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Hydrogen-bonding interactions between 1-methylindole and alcohols
Chemical Physics Letters 401 (2005) 109–114 www.elsevier.com/locate/cplett
Hydrogen-bonding interactions between 1-methylindole and alcohols Marı´a A. Mun˜oz *, Manuel Gala´n, Carmen Carmona, Manuel Balo´n Departamento de Quı´mica Fı´sica, Facultad de Farmacia, Universidad de Sevilla, c/Prof. Garcı´a Gonza´lez s/n, 41012 Sevilla, Spain Received 21 October 2004; in final form 28 October 2004 Available online 26 November 2004
Abstract A theoretical and experimental FTIR study has been undertaken to analyse the p-hydrogen-bonding interactions between 1methylindole (1MI), and the alcohols trifluoroethanol, and hexafluoroisopropanol. Experimental IR measurements performed on the OH stretching band of the alcohols, in hexane as solvent, showed that, after the addition of 1MI, a composite associated red-shifted band is formed. Ab initio calculations (DFT/B3LYP) performed with different starting geometries and stoichiometries led us to propose the initial formation of 1:1 T-shape OH-p hydrogen-bonded complexes which, furtherly, are stabilised by hydrogen-bonding interaction with a second alcohol molecule, giving ternary 1:2 complexes. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction In the last years, the influence that polar solvents exert on the photophysics of indole has attracted considerable interest, particularly those acting as proton donor to the p-cloud of this chromophore. Most of these studies have been carried out by electronic excitation spectra [1–3]. Because the importance of solvating processes of biological systems in its natural environment, experimental and theoretical studies have been reported on small clusters of indole–water complexes. In spite of theoretical calculations have suggested the existence of a p-type complex for indole–water system, no direct experimental evidence could be found, probably due to the dominance of the NH–O bonded complex. Experimental indications of the existence of a p-bonded structure have been reported by R2PI, RIDIR and IR–UV hole burning spectroscopies on 1-methylindole (1MI)– water cluster in supersonic expansion [4], in which the methyl group impedes the formation of a NH–O structure allowing only the p-binding site of indole accessible for water. *
Corresponding author. Fax: +34 954557174. E-mail address: [email protected] (M.A. Mun˜oz).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.156
Semi-empirical analysis brought about by Mons et al. [5] on modelled indole–water and 1MI–water complexes suggested that N-methylation of indole does not influence significantly the geometry and energetic of the p-complex. Furthermore, they measured the binding energy of the methylated indole complex, in gas phase, using a photoionization technique, and assumed this system to be a good model for the unobserved p-complex of indole–water. We recently studied p-complexes of 1-methylpyrrole with alcohols of different H-donor abilities [6]. This Letter, carried out theoretically by DFT/B3LYP/ 6-31++G** and experimentally by FTIR in solvents of different polarity showed an excellent qualitative agreement between theory and experiments, giving stabilisation energies, association constants and band-shifts that decrease as the H-donor strength of the alcohols diminishes. The complexes, of 1:1 stoichiometry, had T-shape geometry, where the OH group of the alcohol points to the C-3 in the aromatic ring of pyrrole. More recently, we have reported a steady-state and time-resolved fluorescence study on the interactions of 1MI with trifluoroethanol (TFE) in hexane [7]. Interestingly, the results revealed, for the first time, the formation of two different complexes. At low alcohol
M.A. Mun˜oz et al. / Chemical Physics Letters 401 (2005) 109–114
concentrations, a ground state hydrogen-bonded complex (HBC), which hardly modify the emission of 1MI, is formed. As the concentration of TFE increased, the formation of a sort of hydrogen-bonded proton transfer exciplex (PTC) is the responsible for the quenching and red-shifted fluorescence of 1MI. In the proposed mechanism two alcohol molecules are, at least, necessary for the PTC to be formed. With the aim to ascertain if different kinds of complexes can be also formed in the ground state, we present in this Letter, a FTIR study on the 1MI–TFE system, in hexane as solvent. Likewise, theoretical calculations at DFT/B3LYP level have been carried out by testing different structures and stoichiometries for the complexes. The study of the interactions between 1MI and the strong H-donor hexafluoroisopropanol (HFIP), has also been included.
0.16
0.12
Absorbance
110
0.08
0.04
0.00 3450
3600
3650
Fig. 1. FTIR spectra, in hexane, of 1MI–HFIP system: [HFIP] = 0.02 M; [1MI] = 0.1–0.4 M.
