Microscopic Roots of AlcoholâKetone Demixing: Infrared

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Microscopic Roots of Alcohol−Ketone Demixing: Infrared Spectroscopy of Methanol−Acetone Clusters Franz Kollipost, Alexandra V. Domanskaya, and Martin A. Suhm* Institut für Physikalische Chemie, Universität Göttingen, Tammannstrasse 6, D-37077 Göttingen, Germany S Supporting Information *

ABSTRACT: Infrared spectra of isolated methanol−acetone clusters up to tetramers are experimentally characterized for the first time. They show evidence for a nanometer-scale demixing trend of the cold species. In combination with quantum calculations, the mutual repulsion is demonstrated to start beyond three molecular units, whereas individual molecules still prefer to form a mixed complex.

and of torsional flexibility can affect the simulation quality in the far-infrared region,8 but the chain-breaking effect of acetone on methanol hydrogen bonding is robust. Simulations neglecting both explicit polarization and torsional flexibility9 are still able to reveal important aspects of the acetone/ methanol mixtures, such as pronounced acetone selfaggregation. It remains to be seen how this picture changes with increasingly realistic molecular modeling.10 In the current work, we investigate small isolated clusters of methanol and acetone at the dispersion corrected hybrid density functional level in the harmonic approximation with the goal of describing the experimental infrared spectra of such clusters. Therefore, we make a move to increasingly realistic molecular modeling at the expense of system size. Experimental infrared spectra of isolated methanol/acetone clusters, based on adiabatic expansions of seeded rare gases, are presented for the first time. Previously, only the mixed dimer was reported in a matrix-isolation study.11 We start with quantum chemical calculations to demonstrate the demixing tendency of acetone and methanol beyond three molecules. After a brief description of the experimental setup, we report the vibrational spectra of the cold aggregates and assign the mixed clusters in the relevant size range. The band positions support demixing tendencies as soon as the cooperativity of the methanol aggregates sets in. The comparison of experimental and theoretical results further validates the energetic considerations.

1. INTRODUCTION Information on the structure and energetics of weakly bound complexes is helpful for insights into the interactions of biologically relevant systems.1 We choose to study complexes of a simple alcohol and a ketone (methanol and acetone) to investigate the competition between carbonyl and hydroxyl groups as hydrogen bond acceptors toward OH donors. Ketones and alcohols show limited mutual miscibility in condensed phases. Given the intrinsic strength of the alcohol− ketone hydrogen bond, which is stronger than the one between two alcohol molecules, the repulsive interaction on the macroscopic scale may appear surprising. Nevertheless, the mixing enthalpy of the two components in the liquid goes through a (repulsive) maximum near the 1:1 composition2 and acetone/methanol forms a minimum boiling point azeotrope.3 Experimental evidence for microscopic heterogeneities was collected by Raman measurements of liquid mixtures of methanol and acetone.4,5 The key to the limited miscibility is to be sought in the strong cooperativity of alcoholic hydrogen bonded chains, which weakens if the chains are terminated by a ketone acceptor molecule. Therefore, the attractive interaction at the molecular pair level switches to less attractive behavior with increasing system size, when compared to the selfassociation. Attempts to decompose vibrational spectra of liquid mixtures into different components4,6 are necessarily model-dependent and particularly problematic in the presence of cooperativity. Neither the spectra nor the molecular motion is sufficiently localized for an unambiguous partitioning. Monte Carlo and molecular dynamics simulations can provide a more realistic representation of such liquid mixtures if they include polarization effects in some reasonable way. In ref 7 this is mimicked by using acetone point charges derived from model cluster calculations instead of the isolated molecules. This drastically increases the abundance of methanol−acetone binary complexes, showing the importance of mutual polarization of the molecules in the mixture.7 Neglect of polarization © XXXX American Chemical Society

2. COMPUTATIONAL DETAILS AND RESULTS In the current work we computed structures, energies, and harmonic IR vibrational spectra of oligomers of acetone and Special Issue: Markku Rasanen Festschrift Received: April 24, 2014 Revised: June 24, 2014

