Chemical Physics 335 (2007) 37–42 www.elsevier.com/locate/chemphys
FTIR study of water clusters in water–triethylamine solutions Marı´a A. Mun˜oz *, 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 9 October 2006; accepted 19 March 2007 Available online 28 March 2007
Abstract The FTIR study presented in this work, on water dissolved in triethylamine (TEA), reveals the formation of water clusters in the TEA liquid phase at tenths of water molar concentrations. In the OH stretching region, the FTIR spectra of water in TEA show, at high frequencies, a narrow band at 3682 cm 1 and, at low frequencies, a wide band that can be resolved into four peaks with maxima at 3249 cm 1, 3348 cm 1, 3440 cm 1 and 3545 cm 1. The results have been rationalised assuming the formation of clusters containing tens of three- and four-coordinated water molecules. TEA molecules surrounding the clusters are hydrogen bonded to one OH of the water molecules at the surface, leaving dangling protons. Further, the analyses of the spectra suggest that, in the used range, the water cluster mean size does not depend strongly on the water concentration. 2007 Elsevier B.V. All rights reserved. Keywords: FTIR; Water clusters; Triethylamine; Water–triethylamine
1. Introduction Vibrational spectroscopy has been for years a potential powerful technique to study hydrogen-bonding interactions. Thus, this technique has been widely used to characterise conventional XH :A and non-conventional XH p hydrogen-bonding interactions between donor and acceptor molecules. In fact, in recent papers we have widely used this spectroscopic technique to study the hydrogen-bonding interactions of biologically interesting molecules, such as indoles and betacarbolines, in apolar solvents [1–12]. However, as it is well known, water hydrogen-bonding interactions play a crucial role in the stabilisation of the biological active molecules in their natural environments. It is, therefore, obvious that the experimental study of the donor/acceptor hydrogen-bonding interactions of these molecules with water is of paramount importance. Unfortunately, IR spectroscopy is scarcely used in bulk water. Thus, the polymeric water structures formed at the
*
Corresponding author. Fax: +34 954557174. E-mail address:
[email protected] (M.A. Mun˜oz).
0301-0104/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2007.03.015
water concentrations usually used in the experiments give rise to so blurred IR signals that the spectra often become useless. High-resolution methods on pair interactions are mainly applied to gas phase conditions and, therefore, the provided fruitful information can not directly be applicable to condensed environments. However, it is worth to note that the (H2O)n clusters have been profusely studied since they can be considered the bridge between the gas and condensed phases. Owing to our interest in the study of the hydrogenbonding interactions in bulk water, we have searched for alternative media in which the hindrance to water polymerisation would make feasible the experimental FTIR measurements. Bearing this idea in mind, we have checked the FTIR spectra of different water–organic co-solvent mixtures. From this scrutiny, we have selected the water– triethylamine (TEA) mixture, a typical partly miscible system. Although this system has been the object of a number of papers focused on the understanding of phase separations [13,14], IR studies of water–TEA solutions have been scarcely reported. Long ago, Braginskaya et al. studied the influence of temperature and water/TEA proportions on the 900 cm 1 and 920 cm 1 bands of TEA [15] and on
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the OH fundamental and combination bands of water [16,17]. Also Schano et al. [18] calculated the formation constants and dipole moments of the 1:1 water–TEA complex in dilute solutions of nonpolar solvents. More recently, Zhanpeisov et al. [19] have theoretically calculated the shifts of the vibrational CH stretching bands of TEA upon water addition. According to the last authors, in the (water)4–TEA system, the water molecules form selfassociated water chains centred in the N site with the tail-end water molecules forming very weak interactions with the methyl and methylene groups. Interestingly, we have found that the FTIR spectra of the water–TEA mixtures in the OH stretching region are relatively simple and resemble those obtained for gas phase water clusters of moderate sizes. Thus, the characteristics of the water–TEA spectra appeared to us of enough importance to be analysed in detail in the light of the previously reported spectra of water in their different states. Apart from their important practical implications concerning the experimental studies of hydrogen bonding interactions in bulk water, the results of the present study can also help to understand the solvation of organic substrates in this medium. 2. Experimental The reagents were commercial products (Aldrich, Fluka) of the best quality (>99%) and used as received.
