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Formation of a ground state twisted-internal-charge-transfer conformer of 4-(dimethylamino)benzaldehyde
Journal of Molecular Structure 516 (2000) 215–223 www.elsevier.nl/locate/molstruc
Formation of a ground state twisted-internal-charge-transfer conformer of 4-(dimethylamino)benzaldehyde G.P. Kushto 1, P.W. Jagodzinski* Department of Chemistry, West Virginia University, Morgantown, WV 26506, USA Received 8 January 1999; accepted 7 April 1999
Abstract Ultraviolet absorption spectra and visible Raman spectra were collected for 4-(dimethylamino)benzaldehyde (DABA) in aqueous solution in the pH range of 9.56 to 21.08. Equilibrium arguments show that only the free base and the dimethylamino protonated species will be present in appreciable concentrations in this pH range. Protonation leads to increased sp 3 hybridization about the amino nitrogen with rotation to form a twisted-internal-charge-transfer (TICT) conformer. Molar absorptivities were calculated for the 352 nm band and these values were used to calculate the angle of rotation of the dimethylamino group out of the ring plane. The Raman spectrum of DABA in concentrated hydrochloric acid solution is consistent with a TICT structure in which the amino group is rotated 908 out of the plane. q 2000 Elsevier Science B.V. All rights reserved. Keywords: 4-(Dimethylamino)benzaldehyde; Twisted-internal-charge-transfer; Ultraviolet spectra; Raman spectra
1. Introduction The anomalous dual fluorescence observed for 4(dimethylamino)benzaldehyde (DABA) in polar solvents has been shown to be due to the existence of a twisted-internal-charge-transfer (TICT) configuration [1,2]. This electronic excited state conformation is the result of the decoupling of the nitrogen lone pair electrons and the p cloud of the benzene ring. The dimethylamino group is rotated between 30 and 908 out of the original molecular plane in the resultant structure (see Fig. 1). Cazeau-Dubroca and coworkers argue that this conformer is the energetically preferred ground state structure when three water molecules are * Tel.: 1 1-303-293-3435; fax: 1 1-303-293-4904. E-mail address: [email protected] (P.W. Jagodzinski). 1 Present address: Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA.
proximate to the dimethylamino group [3]. They also argue that a saturation effect is observed beyond three water molecules with the twisted and planar structures both equally probable in aqueous solutions. Since DABA has a strong electron-donating group para to a strong electron-withdrawing group, the charge separated quinonoid resonance form provides a significant contribution to the ground state configuration. Therefore in non-polar environments, the dimethylamino group would be expected to be in a coplanar arrangement with the benzene ring with the nitrogen in a nearly sp 2 hybridized state. Gorse and Pesquer have calculated that this moiety is only slightly displaced from planarity with the methyl groups moving out of the plane and forming a pyramidal structure about the nitrogen atom [4]. Upon protonation however, the n–p interaction is broken and the nitrogen assumes a more tetrahedral sp 3 type hybridization. Minimizing contributions from the
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2. Experimental
Fig. 1. DABA with the angle of rotation (u ) of the dimethylamino group depicted.
quinonoid resonance structure permits the possible hindered rotation to the TICT configuration. It should be possible to make the TICT structure the dominant ground state conformer by protonating the dimethylamino group, since the barrier to rotation of this moiety has been reported to be in the order of 33 kJ/mol [5]. The protonated form of DABA has been used as the basis for a variety of colorimetric methods for more than 40 years [6,7]. As the strength of the acidic environment increases, the intensity of the 352 nm absorption band decreases and that of the 240 nm band increases. We have collected ultraviolet absorbance and visible Raman spectra of DABA in a variety of acidic environments and find significant changes as the pH is lowered. We interpret them as being due to increasing contributions from the TICT configuration with virtually all molecules having the dimethylamino group twisted by 908 in concentrated hydrochloric acid.
DABA, reagent grade, was obtained from the Eastman Kodak Company (Rochester, NY), and used without further purification. Hydrochloric acid (37.5%) and sodium hydroxide were purchased from the Fisher Scientific Company (Fair Lawn, NJ). The quaternary salt of DABA was produced by dissolving the solid in the acidic medium. Absorption spectra were collected using a Bausch and Lomb Spectronic 2000 spectrophotometer with a 2 nm slit width. Solutions were prepared using doubly distilled water and the pH was adjusted by adding reagent grade hydrochloric acid or an aqueous solution of sodium hydroxide. Concentrations of DABA were between 2 × 10 25 M and 5 × 10 25 M and were chosen so that the absorbance used to calculate e was between 0.2 and 0.6. Spectra were collected for samples that were contained in 1 cm path length quartz Suprasil cuvettes. Raman excitation was provided by a Coherent, Inc. ˚. Innova 70-4 argon ion laser operating at 4880 A Approximately 150 mW of laser power was focused on samples contained in standard melting point capillary tubes. The Raman scattering was collected at 908 by a SPEX 1401 double monochromator (Edison, NJ)
Table 1 Molar absorptivities and dihedral angles for DABA in aqueous solution as a function of pH Solvent Hexane Carbon tetrachloride Benzene Water
Concentrated HCl
pH
e 352 (l/mol cm)
e 240 (l/mol cm)
u 8a
9.56 7.15 4.96 3.87 3.24 2.80 2.40 2.00 1.59 1.28 0.95 0.66 0.21 21.08
74100 50100 38900 27900 28200 27700 29600 27970 29300 24419 21700 13425 8118 3990 1280 422 0
– – – 5690 5750 5650 5730 5877 7170 6765 8920 10,190 11,628 12,600 12,960 13,184 14,130
– – – 44.2 43.9 44.5 42.4 44.2 42.8 47.9 50.8 60.2 67.3 74.3 81.2 84.9 90.0
log [BH 1]/[B]NMe2 b
log [H 1 BH 1]/[BH 1] c
Ref. [18] [18] [18]
27.939 25.530 23.340 22.250 21.620 20.851 20.780 20.380 0.0294 0.340 0.670 0.960 1.410 2.700
215.932 213.520 211.330 210.240 29.611 29.170 28.770 28.370 27.959 27.650 27.320 27.030 26.580 25.290
Calculated using e /54,400 cos 2 u for the NMe2 rotation. Ratio of the molar concentration of DABA protonated on the dimethylamino group to the molar concentration of the free base. c Ratio of the molar concentration of doubly protonated DABA to the molar concentration of DABA protonated on the dimethylamino group. a
b
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Fig. 2. Plot of e 352 and e 240 (l/mol cm) for DABA in aqueous solution as a function of pH.