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dx.doi.org/10.1021/jp503999b | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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methanol OH and OD fundamentals15 is performed and the resulting fit (Supporting Information) is used for mixed cluster predictions. This fit has a standard deviation of 16 cm−1, overestimating the monomer transitions for the abovementioned reason, which has to do with the off-diagonal anharmonicity contributions in hydrogen-bonding clusters. 2.2. Acetone Clusters. Studies of acetone clusters are rather scarce. Their infrared spectra differ much less from each other than in the case of methanol. A multitude of possible structures with similar interaction energies for trimers and higher aggregates complicates the assignment further. A size-selective study, based on tunable VUV photoionization, reports the detection of a dimer and a trimer of acetone.17 The spectrum indicates a preference of the stackedtype (s) dimer over the planar, ring-like (r) one (A2-s and A2-r dimers shown in Figure 1), in robust agreement with the

methanol and their mixed clusters up to the tetramers, using Turbomole v. 6.5 (with TmoleX 3.4 graphical user interface).12 The Becke-3-parameter hybrid density functional (B3LYP), the RI-J approximation, and the def2-TZVP basis set were employed. Dispersion correction (D3), which operates with atom-pairwise specific dispersion coefficients and cutoff radii both computed from first-principles, was added.13 Becke and Johnson damping (BJ) was used.13 Geometry optimizations were run with tight convergence criteria: 10−8 (10−9 in some cases) hartree for the energy convergence and 10−5 (10−6) hartree/bohr for the gradient norm. The B3LYP-D3 approach was chosen as a compromise between a good performance in the description of hydrogen bonds and computational efficiency. No imaginary wavenumbers were found for the presented cluster structures. Basis set superposition errors were not corrected for in geometry optimizations, energy evaluations, harmonic spectra, or zero point energy calculations and could lead to minor quantitative changes despite a similar compactness of all relevant cluster structures of a given size. The calculated distance between the carbon atoms of carbonyl groups in acetone-containing clusters allows for a classification of acetone contacts. If this distance is smaller than 0.4 nm, the acetone molecules adopt a stacked geometry (-s). If the distance exceeds 0.4 nm, the contact between two neighboring subunits is open and denoted ring-like (-r). (-rs) denotes the occurrence of both types of contacts in the neighborhood. 2.1. Methanol Clusters. The structure and dynamics of pure methanol clusters, being at the root of hydrogen bonding in organic compounds, has been a subject of many studies (see, e.g., refs 14−16 and the references therein). In agreement with previous results, the most stable trimers and tetramers are predicted to be cyclic. Energies and strongest infrared peaks of (CH3OH)n and (CH3OD)n are listed in Table 1, the latter in

Figure 1. Lowest minima found on the acetone dimer and trimer potential energy hypersurfaces at B3LYP-D3/def2-TZVP level with corresponding electronic binding energies.

quantum chemical calculations. Although the spectral features due to a trimer were identified in the spectrum, the authors of ref 17 did not report any structural analysis. A DFT calculation, reported in ref 18, finds that the most stable dimer has stackedtype geometry and the most stable trimer is cyclic with C3h symmetry (similar to A3-r, shown in Figure 1). A near-edge Xray absorption fine structure spectroscopic measurement confirms the presence of CO···H−C hydrogen bonding upon clustering, although the observed features are too subtle to unequivocally determine the cluster structure. A recent sizeselective IR-VUV study19 reports the spectra from monomer to tetramer in the region of the carbonyl overtone and claims that the dominant structure of trimers and tetramers is cyclic (C3h and C4h symmetry, respectively). Our calculations confirm that the stacked dimer A2-s has a significant energy advantage over the ring dimer A2-r (Figure 1), and there is little doubt that this dimer dominates at low temperature. Trimers show a competition between two structures: stacked A3-s and cyclic A3-r (Figure 1). In agreement with earlier theoretical results,18,19 the cyclic trimer is found to be somewhat lower in energy by our DFT calculations. Nevertheless, the energy difference between the cyclic and stacked trimers is only 2 kJ/mol, which allows both species to be present in an expansion. The tetramers show a much larger variety of possible geometries with similar energies (Figure 2). Our strategy for the search of the stable configurations was to combine two dimers, or a trimer and a monomer, so that attractive interactions are likely to occur. The lowest structure found (A4-rs) was constructed from a cyclic trimer and a monomer and it is effectively a distorted ring structure with some stacking elements. We were unable to locate a C4h symmetric minimum

Table 1. Electronic Binding Energies ΔEel (kJ/mol) and Wavenumbers (cm−1) of the Most IR-Active OH Stretching Absorption Bands of Methanol Clusters Mn Relative to the Monomer M1a −ΔEel M1 M2 M3