0.30
1MI, HFIP and TFE (commercial products, Aldrich, Merck, Fluka >99 + %) and n-hexane (spectral grade, ˚ molecMerck) were used as received and stored on 4 A ular sieves. The infrared measurements were carried out on a Mattson FTIR Galaxy 2020 spectrometer equipped with a HgCdTe detector. A thermostated cell (25 °C) with ZnSe windows and 0.1 cm pathlength was used. The spectral resolution was 2 cm1 and 200 scans were coadded to obtain each spectrum. The solutions contained fixed concentration (0.02– 0.03 M) of the alcohol and varying concentrations, in great excess (0.08–1 M), of the p-base. The solvent spectrum was always subtracted from those of the mixed solutions. Deconvolution of the spectral bands was achieved with the Peakfit Program, v. 4.0, of Jandel Scientific. The hidden peaks were detected by the second-derivative method, and fitted to an asymmetric logistic peak function given by the program. The calculations have been performed with the GAUSSIAN 98 program [8], at the DFT level with the B3LYP density functional. The 6-31G** and 6-31++G** basis sets were tried in order to know if the use of diffuse functions changes the main conclusions that can be drawn from the calculations. The geometries were constructed with the GAUSVIEW 2.08 program and the interaction energies were corrected from BSSE and from the zero point energy (ZPE).
0.25
Figs. 1 and 2 show the FTIR spectra of 1MI–HFIP and 1MI–TFE systems. As it can be appreciated, the
3550
cm-1
2. Methods
3. Results and discussion
3500
Absorbance
0.20
0.15
0.10
0.05
0.00 3450
3500
3550
3600
3650
cm-1
Fig. 2. FTIR spectra, in hexane, of 1MI–TFE system; [TFE] = 0.03 M, [1MI] = 0.078–0.626 M.
addition of increasing concentrations, in excess, of 1MI to alcohol solutions in hexane, diminishes the intensity of the monomer bands. Simultaneously, new red-shifted bands grow up. The monomer HFIP exhibits two bands with maxima at 3624 and 3589 cm1 in the OH stretching region. These bands can be assigned to the synclinal and antiperiplanar conformers, which, in gas phase, appear at 3667 and 3626 cm1, respectively, [9]. Although, there is debate on the type of forces stabilising the band at lower frequency, the formation of an intramolecular hydrogen bond between the OH group and one of the fluorine atoms seems to be the more generally accepted explanation. In the case of TFE only one band for the monomer is apparent, but the deconvolution of the spectra, Fig. 3, evidences also a dual absorption for this molecule: A weak band at higher frequency, 3644 cm1 and one more intense at 3628 cm1. Numerous studies are being reported concerning the vibration
M.A. Mun˜oz et al. / Chemical Physics Letters 401 (2005) 109–114
111
Ki
TFEcis () TFEtrans
0.25
K1
0.20
Absorbance
ð1Þ
1MI þ TFEtrans () C 1
0.15
ð2Þ
K2
1MI þ TFEtrans () C 2 0.10
ð3Þ
According to this scheme and assuming enough excess of 1MI to consider its initial concentration to be equal to that in equilibrium, the following Benesi–Hildebrand equations for 1:1 stoichiometric complexes can be derived
0.05
0.00 3400
3450
3500
3550
3600
3650
cm-1
1=Akc ¼
K1 þ K2 1 þ ð1=K i Þ þ ð1=½1MIÞ; ekc bK 1 c0TFE ekc bK 1 c0TFE
ð4Þ
1=Akm ¼
1 þ ð1=K i Þ K 1 þ K 2 þ k 0 ½1MI; e m bcTFE ekm bc0TFE
ð5Þ
Fig. 3. Deconvolution of the FTIR spectra of 1MI–TFE system.