Fig. 3. Fractional composition diagram for DABA in acidic environments. Note that the values for the doubly protonated species are nearly zero and lie along the X-axis.
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with a slitwidth of 160 mm resulting in a spectral bandpass of approximately 3.25 cm 21. Each spectrum is an average of 10 scans with data collected for one second at each wave number. All manipulations were carried out on a SPEX Industries DATAMATE spectrometer controller. The concentrations were typically 0.3 mM in aqueous solution and 5 mM in hydrochloric acid solution for collection of the Raman spectra.
3. Results In order to analyze the data, we first consider the pKa’s of the carbonyl oxygen and the dimethylamino nitrogen. To our knowledge these values have not been reported for DABA. The average of the pKa values for the carbonyl group of similar 4-substituted benzaldehydes as reported by Palm and coworkers is 26.37 [8]. The pKa values for the dimethylamino group in N,N-dimethylanilines substituted in the 4position with electron donating groups are reported to be above zero, however the values for molecules sufficiently similar to DABA, to calculate an average have not been reported [9]. The absorbance data that we have collected at 352 and 240 nm is presented in Table 1. Following the method of Davis and Geissman we have plotted e versus pH as shown in Fig. 2 [10]. The inflection points provide the pKa values of 1.55 and 1.80. Limited data reported for DABA in 0.4 M sulfuric acid yields a pKa of 1.5 [11]. We therefore assume that the pKa is 1.62 (the average of these three values) with the associated error being minor within the context of our use of this value. Using equilibrium arguments we have calculated the ratios of the concentrations of DABA to DABA protonated on the dimethylamino group and DABA protonated on both the dimethylamino and carbonyl groups, and present the results in Table 1. We have also calculated the fractional composition of each of these three species and present the results in Fig. 3. It can be seen that only the base form is present at the basic and neutral pH’s, that the dimethylamino protonated acid form is important below pH 2, and that the concentration of the doubly protonated form is not significant down to pH 21. Thus, we interpret the observed spectra by considering increasing equilibrium concentrations of the dimethylamino protonated
species as the pH is lowered, and assume that the doubly protonated species cannot be used to interpret the current data. 3.1. Interpretation of the ultraviolet absorption spectra The ultraviolet absorption spectrum of DABA in neutral aqueous solution consists of bands at 352, 280, 240, 203, and 190 nm. Braude and Sondheimer proposed that the molar absorptivities of ultraviolet bands of the substituted benzenes are correlated with the angle of rotation of the substituent groups out of the plane of the ring [12]. The relationship, based on the overlap intergral of the p orbitals centered on the ring carbon and the a-atom of the substituent, is e / e 0 cos 2 u . Here e is the molar absorptivity for the molecule in a twisted configuration, e 0 is the molar absorptivity for the molecule in the planar configuration, and u is the angle of rotation (the range is 0– 908). The validity of this equation has been challenged, however it has been shown to accurately predict angles in agreement with the experimental results for a variety of molecules [13,14]. We have used this equation to analyze e for the 352 nm band as has been done for other aniline derivatives [15]. We have found that e 352 decreases and e 240 increases with decreasing pH at values lower than pH 3. The 352 nm band is due to the pp p ( 1La ← 1A) transition which involves a molecular orbital primarily composed of the benzyl and dimethylamino portions of the molecule [1]. Polar solvents shift this transition to lower energy than the pp p ( 1Lb ← 1A) transition found at 240 nm [16]. We have assumed that the dimethylamino group will rotate out of the plane of the ring because: (1) protonation occurs at this moiety in the pH range of this study; (2) this rotation is energetically favored over the carbonyl rotation; and (3) the values of n (CyO) suggest that the carbonyl group remains in the plane of the ring (vide infra). Cazeau-Dubroca and coworkers argue that the dimethylamino group is not coplanar with the ring in aqueous solution [3]. Therefore, we cannot use any of the values for our aqueous solutions to calculate e 0. We have chosen to use the average of e 352 for DABA in benzene, hexane and carbon tetrachloride solutions as e 0 since the molecule should be nearly
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Fig. 4. Raman spectra of DABA in aqueous solution (bottom trace) and in 12 M hydrochloric acid solution (top trace).
