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Inhomogeneity Distribution in Methanol/Acetone Mixture: Vibrational and NMR Spectroscopy Analysis Abdenacer Idrissi, Kamil Polok, Bogdan A Marekha, Isabelle De waele, Marc Bria, and Wojciek Gadomski J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp412023g • Publication Date (Web): 13 Jan 2014 Downloaded from http://pubs.acs.org on January 15, 2014

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The Journal of Physical Chemistry

Inhomogeneity Distribution in Methanol/Acetone Mixture: Vibration and NMR Spectroscopy Analysis Abdenacer Idrissi1*, Kamil Polok2*, Bogdan Marekha1,3, Isabelle De waele1, Marc Bria4 and Wojciek Gadomski2 1

University Nord de France, Lille1, LASIR (UMR CNRS A8516) 59655

Villeneuve d’Ascq Cedex, France 2

Laboratory of Physico-chemistry of Dielectrics and Magnetics, Department of

Chemistry, University of Warsaw, Zwirki i Wigury 101, 02-089 Warsaw, Poland 3

Department of inorganic chemistry, V.N. Karazin Kharkiv National University, 4 Svobody sq., 61022, Kharkiv, Ukraine

4

University Nord de France, Lille1, CCM RMN, Bât. C4, Villeneuve d’Ascq

59650 France

*

Electronic mail: [email protected], [email protected];

ACS Paragon Plus Environment


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Abstract

The main target of this paper is to quantify the inhomogeneous distribution of the components of acetone/methanol mixture and to give detailed insight into the interplay between the dipole-dipole and hydrogen bonding interactions inducing this inhomogeneity. To this end, we used the concept of infrared excess molar absorption of a given vibration mode as an observable which contains all the information on the collective interactions in the mixture. Indeed, the changes in the infrared excess molar absorption may be associated with the inhomogeneous (clustering, self-association or high density domains) distribution of the components, and consequently with the interaction between the two components of the mixture. The results show that acetone molecules are not homogeneously distributed in the mixture, particularly, in the mole fraction range of acetone between 0.05 and 0.55. The spectral signature of this inhomogeneity is associated with the appearance of a shoulder in the C=O and C-C stretching vibration profiles of acetone. This inhomogeneity is driven by the prevalence of the dipole-dipole interactions over those of hydrogen bonding between acetone and methanol molecules. The inhomogeneous distribution of methanol molecules is found to occur in the mole fraction range of acetone between 0.55 and 1. In this case the hydrogen bond interactions between methanol molecules are prevailing over those between methanol and acetone. However, the extent of this inhomogeneity is small as compared with that of acetone in the low mole fraction range. The spectral signature of this inhomogeneity is not visible in the O-H stretching vibration mode; however a second peak appears as a shoulder of the C-O stretching vibration mode in this range of mole fraction of acetone.

Keywords:

hydrogen

bonding,

dipole-dipole

interactions,

excess


Infrared

spectra,

local

structure

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I.

Introduction

Studies of the local structure of the acetone/methanol mixture may provide valuable information about the interplay between dipole-dipole, hydrogen bonding and hydrophobic interactions.1 The components of these mixtures are widely used as solvents and reagents in the pharmaceutical and fine industries.2 The research activity on this mixture is associated with the fact that it forms a minimum boiling point pressure azeotrope. The main target of this research is to find a way to separate the azeotropic mixture by using a separating agent or by changing thermodynamic conditions.3-6 In addition, due to interaction between carbonyl group of acetone and the hydroxyl group of methanol, this mixture is a model system whose local structure analysis may give insight into the interactions in bioorganic and organic molecular systems. Indeed, carbonyl and hydroxyl groups are common in biologically interesting molecules. Many experimental methods including thermodynamics7-20, vibration spectroscopy21-27, NMR28 and molecular dynamics simulation29-30 have been applied to analyze the structure and dynamics in this system. Marcus et al.10 used the inverse Kirkwood-Buff integrals method which provides detailed insight into the preferential solvation in many solute-solvent mixtures including the acetone/methanol mixture. The results obtained by Marcus, show that the excess (or deficiency) of acetone over its bulk mole fraction in the environment of acetone molecules has negative values for mole fraction of methanol between 0 and 0.40 and it has positive values for the subsequent methanol mole fraction. The positive values suggest the self-association of acetone molecules. The values of the excess (or deficiency) of acetone over its bulk mole fraction in the environment of methanol were negative in the whole methanol mole fraction range. These negative values suggest that acetone-methanol interactions are unfavorable. Furthermore, in the above mentioned vibration spectroscopic studies, the effect of pressure and temperature on the Raman spectra of the C-C stretching vibration mode of acetone in binary mixture with methanol was analyzed. The results show that this band shifts to higher frequencies with increasing the pressure and with the decrease of acetone mole fraction. This behavior was explained by the decrease of the acetone-acetone interactions.21 In another work by the same authors, the effect of changing the mole fraction and the temperature on the isotropic and anisotropic spectra of the carbonyl mode of acetone was explained by the formation of strong acetone-methanol hydrogen bonds balanced against weaker acetone dipole–acetone dipole interactions. The authors conclude that there is no hard evidence for acetone dimers or clusters in the mixture.21,31 Han and Kim

