the lowest excited singlet state, while 4-cyanoaniline is the weakest one. The derivatives of aniline with two cyano groups in o,o -position show different properties from those characteristic for aniline and other investigated cyanoanilines. In the methanol solution with sodium methanolate the anions of 2,6-dicyano-3,5-dimethylaniline and 2,6-dicyano-3,5-diphenylaniline appear already in the ground state. The electronic ground and excited state charge distributions and dipole moments of all investigated cyanoanilines have been evaluated by ab initio calculations using the GAMESS program. © 2008 Elsevier B.V. All rights reserved. Keywords: Fluorescence; Photoacidity; Cyanoanilines; Anions
1. Introduction Hydroxyarenes and aromatic amines (aniline and its derivatives) have been an object of investigation in the spectroscopy for many years, because they show large enhancement in acidity due to migration of charge from electronegative atom center to the ring in the excited state compared to the ground state [1–7]. Proton dissociation and acid–base equilibria in the excited state are strongly affected by substituents introduced in the aromatic ring [8–10]. Tolbert and Solntsev [11] and Agmon [12] have reported a remarkably increased photoacidity of monocyanonaphtols and dicyanonaphtols in the excited state, which was explained on the basis of the charge migration from the hydroxyl group to the C-5 and C-8 positions in the distal ring in the cyanonaphtol molecule. The pKa value of 2-naphtol drops from 9.5 in the ground state to 2.8 in its first excited singlet state. In the case of cyano-substituted 2-naphtol molecules the presence of the electron-withdrawing CN group makes these molecules more acidic than 2-naphtol. The doubly substituted CN derivatives
∗
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are almost as strong as a mineral acid, pKa = −4.5 in the excited state. Schulman et al. [9] have investigated the proton dissociation processes in the lowest excited singlet state of cyanophenols by fluorimetric titrimetry. They found that in the excited state ocyanophenol was the strongest acid whereas the p-cyanophenol was the weakest one. The proton dissociation processes from the lowest excited singlet state of protonated m- and p-cyanoanilines and m- and pmethoxyanilines in an aqueous solution have been investigated using picosecond time-resolved fluorescence measurements by Shiobara et al. [13,14]. They have found that the position and the electronic character of the substituent strongly influence the rate of proton dissociation. The cyano substitution (with an electron-withdrawing group) at the meta-position influences the rate more strongly in comparison to aniline, while the methoxy substitution (an electron-donating group) reduces the rate remarkably. The pKa of protonated aniline, m- and p-cyanoaniline in the ground state is 4.6, 2.7 and 1.7, respectively. In the first excited state the acidity of these compounds strongly changes, resulting in pKa∗ = −6.5 for aniline, −10.5 for m-cyanoaniline and −3.7 for p-cyanoaniline [15].