planar in these solutions [17,18]. The calculated dihedral angles range from about 44 at pH 9.56 to 90 in the concentrated hydrochloric acid (see Table 1). The exact values of the intermediate angles are valid to the extent that DABA is a planar in the organic solvents chosen to calculate e 0, however the trend is valid and the final value of 908 is valid since e 0 in concentrated acid. It is interesting to note that above pH 3 the calculated angle is nearly 458 which might represent the saturation effect (equal distribution of conformers) predicted by Cazeau-Dubroca [3]. Therefore the ultraviolet absorption data support formation of the TICT state in acidic media. Corroborating evidence can be provided by vibrational spectra. We have collected and analyzed Raman spectra of DABA in acidic solutions. 3.2. Interpretation of the Raman spectra The DABA molecule has C1 point group symmetry since the dimethylamino group is slightly pyramidal for the isolated molecule. The molecule formally adopts Cs point group symmetry
when the dimethylamino group is rotated 90 out of the ring plane. No significant information is available from the depolarization ratios of DABA in concentrated hydrochloric acid due to the complexity of the normal modes and the understanding of them afforded by a normal mode analysis [18]. The Raman spectra of DABA and its dimethylamino protonated analog are presented in Fig. 4 and the spectra are presented as a function of acid concentration in Fig. 5 to allow monitoring of the spectroscopic changes as the equilibrium shifts to the protonated form. All of the Raman bands are polarized and the intensities are independent of exciting wavelength in the visible region of the spectrum. Vibrational assignments are presented in Table 2. Explanation of the changes in selected bands is facilitated by reference to the normal mode analysis of DABA and five isotopic derivatives [18]. A 40 cm 21 blue shift of the Raman signal due to the normal mode with a PED composed of 75% CyO stretching motion (typically called the carbonyl stretch; located at 1643 cm 21 for DABA in water) is observed upon protonation. This band shifts to about 1695 cm 21 in concentrated HCl (actually a Fermi
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Fig. 5. Raman spectra of DABA in distilled water, 1.0 M HCl solution, 3.0 M HCl solution and 12.0 M HCl solution.
doublet with components at 1706 and 1685 cm 21). Protonation of the dimethylamino group is expected to increase the double bond character of the CyO group with concomitant blue shift (n CyO 1694 cm 21 for neat benzaldehyde) [19], whereas protonation of the carbonyl group would favor more single bond character and the band would undergo a red shift. The observed shift is consistent with the breakdown of the extended system thus decreasing the electron density in the limited p p orbital of the carbonyl moiety and increasing the double bond of the aldehyde carbon–oxygen bond. Seehra and Jagodzinski have attributed the Fermi doublet in the vibrational spectrum of uncomplexed DABA to the interaction of the carbonyl CyO stretching fundamental with an overtone of the normal mode at 844 cm 21 [20]. Both the carbonyl stretching mode and the lower wave number mode (now at 849 cm 21) blue shift upon protonation. In the protonated form, the two bands are of approximately equal intensity and are separated by 21 cm 21, while in the
spectrum of DABA in aqueous solution the carbonyl band is comprised of a strong band with a broad shoulder separated by 41 cm 21. The changes that occur in n CyO as the pH is lowered are seen in Fig. 5. The in-plane wagging motion of the formyl proton in the aldehyde moiety also undergoes a significant change upon protonation. In the spectrum of DABA in doubly distilled water, the intense band at 1404 cm 21 is assigned to this vibration [18]. Due to the highly conjugated nature of the DABA molecule, the normal modes exhibit an unusually high degree of delocalization. Upon protonation the majority of conjugation through the benzenoid p system is disrupted and the resultant benzenoid form is expected to produce a vibrational spectrum that is more indicative of the local symmetry of each of the sections of the DABA molecule [21]. Previous studies of para-substituted benzaldehyde systems indicate that the frequency of the formyl proton in-plane wagging mode is relatively independent of the para substituent and usually occurs as a very weak band around 1400 cm 21 in the Raman
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Table 2 Vibrational assignments for the Raman spectra of DABA in aqueous and acidic media DABA in water a
1643 (6) 1592 (10) 1549 (5)
1442 (3) 1420 (2) 1404 (4)
DABA-H1
Approximate mode description
1706(4), 1685(4) b
CyO str. {Fermi doublet} CyO str. {Fermi doublet} Ring mode (n 8a) c Ring mode (n 8b) Ring mode (n 19a) Mixed ring/Me deform. Mixed ring/Me deform.
1609 (10) 1512 (1) 1462 (1) 1444 (1) 1411 (1) 1309 (1)
1254 (1) 1236 (1)
1180 (4) 1135 (1)
948 (1) 844 (4) 732 (2) 636 (2) 598 (2) 381 (,1) 358 (,1)
1246 (2) 1213(4) 1176 (4) 1130 (4) 1016 (1) 994 (1) 898 (2) 849 (5) 737 (3) 644 (2) 632 (3) 613 (2) 435 (2) 381 (,1) 329 (,1) 309 (1)
Aldehyde H in-plane wag Ring mode/Ar–N Str. Mixed ring/Me rock Mixed ring/Me rock Ar–N str./mixed ring Ar–C str./mixed ring Arom. H in-plane wag Ring mode/Me rock Out-of-plane ring mode (n 18a) Out-of-plane ring mode (n 18b) Symmetricf N-methyl str. Mixed ring/CyO bend (n 1) In-plane CyO bend/mixed ring Ring mode/CyO bend (n 6a) Ring mode (n 6b) Mixed mode (n 4) Delocalized mode NC2 deform Mixed ring
All vibrational frequencies are in cm 21 and calibrated with respect to the indene calibration standard. Numbers in parentheses are the relative scattering intensities. c Ring modes have been described using the Wilson numbering system for the analogous vibrations of benzene where ever applicable. a
b
spectrum [19,22,23]. Since it is unlikely that the frequency of this vibration will shift significantly upon protonation, the in-plane wagging vibration of the aldehyde hydrogen is assigned to the weak transition occurring at 1411 cm 21 in the Raman spectrum of protonated DABA. This indicates that the polarizability associated with this normal mode is consistent with previous observations for benzaldehydes. Interestingly, upon protonation of DABA, the relative intensities of all of the bands that occur in this region of the spectrum are significantly diminished. The normal modes associated with the NC2 skeleton of the dimethylamino group are expected to undergo significant shifts due to the loss of electron density on the nitrogen center upon protonation. One example is the symmetric N-methyl stretching