where A is the absorbance at the maximum wave number, c0TFE the initial TFE concentration, e the extinction coefficient, b the path length, and c and m refer to C1 and the monomer TFE trans, respectively. To improve the fits, we have used the integrated areas under the deconvolved bands instead of absorbances. Consistently with the above scheme, plots according to Eqs. (4) and (5) are linear, as typically shown in Fig. 4, but, contrarily to the predictions of the model, the values of the combination of constants, i.e., (K1 + K2)/(1 + 1/Ki), obtained on the monomer and associated bands differ significantly. Thus, values of (0.9 ± 0.2) and (1.1 ± 0.1) M1 were obtained from the bands at 3563 and 3479 cm1, respectively, whereas values of (2.5 ± 0.1) and (2.0 ± 0.1) M1 were calculated from those of the monomer at 3644 and 3628 cm1, respectively. 0.5
0.4
0.3
1/Aλc
spectrum of this important molecule [10–12] and two conformers are described with cis–gauche and trans geometries: the weak component is assigned to the trans conformer and the main band to the gauche one. Discrepancies on the existence of intramolecular hydrogen bonds to stabilise the gauche form have also been reported [13]. To our knowledge, the small band, although described by theoretical calculations, has never been observed in solution. Therefore, on the basis of these previous studies we will assign the 3644 and 3628 cm1 bands to the trans and cis conformers of TFE, respectively. The associated bands in the absorption spectra of both 1MI–HFIP and 1MI–TFE systems have a complex shape. In the first case at least six bands must be considered in the deconvolution process to resolve the spectra and no reliable results were obtained. The analysis of the bands for the 1MI–TFE system was, however, feasible and, as Fig. 3 shows, four main components can be distinguished. The two monomer bands at 3644 and 3628 cm1 which decrease with the increase of 1MI concentration, and two associated bands, at 3563 and 3479 cm1 that grow synchronously. Furthermore, a small band at 3579 cm1 appears at low 1MI concentrations whose intensity furtherly diminishes becoming indistinguishable at high indole concentrations. The existence of two main associated bands in the spectra could be at first rationalised by assuming the formation of two different 1:1 HBC. On this respect we have accounted for the existence of an equilibrium between the cis and trans monomeric TFE. We have also assumed, according to the literature, that only the trans conformer can interact with such a large molecule as 1MI [13]. On this assumption, the OH group of the alcohol would interact at two different sites of the indole ring, possibly at the benzene and the pyrrole rings, respectively. In any case, the scheme would be as follows:
0.2
0.1
0.0 0
1
2
3
4
5
1/[1MI] (M-1)
Fig. 4. Plots of the reciprocal of the areas under the associated bands versus the reciprocal of 1MI concentrations for 1MI–TFE system in hexane: k = 3563 cm1 (); k = 3479 cm1 (n).
112
M.A. Mun˜oz et al. / Chemical Physics Letters 401 (2005) 109–114
The formation of two different 1:1 stoichiometric complexes can also be discarded by theoretical predictions. Thus, fully optimised geometrical calculations searching for possible 1:1 structures, yield always an unique minimum with the OH-bond directed toward the C-3 of the accepting 1MI ring (Fig. 5). As previously reported by other authors for the 1MI–water complex [4,5], attempts to locate minima with the OH-bond pointing to the benzene ring were unsuccessful. In Table 1 they are collected the calculated interaction energies, vibration frequencies, and equilibrium distances of the corresponding OH-bond to the C-3 of 1MI for the 1:1 complexes. These results show discrepancies of only 0.1 kcal/mol in the energies calculated with and without the use of diffuse functions. Likewise, the small differences found in the calculated frequencies (10 cm1) and in the equilibrium distances (10th of ˚ ) led us to conclude that these parameters are rather A insensitive to the basis sets used. At this point and taking into account the structure of the 1:1 complexes, a suggestive hypothesis emerge: the 1:1 complexes could be further stabilised by hydrogen-
bonding interaction with a new alcohol molecule. The 1:2 stoichiometric complexes so formed would explain the existence of the two main associated bands in the FTIR spectra of 1MI–TFE system, i.e., the symmetric and asymmetric stretching vibrations of the two coupled OH bonds in the complex. To substantiate this hypothesis, further theoretical calculations were carried out. Because it was checked that, for the 1:1 complexes, the use of diffuse functions does not significantly modify the results, the less time-consuming 6-31G** basis sets was used for the ternary complexes. Beginning with the optimised geometry of the 1:1 complexes, the 1:2 ones were constructed with the H atom of the OH group of a second alcohol molecule assisting the oxygen atom of the alcohol molecule in the 1:1 complex. Thus, two hydrogen-bonding interactions exist: (1) a p-hydrogen bond between 1MI and the alcohol, and (2) a conventional hydrogen bond between the two alcohol molecules. The optimised geometries of the 1:2 complexes are depicted in Fig. 5. The corresponding interaction energies, vibration frequencies and OH–C-3 distances are reported in Table 2. As can be appreciated, the results show larger energies
Fig. 5. Optimised geometries of the 1:1 and 1:2 complexes of 1MI–HFIP (1(a) and (b)) and 1MI–TFE (2(a) and (b)).
M.A. Mun˜oz et al. / Chemical Physics Letters 401 (2005) 109–114
113
Table 1 1:1 Complexes calculated by DFT and B3LYP functional Complex
DE a (kcal mol1)
mMon (cm1)
˚) d (A
mComplex (cm1)
Theor.
Exp.
Theor.
HFIP
6-31G** 6-31++G**
4.7 4.6
3819.8 3829.1
3624 3589
3657.5 3649.5
TFE
6-31G** 6-31++G**
3.5 3.4
3807.7 3818.5
3644 3628
3687.4 3692.1
Exp. 2.20 2.24 3579
2.25 2.33
Effects of diffuse functions in the energies, frequencies and OH–C-3 equilibrium distances. a DE = E (complex) [E (1MI) + E (alcohol)].