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In the present work, we have studied the protolytic dissociation in the ground and first singlet excited electronic state of aniline derivatives with two cyano groups: 2,6-dicyano3,5-dimethylaniline (oDCA), 3,5-dicyanoaniline (mDCA), 2,6-dicyano-3,5-diphenylaniline (oDCPA) and, for comparison, of o-, m- and p-cyanoaniline (oCA, mCA and pCA) (see Scheme 1). We present: (i) the results of steady-state measurements of absorption and fluorescence in aqueous and methanolic solutions, (ii) ab initio calculations of the electronic structure, the dipole moments and molecular geometry of the investigated molecules in the electronic ground and excited states. 2. Experimental 2.1. Chemicals 2,6-Dicyano-3,5-dimethylaniline (oDCA) was synthetized using the procedure described in Ref. [16]. 2,6-Dicyano-3,5diphenylaniline (5 -amino-1,1 :3 ,1 -terphenyl-4 ,6 -dicarbonitrile) (o-DCPA) was synthetized using procedure described in Ref. [16a]. Spectroscopic data of oDCPA: 1 H NMR (DMSOd6 , 300 MHz): 6.79 (s, 1H); 6.81 (s, 2H, NH2 ); 7.49–7.54 (m, 6H); 7.60–7.65 (m, 4H); 13 C NMR (DMSO-d6 , 75 MHz): 93.97, 115.90, 118.44, 128.44, 128.58, 129.34, 137.34, 149.57, 154.02. Synthesis of 3,5-dicyanoaniline (mDCA) is described in Ref. [17]. Spectroscopic data of mDCA: 1 H NMR (500 MHz, DMSO-d6 ) 6.12 (s, 2H), 7.15 (d, 2H, J = 1.4 Hz), 7.39 (t, 1H, J = 1.4 Hz); 13 C NMR (125 MHz, DMSO-d6 ) 113.08, 117.72, 120.08, 121.15, 150.13; IR max/cm−1 : 435, 672, 865, 1328, 1343, 1443, 1599, 1630, 2230, 3344, 3463. o-Cyanoaniline (oCA), m-cyanoaniline (mCA) and pcyanoaniline (pCA) are commercial (Aldrich). The solvents were purchased from Merck and Aldrich and were spectroscopic or HPLC grade. The perchloric acid was purchased from Apolda (Germany), and sodium and sodium hydroxide from P.O.Ch. Gliwice (Poland).
3. Results 3.1. Protolytic dissociation of investigated anilines in aqueous solutions Figs. 1 and 2 show the steady-state absorption and fluorescence spectra of oCA, mCA, pCA, oDCA and mDCA measured in aqueous solutions under different pH conditions at 293 K. Detailed data for all amines are presented in Table 1. The absorption spectra obtained in neutral and alkaline solutions (3 M NaOH) are attributed to neutral molecules of all investigated amines (Fig. 1). Alkalization of the solutions does not lead to a change of the absorption spectrum, indicating the lack of cyanoanilines anions formation in aqueous solution in the ground state. The first absorption bands of oCA and mCA appear at similar energies (∼32,000 cm−1 ). In the case of pCA this band is shifted into the blue about 5000 cm−1 . The absorption spectra of derivatives with two cyano groups are shifted into the red in comparison to the derivatives with one cyano substituent (oCA and mCA) about 2000 cm−1 . The fluorescence spectra obtained in the neutral solution for all above mentioned amines are ascribed to the neutral form of cyanoanilines, but in the case of pCA we observe at the
2.2. Methods The absorption spectra were recorded on a SPECORD M500 UV–VIS spectrophotometer (Carl-Zeiss Jena). The steady-state fluorescence spectra were measured on the KONTRON SFM 25 spectrofluorimeter (Kontron Instruments Germany). The quantum yields were obtained using quinine bisulphate in 0.1 N H2 SO4 (Φf = 0.52) and 2-amino-pyridine in 0.05 M H2 SO4 (Φf = 0.74) as standards [18,19]. 2.3. Quantum chemical calculations The distributions of electronic densities and the dipole moments of the cyanoanilines molecules in the ground and first excited state were obtained on the basis of ab initio calculations in the base 6-31G*, using GAMESS program [20]. Additionally the CI calculations taking into consideration the one- and two-electron excitation were performed. The geometries optimization was obtained on the basis of ab initio calculations in the base STO-3G with the application of the AM1 method [21].
Fig. 1. Absorption and corrected fluorescence spectra of oCA, mCA, pCA, oDCA and mDCA in water and in alkaline solution (3 M NaOH) at 298 K (the absorption spectrum of mDCA was normalized to the second band in water).