The solutions were prepared by adding different volumes of water to TEA in a range of 0.05–0.20 M concentrations. The measurements were carried out at room temperature and atmospheric pressure on a Mattson FTIR Galaxy 2020 spectrometer equipped with a HgCdTe detector. A Specac cell with ZnSe windows and 0.1 cm pathlength was used. The spectral resolution was 2 cm 1 and 200 scans were coadded to obtain each spectrum. The TEA spectrum was always subtracted from those of the mixed water–TEA solutions. Deconvolution of the spectral bands was achieved with the Peakfit Program, v.4 of Jandel Scientific. The hidden peaks were detected by the second-derivative method and, after checking different functions, they were fitted to an asymmetric logistic peak function given by the program. An example of the detailed fitting results is presented in Fig. 1. 3. Results and discussion The IR spectra of water in TEA, in the OH stretching region, are shown in Fig. 2. As can be appreciated, in the water concentration range from 0.05 to 0.2 M, the intensity of the whole band increases but the shape of the spectra is maintained. Deconvolutions of the spectra give rise to five peaks at 3249 cm 1, 3348 cm 1, 3440 cm 1, 3545 cm 1 and 3682 cm 1 (Fig. 3). The 3249 cm 1 band,
1.2
1.0
Absorbance
0.8
0.6
0.4
0.2
0.0 3200
3300
3400
3500
3600
3700
3800
Wavenumber
+ 0.01
R - 0.01
Fig. 1. Deconvolution fitting of the water–TEA mixture containing 0.1 M water (R2 = 0.9997; Std. Err. 0.0056; F = 2.47 · 105). Experimental (—) and fitted ( ) spectra.
M.A. Mun˜oz et al. / Chemical Physics 335 (2007) 37–42
Absorbance
2
1
0 3300
3400
3500
3600
3700
Wavenumber
Absorbance
Fig. 2. FTIR spectra of water in TEA. [H2O] = 0.05 M (—), 0.067 M (- - -), 0.10 M (––), 0.15 M ( ) and 0.20 M (––).
1
0 3300
3400
3500
3600
3700
Wavenumber Fig. 3. Deconvolution of the FTIR spectra in Fig. 2.
although incomplete in the spectra, has physical significance and it is not an artefact of the deconvolution process. Thus, it is wholly taken into account in the numerical fitting. The reason to cut off the spectra around 3200 cm 1 is the existence of a shoulder at 3177 cm 1 belonging to TEA, which makes the subtraction of TEA faulty. Below 3080 cm 1 the CH bonds of TEA have an intense absorption band. Interestingly, at 3080 cm 1 the subtracted bands go down to the baseline. Therefore, the only water bands in the spectra are those shown in Fig. 3. The shape of the spectra in Fig. 2 and, particularly, the observation of a sharp peak at 3682 cm 1, are very uncommon for bulk-water conditions. To our knowledge, similar spectra have only been previously reported by Braginskaya et al. and Schano et al. [16–18]. Thus, Braginskaya et al. [16] obtained, in 4% water/TEA mole fraction solutions, a broad spectral band with a maximum at 3360 cm 1, two shoulders at 3430 cm 1 and 3513 cm 1 and the sharp free OH peak at 3682 cm 1. Conversely, the IR spectrum of neat water presents a broad band in the
39
3000–3600 cm 1 region containing three peaks at 3215 cm 1, 3450 cm 1 and 3571 cm 1 [20]. Also, the picosecond double resonance transient spectra of dilute HDO in D2O [21,22] show similar components of hydrogenbonded water species. Curiously, the attempts to reproduce similar spectra to those of water in TEA in other water–cosolvent mixtures were unsuccessful. Thus, the spectrum of water in dioxane gives only two maxima, at 3585 cm 1 and 3515 cm 1. These bands have been ascribed to the symmetric and antisymmetric vibrations of water–(dioxane)2 complexes [23]. Dilute water in other aprotic solvents with H-bond acceptor groups as tetrahydrofurane, acetonitrile or diethyl ether [24], at similar concentration than those used in Fig. 2, presents more or less structured broad bands in the IR spectra. But, in any case, a free non-bonded OH stretch transition appears. Only in inert solvents as carbon tetrachloride or alkanes [25], the symmetric and antisymmetric peaks of the water monomer can be seen at very low water concentrations. Monomeric water bands in the liquid state have been also achieved in benzene and its derivatives [26,27]. On the other hand, water molecules with an unoccupied H-bonding site can only be found in gas phase water clusters [28], at the surfaces of liquid water [20,29], in ices obtained in low temperature vapour deposits [30], and in restricted environments such as water complexes trapped in rare gas matrices [28]. As before mentioned, the exploration of (H2O)n clusters has provided fundamental information for the understanding of the bulk water properties. In fact, the structures of small water polymers (n < 10) are nowadays well understood [28]. All these