28.6 85.2

M4

147.9

ωOH 3811 3649 3548 3543 3355

{25} {473} {828} {886} {3880}

ν̃OH 3686 3575 3474 3469 3294

(125) (74) (74) (74) (61)

ωOD 2782 2665 2591 2588 2453

ν̃OD 2718 2638 2571 2567 2444

(64) (27) (20) (21) (9)

The calculated harmonic wavenumbers ωOH are unscaled. Values for CH3OD are provided as well. Band strengths for CH3OH aggregates in the double harmonic approximation are given in braces in km/mol. The experimental anharmonic wavenumbers ν̃OH and ν̃OD are from ref 15. The deviation of ν̃ from the harmonic prediction is given in parentheses. a

comparison with experiment. According to the experimental estimates from ref 15, the difference between harmonic and anharmonic wavenumbers is 174 cm−1 for the monomer. As one can see from Table 1, the discrepancy between harmonic theory and anharmonic experiment is too small for the monomer, indicative of an OH oscillator that is too soft. The discrepancy for clusters is even smaller, but fortuitously nearly size-independent. We exploit this error compensation to assign anharmonic spectra with harmonic predictions. For this purpose, a linear scaling to the 16 experimentally available B



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methanol subunits show alternation ((MA)2) or no alternation (M2A2) in the molecular sequence. M2A2 is energetically favored over (MA)2 and exists in two iso-energetic configurations, which differ in some weak CH···O contacts. Numerous tetramers with three acetone subunits can be constructed from acetone stack and ring dimers (Figure 4). 2.4. Segregation Process in a Nanoscopic Alcohol− Ketone Mixture. The minimum boiling point azeotrope for acetone/methanol mixtures3 indicates that the interaction between the two molecules is unfavorable in the liquid, despite a strong OH···OC hydrogen bond interaction. At lower temperatures, limited miscibility in the solid state and a eutectic melting point are thus expected. Indeed, Sapgir reported a eutectic point at an acetone mole fraction of about xA = 0.64,20 which was also tabulated in the handbook by Timmermans21 and used in later work.22 However, a close inspection of the original data20 reveals that the tabulated columns for mass and mole fractions must have been interchanged. Therefore, the eutectic composition occurs at smaller concentrations of acetone (xA = 0.50). This correction also brings the ideal melting point depression of acetone by methanol RTm2/ΔmH = 46 K (using Tm = 178.7 K23 and ΔmH = 5.72 kJ/mol24) into better agreement with the observed initial slope of 60 K, indicative of little solubility of methanol in solid acetone. With the uncorrected data, the experimental slope would be 100 K. On the methanol side, the agreement between the expected melting point depression of 65−80 K (with Tm = 175.6 K and ΔmH = 3.22 or 3.86 kJ/mol depending on inclusion or exclusion of the latent heat of 0.64 kJ/mol for a solid crystalline-II to crystalline-I phase transition at 157.4 K25) and the observed one of 30 K is worse, but still better than for the 20 K extracted from the nominal data of Sapgir. The residual deviations are well within those expected from the nonideal liquid mixing behavior, so that there is no experimental evidence for pronounced solid state mixing from the melting diagram. The solid−liquid phase diagrams for mixtures of the alcohols and ketones with considerably longer hydrocarbon tails are also essentially eutectic.26 The demixing tendency is pronounced in the mixture of 1-dodecanol/2-tridecanone and 1-dodecanol/2dodecanone. Upon fast cooling, it freezes as a metastable solid and later segregates to dodecanol-rich and tridecanone-rich regions on a time scale of days (unpublished work). In a theoretical analysis of the experimental solid−liquid phase diagrams, the interaction energy between ketones (acetone and butanone) and 1-alcohols (methanol, ethanol, and four heavier alcohols) in the solid was determined to be 13 kJ/mol,22 which is significantly smaller than the interaction energy between 1-alcohols (23−28 kJ/mol) found earlier by the same method.27 Despite the distorted methanol−acetone phase diagram used in ref 22 due to the typographical error in the original data (see above), the difference in the interaction energies is consistent with the demixing tendency. Additional support for this theoretical finding comes from the Raman studies of acetone−methanol solutions. The noncoincidence effect for the CO stretching mode (difference between the isotropic and anisotropic Raman modes) persists longer upon dilution in methanol than expected in a statistical model, in particular at low temperature.5 This is consistent with acetone clustering in methanol-rich solutions, indicating that the methanol−methanol interactions are strong compared with methanol−acetone interactions.4