23

conducted an infrared matrix isolation study of this mixture in solid argon, supported by ab

initio calculations, and found that acetone and methanol form a 1:1 binary complex. Kun et al.25 studied the variation of the C=O stretching frequency of acetone in a variety of solvents including methanol. They show that this band may be resolved into two contributions. The lowest frequency contribution was assigned to hydrogen bonded complex of methanol and acetone confirming the finding of Han and Kim. The highest frequency contribution was assigned to acetone-acetone interactions. However factor analysis of the infrared spectra of the mixture concluded that this mixture is almost but not exactly random and no acetone-methanol complex is formed in the mixture.25 In the latter study the authors also mention 21% non-ideality of these mixtures with methanol being 1.5 times better hydrogen-bond acceptor than acetone thus influencing hydrogen-bond speciation. The observed red-shift of acetone C=O stretching upon dilution with methanol is interpreted as outweighing of the effect of dipole-dipole interactions by hydrogen-bonding. Raman spectroscopic study of this



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mixture shows that the non-coincidence effect of both the C=O stretch of acetone and the OH stretch of methanol has a convex concentration behavior indicating the occurrence of microscopic inhomogeneity (clustering, self-association or high density domains).27 It should be noted that early Raman spectroscopic study of the same system and which also pointed the occurrence of the non coincidence effect didn’t report any inhomogeneity in the system.32 The association constant between methanol and acetone was determined using the infrared intensity of the OH stretching spectral profile of methanol.26 The value of this constant was not commented by the authors of this paper. However, the comparison between the values of the association constant obtained for methanol mixtures with various solvents (acetone, acetonitrile, ethyl acetate, pyridine, …) allows us to conclude that the methanol-acetone association is weak. It should be noticed that in this approach the authors made the hypothesis that the self-association of acetone, which may be driven by dipole-dipole interactions, is negligible. Molecular dynamics results provide information on the local structure particularly on the state of hydrogen bonding of methanol molecules. Molecular dynamics simulations contribute in a large part to shed light on the microscopic structure of the methanol/acetone mixture. For instance, Venables et al.30 used molecular dynamics simulation to calculate dynamical properties (translational and rotational motions) related to the infrared spectra of these mixtures. The results show that the ability of acetone to accept hydrogen bonds is the primary determinant of the structure and dynamics in the methanol/acetone mixture. They also show that methanol molecules have a remarkably strong tendency to remain in chains, and the chains shorten as the methanol concentration decreases. In another work by Gupta et al.33 the simulation results show that the dynamical properties (diffusion coefficient, energies and life time of hydrogen bonds between like and unlike molecules) in methanol/acetone mixture have essentially linear dependence on mixture composition, thus behave ideally with respect to changes in the composition of the mixture. Kamath et al.29 used Monte Carlo simulation to determine the phase diagram of the acetone/methanol mixture and particularly to reproduce the minimum pressure azeotropy found in this system. The analysis of the structure is given for an equimolar mixture of acetone and methanol. The results show that the limited amount of interspecies association that occurs in this mixture has only a small effect on the self-association behavior of methanol molecules. In other work by Perera et al. 34, the analysis of structure of methanol/water and methanol/acetone mixtures using Kirkwood-Buff integrals suggests that methanol molecules are more clustered in acetone than in water. Polok et al.35 analyzed the hydrogen bonding in this mixture. The MD results suggest that for low methanol mole fractions, methanol molecules forms small structures of 1-3 molecules, surrounded by acetone molecules. However, for high mole fraction (>0.3) a network of methanol chains were observed. Acetone molecules were assumed in this study to be uniformly distributed in the system. It is obvious from the previous data on the methanol/acetone mixture that the quantitative description of the inhomogeneity and the effect of the solute on the structure of the solvent is a key point to our rational understanding of the local structure in this mixture. The main difficulty is to find an observable which is specifically sensitive to the interactions between the constituents of the mixture. In our recent simulation work,36 we have chosen the Voronoi polyhedra (VP) as an observable which characterizes the local structure in the methanol/acetone mixture. The composition of the systems investigated covered the entire range from neat acetone to neat methanol. Distribution of the volume, reciprocal volume and asphericity parameter of the VP as well as that of the area of the individual VP faces and of the radius of the empty voids located between the