small clusters, up to decamers, have twofold and/or threefold coordinated water molecules acting, in the last case, as double donor-single acceptor (DDA) or single donor-double acceptor (DAA) but, excepting the heptamer, none of these structures present bands in the range 3200–3450 cm 1. The largest clusters (n = 8–10) have cubic water structures characterised by three isolated spectral bands: At above 3700 cm 1 for free OH and bands in the regions of 3530–3570 cm 1 and around 3100 cm 1 for DDA and DAA species, respectively. Data for higher size-selected water clusters are not yet available. However, infrared pre-dissociation spectra of clusters formed by expanding water vapour in helium, in a range of pressures and temperatures, show a distribution of different size clusters. Thus, as the water concentration increases, a transition from the dimer spectrum to one consisting of a sharp peak at 3710 cm 1 and a broad band resembling that of liquid water is observed [31]. Nevertheless, a difference exists in the position of the maximum of the broad band, which lies at 3470 cm 1 and 3375 cm 1 for vapour and liquid water, respectively. Substitution of bolometric by mass-spectrometric detection provides narrower range of cluster sizes. Thus, Page et al. [32] and Huisken et al. [33] obtained clusters with a mean size of 15–20 u. The IR spectra of these clusters show an OH-free peak at frequencies higher than 3700 cm 1 and a structured band
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M.A. Mun˜oz et al. / Chemical Physics 335 (2007) 37–42
whose maxima roughly correspond with those of the deconvolved spectra in Fig. 3. Recently, Buch et al. have isolated solid water clusters with an estimated size range from tens to thousand molecules in an interesting work that combines theoretical and experimental tools [30]. The ice particles generated in a collisional cooling cell (1–100 ppm of water in He at different low temperatures) provide FTIR spectra of clusters with tens to few hundreds of particles. In these spectra, the ratio, R = OH bonded/OH free, measured as the quotient between the areas under the spectra of OH bonded and OH free bands, increases as the mean cluster size increases. Thus, above R 100, the OH free band becomes practically imperceptible. Moreover, for clusters from 20 to several hundred of molecules, the maximum of the OH bonded broad band shifts from 3450 cm 1 to 3270 cm 1. The similarity between these and the water–TEA spectra leads us to propose the formation of water clusters in TEA, even at tenths of molar water concentrations. Thus, taking into account the above information, we will tentatively assign the deconvolved bands in the spectra of Fig. 3. The higher frequency peak at 3682 cm 1 clearly belongs to a free OH bond. The narrowness of this peak indicates that in the water–TEA clusters a sole kind of free OH bond exists. It is interesting to note that in medium-sized water clusters such as (H2O)19 and nanometer-sized water clusters, the OH-bonded absorptions become essentially featureless in the 3000–3600 cm 1 region, but the free-OH stretching absorption maintains its sharp resonance above 3700 cm 1. This OH-free stretch frequency has been generally attributed to twofold coordinated H2O molecules, although in benzene–(water)n complexes, threefold coordinated water molecules also absorb at wavenumbers above 3700 cm 1. In contrast, in the IR spectra of ice or in measurements by vibrational sum frequency generation (SFG) spectroscopy of air/water interface, this vibrational mode appears at lower frequency (3695 cm 1). Jiang et al. [34] proved that the frequency of this band depends on the type of neighbouring molecules and on the directionality of the involved hydrogen bonds. In the clusters, because the directionality of the hydrogen bond is strained and the neighbouring water molecules are primarily three-coordinated, the OH-free appears at higher frequency than in bulk water or ice, which contain much more four-coordinated water molecules. Thereafter, Buch et al. [30] reported that the recovering of solid water clusters by the N2 contaminating the fluxing gas in their experiments shifts the OH-free band from 3700 cm 1 to 3682 cm 1. The presence of a similar band at 3682 cm 1 in the water–TEA spectra strongly suggests that TEA molecules surround the cluster surface occupying the nearby sites to the free OH bonds. Similar examples of water hydrogen-bonded cage structures with solutes attached to the surface are found in medium-size (n = 15–20) water clusters doped with methanol [33], in benzene–(H2O)8complexes [35] and benzene–(H2O)20 cluster cation complexes [36].