Figure 2. Lowest minima found on the acetone tetramer potential energy hypersurface at the B3LYP-D3/def2-TZVP level with corresponding electronic binding energies.

structure, mentioned in ref 19 as the energetically most stable; it always collapsed to other structures during the geometry optimization. The most persistent structure that resulted from different optimizations was the asymmetric stack structure A4-s, which is only slightly less stable than A4-rs. 2.3. Mixed Clusters. A binary complex of acetone and methanol was observed experimentally in matrix isolation experiments in solid argon and characterized computationally.11 To the best of our knowledge, no further studies of mixed complexes are available. It was reported that the binary complex exists in two forms in the cryogenic host. The most stable configuration has two hydrogen bonds: a strong one between the carbonyl oxygen and the hydroxyl hydrogen atom and a much weaker interaction between the alcohol oxygen and a methyl hydrogen of acetone, closing a near planar sixmembered ring.11 Our calculations converged to a similar structure (Figure 3).

Figure 3. Lowest minima found on the methanol−acetone dimer and trimer potential energy hypersurfaces at the B3LYP-D3/def2-TZVP level with corresponding electronic binding energies.

The most stable mixed trimer with two acetones (M1A2) may be viewed as being formed from a stacked acetone dimer and a methanol monomer or from a mixed dimer and an acetone monomer (Figure 3). Mixed trimers with similar binding energies have two methanol subunits (M2A1 and M2A1′ in Figure 3). They involve a methanol dimer terminated by an acetone subunit and differ slightly in their methyl group orientation. Mixed tetramers offer a variety of different structures and their diversity increases with acetone content (Figure 4). There is only one leading configuration for the tetramer with three methanol subunits (M3A1). Tetrameric structures with two C



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Figure 4. Lowest minima found on the methanol−acetone tetramer potential energy hypersurfaces at the B3LYP-D3/def2-TZVP level with corresponding electronic binding energies.

situation changes qualitatively for tetramers: homotetramers are predicted to be more stable than the mixed complexes. Only the M2A1 cooperativity gain in M2A2 is still able to more or less compensate for one-half of the cooperativity gain in M4 in this simplified picture. Though quantitative changes with the level of theory must be expected, the qualitative trend appears to be robust, as all essential interaction mechanisms, including dispersion interaction, are accounted for. The main reason for the change in binding preference is to be sought in the strong cooperativity of alcoholic hydrogen bonds, which exceeds the pairwise preference for the alcohol− ketone interaction. This predicted energy trend is also reflected in the unscaled harmonic red shifts with respect to the methanol monomer. Despite a larger bathochromic shift for the M1A1 complex (calc: 187 cm−1) than for the methanol dimer M2 (calc: 162 cm−1), none of the tetrameric OH vibrations, with bathochromic shifts from 261 to 458 cm−1, exceeds the shift of the Raman active band of methanol tetramer (calc: 567 cm−1). This shows that methanol cooperativity is slowed down by addition of acetone, as expected for a hydrogen bond terminus.28 Actually, the bathochromic shift is so sensitive to cooperativity, that even the OH wavenumbers of mixed trimers are predicted to fall short of the concerted OH stretch of methanol trimer (Supporting Information).15 This is also a consequence of focusing on the analysis of methanol only. Therefore, the energy analysis is more relevant for the thermodynamic behavior. In summary, the cohesion forces in the mixture do not reach those in the pure components, rationalizing the azeotropic behavior and the positive excess enthalpy of the liquid mixture as well as the solid state demixing. It remains to be seen whether increased alkyl groups and thus London dispersion forces will remove or at least delay this demixing tendency.

We therefore have the theoretical prediction that acetone− methanol mixing is favorable at the molecular pair level and the experimental observation that it is not favorable at the bulk solid and liquid level. This does not reflect a theoretical deficiency, but rather a system-size dependent onset of demixing, as we will show in the following. For this purpose we analyze weighted energy differences δ(n,k), which can be defined for a mixed complex formed by n methanol subunits and k acetone subunits as follows: δ(n ,k) = ΔE(M nA k) −

n·ΔE(M n + k) + k·ΔE(A n + k) n+k (1)

Here ΔE(MnAk) is the complex binding energy and ΔE(Mn+k) and ΔE(An+k) are the binding energies of corresponding pure clusters of methanol and acetone, respectively. They are tabulated in the Supporting Information. δ(n,k) values reflect the energy change upon forming a mixed cluster from pure clusters of the same size. Positive values of δ(n,k) signal a demixing tendency at the n+k cluster size level. The calculated values for the energetic preference function δ(n,k) for acetone/methanol aggregates are shown in Figure 5.