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molecules were calculated. To investigate the tendency of the like molecules for self-association, the analyses were repeated with disregarding one of the two components. The self-association of the disregarded component thus turns to large empty voids, which were easily detectable in VP analysis. The obtained results reveal that both molecules show self-association, but this behavior is considerably stronger among acetone than among methanol molecules. The strongest self-association of the acetone and methanol molecules is found at their mole fraction ranges of 0.2 – 0.5 and 0.5 – 0.6, respectively. In a continuation of this effort to characterize the inhomogeneity distribution of molecules in the methanol/acetone mixture, we present in this paper, how we can tackle these complex interactions using IR spectroscopy. The approach used in this paper is that developed by Li et al.37-38 and Koga et al.39 Indeed, Li et al., following the idea of excess thermodynamic functions, proposed to use the concept of infrared excess molar absorption as an observable which contains all the information on the collective interactions in the mixture resulting in non-ideality. Indeed, the changes in the infrared excess molar absorption may be associated with the inhomogeneity distribution of the components, and then with the interaction between the two components of the mixture. Furthermore, Koga et al.39 showed that detailed insights are gained when analyzing the derivative of the excess molar absorption with respect to the change of the mole fraction of one of the components of the mixture. Accordingly, we report in the paper a detailed analysis of the local structure of the methanol/acetone mixture in the whole mole fraction range. The main target of this study is to quantify the inhomogeneous distribution of the components of this mixture and to give detailed insight into the interplay between the dipole-dipole and hydrogen bonding interactions. These interactions are modulated by the change of the mole fraction of the components. The paper then is organized as follows: First, details on the experiment are given as well as on the approach of the excess molar absorption used to analyze the data. In a second step the results and their analysis are presented and finally a conclusion is given.

II. Experimental and excess molar absorption A series of acetone -methanol mixtures were prepared by weighing. The mole fractions of acetone in methanol were between 0 and 1 with a step of 0.05. FTIR spectra in the frequency range between 350 to 4000 cm-1 were collected at room temperature (~24°C) using Bruker Tensor 27 670 FT-IR spectrometer. It is equipped with a Pike Miracle single-bounce attenuated total reflectance (ATR) cell equipped with diamond single crystal, with DTGS detector and KBr beam splitter. All the spectra were recorded with a resolution of 2 cm-1 using 60 scans. In general the change in the local structure is characterized through the shift of a given vibration mode as well as the behavior of the corresponding width of its profile. The determination of these two parameters is given by a fitting procedure which has the advantage of using strongly established mathematical methods. However, it has the disadvantage to be model dependent. Based on the idea of excess thermodynamic functions, Li et al.37-38 proposed an excess infrared absorption function (which is a function of wavenumber and mole fraction of the mixture) in order to investigate the intermolecular interactions in the mixture and the possible formation of new complexes. Later on, Koga et al. proposed to use the derivative of the excess absorption with respect to the mole fraction of one of the components, in order to analyze the perturbing effect of this component on the interactions in the mixture. The procedure to obtain the excess absorption and partial molar absorption is described in details in references.37-39 The excess absorption is defined as the difference between the molar absorption spectrum at a



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given mixture composition and those of neat components of the mixture weighted by the corresponding mole fractions. Indeed, starting from the Beer- Lambert law, the absorbance A (ν

A(ν ) N = ε (ν )C = ε (ν ) l V

)

is written as:

(1)

where, ν is the wavenumber, ε (ν ) is the molar absorption coefficient, l is the optical path length, C is the molar concentration, V is the volume of the sample and N is the molar amount of the sample. Following Li et al.37-39, Eq. 1 may be rewritten for a mixture containing nA moles of acetone and nM moles of methanol as follows:

A(v ) nAε A0 (v ) nM ε M0 (v ) (nA + nM )ε E (v ) = + + l V V V

(2)

where ε A0 (v ) and ε M0 (v ) are the molar absorptions for pure acetone and methanol, respectively and ε E (v ) is the excess molar absorption. Introducing the mole fractions xA and xM of acetone and methanol, respectively, Eq. 2 is rewritten as:

ε (v ) = x Aε A0 (v ) + xM ε M0 (v ) + ε E (v )

(3)