motion. Guichard et al. have shown that for N,Ndimethylaniline, this mode shifts approximately 47 cm 21 towards the red upon protonation [21]. In the spectrum of DABA in water, this stretching mode can be assigned to the broad, weak band at 948 cm 21 based on the vibrational assignments for dimethylaniline and the normal coordinate analysis of DABA [18,21]. Upon protonation, the band at 948 cm 21 is no longer observed, but a new band arises at 898 cm 21. Assigning this band to the symmetric stretching motion of the dimethylamino nitrogen– methyl carbon bonds is consistent with the observation for N,N-dimethylaniline and with protonation at the NMe2 group. Due to the inherent stability of the benzenoid ring, the energies of the ring vibrations are expected to
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remain relatively unchanged upon protonation [21]. However, the relative Raman intensities of each band associated with these ring modes are subject to significant change. This effect is exhibited by the bands at 1549 and 1442 cm 21 in the spectrum of the parent molecule in water (Fig. 4). Upon ligation of the acidic proton, the intensities of these bands are significantly decreased. As a result, the Raman spectrum of protonated DABA appears remarkably similar to the spectra of neat N,N-dimethylaniline or benzaldehyde as previously reported [19,21]. The change in the intensity of these lines indicates a reduction of the extended conjugation in the DABA molecule. Additional bands that undergo noticeable changes upon protonation are found at 1549, 1246, 1213, and 1130 cm 21. The 1549 cm 21 band is due to a ring mode (Wilson number n 8b) and is completely absent in the spectrum of the protonated molecule. The form of this mode shows that it is localized at the amino end of the ring and includes a contribution from the Ar–N stretching motion [18,22,23]. The disappearance of this signal strongly supports the decoupling of the dimethylamino group from the aromatic ring system. The band at 1246 cm 21 in the spectrum of the protonated species corresponds to either 1254 or 1236 cm 21 in the spectrum of the parent molecule (see Table 2). The increase in intensity would indicate that the methyl rocking motion no longer contributes to the mode leaving it as primarily a ring mode. The 1213 cm 21 band in the spectrum of protonated DABA (with no counterpart in the spectrum of the protonated form) is analogous to the benzaldehyde vibration found at 1206 cm 21. Both are reasonably intense signals. Zwarich, et al. have calculated that 41% of the PED of this mode is due to the Ar–C stretching coordinate [19]. The 1135 cm 21 band in the spectrum of the unprotonated DABA is due to a mixed ring-methyl rocking mode. The signal shifts to 1130 cm 21 and becomes much more intense probably indicating that it has increasing contributions from ring motions. In the frequency region below 1000 cm 21 there are several signals due to ring deformation modes. The band at 844 cm 21 in the parent molecule spectrum shifts to 849 cm 21 and becomes more intense upon protonation (see Fig. 5). This band is due to in-plane bending motions of the CHO moiety and a ring deformation localized near the CHO group. The increase in
intensity is consistent with the relative intensity of this mode in benzaldehyde [19,22]. The changes in the Raman spectra are consistent with protonation of the dimethylamino group and subsequent decoupling from the extended p system. This data supports the interpretation of the absorption spectra. 4. Conclusions In this study, we have attempted to model the TICT conformer by protonating the dimethylamino nitrogen in the DABA molecule. Upon protonation, the dimethylamino group is twisted out of the plane of the aromatic ring into a conformation identical to that of the TICT state. This disruption of the extended p system has profound effects on the ultraviolet absorption and visible Raman spectrum of DABA. The absorption bands at 352 and 240 nm allow us to predict a pKa of 1.62 for the dimethylamino moiety. The analysis of e 352 allows us to predict the average dihedral angle for rotation out of the plane of the ring at a particular pH. It was found that all molecules have a dihedral angle of 908 in the concentrated hydrochloric acid. Owing to the loss of the interaction between the nitrogen lone pair electrons and the p system of the benzenoid ring, the Raman spectrum of protonated DABA exhibits spectral characteristics that are similar to para-substituted benzaldehyde derivatives that contain substituents having no p base qualities. The carbonyl CyO stretching vibration exhibits a significant blue shift upon protonation supporting protonation of the dimethylamino nitrogen. Due to the loss of electron density at the amino nitrogen, modes such as the aromatic ringdimethylamino nitrogen stretching coordinate, the dimethylamino nitrogen–methyl carbon stretching motions and the deformations of the NC2 skeleton all change upon protonation.
Acknowledgements The authors would like to acknowledge useful discussions with K.M. Brummond and the assistance of S.K. Sneckenberger and C.-M. Coyle.
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References [1] J. Dobkowski, E. Kirkor-Kaminska, J. Koput, A.J. Siemiarczuk, Lumin. 27 (1982) 339–355. [2] W. Rettig, Angew. Chem. Int. Ed. Engl. 25 (1986) 971–988. [3] C. Cazeau-Dubroca, A.S. Lyazidi, P. Cambou, Ph. Perigua, M. Pesquer, J. Phys. Chem. 93 (1989) 2347–2358. [4] A.D. Gorse, M. Pesquer, J. Mol. Struct. (Theochem) 281 (1993) 21–32. [5] R.K. MacKenzie, D.D.L. MacNicol, Chem. Soc. Chem. Commun. (1970) 1299–1300. [6] F.D. Snell, C.T. Snell, Colorimetric Methods of Analysis, Van Nostrand Reinhold, New York, 1970 (and following volumes). [7] M. Perez, J. Bartos, Colorimetric and Fluorimetric Analysis of Organic Compounds and Drugs, Marcel Dekker, New York, 1974. ¨ .L. Haldna, A.J. Talvik, in: S. Patai (Ed.), The [8] V.A. Palm, U Chemistry of the Carbonyl Group, Interscience Publishers, London, 1966, p. 421. [9] D.D. Perrin, Dissociation Constants of Organic Bases in Aqueous Solution, Butterworths, London, 1965 Supplement 1972. [10] C.T. Davis, T.A. Geissman, J. Am. Chem. Soc. 76 (1954) 3507–3511. [11] R.E. Cline, R.M. Fink, Anal. Chem. 28 (1956) 47–52.