Moreover, as can be seen in Fig. 3, a band at 3348 cm 1, absent in the reported water spectra, is present in the water–TEA system. It is worth to mention the study of DOH in TEA, carried out by Glew and Rath [37], in which bands at 3678 cm 1 and 3374 cm 1 were assigned to monomeric DO–H non-bonded and bonded, respectively, to the TEA nitrogen. Therefore, the absorption band at 3348 cm 1 must be attributed to the OH stretching vibration of water-bounded to the N atom of TEA. This is, on the other hand, in excellent agreement with the theoretical calculations of Zhanpeisov et al. [19], which report a 290 cm 1 red shift for the OH-free water vibration band upon hydrogen-bonding interaction with TEA. Accordingly, we will assign the band at 3682 cm 1 to the OH-free of the three-coordinated water molecules. These molecules interact as an acceptor–acceptor through their oxygen atoms with two adjacent water molecules of the cluster, and as an H-donor with the electron pair of the nitrogen atom of TEA. These assignments are also supported, as we will see in a companion paper, by the red-shifts of 30 cm 1 that the 3682 cm 1 and 3348 cm 1 bands experience upon the addition of N-methylindole to a water–TEA mixture. These shifts are rationalised on the basis of an OH–p hydrogen bond between the free OH bond of water molecules in the water–TEA system and the p-cloud of the indole ring. The remainder bands in the spectra at 3249 cm 1, 3440 cm 1, and 3545 cm 1 can be identified with those found in the ATR-IR and SFG spectra of neat water [20] and in solid water clusters [30]. These systems invariably present a broad absorption in the 3000–3600 cm 1 region containing two relatively intense bands at 3215– 3220 cm 1 and 3400–3450 cm 1 and one weaker band at 3512–3550 cm 1. Therefore, according to the literature assignments, the peaks at the high frequency end, at 3545 cm 1 and 3440 cm 1, Fig. 3, are unambiguously assigned to three-coordinated double proton-donor single proton-acceptor, DDA, and to four-coordinated, DDAA, water molecules, respectively. However, the band at lower frequency, above 3200 cm 1, has been differently interpreted in the literature. Thus, in water clusters [30] and neat water SFG spectra [20], it is attributed to the companion bonded-OH of the molecules with dangling protons, but in neat water [20], to the vibrational mode of the oscillating dipoles of four-coordinated hydrogen bonded water molecules. In the opinion of the last authors, these water molecules possess more symmetric character than those vibrating at 3440 cm 1. On the other hand, it is evident that the relative proportion of three- and four-coordinated water molecules would determine the size and stability of the clusters. Thus, to get the greater stability, the cluster would maximise the number of hydrogen bonds by optimising the number of tetrahedral hydrogen bonded water structures. This process would expand the cluster surface and, therefore, its size. Although on the basis of the IR spectra alone it is difficult to predict the cluster size, the number of water molecules
M.A. Mun˜oz et al. / Chemical Physics 335 (2007) 37–42 Table 1 Integrated absorbances of the water–TEA FTIR spectra resolved bands and OH bonded/OH free band ratio (R)
0.050 0.067 0.100 0.150 0.200
R
3249
3348
3440
3545
3682
38.6 54.0 100.7 159.3 199.0
53.9 74.2 101.9 156.1 199.0
46.4 64.3 99.0 159.3 217.0
16.5 23.2 35.9 54.4 65.0
2.6 3.8 5.3 7.8 9.1
61 58 64 68 75
12 10
Absorbance / [H2O]0
Peak position (cm 1)