Table 2 1:2 Complexes calculated by DFT, B3LYP functional and 6-31G** basis sets DE a (kcal mol1)
Complex
mComplex(asym) (cm1)
˚) d (A
mComplex(sym) (cm1)
Theor.
Exp.
Theor.
Exp.
HFIP
6.05
3697.9
3531
3503.7
3490
2.08
TFE
4.75
3665.6
3563
3582.0
3479
2.15
a
DE = E (1:2 complex) minus [E (1:1 complex) + E (alcohol)].
as well as smaller OH–C-3 distances for 1:2 than for 1:1 complexes. Furthermore, the theoretically calculated distance between the symmetric and asymmetric associated bands in the 1:2 1MI–TFE complex, 84 cm1, exactly coincides with that observed in the experimental FTIR spectra. Thus, the theoretical results plentifully support the formation of 1:2 complexes. The formation of these ternary complexes could result from two alternative pathways, one of them could be the TFE dimer interaction with a 1MI molecule. In this case, a more complex dependence of the measured absorbances with concentrations than the observed should be expected. Moreover, it is to be noted that dimeric TFE species are not present in solution at the used concentrations, since, in accordance with other authors [9], we found this band at 3530 cm1 only at the TFE concentrations above 0.03 M. Alternatively, the formation of the 1:2 complex could be viewed as a two step process K1
1MI þ TFEtrans () 1MI TFE K2
1MI TFE þ TFEtrans () 1MI TFE2
ð6Þ ð7Þ
In this case, to explain the experimentally observed 1:1 stoichiometry, it would be assumed K2 K1, i.e., once the 1:1 complex is formed, it quickly reacts to give the 1:2 complex, remaining the concentration of the binary complex practically stationary. This model seems to be supported by the existence, in the deconvoluted spectra, of the small band at 3579 cm1 which is patent only at low 1MI concentrations and disappears as this concentration increases. Its location, at higher frequency than the other associated bands, qualitatively agrees
with the calculated frequency for the 1:1 complex. In fact, as it can be appreciated, the frequencies for the complexes in Tables 1 and 2 match the experimentally observed bands with a scale factor of 0.97. The no complete coupling of the above equilibria could be responsible for the discrepancies observed when the experimental results are analysed in the monomer and associated bands.
4. Summary and conclusions Experimental FTIR measurements provide evidences on the formation of OH–p HBC of 1MI with HFIP and TFE in hexane. The existence of two main associated bands in the spectra of 1MI–TFE system and the apparent 1:1 stoichiometry of the complexation process, let us to initially suspect the coupled formation of two different 1:1 1MI–alcohol complexes with the OH group of the alcohol attached at different sites of the indole ring. However, the theoretical results only support the existence of an unique site for OH–p hydrogen-bonding interactions in the 1MI ring. Thus, DFT calculations only found minima for 1:1 complexes with T-shape geometries, where the OH group of the alcohol points to the C-3 atom of 1MI. These complexes can be further stabilised by a second alcohol molecule hydrogen bonded to the oxygen atom of the OH bridge. Interestingly, in the 1MI–TFE system, the frequencies of the theoretical symmetric and asymmetric stretching vibrations of these coupled OH bonds excellently match those of the associated bands observed in the experimental spectra.
114
M.A. Mun˜oz et al. / Chemical Physics Letters 401 (2005) 109–114
These results let us to propose that HFIP and TFE form 1:1 and 1:2 stoichiometric HBC with 1MI. Since 1:2 complexes are much more stable, the concentration of the 1:1 complexes remains practically stationary. This model is further supported by the experimental observation of a very weak band in the spectra of the TFE system. This band is distinguishable only at low 1MI concentration, and it is located in the spectral region theoretically predicted for the associated band of the 1:1 complexes.
References [1] M.J. Tubergen, D.H. Levy, J. Phys. Chem. 95 (1991) 2175. [2] Y. Huang, M. Sulkes, J. Phys. Chem. 100 (1996) 16479.