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[12] E.A. Braude, F. Sondheimer, J. Chem. Soc. (1955) 3754– 3766. [13] N.L. Allinger, E.S. Jones, J. Org. Chem. 30 (1965) 2165– 2169. [14] L.N. Ferguson, Organic Molecular Structure, Willard Grant Press, Boston, MA, 1975, p. 419. [15] H.H. Jaffe´, M. Orchin, Theory and Applications of Ultraviolet Spectroscopy, Wiley, New York, 1962, p. 414. [16] E. Lippert, in: J.B. Birks (Ed.), Organic Molecular Photophysics, 2, Wiley, New York, 1975, p. 16. [17] J.P. Phillips, D. Bates, H. Feuer, B.S. Thyagarajan (Eds.), Organic Electronic Spectral Data, XVII, Wiley, New York, 1975, p. 127. [18] G.P. Kushto, P.W. Jagodzinski, Spectrochim. Acta 54A (1998) 799–819. [19] R. Zwarich, J. Smolerek, L. Goodman, J. Mol. Spectrosc. 38 (1971) 336–357. [20] J.K. Seehra, P.W. Jagodzinski, J. Raman Spectrosc. 21 (1990) 31–36. [21] V. Guichard, A. Bourkba, M. Lautie, O. Poizat, Spectrochim. Acta 45A (1989) 187–201. [22] S.H.W. Hankin, O.S. Khalil, L. Goodman, J. Mol. Spectrosc. 72 (1978) 383–405. [23] J.H.S. Green, D.J. Harrison, Spectrochim. Acta 32A (1976) 1265–1277.
Formation of a ground state twisted-internal-charge-transfer conformer of 4-(dimethylamino)benzaldehyde G.P. Kushto 1, P.W. Jagodzinski* Department of Chemistry, West Virginia University, Morgantown, WV 26506, USA Received 8 January 1999; accepted 7 April 1999
Abstract Ultraviolet absorption spectra and visible Raman spectra were collected for 4-(dimethylamino)benzaldehyde (DABA) in aqueous solution in the pH range of 9.56 to 21.08. Equilibrium arguments show that only the free base and the dimethylamino protonated species will be present in appreciable concentrations in this pH range. Protonation leads to increased sp 3 hybridization about the amino nitrogen with rotation to form a twisted-internal-charge-transfer (TICT) conformer. Molar absorptivities were calculated for the 352 nm band and these values were used to calculate the angle of rotation of the dimethylamino group out of the ring plane. The Raman spectrum of DABA in concentrated hydrochloric acid solution is consistent with a TICT structure in which the amino group is rotated 908 out of the plane. q 2000 Elsevier Science B.V. All rights reserved. Keywords: 4-(Dimethylamino)benzaldehyde; Twisted-internal-charge-transfer; Ultraviolet spectra; Raman spectra
1. Introduction The anomalous dual fluorescence observed for 4(dimethylamino)benzaldehyde (DABA) in polar solvents has been shown to be due to the existence of a twisted-internal-charge-transfer (TICT) configuration [1,2]. This electronic excited state conformation is the result of the decoupling of the nitrogen lone pair electrons and the p cloud of the benzene ring. The dimethylamino group is rotated between 30 and 908 out of the original molecular plane in the resultant structure (see Fig. 1). Cazeau-Dubroca and coworkers argue that this conformer is the energetically preferred ground state structure when three water molecules are * Tel.: 1 1-303-293-3435; fax: 1 1-303-293-4904. E-mail address: [email protected] (P.W. Jagodzinski). 1 Present address: Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA.
proximate to the dimethylamino group [3]. They also argue that a saturation effect is observed beyond three water molecules with the twisted and planar structures both equally probable in aqueous solutions. Since DABA has a strong electron-donating group para to a strong electron-withdrawing group, the charge separated quinonoid resonance form provides a significant contribution to the ground state configuration. Therefore in non-polar environments, the dimethylamino group would be expected to be in a coplanar arrangement with the benzene ring with the nitrogen in a nearly sp 2 hybridized state. Gorse and Pesquer have calculated that this moiety is only slightly displaced from planarity with the methyl groups moving out of the plane and forming a pyramidal structure about the nitrogen atom [4]. Upon protonation however, the n–p interaction is broken and the nitrogen assumes a more tetrahedral sp 3 type hybridization. Minimizing contributions from the
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2. Experimental
Fig. 1. DABA with the angle of rotation (u ) of the dimethylamino group depicted.
quinonoid resonance structure permits the possible hindered rotation to the TICT configuration. It should be possible to make the TICT structure the dominant ground state conformer by protonating the dimethylamino group, since the barrier to rotation of this moiety has been reported to be in the order of 33 kJ/mol [5]. The protonated form of DABA has been used as the basis for a variety of colorimetric methods for more than 40 years [6,7]. As the strength of the acidic environment increases, the intensity of the 352 nm absorption band decreases and that of the 240 nm band increases. We have collected ultraviolet absorbance and visible Raman spectra of DABA in a variety of acidic environments and find significant changes as the pH is lowered. We interpret them as being due to increasing contributions from the TICT configuration with virtually all molecules having the dimethylamino group twisted by 908 in concentrated hydrochloric acid.
