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Microscopic Roots of AlcoholKetone Demixing: Infrared Spectroscopy of MethanolAcetone Clusters Franz Kollipost, Alexandra V. Domanskaya, and Martin A. Suhm* Institut fü r Physikalische Chemie, Universitä t Gö ttingen, Tammannstrasse 6, D-37077 Gö ttingen, Germany * S Supporting Information ABSTRACT: Infrared spectra of isolated methanolacetone clusters up to tetramers are experimentally characterized for the rst 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. 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 alcoholketone 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 composition 2 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 self- association. Attempts to decompose vibrational spectra of liquid mixtures into dierent components 4,6 are necessarily model-dependent and particularly problematic in the presence of cooperativity. Neither the spectra nor the molecular motion is suciently 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 eects 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 methanolacetone binary complexes, showing the importance of mutual polar- ization of the molecules in the mixture. 7 Neglect of polarization and of torsional exibility can aect the simulation quality in the far-infrared region, 8 but the chain-breaking eect of acetone on methanol hydrogen bonding is robust. Simulations neglecting both explicit polarization and torsional exibility 9 are still able to reveal important aspects of the acetone/ methanol mixtures, such as pronounced acetone self- aggregation. 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 rst 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. 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 Article pubs.acs.org/JPCA © XXXX American Chemical Society A dx.doi.org/10.1021/jp503999b | J. Phys. Chem. A XXXX, XXX, XXXXXX

Microscopic Roots of Alcohol–Ketone Demixing: Infrared Spectroscopy of Methanol–Acetone Clusters

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Microscopic Roots of Alcohol−Ketone Demixing: InfraredSpectroscopy of Methanol−Acetone ClustersFranz Kollipost, Alexandra V. Domanskaya, and Martin A. Suhm*

Institut fur Physikalische Chemie, Universitat Gottingen, Tammannstrasse 6, D-37077 Gottingen, Germany

*S Supporting Information

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

1. INTRODUCTION

Information on the structure and energetics of weakly boundcomplexes is helpful for insights into the interactions ofbiologically relevant systems.1 We choose to study complexes ofa simple alcohol and a ketone (methanol and acetone) toinvestigate the competition between carbonyl and hydroxylgroups 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 betweentwo alcohol molecules, the repulsive interaction on themacroscopic scale may appear surprising. Nevertheless, themixing enthalpy of the two components in the liquid goesthrough a (repulsive) maximum near the 1:1 composition2 andacetone/methanol forms a minimum boiling point azeotrope.3

Experimental evidence for microscopic heterogeneities wascollected by Raman measurements of liquid mixtures ofmethanol and acetone.4,5 The key to the limited miscibility isto be sought in the strong cooperativity of alcoholic hydrogenbonded chains, which weakens if the chains are terminated by aketone acceptor molecule. Therefore, the attractive interactionat the molecular pair level switches to less attractive behaviorwith increasing system size, when compared to the self-association.Attempts to decompose vibrational spectra of liquid mixtures

into different components4,6 are necessarily model-dependentand particularly problematic in the presence of cooperativity.Neither the spectra nor the molecular motion is sufficientlylocalized for an unambiguous partitioning. Monte Carlo andmolecular dynamics simulations can provide a more realisticrepresentation of such liquid mixtures if they includepolarization effects in some reasonable way. In ref 7 this ismimicked by using acetone point charges derived from modelcluster calculations instead of the isolated molecules. Thisdrastically increases the abundance of methanol−acetonebinary complexes, showing the importance of mutual polar-ization of the molecules in the mixture.7 Neglect of polarization

and of torsional flexibility can affect the simulation quality inthe far-infrared region,8 but the chain-breaking effect of acetoneon methanol hydrogen bonding is robust. Simulationsneglecting both explicit polarization and torsional flexibility9

are still able to reveal important aspects of the acetone/methanol mixtures, such as pronounced acetone self-aggregation. It remains to be seen how this picture changeswith increasingly realistic molecular modeling.10