[3] K.W. Short, P.R. Callis, J. Chem. Phys. 108 (1998) 10189. [4] J.R. Carney, T.S. Zwier, J. Phys. Chem. A 103 (1999) 9943. [5] M. Mons, I. Dimicoli, B. Tardivel, F. Piuzzi, V. Brenner, P. Mille´, J. Phys. Chem. A 103 (1999) 9958. [6] M.A. Mun˜oz, M. Gala´n, L. Go´mez, C. Carmona, P. Guardado, M. Balo´n, Chem. Phys. 290 (2003) 69. [7] M.A. Mun˜oz, C. Carmona, M. Balo´n, Chem. Phys. Lett. 393 (2004) 217. [8] M.J. Frisch et al., GAUSSIAN 98, (Revision A.11.3), Gaussian Inc., Pittsburgh, PA, 1998. [9] H. Schaal, T. Ha¨ber, M.A. Suhm, J. Phys. Chem. A 104 (2000) 265. [10] V.F. Kalasinsky, H.V. Anjaria, J. Phys. Chem. 84 (1980) 1940. [11] J.R. Durig, R.A. Larsen, J. Mol. Struct. 238 (1989) 195. [12] J. Marco, J.M. Orza, J. Mol. Struct. 267 (1992) 33. [13] M.L. Senent, A. Nin˜o, C. Mun˜oz-Caro, Y.G. Smeyers, R. Domı´nguez-Go´mez, J.M. Orza, J. Phys. Chem. A 106 (2002) 10673.
Hydrogen-bonding interactions between 1-methylindole and alcohols Marı´a A. Mun˜oz *, Manuel Gala´n, Carmen Carmona, Manuel Balo´n Departamento de Quı´mica Fı´sica, Facultad de Farmacia, Universidad de Sevilla, c/Prof. Garcı´a Gonza´lez s/n, 41012 Sevilla, Spain Received 21 October 2004; in final form 28 October 2004 Available online 26 November 2004
Abstract A theoretical and experimental FTIR study has been undertaken to analyse the p-hydrogen-bonding interactions between 1methylindole (1MI), and the alcohols trifluoroethanol, and hexafluoroisopropanol. Experimental IR measurements performed on the OH stretching band of the alcohols, in hexane as solvent, showed that, after the addition of 1MI, a composite associated red-shifted band is formed. Ab initio calculations (DFT/B3LYP) performed with different starting geometries and stoichiometries led us to propose the initial formation of 1:1 T-shape OH-p hydrogen-bonded complexes which, furtherly, are stabilised by hydrogen-bonding interaction with a second alcohol molecule, giving ternary 1:2 complexes. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction In the last years, the influence that polar solvents exert on the photophysics of indole has attracted considerable interest, particularly those acting as proton donor to the p-cloud of this chromophore. Most of these studies have been carried out by electronic excitation spectra [1–3]. Because the importance of solvating processes of biological systems in its natural environment, experimental and theoretical studies have been reported on small clusters of indole–water complexes. In spite of theoretical calculations have suggested the existence of a p-type complex for indole–water system, no direct experimental evidence could be found, probably due to the dominance of the NH–O bonded complex. Experimental indications of the existence of a p-bonded structure have been reported by R2PI, RIDIR and IR–UV hole burning spectroscopies on 1-methylindole (1MI)– water cluster in supersonic expansion [4], in which the methyl group impedes the formation of a NH–O structure allowing only the p-binding site of indole accessible for water. *
Corresponding author. Fax: +34 954557174. E-mail address: [email protected] (M.A. Mun˜oz).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.156
Semi-empirical analysis brought about by Mons et al. [5] on modelled indole–water and 1MI–water complexes suggested that N-methylation of indole does not influence significantly the geometry and energetic of the p-complex. Furthermore, they measured the binding energy of the methylated indole complex, in gas phase, using a photoionization technique, and assumed this system to be a good model for the unobserved p-complex of indole–water. We recently studied p-complexes of 1-methylpyrrole with alcohols of different H-donor abilities [6]. This Letter, carried out theoretically by DFT/B3LYP/ 6-31++G** and experimentally by FTIR in solvents of different polarity showed an excellent qualitative agreement between theory and experiments, giving stabilisation energies, association constants and band-shifts that decrease as the H-donor strength of the alcohols diminishes. The complexes, of 1:1 stoichiometry, had T-shape geometry, where the OH group of the alcohol points to the C-3 in the aromatic ring of pyrrole. More recently, we have reported a steady-state and time-resolved fluorescence study on the interactions of 1MI with trifluoroethanol (TFE) in hexane [7]. Interestingly, the results revealed, for the first time, the formation of two different complexes. At low alcohol
M.A. Mun˜oz et al. / Chemical Physics Letters 401 (2005) 109–114
concentrations, a ground state hydrogen-bonded complex (HBC), which hardly modify the emission of 1MI, is formed. As the concentration of TFE increased, the formation of a sort of hydrogen-bonded proton transfer exciplex (PTC) is the responsible for the quenching and red-shifted fluorescence of 1MI. In the proposed mechanism two alcohol molecules are, at least, necessary for the PTC to be formed. With the aim to ascertain if different kinds of complexes can be also formed in the ground state, we present in this Letter, a FTIR study on the 1MI–TFE system, in hexane as solvent. Likewise, theoretical calculations at DFT/B3LYP level have been carried out by testing different structures and stoichiometries for the complexes. The study of the interactions between 1MI and the strong H-donor hexafluoroisopropanol (HFIP), has also been included.