Figure 5. Weighted energy differences δ(n,k) of mixed clusters MnAk relative to pure clusters based on ZPE-corrected binding energies ΔE0 (see text for details).

3. EXPERIMENTAL DETAILS Diluted gas mixtures of methanol (CH3OH: VWR, 99.9%, CH 3 OD: euriso-top, 99.9%, 99% OD) and acetone ((CH3)2CO: Roth, 99.8%, (CD3)2CO: Roth, euriso-top, 99.98%, 99.8% D) in helium (Linde, 99.996%) were prepared in a 67 l reservoir by filling it through three individual gas lines up to a pressure of 0.6 bar. In two of these lines helium flowed through cooled saturators picking up the molecules. The third line was used for further dilution with helium. The concentration of the substances was controlled by changing their vapor pressure via the saturator temperature and by a pulsed admission of He gas through the third line.

Only the most stable structures for each cluster size were selected. The binding energies are harmonically ZPE corrected. It is noteworthy that the ZPE correction does not change the result qualitatively (Supporting Information). Figure 5 demonstrates that the formation of a mixed dimer is energetically favorable with respect to methanol and acetone homodimers, in agreement with expectations. Mixed trimers are also favorable, especially in the case of the M1A2 complex. The D



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hydrogen bond acceptor (CD3)2CO), must be also attributed to the M1A1 dimer. It is further shifted than the methanol dimer (3575 cm−1) by 40%. Any remaining doubt can be removed by deuteration of the methanol OH group. An analogous transition is observed at 2608 cm−1 (showing virtually no shift with (CD3)2CO as a binding partner), which corresponds to a dimerization shift of −110 cm−1, again almost 40% larger than for the deuterated methanol dimer15 (−80 cm−1). We note that the OH/OD isotope ratio is within expectations for M1 (1.36), M2 (1.36), and M1A1 (1.35) and even for the M1A1 shift from M1 (1.42), which is more sensitive to anharmonic effects. In the matrix, there is more scattering due to site effects (1.36 for M1, 1.35 for M2, 1.36−1.37 for M1A1) and hence no stable ratio for the monomer−mixed dimer shift from M1 (1.12−1.20). This shows that matrix site effects complicate the analysis. One possible interpretation of the unstable isotope ratio in matrix measurements is an anharmonic resonance in one of the two isotopomers, which is intensified by matrix embedding due to nonuniform shifts of all levels. Indeed, we will see such a resonance for the M1A2 trimer later on. 4.2. Mixed Trimers. M1A2 leaves a distinct trace in expansions with high acetone and low methanol content (solid line in Figure 6): a single band appears at 3462 cm−1. It is further red-shifted than M1A1 and shifts to 3460 cm−1 for the better acceptor (CD3)2CO. The enhanced red shifts caused by the second acetone unit are remarkable and believed to be due to further polarization in the stacked antiparallel acetone dimer. The mixed trimer is close to M3 transitions at 3469 and 3474 cm−1. The same pattern is found for deuterated methanol, where the mixed trimer transition appears at 2563 cm−1 (2561 cm−1 for (CD3)2CO), whereas the deuterated methanol trimer absorbs at 2567 and 2571 cm−1. For the other mixed trimer, M2A1, one expects two OH stretching bands that scale more strongly with methanol content than M1A2 and are further red-shifted than M1A1. Inspection of the methanol-rich trace (dotted) in Figure 6 suggests the presence of three such bands, a single peak at 3456 cm−1 and a doublet at 3485/3494 cm−1. The deuteration experiment gives an immediate explanation for the extra band. After appropriate scaling of the wavenumber axis to match the position of the methanol monomer and M1A1 cluster bands, one can see that the doublet merges into one more intense band (Figure 7). This spectral behavior is characteristic for a Fermi resonance, possibly with the CO stretching overtone of acetone through the hydrogen bond, which is lost for the deuterated complex. The CO stretch fundamental of acetone is itself affected by (weak) Fermi resonances. An early study traces the resonance to the combination band of C−C−C symmetric stretch and CO bending modes,33 but recent jet experiments suggest that there are more energy levels involved [to be published]. These resonances are also sensitive to the environment.34 One might expect that the CO overtone is prone to similar anharmonic resonances that are sensitive to the complexation partner. In our measurements of pure acetone expansions two weak peaks at 3433 and 3467 cm−1 are visible in the 2ν(CO) region (lower trace in Figure 8). A relatively small difference between the average wavenumber of the M2A1 doublet and the 2ν(CO) bands of acetone (23 and 57 cm−1) is favorable for a Fermi resonance between 2ν(CO) and ν(OH) modes.