B. Szczepanik et al. / Spectrochimica Acta Part A 71 (2008) 403–409
Fig. 2. Absorption and corrected fluorescence spectra of oCA, oDCA and mDCA in water and in acidic solution (HClO4 , pH about 1) at 293 K (the absorption spectrum of mDCA was normalized to the second band in water).
long wavelength side the trace of an additional band of fluorescence. In the alkaline solution a new band appears at the long wavelength side for oCA, pCA and mDCA, peaking at 21,000, 23,200 and 18,500 cm−1 , respectively (Fig. 1). These bands can be ascribed to anionic forms of investigated compounds. The fluorescence spectra of mCA in neutral and alkaline solutions are similar, but further alkalization of the solution leads to a Table 1 Spectroscopic data of oCA, mCA, pCA, oDCA and mDCA at 293 K in water, alkaline and acidic solutions: molar extinction coefficient ε (dm3 mol−1 cm−1 ), absorption and fluorescence transition energy maxima ν˜ amax and ν˜ fmax (cm−1 ) and Stokes shifts ˜νSt
405
Scheme 1. Chemical structure of the compounds investigated.
change of the fluorescence spectrum with the trace of a new band appearing at the long wavelength side. This trace can confirm that also in the case of mCA an anion is formed in the excited state. In the case of oDCA we do not observe a fluorescence band corresponding to the neutral molecule, but at the long wavelength side a new band with maximum at 20,100 cm−1 appears, suggesting the emission only from anionic form of oDCA in the alkaline solution (Fig. 1). In highly acidic conditions (5 M HClO4 ) the absorption spectra of protonated form of oCA and mDCA is are observed at 36,000 and 34,500 cm−1 , respectively (Fig. 2). In the case of mCA and pCA investigated earlier by Shiobara et al. [14] under similar conditions the protonated forms of these amines absorb at similar energy like oCA (about 36,000 cm−1 ). The fluorescence spectra of oCA and mDCA show bands resulting from the presence of the deprotonated species of investigated amines (Fig. 2). oDCA is not protonated even in 5 M HClO4 . We can observe only the absorption and fluorescence bands of the neutral molecule (Fig. 2). The fluorescence quantum yield of oDCA is relatively large in water and acidic solutions (0.22 and 0.18, respectively). In the alkaline solution the anion fluoresces stronger that the neutral molecule, Φf = 0.41.
Solvent
ε
ν˜ amax
ν˜ fmax
˜νSt
oCA Water 5 M HClO4 3 M NaOH
3,200 1,000 3,100
32,000 36,000 31,700
26,200 25,800 26,200, 21,000
5,800 10,200 6,500
mCA Water 5 M HClO4 3 M NaOH
2,500 600 2,700
32,500 36,000 32,500
24,200
8,300
24,500
8,000
21,400
37,200
28,400
8,800
20,400
37,000
28,300, 23,200
8,700
3.2. Protolytic dissociation in methanol solutions
oDCA Water 5 M HClO4 3 M NaOH
8,100 7,800 4,000
29,400 29,300 29,100
26,100 26,100 20,100
3,300 3,200 9,000
mDCA Water 5 M HClO4 3 M NaOH
2,800 800 2,600
30,300 34,500 31,000
24,700 25,200 24,600, 18,500
5,600 9,300 6,400
Absorption and fluorescence spectra of oDCPA were measured in the methanol–water solution (60/40%, v/v) because of very weak solubility of this compound in neat water. Fig. 3 shows the steady-state absorption and fluorescence spectra of oDCPA recorded in methanol–water solutions (60/40%, v/v) with sodium hydroxide, in methanol with sodium hydroxide and in methanol with sodium at 293 K.
pCA Water 5 M HClO4 3 M NaOH
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Fig. 3. Absorption and corrected fluorescence spectra of oDPCA in methanol–water (6:4, v/v)—1 M NaOH solution, in methanol with NaOH and in methanol with sodium at 293 K.