0.16
0.12
Absorbance
110
0.08
0.04
0.00 3450
3600
3650
Fig. 1. FTIR spectra, in hexane, of 1MI–HFIP system: [HFIP] = 0.02 M; [1MI] = 0.1–0.4 M.
0.30
1MI, HFIP and TFE (commercial products, Aldrich, Merck, Fluka >99 + %) and n-hexane (spectral grade, ˚ molecMerck) were used as received and stored on 4 A ular sieves. The infrared measurements were carried out on a Mattson FTIR Galaxy 2020 spectrometer equipped with a HgCdTe detector. A thermostated cell (25 °C) with ZnSe windows and 0.1 cm pathlength was used. The spectral resolution was 2 cm1 and 200 scans were coadded to obtain each spectrum. The solutions contained fixed concentration (0.02– 0.03 M) of the alcohol and varying concentrations, in great excess (0.08–1 M), of the p-base. The solvent spectrum was always subtracted from those of the mixed solutions. Deconvolution of the spectral bands was achieved with the Peakfit Program, v. 4.0, of Jandel Scientific. The hidden peaks were detected by the second-derivative method, and fitted to an asymmetric logistic peak function given by the program. The calculations have been performed with the GAUSSIAN 98 program [8], at the DFT level with the B3LYP density functional. The 6-31G** and 6-31++G** basis sets were tried in order to know if the use of diffuse functions changes the main conclusions that can be drawn from the calculations. The geometries were constructed with the GAUSVIEW 2.08 program and the interaction energies were corrected from BSSE and from the zero point energy (ZPE).
0.25
Figs. 1 and 2 show the FTIR spectra of 1MI–HFIP and 1MI–TFE systems. As it can be appreciated, the
3550
cm-1
2. Methods
3. Results and discussion
3500
Absorbance
0.20
0.15
0.10
0.05
0.00 3450
3500
3550
3600
3650
cm-1
Fig. 2. FTIR spectra, in hexane, of 1MI–TFE system; [TFE] = 0.03 M, [1MI] = 0.078–0.626 M.
addition of increasing concentrations, in excess, of 1MI to alcohol solutions in hexane, diminishes the intensity of the monomer bands. Simultaneously, new red-shifted bands grow up. The monomer HFIP exhibits two bands with maxima at 3624 and 3589 cm1 in the OH stretching region. These bands can be assigned to the synclinal and antiperiplanar conformers, which, in gas phase, appear at 3667 and 3626 cm1, respectively, [9]. Although, there is debate on the type of forces stabilising the band at lower frequency, the formation of an intramolecular hydrogen bond between the OH group and one of the fluorine atoms seems to be the more generally accepted explanation. In the case of TFE only one band for the monomer is apparent, but the deconvolution of the spectra, Fig. 3, evidences also a dual absorption for this molecule: A weak band at higher frequency, 3644 cm1 and one more intense at 3628 cm1. Numerous studies are being reported concerning the vibration
M.A. Mun˜oz et al. / Chemical Physics Letters 401 (2005) 109–114
111
Ki
TFEcis () TFEtrans
0.25
K1
0.20
Absorbance
ð1Þ
1MI þ TFEtrans () C 1
0.15
ð2Þ
K2
1MI þ TFEtrans () C 2 0.10
ð3Þ
According to this scheme and assuming enough excess of 1MI to consider its initial concentration to be equal to that in equilibrium, the following Benesi–Hildebrand equations for 1:1 stoichiometric complexes can be derived
0.05
0.00 3400
3450
3500
3550
3600
3650
cm-1
1=Akc ¼
K1 þ K2 1 þ ð1=K i Þ þ ð1=½1MIÞ; ekc bK 1 c0TFE ekc bK 1 c0TFE
ð4Þ
1=Akm ¼
1 þ ð1=K i Þ K 1 þ K 2 þ k 0 ½1MI; e m bcTFE ekm bc0TFE
ð5Þ
Fig. 3. Deconvolution of the FTIR spectra of 1MI–TFE system.