The qualitative analysis of the previous equation indicates that for an ideal mixture, the intermolecular interactions, experienced by the molecules of the two components are the same as in the corresponding pure states and the excess absorption ε E (v ) is equal to zero. For a given vibration mode, the departure of ε E (v ) from zero is then a clear indication of the change of the intermolecular interactions in the mixture with respect to those of pure systems. Indeed, for the spectral region associated mainly with a given vibration mode of one of the two components of the mixture, the excess molar absorption ε E (v ) has negative and positive regions, the order of occurrence of which indicates the direction of the red or blue frequency shift. Furthermore, Li et al.37 showed that the number of minima and maxima in the excess molar absorption can help to rationalize the choice of the number of contributions in a fitting procedure of a given spectral profile. To get more insight from the behavior of the excess absorption spectra, ε E (v ) , Koga proposed an approach to calculate the excess partial molar absorption which quantifies the perturbing effect of increasing the concentration of one of the components on the vibration mode associated with the other component. This approach then allows to quantify the changes in the local structure.39 In this approach the area, ε YE of the negative and positive regions of the excess absorption spectra ε E (v ) of the chosen vibration mode Y (Y=O-H str., C-O str. of methanol or C-C str., C=O str. of acetone) were calculated for each mole fraction. The values of the excess partial molar absorptions ε AE (Y ) of the O-H and C-O vibration modes of methanol with respect to the increase of xA, and of ε ME (Y ) of C=O and C-C vibration modes of acetone with respect to the increase of xM, are obtained using the following equations:


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ε AE (Y ) = (1 − x A )

∂ε YE ∂x A

,

(4)

where Y is the O-H or C-O stretching vibration mode of methanol, xA is the mole fraction of acetone and

ε ME (Y ) = (1 − xM )

∂ε YE ∂x M

(5)

where Y is the C=O or C-C stretching vibration mode of acetone, xM is the mole fraction of methanol. The behavior of ε Y then quantifies the effect of changing the mole fraction of one of the components on the spectral profile of a vibration mode Y associated with another component and then gives insight into the change of the local structure.

III. Results and discussion

Figures 1a, 1b show the infrared spectra of pure methanol, and acetone respectively. The assignment of the spectra is given in the same figures and is based on already published data.25,40 The compilation of the various assignments is summarized in table 1. The changes in the IR spectral profiles of chosen vibration modes of acetone and methanol of the mixture as a function of the acetone mole fraction are illustrated in Fig. 2a, 2b for the O-H, CH3 and C-O vibration modes of methanol and in Fig 2a ,3a, 3b for the CH3, C=O, C-C vibration modes of acetone, respectively. The analysis of the IR spectra of acetone/methanol mixture shows that the C=O spectral profile is characterized by two peaks at low mole fraction of acetone, while only one peak is observed for pure acetone. The same behavior is observed for the C-C spectral profile of acetone. In the case of methanol, the visual inspection of the C-O vibration profile clearly shows that this profile is not resolved in two peaks, however, a shoulder is observed at the high frequency side of this profile. The two spectral contributions may be associated with the occurrence of two types of local environment around acetone and methanol molecules. In the case of methanol, the O-H spectral profile undergoes a blue shift, however, the O-H profile didn’t show any indication of two distinct spectral contributions. The first observation is that the O-H stretching mode is blue shifted upon the increase of xA. Both a weakening of the existing hydrogen bonds between methanol molecules and the formation of new weak hydrogen bonds are expected to blue shift the O-H frequency. Fig. 2a shows that the broad O-H spectral profile skews to higher frequencies as xA increases. The low frequency side of the O-H vibration profile is usually assigned to the vibration of hydrogen bonded O-H groups.41 This indicates that the blue shift of the O-H frequency is rather associated with the weakening of hydrogen bonding between methanol molecules. The changes in the O-H spectral profile are then connected to the change in the equilibrium populations of hydrogen bonded methanol molecules, which is modulated by the increase of xA. In order to get complementary information on the hydrogen bonding interactions in acetone methanol system, we used NMR spectroscopy. Indeed, the 1H NMR chemical shift, δ, is a direct probe and can be used for elucidating the changes in electronic environment of various protons. Hydrogen bonding decreases the electron density around the proton, hence the proton peak moves towards higher δ values. As we can see from Fig. 4, the δ of 1H-OH decreases with increasing xA which is in accordance with earlier NMR analysis of the methanol/acetone mixture.28 Consistently with the behavior of the O-H frequency shift, this is a consequence of the increase of



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electron density of O-H bond of methanol which is due to (i) breaking of hydrogen bonding between methanol molecules and/or (ii) to the formation of weaker hydrogen bonding between methanol and acetone.