The absorption spectrum measured in methanol solutions with NaOH shown in Fig. 3 is attributed to a neutral molecule of oDCPA (maximum 27,200 cm−1 ). As a result of alkalization of methanol–water solution (60/40%, v/v) very weak absorption appears at the long wavelength side, which could be ascribed to the anion of oDCPA. Addition of sodium to the methanol solution leads to the change of the absorption spectrum with a new band appearing at the long wavelength side, with the maximum at 22,000 cm−1 . This band can be ascribed to the anionic form of oDCPA. The fluorescence spectra recorded in methanol–water and methanol solutions with NaOH show two bands: the band with maximum at 23,200 cm−1 which can be ascribed to the neutral molecule of oDCPA and the band at the long wavelength side, with the maximum at 17,400 cm−1 ascribed of the oDCPA anion. In methanol with addition of sodium only one band appears peaking at 17,400 cm−1 ). The absorption and fluorescence spectra were measured under the same conditions for other derivatives of aniline with two cyano groups—oDCA and mDCA. Fig. 4 shows the absorption and fluorescence spectra of oDCA in methanol, methanol with sodium and the alkaline water solution. Absorption spectra of oDCA in the alkaline water solution and in methanol are blue shifted about 2000 cm−1 in comparison to oDCPA and correspond to the neutral molecule. In the methanolate solution a new band appears on the long wavelength side similarly to oDCPA, and is shifted also about 2000 cm−1 compared with the anion of oDCPA (Fig. 4). The fluorescence spectrum recorded in methanol with maximum at 26,100 cm−1 was ascribed to the neutral molecule of oDCA. In methanol–water solutions with addition of sodium hydroxide and in methanol with sodium, only one band appears peaking at about 20,100 cm−1 , which can be ascribed to the anion (Fig. 4).
Fig. 4. Absorption and corrected and normalized fluorescence spectra of oDCA in methanol with sodium, alkaline water solution (1 M NaOH) and methanol at 293 K. Excitation fluorescence spectrum was measured in methanol + Na solution, λobs = 20,200 cm−1 .
In the case of mDCA we do not obtain the anion in the ground state in the methanolate solution. We observed only the absorption spectrum corresponding to the neutral molecule. 3.3. Acidity constants The change of acidity constant −pKa value in the excited state, was calculated on the basis of the F¨orster cycle according to Eq. (1) [22]: pKa =
0.625 − Nhc A− HA A HA = − ν˜ 00 ν˜ 00 − ν˜ 00 ν˜ 00 2.3RT T
(1)
where A− ν˜ 00 =
a +ν f ˜ A− ν˜ A− 2
HA ν˜ 00 =
a +ν f ˜ HA ν˜ HA 2
A− HA are the energies and ν˜ 00 T is the temperature on the Kelvin, ν˜ 00 (cm−1 ) of the 0–0 transitions for the anion A− and the neutral a and ν a are the wavenumbers of the absorption ˜ HA acid HA, ν˜ A− f and band maxima of the anion A− and the neutral acid HA, ν˜ A− f ν˜ HA are the wavenumbers of the fluorescence maxima of these species. In the case of the estimation of pKa for investigated amines in aqueous solutions the problem is the lack of absorption spectra of the anion in the alkaline solution. In this situation, we have used another approach of the calculation of pKa , which can be performed using the difference in energy between the
B. Szczepanik et al. / Spectrochimica Acta Part A 71 (2008) 403–409 Table 2 Wavenumbers of fluorescence maxima for o-, m-, pCA, oDCA and mDCA in water (pH ∼ 6) and strongly basic aqueous solution (293 K): ν˜ fmax (cm−1 ), pKa calculated from Eq. (2) Compound
ν˜ fmax (cm−1 ) pH ∼ 6
ν˜ fmax (cm−1 ) 2 M NaOH
pKa
oCA pCA oDCA mDCA
26,200 28,400 26,100 24,700
21,000 23,200 20,100 18,500
−11.1 −11.1 −12.8 −13.2
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including alcohols can be sometimes considerably larger than that for aqueous solutions [23]. pKa estimated from the F¨orster cycle in methanolic solutions (calculated from the 0–0 transition wavenumbers) is found to be −11.7 for oDCA and oDCPA. The value obtained basing on the energy differences of the fluorescence maxima is about one unit greater and identical in aqueous and methanolic solutions for oDCA (Tables 2 and 3). We can suppose that the photoacidity enhancement in the excited state is comparable in both solutions for the investigated cyanoanilines.
fluorescence maxima of the anionic and neutral forms (Eq. (2)).