where A is the absorbance at the maximum wave number, c0TFE the initial TFE concentration, e the extinction coefficient, b the path length, and c and m refer to C1 and the monomer TFE trans, respectively. To improve the fits, we have used the integrated areas under the deconvolved bands instead of absorbances. Consistently with the above scheme, plots according to Eqs. (4) and (5) are linear, as typically shown in Fig. 4, but, contrarily to the predictions of the model, the values of the combination of constants, i.e., (K1 + K2)/(1 + 1/Ki), obtained on the monomer and associated bands differ significantly. Thus, values of (0.9 ± 0.2) and (1.1 ± 0.1) M1 were obtained from the bands at 3563 and 3479 cm1, respectively, whereas values of (2.5 ± 0.1) and (2.0 ± 0.1) M1 were calculated from those of the monomer at 3644 and 3628 cm1, respectively. 0.5
0.4
0.3
1/Aλc
spectrum of this important molecule [10–12] and two conformers are described with cis–gauche and trans geometries: the weak component is assigned to the trans conformer and the main band to the gauche one. Discrepancies on the existence of intramolecular hydrogen bonds to stabilise the gauche form have also been reported [13]. To our knowledge, the small band, although described by theoretical calculations, has never been observed in solution. Therefore, on the basis of these previous studies we will assign the 3644 and 3628 cm1 bands to the trans and cis conformers of TFE, respectively. The associated bands in the absorption spectra of both 1MI–HFIP and 1MI–TFE systems have a complex shape. In the first case at least six bands must be considered in the deconvolution process to resolve the spectra and no reliable results were obtained. The analysis of the bands for the 1MI–TFE system was, however, feasible and, as Fig. 3 shows, four main components can be distinguished. The two monomer bands at 3644 and 3628 cm1 which decrease with the increase of 1MI concentration, and two associated bands, at 3563 and 3479 cm1 that grow synchronously. Furthermore, a small band at 3579 cm1 appears at low 1MI concentrations whose intensity furtherly diminishes becoming indistinguishable at high indole concentrations. The existence of two main associated bands in the spectra could be at first rationalised by assuming the formation of two different 1:1 HBC. On this respect we have accounted for the existence of an equilibrium between the cis and trans monomeric TFE. We have also assumed, according to the literature, that only the trans conformer can interact with such a large molecule as 1MI [13]. On this assumption, the OH group of the alcohol would interact at two different sites of the indole ring, possibly at the benzene and the pyrrole rings, respectively. In any case, the scheme would be as follows:
0.2
0.1
0.0 0
1
2
3
4
5
1/[1MI] (M-1)
Fig. 4. Plots of the reciprocal of the areas under the associated bands versus the reciprocal of 1MI concentrations for 1MI–TFE system in hexane: k = 3563 cm1 (); k = 3479 cm1 (n).
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The formation of two different 1:1 stoichiometric complexes can also be discarded by theoretical predictions. Thus, fully optimised geometrical calculations searching for possible 1:1 structures, yield always an unique minimum with the OH-bond directed toward the C-3 of the accepting 1MI ring (Fig. 5). As previously reported by other authors for the 1MI–water complex [4,5], attempts to locate minima with the OH-bond pointing to the benzene ring were unsuccessful. In Table 1 they are collected the calculated interaction energies, vibration frequencies, and equilibrium distances of the corresponding OH-bond to the C-3 of 1MI for the 1:1 complexes. These results show discrepancies of only 0.1 kcal/mol in the energies calculated with and without the use of diffuse functions. Likewise, the small differences found in the calculated frequencies (10 cm1) and in the equilibrium distances (10th of ˚ ) led us to conclude that these parameters are rather A insensitive to the basis sets used. At this point and taking into account the structure of the 1:1 complexes, a suggestive hypothesis emerge: the 1:1 complexes could be further stabilised by hydrogen-
bonding interaction with a new alcohol molecule. The 1:2 stoichiometric complexes so formed would explain the existence of the two main associated bands in the FTIR spectra of 1MI–TFE system, i.e., the symmetric and asymmetric stretching vibrations of the two coupled OH bonds in the complex. To substantiate this hypothesis, further theoretical calculations were carried out. Because it was checked that, for the 1:1 complexes, the use of diffuse functions does not significantly modify the results, the less time-consuming 6-31G** basis sets was used for the ternary complexes. Beginning with the optimised geometry of the 1:1 complexes, the 1:2 ones were constructed with the H atom of the OH group of a second alcohol molecule assisting the oxygen atom of the alcohol molecule in the 1:1 complex. Thus, two hydrogen-bonding interactions exist: (1) a p-hydrogen bond between 1MI and the alcohol, and (2) a conventional hydrogen bond between the two alcohol molecules. The optimised geometries of the 1:2 complexes are depicted in Fig. 5. The corresponding interaction energies, vibration frequencies and OH–C-3 distances are reported in Table 2. As can be appreciated, the results show larger energies
Fig. 5. Optimised geometries of the 1:1 and 1:2 complexes of 1MI–HFIP (1(a) and (b)) and 1MI–TFE (2(a) and (b)).