DABA, reagent grade, was obtained from the Eastman Kodak Company (Rochester, NY), and used without further purification. Hydrochloric acid (37.5%) and sodium hydroxide were purchased from the Fisher Scientific Company (Fair Lawn, NJ). The quaternary salt of DABA was produced by dissolving the solid in the acidic medium. Absorption spectra were collected using a Bausch and Lomb Spectronic 2000 spectrophotometer with a 2 nm slit width. Solutions were prepared using doubly distilled water and the pH was adjusted by adding reagent grade hydrochloric acid or an aqueous solution of sodium hydroxide. Concentrations of DABA were between 2 × 10 25 M and 5 × 10 25 M and were chosen so that the absorbance used to calculate e was between 0.2 and 0.6. Spectra were collected for samples that were contained in 1 cm path length quartz Suprasil cuvettes. Raman excitation was provided by a Coherent, Inc. ˚. Innova 70-4 argon ion laser operating at 4880 A Approximately 150 mW of laser power was focused on samples contained in standard melting point capillary tubes. The Raman scattering was collected at 908 by a SPEX 1401 double monochromator (Edison, NJ)
Table 1 Molar absorptivities and dihedral angles for DABA in aqueous solution as a function of pH Solvent Hexane Carbon tetrachloride Benzene Water
Concentrated HCl
pH
e 352 (l/mol cm)
e 240 (l/mol cm)
u 8a
9.56 7.15 4.96 3.87 3.24 2.80 2.40 2.00 1.59 1.28 0.95 0.66 0.21 21.08
74100 50100 38900 27900 28200 27700 29600 27970 29300 24419 21700 13425 8118 3990 1280 422 0
– – – 5690 5750 5650 5730 5877 7170 6765 8920 10,190 11,628 12,600 12,960 13,184 14,130
– – – 44.2 43.9 44.5 42.4 44.2 42.8 47.9 50.8 60.2 67.3 74.3 81.2 84.9 90.0
log [BH 1]/[B]NMe2 b
log [H 1 BH 1]/[BH 1] c
Ref. [18] [18] [18]
27.939 25.530 23.340 22.250 21.620 20.851 20.780 20.380 0.0294 0.340 0.670 0.960 1.410 2.700
215.932 213.520 211.330 210.240 29.611 29.170 28.770 28.370 27.959 27.650 27.320 27.030 26.580 25.290
Calculated using e /54,400 cos 2 u for the NMe2 rotation. Ratio of the molar concentration of DABA protonated on the dimethylamino group to the molar concentration of the free base. c Ratio of the molar concentration of doubly protonated DABA to the molar concentration of DABA protonated on the dimethylamino group. a
b
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Fig. 2. Plot of e 352 and e 240 (l/mol cm) for DABA in aqueous solution as a function of pH.
Fig. 3. Fractional composition diagram for DABA in acidic environments. Note that the values for the doubly protonated species are nearly zero and lie along the X-axis.
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with a slitwidth of 160 mm resulting in a spectral bandpass of approximately 3.25 cm 21. Each spectrum is an average of 10 scans with data collected for one second at each wave number. All manipulations were carried out on a SPEX Industries DATAMATE spectrometer controller. The concentrations were typically 0.3 mM in aqueous solution and 5 mM in hydrochloric acid solution for collection of the Raman spectra.
3. Results In order to analyze the data, we first consider the pKa’s of the carbonyl oxygen and the dimethylamino nitrogen. To our knowledge these values have not been reported for DABA. The average of the pKa values for the carbonyl group of similar 4-substituted benzaldehydes as reported by Palm and coworkers is 26.37 [8]. The pKa values for the dimethylamino group in N,N-dimethylanilines substituted in the 4position with electron donating groups are reported to be above zero, however the values for molecules sufficiently similar to DABA, to calculate an average have not been reported [9]. The absorbance data that we have collected at 352 and 240 nm is presented in Table 1. Following the method of Davis and Geissman we have plotted e versus pH as shown in Fig. 2 [10]. The inflection points provide the pKa values of 1.55 and 1.80. Limited data reported for DABA in 0.4 M sulfuric acid yields a pKa of 1.5 [11]. We therefore assume that the pKa is 1.62 (the average of these three values) with the associated error being minor within the context of our use of this value. Using equilibrium arguments we have calculated the ratios of the concentrations of DABA to DABA protonated on the dimethylamino group and DABA protonated on both the dimethylamino and carbonyl groups, and present the results in Table 1. We have also calculated the fractional composition of each of these three species and present the results in Fig. 3. It can be seen that only the base form is present at the basic and neutral pH’s, that the dimethylamino protonated acid form is important below pH 2, and that the concentration of the doubly protonated form is not significant down to pH 21. Thus, we interpret the observed spectra by considering increasing equilibrium concentrations of the dimethylamino protonated
species as the pH is lowered, and assume that the doubly protonated species cannot be used to interpret the current data. 3.1. Interpretation of the ultraviolet absorption spectra The ultraviolet absorption spectrum of DABA in neutral aqueous solution consists of bands at 352, 280, 240, 203, and 190 nm. Braude and Sondheimer proposed that the molar absorptivities of ultraviolet bands of the substituted benzenes are correlated with the angle of rotation of the substituent groups out of the plane of the ring [12]. The relationship, based on the overlap intergral of the p orbitals centered on the ring carbon and the a-atom of the substituent, is e / e 0 cos 2 u . Here e is the molar absorptivity for the molecule in a twisted configuration, e 0 is the molar absorptivity for the molecule in the planar configuration, and u is the angle of rotation (the range is 0– 908). The validity of this equation has been challenged, however it has been shown to accurately predict angles in agreement with the experimental results for a variety of molecules [13,14]. We have used this equation to analyze e for the 352 nm band as has been done for other aniline derivatives [15]. We have found that e 352 decreases and e 240 increases with decreasing pH at values lower than pH 3. The 352 nm band is due to the pp p ( 1La ← 1A) transition which involves a molecular orbital primarily composed of the benzyl and dimethylamino portions of the molecule [1]. Polar solvents shift this transition to lower energy than the pp p ( 1Lb ← 1A) transition found at 240 nm [16]. We have assumed that the dimethylamino group will rotate out of the plane of the ring because: (1) protonation occurs at this moiety in the pH range of this study; (2) this rotation is energetically favored over the carbonyl rotation; and (3) the values of n (CyO) suggest that the carbonyl group remains in the plane of the ring (vide infra). Cazeau-Dubroca and coworkers argue that the dimethylamino group is not coplanar with the ring in aqueous solution [3]. Therefore, we cannot use any of the values for our aqueous solutions to calculate e 0. We have chosen to use the average of e 352 for DABA in benzene, hexane and carbon tetrachloride solutions as e 0 since the molecule should be nearly
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Fig. 4. Raman spectra of DABA in aqueous solution (bottom trace) and in 12 M hydrochloric acid solution (top trace).