3.4. Quantum chemical calculations
Nhc max − ν˜ f (A ) − ν˜ fmax (HA) 2.3RT 0.625 max − (2) = ν˜ f (A ) − ν˜ fmax (HA) T The data used for the calculation of pKa in aqueous solutions are collected in Table 2. In methanolic solutions pKa for oDCA and oDCPA have been calculated using the 0–0 energy, because we have obtained absorption spectra of anilines anions in these conditions. For comparison, pKa was determined also from the fluorescence maxima of the neutral and anionic form of investigated anilines. Results are collected in Table 3. The acidity of the investigated amines in the aqueous solution strongly changes in the excited state, resulting in pKa from −11.1 for oCA and pCA to −12.8 and −13.2 for oDCA and mDCA (Table 2). The photoacidity enhancement obtained on the basis of the F¨orster cycle from measurements in non-aqueous solvents
Ab initio calculations of the molecular geometry for the ground and lowest excited states of the mono- and disubstituted cyanoanilines were carried out by Kolek et al. [17,24–27]. The authors have compared the geometry change upon electronic excitation for aniline and 2-, 3-, 4-cyanoaniline, 2,6-, 3,5-dicyanoaniline and 2,6-dicyano-3,5-dimethyl-aniline. The aniline molecule is non-planar in the ground electronic state, with the NH2 plane at 42◦ with respect to the ring plane. In the lowest singlet excited state the aniline molecule becomes planar [27]. For mono-cyanoanilines and 3,5-dicyanoaniline the ab initio calculations predict a non-planar equilibrium geometry in the ground electronic state (all these molecules have a pyramidal amino group in ground state) like in the case of aniline. Geometric optimization of these aniline derivatives gives almost planar structure in the S1 excited state. The situation is different only in the case of 2,6-dicyanoaniline and 2,6-dicyano3,5-dimethylaniline. The molecules having two cyano groups in ortho positions reveal different properties from those character-
pKa =
Fig. 5. The distributions of the charge densities (in the area of amino- and cyanogroup) for oDCA, mDCA, o-, m- and pCA in the S0 and S1 (values in brackets).
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Table 3 Wavenumbers of absorption and fluorescence maxima for oDCA and oDCPA in methanol (MeOH) and in methanolate solution (methanol + Na) (MeONa) (293 K): abs and ν f ˜ max ν˜ max (cm−1 ); 0–0 transition wavenumbers of the neutral molecule and the anion: ν˜ 00 (cm−1 ); pKa as calculated from Eq. (1) Compound
oDCA oDCPA a b c
abs (cm−1 ) ν˜ max
f ν˜ max (cm−1 )
pKa a
MeOH
MeONa
MeOH
MeONa
29, 000 27, 200
24,000 22,000
26, 200 23, 200
20,200 17,400
−12.8 −12.4
ν˜ 00 (cm−1 )b
pKa c
MeOH
MeONa
27, 600 25, 200
22,100 19,700
−11.7 −11.7
pKa determined from the fluorescence band maxima. ν˜ 00 determined from the mean of absorption and fluorescence band maxima. pKa calculated from the 0–0 transition wavenumbers.