M.A. Mun˜oz et al. / Chemical Physics Letters 401 (2005) 109–114
113
Table 1 1:1 Complexes calculated by DFT and B3LYP functional Complex
DE a (kcal mol1)
mMon (cm1)
˚) d (A
mComplex (cm1)
Theor.
Exp.
Theor.
HFIP
6-31G** 6-31++G**
4.7 4.6
3819.8 3829.1
3624 3589
3657.5 3649.5
TFE
6-31G** 6-31++G**
3.5 3.4
3807.7 3818.5
3644 3628
3687.4 3692.1
Exp. 2.20 2.24 3579
2.25 2.33
Effects of diffuse functions in the energies, frequencies and OH–C-3 equilibrium distances. a DE = E (complex) [E (1MI) + E (alcohol)].
Table 2 1:2 Complexes calculated by DFT, B3LYP functional and 6-31G** basis sets DE a (kcal mol1)
Complex
mComplex(asym) (cm1)
˚) d (A
mComplex(sym) (cm1)
Theor.
Exp.
Theor.
Exp.
HFIP
6.05
3697.9
3531
3503.7
3490
2.08
TFE
4.75
3665.6
3563
3582.0
3479
2.15
a
DE = E (1:2 complex) minus [E (1:1 complex) + E (alcohol)].
as well as smaller OH–C-3 distances for 1:2 than for 1:1 complexes. Furthermore, the theoretically calculated distance between the symmetric and asymmetric associated bands in the 1:2 1MI–TFE complex, 84 cm1, exactly coincides with that observed in the experimental FTIR spectra. Thus, the theoretical results plentifully support the formation of 1:2 complexes. The formation of these ternary complexes could result from two alternative pathways, one of them could be the TFE dimer interaction with a 1MI molecule. In this case, a more complex dependence of the measured absorbances with concentrations than the observed should be expected. Moreover, it is to be noted that dimeric TFE species are not present in solution at the used concentrations, since, in accordance with other authors [9], we found this band at 3530 cm1 only at the TFE concentrations above 0.03 M. Alternatively, the formation of the 1:2 complex could be viewed as a two step process K1
1MI þ TFEtrans () 1MI TFE K2
1MI TFE þ TFEtrans () 1MI TFE2
ð6Þ ð7Þ
In this case, to explain the experimentally observed 1:1 stoichiometry, it would be assumed K2 K1, i.e., once the 1:1 complex is formed, it quickly reacts to give the 1:2 complex, remaining the concentration of the binary complex practically stationary. This model seems to be supported by the existence, in the deconvoluted spectra, of the small band at 3579 cm1 which is patent only at low 1MI concentrations and disappears as this concentration increases. Its location, at higher frequency than the other associated bands, qualitatively agrees
with the calculated frequency for the 1:1 complex. In fact, as it can be appreciated, the frequencies for the complexes in Tables 1 and 2 match the experimentally observed bands with a scale factor of 0.97. The no complete coupling of the above equilibria could be responsible for the discrepancies observed when the experimental results are analysed in the monomer and associated bands.
4. Summary and conclusions Experimental FTIR measurements provide evidences on the formation of OH–p HBC of 1MI with HFIP and TFE in hexane. The existence of two main associated bands in the spectra of 1MI–TFE system and the apparent 1:1 stoichiometry of the complexation process, let us to initially suspect the coupled formation of two different 1:1 1MI–alcohol complexes with the OH group of the alcohol attached at different sites of the indole ring. However, the theoretical results only support the existence of an unique site for OH–p hydrogen-bonding interactions in the 1MI ring. Thus, DFT calculations only found minima for 1:1 complexes with T-shape geometries, where the OH group of the alcohol points to the C-3 atom of 1MI. These complexes can be further stabilised by a second alcohol molecule hydrogen bonded to the oxygen atom of the OH bridge. Interestingly, in the 1MI–TFE system, the frequencies of the theoretical symmetric and asymmetric stretching vibrations of these coupled OH bonds excellently match those of the associated bands observed in the experimental spectra.
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These results let us to propose that HFIP and TFE form 1:1 and 1:2 stoichiometric HBC with 1MI. Since 1:2 complexes are much more stable, the concentration of the 1:1 complexes remains practically stationary. This model is further supported by the experimental observation of a very weak band in the spectra of the TFE system. This band is distinguishable only at low 1MI concentration, and it is located in the spectral region theoretically predicted for the associated band of the 1:1 complexes.
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