planar in these solutions [17,18]. The calculated dihedral angles range from about 44 at pH 9.56 to 90 in the concentrated hydrochloric acid (see Table 1). The exact values of the intermediate angles are valid to the extent that DABA is a planar in the organic solvents chosen to calculate e 0, however the trend is valid and the final value of 908 is valid since e 0 in concentrated acid. It is interesting to note that above pH 3 the calculated angle is nearly 458 which might represent the saturation effect (equal distribution of conformers) predicted by Cazeau-Dubroca [3]. Therefore the ultraviolet absorption data support formation of the TICT state in acidic media. Corroborating evidence can be provided by vibrational spectra. We have collected and analyzed Raman spectra of DABA in acidic solutions. 3.2. Interpretation of the Raman spectra The DABA molecule has C1 point group symmetry since the dimethylamino group is slightly pyramidal for the isolated molecule. The molecule formally adopts Cs point group symmetry
when the dimethylamino group is rotated 90 out of the ring plane. No significant information is available from the depolarization ratios of DABA in concentrated hydrochloric acid due to the complexity of the normal modes and the understanding of them afforded by a normal mode analysis [18]. The Raman spectra of DABA and its dimethylamino protonated analog are presented in Fig. 4 and the spectra are presented as a function of acid concentration in Fig. 5 to allow monitoring of the spectroscopic changes as the equilibrium shifts to the protonated form. All of the Raman bands are polarized and the intensities are independent of exciting wavelength in the visible region of the spectrum. Vibrational assignments are presented in Table 2. Explanation of the changes in selected bands is facilitated by reference to the normal mode analysis of DABA and five isotopic derivatives [18]. A 40 cm 21 blue shift of the Raman signal due to the normal mode with a PED composed of 75% CyO stretching motion (typically called the carbonyl stretch; located at 1643 cm 21 for DABA in water) is observed upon protonation. This band shifts to about 1695 cm 21 in concentrated HCl (actually a Fermi
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Fig. 5. Raman spectra of DABA in distilled water, 1.0 M HCl solution, 3.0 M HCl solution and 12.0 M HCl solution.
doublet with components at 1706 and 1685 cm 21). Protonation of the dimethylamino group is expected to increase the double bond character of the CyO group with concomitant blue shift (n CyO 1694 cm 21 for neat benzaldehyde) [19], whereas protonation of the carbonyl group would favor more single bond character and the band would undergo a red shift. The observed shift is consistent with the breakdown of the extended system thus decreasing the electron density in the limited p p orbital of the carbonyl moiety and increasing the double bond of the aldehyde carbon–oxygen bond. Seehra and Jagodzinski have attributed the Fermi doublet in the vibrational spectrum of uncomplexed DABA to the interaction of the carbonyl CyO stretching fundamental with an overtone of the normal mode at 844 cm 21 [20]. Both the carbonyl stretching mode and the lower wave number mode (now at 849 cm 21) blue shift upon protonation. In the protonated form, the two bands are of approximately equal intensity and are separated by 21 cm 21, while in the
spectrum of DABA in aqueous solution the carbonyl band is comprised of a strong band with a broad shoulder separated by 41 cm 21. The changes that occur in n CyO as the pH is lowered are seen in Fig. 5. The in-plane wagging motion of the formyl proton in the aldehyde moiety also undergoes a significant change upon protonation. In the spectrum of DABA in doubly distilled water, the intense band at 1404 cm 21 is assigned to this vibration [18]. Due to the highly conjugated nature of the DABA molecule, the normal modes exhibit an unusually high degree of delocalization. Upon protonation the majority of conjugation through the benzenoid p system is disrupted and the resultant benzenoid form is expected to produce a vibrational spectrum that is more indicative of the local symmetry of each of the sections of the DABA molecule [21]. Previous studies of para-substituted benzaldehyde systems indicate that the frequency of the formyl proton in-plane wagging mode is relatively independent of the para substituent and usually occurs as a very weak band around 1400 cm 21 in the Raman
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Table 2 Vibrational assignments for the Raman spectra of DABA in aqueous and acidic media DABA in water a
1643 (6) 1592 (10) 1549 (5)
1442 (3) 1420 (2) 1404 (4)
DABA-H1
Approximate mode description
1706(4), 1685(4) b
CyO str. {Fermi doublet} CyO str. {Fermi doublet} Ring mode (n 8a) c Ring mode (n 8b) Ring mode (n 19a) Mixed ring/Me deform. Mixed ring/Me deform.