istic for aniline. The interactions between the amino group and the cyano groups in these molecules induce significant rehybridization at the nitrogen atom, thus in the ground S0 and excited S1 states the molecules of these compounds have an almost planar structure [27]. In this paper we have performed ab initio calculations for all the investigated cyanoanilines molecules in the ground and first excited state. The charge distribution in the ground electronic state S0 and the lowest S1 electronic excited state are presented in Fig. 5. The results presented in Fig. 5 show that the sum of charge densities on atoms of the amino group becomes more positive in the first excited state in comparison to the ground state for all investigated amines. The charge densities on the amino group decrease after excitation in the following order: oDCA > mDCA > oCA > mCA > pCA The charge densities on atoms of the cyano groups have considerably negative values. In the excited state these values become more negative in the same order in which the charge densities on the amino group become more positive. The decrease of charge density on the amino group in the excited state corresponds to the direction of the decrease of the investigated amines basicity with the pKa of the dicyano-derivatives being higher than that for the monocyano-derivatives of aniline. The dipole moments in the ground state show values from 3.72 for oDCA to 5.70 D for pCA (Table 4). The dipole moments change rather little upon excitation, μ is from 0.14 D for pCA to 1.14 D for mCA. The highest increase is observed for meta cyanoderivatives of aniline while the weakest for pCA, which stands in correlation with the pKa values for these compounds (Table 2).
Table 4 Ground state dipole moments (D) (μg ) and the changes of the dipole moment upon excitation (in the first excited singlet state) (μ (D)) Compound
μg (D)
μ (D)
oCA mCA pCA mDCA oDCA
4.43 5.07 5.70 5.31 3.72
0.84 1.14 0.14 0.96 0.57
4. Conclusions The presented study provides new information concerning the protolytic reaction of mono- and dicyanoanilines in the ground and first excited singlet state. In alkaline aqueous solutions the anions of all monocyanoanilines and mDCA were not formed in the ground state even in strong alkaline solutions, which remains in accordance with the character of aromatic amines which can form anions only in strong bases like ammonia in the presence of sodium or potassium amides [28]. In the excited state the acidity of amino group increases strongly: pKa ∼ −11 for o- and pCA and ∼−13 for derivatives with two cyano groups and the anions are formed for all amines. mDCA is a stronger acid in the S1 state than oDCA while oDCA is a stronger acid than oCA and pCA. The order of acidity in the S1 state suggests that the inductive effect of cyano substituents is more important in the case of cyanoanilines than the resonance effect. In the acidic aqueous solution the monocyanoanilines and the derivative with two cyano groups in meta position form the cations in the ground state in accordance with the alkaline character of the amino group, so we can observe the cation absorption for oCA and mDCA. In the excited state the fluorescence of the neutral molecule is observed only, like in the case of m- and p-cyanoaniline [14], suggesting the proton dissociation reaction of the protonated cyanoaniline derivatives in the excited state. The derivatives of aniline with two cyano groups in o,o position show different properties from those characteristic for mono-cyanoanilines and mDCA. oDCA and oDPCA did not form the cations in the ground state nor in the excited state. The two substituents in the proximity of amino group act as steric hindrances, making the protonation of amino group difficult. In the methanol solution with addition of sodium we obtained the anions of all studied anilines in the excited state, but only in the case of oDCA and oDCPA the anions appeared already in the ground state in these conditions. The presence of two cyano groups in ortho position in the case of oDCA and oDCPA causes considerable migration of negative charge from the nitrogen atom to the aromatic ring and as a result the basicity of the amino group can decrease more strongly in the ground state than for derivatives with one cyano group and aniline. This decrease of basicity is so considerable that the anion formation is possible in the ground state in methanol solution containing the sodium methanolate.
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The charge migration from the amino group to the benzene ring is the result of the structural change of the amino group in the S1 state. Amino nitrogen of monocyanoanilines in the ground state sp3 has the configuration with almost pyramidal structure, while upon excitation the configuration of the amino group changes to sp2 with almost planar structure [14]. The unusual behaviour of oDCA and oDCPA in protolytic reactions results not only from the strongest interactions between the amino and cyano groups in comparison to all the other aniline derivatives examined, but also from the presence of two cyano groups in o,o -position which make the recombination of the anion with the proton difficult, leading to the stabilization of the anion already in the ground state. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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