The gas mixtures were expanded in 120 ms pulses from the reservoir through a 600 × 0.2 mm2 slit nozzle into a 23 m3 vacuum system (0.1 mbar) that was continuously evacuated at 2500 m3/h. Pumping for 35 s was needed after each expansion to regenerate the vacuum. The pulses were synchronized to 2 cm−1 spectral resolution scans of a Bruker IFS 66v/S FTIR instrument with its IR beam parallel to the slit nozzle. High light throughput was achieved by a 4 mm aperture for the tungsten or globar source in combination with CaF2 optics and suitable band-pass filters in front of a l-N2 cooled InSb detector. Typically the interferograms from 50 pulses were coadded to improve the signal-to-noise ratio. More experimental details can be found in ref 29.

4. JET SPECTRA AND EXPERIMENTAL ASSIGNMENTS Supersonic jet spectra with a systematic variation of absolute and relative concentrations as well as isotopic substitution usually yield unambiguous size assignments for dimers and most mixed trimers in the OH stretching range. The reason is the wide spectral separation of bands as a function of hydrogen bond interaction, spanning more than 10% of the vibrational wavenumber. Together with the expected cluster abundances, which should drop with increasing cluster size and with decreasing content of a given component in the expansion, the only remaining uncertainties are coincidental band overlap or splitting due to resonances or subtle isomerism. For the mixed tetramers of methanol and acetone, this sizing approach, which falls short of rigorously size-resolved techniques for aromatic compounds,30 reaches its limit as we will see. 4.1. Mixed Dimer. In matrix isolation, the mixed dimer M1A1 shows two OH stretching bands (3503 and 3518 cm−1) shifted from the monomer by −149 and −164 cm−1.11 The observation of two bands was attributed to two different conformers. They are further shifted than the OH donor band in the methanol dimer by 1−30% (the large uncertainty stems from extensive matrix splitting in particular for methanol dimer11,31). A related system, in which methanol is replaced by HF, has also been measured in the room temperature gas phase32 and an HF wavenumber red shift of about 160 cm−1 due to complexation with acetone has been observed. The single strong cluster absorption found in the jet spectra (Figure 6) at 3530 cm−1 (3529 cm−1 for the slightly better

Figure 6. FTIR jet spectra of methanol and acetone mixtures (He is the carrier gas) in the ν(OH) region showing absorption bands of homo- and heteromultimers up to trimers (solid and dotted lines). The spectrum of a methanol expansion is shown by a dashed line. E



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Table 2. Experimental ν̃OH/ν̃OD Wavenumbers for the Mixed Dimer and Trimers of Acetone with Methanol and Their Deuterated Analoguesa ν̃OH/cm−1

M1A1 M2A1

M1A2

ν̃OD/cm−1

CH3OH/ (CH3)2CO

CH3OH/ (CD3)2CO

CH3OD/ (CH3)2CO

CH3OD/ (CD3)2CO

3530(−9) 3494(−0)b 3485(−9)b 3456(+5) 3462(+5)

3529 c

2608(−17) 2580(−13)

2608 2579

c 3460

2558 (−7) 2563(−6)

2558 2561

a Differences between experimental and calculated (methanol-scaled) wavenumbers are given in parentheses. bFermi doublet. See text for details. cVibrational wavenumbers for M2A1 in the CH3OH/ (CD3)2CO expansion were not assigned due to a small concentration of methanol in the original spectrum and the limited number of available data.