III-1 O-H vibration mode of methanol

In order to get insight into the change of the local structure around the O-H group, we calculated the excess infrared absorption spectra (see Fig. 5). It is evident from this figure, that there are at least two contributions to the spectral profile of ε OE– H (ν ) . Indeed, many assignments for the O-H vibration mode are possible.25,42-45 For instance, the low frequency side of ε OE– H (ν ) may be associated with the contribution of methanol molecules within polymeric chains (except terminal ones). Each of these molecules forms one hydrogen bond via the O-H proton and one via the oxygen lone pair. The high frequency side may be associated with the contribution of methanol molecules at the end of methanol polymeric chains. In order to have a rational assignment of these two contributions and particularly to assign the spectral contribution of hydrogen bonding between acetone and methanol molecules, we carried out quantum calculations using Spartan program package46. Geometry optimization calculations were performed on the various configurations of acetone and/or methanol molecules that were obtained from classical molecular dynamics simulations (see Fig. 6a). We used the density functional theory, which incorporates Becke’s three parameter exchange with the Lee, Yang and Parr correlation functional method (B3LYP) with the 6-31G(d,p) basis set. The optimized geometries were confirmed to be the minima on the potential energy surface by analyzing the vibration frequencies, which were found to have no imaginary components. The geometry optimization of the configuration of eight methanol molecules shows that the ν O – H of molecules inside polymeric chain has lower values than those located at the end of chain of hydrogen bonded methanol molecules. Furthermore, the same procedure was used to obtain a configuration of one acetone molecule solvated by 7 methanol molecules (see Fig. 6b). The ν O – H vibration mode frequencies of methanol molecules hydrogen bonded to acetone molecule are closer to those of molecules located at the end than those of molecules located inside polymeric chains. As a consequence, the behavior of the low frequency side of

ε OE– H (ν ) is considered to be sensitive to the hydrogen bonding between methanol molecules. This contribution will be referred here after as HB-MM. The highest frequency side is considered to be sensitive to both hydrogen bonding between O-H and acetone molecules and to O-H located at the end of chain of hydrogen bonded methanol molecules. This contribution will be referred here after as HB-MA. In order to quantify the effect of adding acetone on these two contributions, we used the approach developed by Koga. Indeed, after calculating the areas of the negative and positive spectral regions of ε OE– H (ν ) , we then calculate their derivatives with respect to the acetone mole fraction. Using Eq. 5, we determined the excess partial molar absorptions of the HBMM and HB-MA spectral contributions to the OH vibration mode with respect to the change of acetone mole

fraction ε AE ( HB − MM ) and ε AE ( HB − MA) the behavior of which is reported in Fig. 7. At high xM,

ε AE ( HB − MM ) has a large negative value which indicates that the effect of adding acetone in the solution is to reduce the contribution of hydrogen bonded methanol molecules.39 Concomitantly, the large positive value of

ε AE ( HB − MA) indicates that the population of hydrogen bonding between methanol and acetone molecules increases. This also may correlate with the increase of the population of hydrogen bonded methanol molecules


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with O-H located at the end of chain (free OH). With decreasing xM (below 0.45), the negative values of the

ε AE ( HB − MM ) increase reaching zero value at xM ~0.45. This behavior suggests that the effect of adding acetone on the reduction of the HB-MM population diminishes gradually. The positive values of ε AE ( HB − MA) decrease and this indicates that adding acetone induces a decrease of the rate of increase of the HB-MA population. The physical picture which may emerge from these results is that large part of the added acetone molecules do not establish hydrogen bonding with methanol molecules, but prefer to form domains of acetone molecules. The formation of these domains is driven by dipole-dipole interactions between acetone molecules. These results suggest that for xM higher than 0.45, the distribution of acetone molecules is not homogeneous. For xM lower than 0.45, the positive values of ε AE ( HB − MM ) indicate that adding acetone molecules to the mixture

induces an increase of the HB-MM population, while, the concomitant negative values of ε AE ( HB − MA) suggest a decrease of HB-MA population. This means that in this range of xM, methanol molecules are not randomly distributed in the mixture, but prefer to self-associate forming microscopic domains. However, as the positive values of ε AE ( HB − MM ) are small, we can expect that the extent of this inhomogeneous distribution of methanol molecules is small. The overall changes of both ε AE ( HB − MM ) and ε AE ( HB − MA) contributions are compatible with the shift of OH vibration to higher frequencies. Furthermore, the acetone/methanol mixture is characterized by the occurrence of minimum temperature azeotrope at xM~0.25. This means that at this mole fraction range like molecules tend to preferentially interact with themselves than with unlike molecules of the mixture. Our interpretation of the behavior of both ε AE ( HB − MM ) and ε AE ( HB − MA) contributions with respect to the change of xA is consistent with this main physical property of the acetone/methanol mixture.

III-2 C-O vibration mode of methanol

Hydrogen bond formation involving O-H group is expected to considerably redistribute the electronic density within the methanol molecule. This is confirmed by the blue shift of the C-O stretching vibration mode of methanol with respect to that in pure methanol. Indeed when a small amount of acetone is added to methanol, the band is slightly shifted toward high frequencies (Fig. 2b). As xA increases, a shoulder gradually appears on the high frequency side of the C-O vibration profile. Our quantum calculations suggest analyzing the C-O spectral profile in terms of the hydrogen bond donor or acceptor character of methanol molecules. Indeed, the donor character of the O-H bond induces a polarization of the C-O bond and a decrease of its length and then results in a contribution in the high frequency side of the C-O vibration mode. When hydrogen bonding is established between methanol and acetone molecules, O-H is acting as a donor. Our calculations then show that this contribution is located in the high frequency side of the C-O vibration mode. This contribution will be referred here after as D-MA. We also include in this contribution the methanol molecules which are both donor and acceptor of hydrogen bonding. When the O-H bond is acting exclusively as an acceptor of hydrogen bond, the frequency of the C-O is located at the low frequency side of the C-O vibration mode. This contribution will be referred here after as A-MM. In order to analyze the effect of increasing xA on the changes of D-MA and A-MM spectral contribution to the C-O vibration profile, we calculated the excess infrared spectra ε CE– O (ν ) (see Fig. 8).