C (M)
41
8 6 4 2
engaged in it could be roughly estimated by comparing the water–TEA experimental spectra with those reported by Buch et al. for model solid water clusters [30]. Thus, the R values of the integrated absorbances of OH bonded/ OH free bands, obtained from the deconvolved spectra in Fig. 3, go from around 60 to 75 (Table 1) upon increasing the water concentration from 0.05 M to 0.2 M, respectively. This value, together with the maximum, lying around 3400 cm 1, of the broad band in Fig. 2, suggest tens of water molecules engaged in the cluster. It is also worth to note that, according to the R values, the number of water molecules composing the clusters does not importantly change despite the water concentration is fourfold increased. Therefore, it can be inferred that the increasing of water concentration increases the number of clusters better than the cluster size. In fact, the fulfilment of the Beer law for all the bands up to 0.20 M water concentration, Fig. 4, together with the comparison of the spectra normalised over concentration, presented in Fig. 5, plentifully support this assumption. This result has very important practical implications, because it makes feasible to experimentally study the hydrogen bonding interactions of water protons with organic substrates under controlled conditions. Thus, by modifying the water concentration of the water–TEA mixtures the cluster concen-
Absorbance
2
1
0 0.0
0.1
0.2
c (M) Fig. 4. Plots of absorbance versus water concentration at 3682 cm 1 (j), 3545 cm 1 ($), 3440 cm 1 (.), 3348 cm 1 (s), and 3249 cm 1 (d).
0 3300
3400
3500
3600
3700
Wavenumber Fig. 5. Concentration normalised spectra of the water–TEA mixtures in Fig. 2.
tration can be adequately monitored and, therefore, the dangling protons available for hydrogen-bonding. Acknowledgements We gratefully acknowledge financial support from the Direccio´n General Cientı´fica y Te´cnica (BQU2002-01582) and Junta de Andalucı´a (2005/FQM-106 and 2005/FQM368). References [1] M. Balo´n, M.A. Mun˜oz, P. Guardado, C. Carmona, Photochem. Photobiol. 64 (1996) 531. [2] M. Balo´n, P. Guardado, M.A. Mun˜oz, C. Carmona, Biospectroscopy 4 (1998) 185. [3] M. Balo´n, C. Carmona, P. Guardado, M.A. Mun˜oz, Photochem. Photobiol. 67 (1998) 414. [4] M.A. Mun˜oz, O. Sama, M. Gala´n, P. Guardado, C. Carmona, M. Balo´n, J. Phys. Chem. B 103 (1999) 8794. [5] M.A. Mun˜oz, P. Guardado, M. Gala´n, C. Carmona, M. Balo´n, Biophys. Chem. 83 (2000) 101. [6] C. Carmona, M. Gala´n, G. Angulo, M.A. Mun˜oz, P. Guardado, M. Balo´n, Phys. Chem. Chem. Phys. 2 (2000) 5076. [7] M.A. Mun˜oz, O. Sama, M. Gala´n, P. Guardado, C. Carmona, M. Balo´n, Spectrochim. Acta A 57 (2001) 1049. [8] M. Gala´n, C. Carmona, P. Guardado, M.A. Mun˜oz, M. Balo´n, J. Photochem. Photobiol. A 147 (2002) 103. [9] M.A. Mun˜oz, M. Gala´n, L. Go´mez, C. Carmona, P. Guardado, M. Balo´n, Chem. Phys. 290 (2003) 69. [10] M.A. Mun˜oz, R. Ferrero, C. Carmona, M. Balo´n, Spectrochim. Acta A 60 (2004) 193. [11] M.A. Mun˜oz, C. Carmona, M. Balo´n, Chem. Phys. Lett. 393 (2004) 217. [12] M.A. Mun˜oz, M. Gala´n, C. Carmona, M. Balo´n, Chem. Phys. Lett. 401 (2005) 109. [13] J. Hobley, S. Kajimoto, A. Takamizawa, K. Ohta, Q. Tran-Cong, H. Fukumura, J. Phys. Chem. B 107 (2003) 11411. [14] A. Ikehata, C. Hashimoto, Y. Mikami, Y. Ozaki, Chem. Phys. Lett. 393 (2004) 403. [15] T.G. Braginskaya, A.I. Sibilev, Molekulyar. Fiz. i Biofiz. Vod. System 5 (1983) 34.
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