1609 (10) 1512 (1) 1462 (1) 1444 (1) 1411 (1) 1309 (1)
1254 (1) 1236 (1)
1180 (4) 1135 (1)
948 (1) 844 (4) 732 (2) 636 (2) 598 (2) 381 (,1) 358 (,1)
1246 (2) 1213(4) 1176 (4) 1130 (4) 1016 (1) 994 (1) 898 (2) 849 (5) 737 (3) 644 (2) 632 (3) 613 (2) 435 (2) 381 (,1) 329 (,1) 309 (1)
Aldehyde H in-plane wag Ring mode/Ar–N Str. Mixed ring/Me rock Mixed ring/Me rock Ar–N str./mixed ring Ar–C str./mixed ring Arom. H in-plane wag Ring mode/Me rock Out-of-plane ring mode (n 18a) Out-of-plane ring mode (n 18b) Symmetricf N-methyl str. Mixed ring/CyO bend (n 1) In-plane CyO bend/mixed ring Ring mode/CyO bend (n 6a) Ring mode (n 6b) Mixed mode (n 4) Delocalized mode NC2 deform Mixed ring
All vibrational frequencies are in cm 21 and calibrated with respect to the indene calibration standard. Numbers in parentheses are the relative scattering intensities. c Ring modes have been described using the Wilson numbering system for the analogous vibrations of benzene where ever applicable. a
b
spectrum [19,22,23]. Since it is unlikely that the frequency of this vibration will shift significantly upon protonation, the in-plane wagging vibration of the aldehyde hydrogen is assigned to the weak transition occurring at 1411 cm 21 in the Raman spectrum of protonated DABA. This indicates that the polarizability associated with this normal mode is consistent with previous observations for benzaldehydes. Interestingly, upon protonation of DABA, the relative intensities of all of the bands that occur in this region of the spectrum are significantly diminished. The normal modes associated with the NC2 skeleton of the dimethylamino group are expected to undergo significant shifts due to the loss of electron density on the nitrogen center upon protonation. One example is the symmetric N-methyl stretching
motion. Guichard et al. have shown that for N,Ndimethylaniline, this mode shifts approximately 47 cm 21 towards the red upon protonation [21]. In the spectrum of DABA in water, this stretching mode can be assigned to the broad, weak band at 948 cm 21 based on the vibrational assignments for dimethylaniline and the normal coordinate analysis of DABA [18,21]. Upon protonation, the band at 948 cm 21 is no longer observed, but a new band arises at 898 cm 21. Assigning this band to the symmetric stretching motion of the dimethylamino nitrogen– methyl carbon bonds is consistent with the observation for N,N-dimethylaniline and with protonation at the NMe2 group. Due to the inherent stability of the benzenoid ring, the energies of the ring vibrations are expected to
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remain relatively unchanged upon protonation [21]. However, the relative Raman intensities of each band associated with these ring modes are subject to significant change. This effect is exhibited by the bands at 1549 and 1442 cm 21 in the spectrum of the parent molecule in water (Fig. 4). Upon ligation of the acidic proton, the intensities of these bands are significantly decreased. As a result, the Raman spectrum of protonated DABA appears remarkably similar to the spectra of neat N,N-dimethylaniline or benzaldehyde as previously reported [19,21]. The change in the intensity of these lines indicates a reduction of the extended conjugation in the DABA molecule. Additional bands that undergo noticeable changes upon protonation are found at 1549, 1246, 1213, and 1130 cm 21. The 1549 cm 21 band is due to a ring mode (Wilson number n 8b) and is completely absent in the spectrum of the protonated molecule. The form of this mode shows that it is localized at the amino end of the ring and includes a contribution from the Ar–N stretching motion [18,22,23]. The disappearance of this signal strongly supports the decoupling of the dimethylamino group from the aromatic ring system. The band at 1246 cm 21 in the spectrum of the protonated species corresponds to either 1254 or 1236 cm 21 in the spectrum of the parent molecule (see Table 2). The increase in intensity would indicate that the methyl rocking motion no longer contributes to the mode leaving it as primarily a ring mode. The 1213 cm 21 band in the spectrum of protonated DABA (with no counterpart in the spectrum of the protonated form) is analogous to the benzaldehyde vibration found at 1206 cm 21. Both are reasonably intense signals. Zwarich, et al. have calculated that 41% of the PED of this mode is due to the Ar–C stretching coordinate [19]. The 1135 cm 21 band in the spectrum of the unprotonated DABA is due to a mixed ring-methyl rocking mode. The signal shifts to 1130 cm 21 and becomes much more intense probably indicating that it has increasing contributions from ring motions. In the frequency region below 1000 cm 21 there are several signals due to ring deformation modes. The band at 844 cm 21 in the parent molecule spectrum shifts to 849 cm 21 and becomes more intense upon protonation (see Fig. 5). This band is due to in-plane bending motions of the CHO moiety and a ring deformation localized near the CHO group. The increase in
intensity is consistent with the relative intensity of this mode in benzaldehyde [19,22]. The changes in the Raman spectra are consistent with protonation of the dimethylamino group and subsequent decoupling from the extended p system. This data supports the interpretation of the absorption spectra. 4. Conclusions In this study, we have attempted to model the TICT conformer by protonating the dimethylamino nitrogen in the DABA molecule. Upon protonation, the dimethylamino group is twisted out of the plane of the aromatic ring into a conformation identical to that of the TICT state. This disruption of the extended p system has profound effects on the ultraviolet absorption and visible Raman spectrum of DABA. The absorption bands at 352 and 240 nm allow us to predict a pKa of 1.62 for the dimethylamino moiety. The analysis of e 352 allows us to predict the average dihedral angle for rotation out of the plane of the ring at a particular pH. It was found that all molecules have a dihedral angle of 908 in the concentrated hydrochloric acid. Owing to the loss of the interaction between the nitrogen lone pair electrons and the p system of the benzenoid ring, the Raman spectrum of protonated DABA exhibits spectral characteristics that are similar to para-substituted benzaldehyde derivatives that contain substituents having no p base qualities. The carbonyl CyO stretching vibration exhibits a significant blue shift upon protonation supporting protonation of the dimethylamino nitrogen. Due to the loss of electron density at the amino nitrogen, modes such as the aromatic ringdimethylamino nitrogen stretching coordinate, the dimethylamino nitrogen–methyl carbon stretching motions and the deformations of the NC2 skeleton all change upon protonation.
Acknowledgements The authors would like to acknowledge useful discussions with K.M. Brummond and the assistance of S.K. Sneckenberger and C.-M. Coyle.
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