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The negative part is located at the low frequency side of the C-O vibration mode and this is in agreement with the shift of the C-O vibration mode to higher frequencies. We then calculate their derivatives with respect to the acetone mole fraction. Using Eq. 5, we determined the excess partial molar absorptions of the D-MA and A-MM spectral contributions to the C-O vibration mode with respect to the change of acetone mole fraction

ε AE ( D − MA) and ε AE ( A − MM ) (see Fig. 9). The positive values of ε AE ( D − MA) indicate that adding acetone molecule induces an increase of the hydrogen bonding between methanol and acetone molecules while the negative values of ε AE ( A − MM ) indicate that adding acetone molecules reduces the hydrogen bonding between methanol molecules. For the range of xM higher than 0.45, adding acetone induces a decrease of both the values of ε AE ( D − MA) and ε AE ( A − MM ) indicating the decrease of the corresponding populations. This behavior is consistent with the previously proposed physical picture that for xM below 0.45, part of acetone molecules are not homogeneously distributed in the solution but form domains (as we will see later, this gives rise to a spectral contribution in the low frequency side of the C=O vibration mode of acetone). At xM lower than 0.45, adding acetone induces a small constant increase of the D-MM population while the A-MA population decreases reaching its maximum decrease rate at xM~0.25. Furthermore, as xA increases, the overall shift of the position of the CH3 vibration mode of methanol is very small, however a noticeable shift to lower frequency is observed at high xA values (see Fig. 2a). The position of the CH3 vibration mode of acetone is almost constant at the high xA values and because of the overlap with the contribution of methanol we were not able to quantify the change of its position at low xA. In order to overcome this limitation, we recorded the 1H-CH3 chemical shift of CH3 groups in acetone and in methanol. The behavior of 1H chemical shift of CH3 of methanol and acetone is illustrated in Figure 4. An obvious small monotonic increase of the 1H-CH3 is observed with increasing xA, indicating a small decrease of the electron density around the proton of the methyl groups which is consistent with the low frequency shift of the CH3 vibration modes. Complementary information on the interactions involving the CH3 groups of acetone and methanol were obtained from quantum calculations. Indeed, the optimized acetone-methanol, methanol-methanol, and acetoneacetone47 configurations show that, in the case of the optimized methanol-methanol configuration, hydrogen bonding, in addition to the standard O-H···O interactions, is also established between H-O···H-C. The CH3 vibration mode is then modulated by these interactions, while in the case of acetone-acetone optimized configuration, the dipole-dipole interaction has a small effect on the CH3 vibration mode.

III-3 C=O vibration mode of acetone

The microscopic distribution of acetone molecules in the mixture may be addressed by analyzing the effect of increasing the mole fraction of methanol, xM, on the C=O vibration mode. As it is shown in Fig. 3a, with increasing xM, the C=O vibration mode of acetone is red shifted. As it was reported previously,23,40 the overall shift of the C=O vibration mode is associated with the involvement of the C=O group in hydrogen bonding. Moreover, at high xM, Fig. 3a shows that the C=O vibration mode consists of at least two spectral contributions located at 1707cm-1 and 1716 cm-1, respectively. In this paper the contribution located in the low frequency side is assigned to acetone hydrogen bonded to methanol (HB); the second one located at the high frequency side is assigned to dipole-dipole interacting pairs of non-hydrogen-bonded acetone molecules (DD). This assignment is in agreement with that proposed by Musso et al.27 Consistently, the C=O bending vibration mode (at around 530


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cm-1) of acetone is also characterized by the occurrence of at least two contributions which are assigned to the two previous cited configurations of acetone molecules (Results not shown ). The behavior of HB and DD contributions to the C=O stretching vibration profile as a function of xA is clearly shown in the change of profile E of the excess spectra ε C=O (ν ) (see Fig. 10). This figure also indicates that increasing xM has more effect on the