Figure 7. Influence of methanol deuteration on the coexpansion spectra of methanol and acetone. The spectrum of the deuterated compound (dotted line) was superimposed on the nondeuterated one (solid line) so that the methanol monomer and heterodimer bands match.

new peaks are due to mixed tetramers, but larger clusters cannot be ruled out, in particular for high acetone content. Although several of the mixed peaks show a systematic evolution as a function of methanol and acetone concentration, first rising and then leveling off or dropping from top to bottom, a firm assignment without size-selective measurements is difficult due to a number of reasons. The probability of band overlap among methanol oligomers and mixed clusters is substantial, the competition from pure methanol clusters is strong, the expected cluster isomerism is large, the difference in concentration scaling between neighboring cluster compositions is not as large as for dimers and trimers, and the region contains acetone modes such as the complex Fermi resonance pattern related to the overtone of the CO stretching mode near 3450 cm−1 and other combination bands near 2550 cm−1. These modes, which shift as a function of acetone cluster size,19 may share intensity with nearby OH stretching modes of acetone-rich mixed clusters and complicate the assignment. In particular the two bands marked with a dagger (Figure 8) look like blue-shifted M1An counterparts of the corresponding pure acetone CO overtone modes in the lowest trace. The blue shift may even reflect the limited miscibility of the two compounds. The experimental observations allow us to determine the tetramer stoichiometry, but the success of the uniform linear scaling of predicted harmonic wavenumbers for mixed dimers and trimers in the preceding sections (Table 2) encourages a quantum chemistry-driven tentative assignment. For this purpose, we use the scaled harmonic OH and OD wavenumbers listed in the Supporting Information. In Table 3, the resulting list of best predictions based on pure methanol oligomers (scaled) is mapped on the observed additional peaks in the spectra, also considering relative intensities in the presence of multiple bands. Five things may be noted. Although the difference of the M2A2 and M2A2′ binding energies is vanishingly small, the predicted band positions differ noticeably (Table S1 in the Supporting Information). Significantly better agreement between predicted and observed wavenumbers for the M2A2 structure rules out M2A2′ as a probable candidate for the assignment. Strongly shifted bands corresponding to concerted OH (OD) stretching at 3302 cm−1 (2450 cm−1) seem weak or broad, which may be related to their partial proton-transfer character. Raman spectroscopy would be better suited to

Figure 8. Overview of the spectral changes in the ν(OH) region upon variation of methanol and acetone concentrations. The assignment of the observed tetrameric peaks is tentative. The weak bands marked with an asterisk do not find straightforward assignment. The daggers mark two features that may have contributions from the 2ν(CO) vibrational mode. See text for details.

This is as far as robust assignment entirely based on isotope effects and a comparison of cluster band intensities reaches. The resulting ν(OH) band positions of the heterodimer and the heterotrimers are collected in Table 2. The close agreement and systematic trend of residual deviations from the methanolscaled calculated harmonic wavenumbers (in parentheses) lends high confidence to the experimental assignments. 4.3. Mixed Tetramers. At higher concentrations of acetone and methanol, numerous new peaks appear in the region of the methanol trimer and tetramer OH stretching bands (Figure 8 and also corresponding spectra of deuterated methanol in Figure S4 in the Supporting Information). The spectra in the figures are ordered such that the methanol content increases from bottom to top, whereas the acetone content decreases. The limiting spectra correspond to pure methanol and pure acetone expansions, respectively. It is likely that many of the F



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imental spectra only confirm the essential correctness of the computational model, which in turn shows demixing to be energetically favorable. On the way toward this goal, a number of thermodynamic and spectroscopic findings for methanol/acetone are worth being mentioned: I. A previously overlooked error in the tabulated solid− liquid phase diagram for this model system has been uncovered. II. A unique structure for the mixed dimer has been identified. III. A Fermi resonance, presumably transmitted through the OH···OC hydrogen bond, was found for the trimeric complex built from two methanol and one acetone unit. IV. Some cooperativity was also evidenced by attaching a second acetone unit to the mixed dimer, possibly due to the polarization of the CO group in the stacked acetone dimer. V. Some mixed tetramers, although thermodynamically disfavored relative to pure clusters, were tentatively assigned despite the lack of rigorous size selection in the linear FTIR spectroscopy. Whenever more than one methanol is present in the clusters, OH···OH-based cooperativity is preferred. The switch from isolated OH···OC to cooperative OH···OH···OH patterns is also responsible for one of the most unusual supramolecular recognition processes found for neutral molecules in the gas phase: depending on the relative chirality of the building blocks, the tetramer of methyl lactate occurs in one or the other form.36 The quantitative success of the present work implies that B3LYP-D3 should be a suitable method to study this chirality recognition phenomenon in more detail. Future work will have to address the effect of alkyl chain length and thus London dispersion forces on the demixing tendency of ketones and alcohols and may also want to have a look at the even more elementary water−acetone system. As the mixed dimer with methanol can be produced in high abundance in a supersonic jet expansion, it would be interesting to study its OH stretching overtone29 and to reveal the change of OH bond anharmonicity and transition dipole moment upon acetone binding. Finally, it is worth addressing the rather poorly studied acetone dimer by vibrational spectroscopy. The absence of a dipole moment and of an interaction-sensitive IRchromophore has turned this system into one of the least studied basic supramolecular complexes, but a combination of IR and Raman spectroscopy promises to unravel some of its dynamical details.