shape (intensity, width) of the HB contribution than that of the DD contribution. In order to quantify the change of these two contributions with increasing methanol mole fraction, we used Eq. 5 to calculate excess partial molar absorptions ε ME ( DD) and ε ME ( HB) with respect to the change of xM. In Fig. 11 their xM dependence is shown. At low xM , the positive values of ε ME ( HB) and the negative values of ε ME ( DD) indicate that adding methanol molecules in pure acetone induces a reduction of dipole-dipole interactions between acetone molecules and concomitantly an increase of the hydrogen bonding between acetone and methanol molecules. For xM lower than 0.35, the value of ε ME ( DD) increases while that of ε ME ( HB) decreases. This can be interpreted as a signature of a decrease of the effect of adding methanol molecules in the mixture on both the reduction of the DD interactions and the formation of hydrogen bonding between acetone and methanol. For the subsequent xM

between 0.35 and 0.75, the positive values of ε ME ( DD) suggest that the DD contribution to the spectral profile is increasing while the negative values of ε ME ( HB) indicates that the HB contribution is decreasing. At xM higher than 0.75, the small positive values of ε ME ( DD) decrease toward zero indicating that the DD contribution becomes constant. The negative values of ε ME ( HB) go through a minimum indicating the location of xM value (~0.65) at which the rate of decrease of the contribution of the HB is the maximum. For subsequent mole fractions, the increase of the values of ε ME ( HB) indicates that the effect of adding methanol on rate of reduction of the hydrogen bonding between methanol and acetone is slightly decreasing. This is consistent with the previously explained physical picture that in this range of mole fraction, part of acetone molecules are not homogeneously distributed in the solution but form domains which gives rise to a spectral contribution at the high frequency side of the C=O vibration mode. The other part of acetone molecules are hydrogen bonded with methanol and they give rise to spectral contribution at the low frequency side of the C=O vibration mode. These results show that in the whole acetone mole fraction range, the spectral profile is shaped by the balance between the dipole-dipole and hydrogen bonding interactions contribution involving acetone molecules. More precisely they show clearly that at low mole fractions of acetone the dipole-dipole interactions are enough strong to induce a self-association or clustering of acetone molecules which coexists with acetone molecules that are hydrogen bonded or solvated with methanol molecules. This means that at a microscopic level, there are domains formed by acetone molecules which coexist with domains formed by acetone molecule solvated by methanol molecules. This suggests that the driving force for the occurrence of this inhomogeneous distribution may be correlated with the disruption of the balance between the hydrogen bond and dipole-dipole interactions in favor of the latter. The extent of this in homogeneity may be then quantified by the extent to which the vibration modes of acetone (particularly the C=O and C-C vibration modes) are resolved into many (at least two) spectral contributions. With this respect, the analysis of the spectral profile of the C=O and C-C stretching vibrations indicates that the inhomogeneous character of the distribution of acetone molecules is higher at the mole fraction of acetone between 0.05 and 0.55. In the case of methanol, the spectral profiles of the vibration modes associated with methanol are not clearly resolved in two spectral contributions. Although, a careful analysis of the C=O spectral



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profile (see Fig. 2b), indicates that a second peak appears as a shoulder in the low mole fraction range. This result suggests that the extent of the inhomogeneity in the distribution of methanol molecules is lower than that of acetone molecules. IV. Conclusion In this paper we analyzed the structure of acetone methanol mixture in the whole mole fraction range. This mixture provides the opportunity to get insight into the consequence of the interplay between dipole-dipole and the hydrogen bond interactions on the inhomogeneous distribution of acetone and methanol molecules in the mixture. The balance between these two interactions is modulated by the progressive change of the concentration of the components. The excess molar absorptions associated with the vibration modes of acetone and methanol are the observables that were used to quantify the changes of the dipole-dipole and hydrogen bond interactions occurring in the mixture. The behavior of the excess molar absorption as a function of the mole fractions is used to quantify the inhomogeneity distribution of molecules in the mixture. The analysis of their behavior shows that at low mole fractions of acetone the dipole-dipole interactions are enough strong to induce an inhomogeneity in the distribution (or self-association or clustering) of acetone molecules which coexists with acetone molecules that are hydrogen bonded or solvated with methanol molecules. This suggests that the driving force for the occurrence of this inhomogeneity may be correlated with the disruption of the balance between the hydrogen bond and dipole-dipole interactions in favor of the latter. In the case of methanol molecules, the inhomogeneity in their distribution is observed at high acetone mole fractions. However, the extent of this inhomogeneity is lower than that of acetone molecules. ACKNOWLEDGMENTS

K. P. thank for the LASIR laboratory of the USTL University, Lille, for its hospitality. This project was supported by the foundation for Polish science MPD program CO-financed by the EU Regional Development fund and by the Marie Curie program IRSES (International Research Staff Exchange Scheme, GAN°247500).


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TABLE 1. Spectral assignment of the IR spectra of pure methanol and acetone. Methanol Vibration

Assignment based on the data found in the literature

366723; 368240 333748

ν(O-H) (O-H)str.

300523,40 284723,40

ν(CH3) (CH3) asym str. ν(CH3) (CH3) sym str.

1332; 133523,40

δ(O-H) (O-H) bending

103423,40, 102948

ν(C-O) (C-O) str.

Acetone Vibration

Assignment based on the data found in the literature

301823 2932; 297223 172149

ν(CH3) (CH3) asym str. ν(CH3) (CH3) sym str. ν(C=O) (C=O) str.

121623,49, 122150

ν(C-C) (C-C) asym str.

78149, 78550;78023

ν(C-C) (C-C) sym str.

52823,49

δ(C=O) (C=O) str.



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Figures Fig. 1a. ATR-IR spectrum of neat methanol. See Table 1 for the assignment. Fig. 1b. ATR-IR spectrum of neat acetone. See Table 1 for the assignment Fig. 2a. ATR-IR spectra of the ν(O-H) and ν(CH3) spectral region of acetone-methanol mixtures as a function of acetone mole fraction. The arrow indicates the change of mole fraction from 0.0 to 1.0 with a step of 0.05. Fig. 2b. ATR-IR spectra of the ν(C-O) spectral region of acetone-methanol mixtures as a function of acetone mole fraction. The arrow indicates the change of mole fraction from 0.0 to 1.0 with a step of 0.05. The dashed arrow indicates the position of the shoulder which appears at mole fraction range between 0.55 and 1. Fig. 3a. ATR-IR of the ν(C=O) spectral region of acetone-methanol mixtures as a function of methanol mole fraction. The arrow indicates the change of mole fraction from 0.0 to 1.0 with a step of 0.05. The dashed arrow indicates the position of the second peak which appears in the high mole fraction range of methanol.

Fig. 3b. ATR-IR of the ν(C-C) spectral region of acetone-methanol mixtures as a function of methanol mole fraction. The arrow indicates the change of mole fraction from 0.0 to 1.0 with a step of 0.05. The dashed arrow indicates the position of the second peak which appears at the high mole fraction range of methanol. Fig. 4. Behavior of the 1H-NMR chemical shift for different proton as a function of acetone mole fraction. (black) Proton of CH3 in acetone, (blue) Proton of CH3 in methanol and (red) proton of OH.

Fig. 5. Excess infrared spectra of the O-H stretching vibration mode in acetone/methanol mixtures. The arrow indicates the increase of acetone mole fraction from 0 to 0.55 with an increment of 0.10 starting from 0.05. The inset: the same legend, however the mole fraction of acetone is between 0.65 and 0.95.

Fig. 6a (left) the optimized configuration of 8 methanol molecules (red: Oxygen; gray: Carbon and white: hydrogen). Fig. 6b (right) the optimized configuration of a mixture of one acetone molecule and 7 methanol molecules. (the dashed lines indicate the hydrogen bonded molecules) Fig. 7. Behavior of the excess partial molar absorption of O-H stretching vibration in acetone/methanol mixtures as function of acetone mole fraction. HB-MM is associated with the contribution of hydrogen bonding between methanol molecules. HB-MA is associated with both hydrogen bonding between O-H and acetone molecules and with O-H located at the end of chain of hydrogen bonded methanol molecules. Fig.8. Excess infrared spectra of C-O stretching vibration in acetone/methanol mixtures. The arrow indicates the increase of acetone mole fraction from 0 to 0.55 with an increment of 0.10 starting from 0.05. The inset: the same legend, however the mole fraction of acetone is between 0.65 and 0.95.


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Fig. 9. Behavior of the excess partial molar absorption of C-O stretching vibration in acetone/methanol mixtures as function of acetone mole fraction. D-MA is associated with the contribution of methanol hydrogen bonded to acetone when the former acts as a hydrogen bond donor. A-MM is associated with self-associated methanol molecules which act as a hydrogen bond acceptor. Fig. 10. Excess infrared spectra of C=O stretching vibration in acetone/methanol mixtures. The arrow indicates the increase of methanol mole fraction from 0.45 to 0.95 with an increment of 0.10. The inset: the same legend, however the mole fraction of methanol is between 0.05 and 0.35. Fig. 11 Behavior of the excess partial molar absorption of C=O stretching vibration in acetone/methanol mixtures as a function of methanol mole fraction xM. HB is associated with the contribution of acetone hydrogen bonded to methanol. DD is associated with dipole-dipole interacting (DD) pairs of non hydrogen bonded acetone molecules.



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Fig. 1a

Fig. 1b



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Fig. 2a


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Fig. 2b



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Fig. 3a


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Fig. 3b



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Fig. 4


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Fig. 5



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Fig. 6a , 6b


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Fig. 7

Fig.8



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Fig. 9


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Fig. 10



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Fig. 11


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For Table of Contents Only:

Scheme of the balance between the dipole-dipole and hydrogen bonding interactions in methanol/acetone mixture.


Acetone Mixture - American

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