Table 3. Tentative Assignment of the Spectral Features in the ν(OH)/ν(OD) Region of the Heterotetramers of CH3OH/CH3OD with (CH3)2COa ν̃OH/ν̃OD tentative assignment M3A1

M2A2 M2A2′ M1A3-s(′) a

scaled

experiment

3407/2529 3366/2501 3285/2445 3428/2538 3366/2496 3394/2514 3333/2473 3397/2517 3439/2548

3409/2527 3370/2502 3302/2451 3423/2533 3370/2502

3409/2527 2ν(CO)/CH3 def + ν(CC)

Experimental and calculated wavenumbers are in cm−1.

identify these bands. Bands which find multiple explanations (M3A1 and M1A3: 3409, 2527 cm−1; M3A1 and M2A2: ≈3770, ≈2502 cm−1) indeed show a more complex concentration evolution pointing to band overlap. Some stronger bands that find no assignment (3438, 3475/78 cm−1) fall in the region of pure acetone bands. Other bands that find no assignment (3351, 3373, 3384 cm−1) continue to gain intensity with increasing acetone concentration, indicating a size larger than tetramers. The resulting assignments of M3A1 and M2A2 are thus plausible, but tentative (Table 3), whereas an assignment of M1A3 appears difficult due to the multitude of structures and overlap with the CO overtone. It would be desirable to have more accurate spectral predictions35 to verify the tetramer assignment. In summary, the combination of theory and firm experimental mixed trimer assignments allows for some tentative proposals for mixed tetramer bands building on two or three methanol units, consistent with qualitative concentration scaling.

5. CONCLUDING REMARKS The intermolecular interaction of two important molecular functionalities, hydroxyl and carbonyl groups, is ambiguous. Intrinsically, their combination in an OH···OC hydrogen bond is preferred over self-interactions, but the polarization of alcoholic OH groups in cooperative hydrogen bond chains is stopped by CO groups, therefore building up a demixing tendency between alcohols and ketones with increasing system size. For methanol and acetone, we have presented combined quantitative experimental and theoretical evidence for this phenomenon as a function of the number of interacting molecules. Theoretically, we have shown that up to a cluster size of three, it is energetically more favorable to have mixed complexes, whereas starting at a cluster size of four a partitioning into pure clusters is lower in energy. To back this quantum chemical prediction, we have for the first time studied vacuum-isolated mixed methanol−acetone clusters by infrared spectroscopy and assigned them on the basis of their OH stretching signature, confirming the predicted harmonic fundamental wavenumbers in combination with a smooth scaling based only on pure methanol clusters. This underscores a fully self-consistent description of energetical and spectroscopic aspects of this important model system. We emphasize that the jet expansions themselves do not show cluster demixing tendencies due to kinetic barriers for significant composition exchange in cold cluster collisions. The exper-

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ASSOCIATED CONTENT

S Supporting Information *

Calculated wavenumbers of the ν(OH) mode and binding energies of homo- and heteroclusters, quality of the scaling function (Figure S1), change of binding preference based on binding energies and the ν(OH) shifts (Figures S2 and S3), spectra of deuterated methanol−acetone clusters (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M. A. Suhm. Electronic mail: [email protected]. Phone: +49 551 39-33112. Fax: +49 551 39-33117. G



Article

Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Anja Poblotzki for complementary studies on long chain alcohol/ketone mixtures. Funding is through Grant Su 121/4, DFG (Deutsche Forschungsgemeinschaft).

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