In the current work, we investigate small isolated clusters ofmethanol and acetone at the dispersion corrected hybriddensity functional level in the harmonic approximation with thegoal of describing the experimental infrared spectra of suchclusters. Therefore, we make a move to increasingly realisticmolecular modeling at the expense of system size.Experimental infrared spectra of isolated methanol/acetone

clusters, based on adiabatic expansions of seeded rare gases, arepresented for the first time. Previously, only the mixed dimerwas reported in a matrix-isolation study.11

We start with quantum chemical calculations to demonstratethe demixing tendency of acetone and methanol beyond threemolecules. After a brief description of the experimental setup,we report the vibrational spectra of the cold aggregates andassign the mixed clusters in the relevant size range. The bandpositions support demixing tendencies as soon as thecooperativity of the methanol aggregates sets in. Thecomparison of experimental and theoretical results furthervalidates the energetic considerations.

2. COMPUTATIONAL DETAILS AND RESULTS

In the current work we computed structures, energies, andharmonic IR vibrational spectra of oligomers of acetone and

Special Issue: Markku Rasanen Festschrift

Received: April 24, 2014Revised: June 24, 2014

Article

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

methanol and their mixed clusters up to the tetramers, usingTurbomole v. 6.5 (with TmoleX 3.4 graphical user interface).12

The Becke-3-parameter hybrid density functional (B3LYP), theRI-J approximation, and the def2-TZVP basis set wereemployed. Dispersion correction (D3), which operates withatom-pairwise specific dispersion coefficients and cutoff radiiboth computed from first-principles, was added.13 Becke andJohnson damping (BJ) was used.13 Geometry optimizationswere run with tight convergence criteria: 10−8 (10−9 in somecases) hartree for the energy convergence and 10−5 (10−6)hartree/bohr for the gradient norm. The B3LYP-D3 approachwas chosen as a compromise between a good performance inthe description of hydrogen bonds and computationalefficiency. No imaginary wavenumbers were found for thepresented cluster structures. Basis set superposition errors werenot corrected for in geometry optimizations, energy evalua-tions, harmonic spectra, or zero point energy calculations andcould lead to minor quantitative changes despite a similarcompactness 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 aclassification of acetone contacts. If this distance is smaller than0.4 nm, the acetone molecules adopt a stacked geometry (-s). Ifthe distance exceeds 0.4 nm, the contact between twoneighboring subunits is open and denoted ring-like (-r). (-rs)denotes the occurrence of both types of contacts in theneighborhood.2.1. Methanol Clusters. The structure and dynamics of

pure methanol clusters, being at the root of hydrogen bondingin organic compounds, has been a subject of many studies (see,e.g., refs 14−16 and the references therein). In agreement withprevious results, the most stable trimers and tetramers arepredicted to be cyclic. Energies and strongest infrared peaks of(CH3OH)n and (CH3OD)n are listed in Table 1, the latter in

comparison with experiment. According to the experimentalestimates from ref 15, the difference between harmonic andanharmonic wavenumbers is 174 cm−1 for the monomer. Asone can see from Table 1, the discrepancy between harmonictheory and anharmonic experiment is too small for themonomer, indicative of an OH oscillator that is too soft. Thediscrepancy for clusters is even smaller, but fortuitously nearlysize-independent. We exploit this error compensation to assignanharmonic spectra with harmonic predictions. For thispurpose, a linear scaling to the 16 experimentally available

methanol OH and OD fundamentals15 is performed and theresulting fit (Supporting Information) is used for mixed clusterpredictions. This fit has a standard deviation of 16 cm−1,overestimating the monomer transitions for the above-mentioned reason, which has to do with the off-diagonalanharmonicity contributions in hydrogen-bonding clusters.

2.2. Acetone Clusters. Studies of acetone clusters arerather scarce. Their infrared spectra differ much less from eachother than in the case of methanol. A multitude of possiblestructures with similar interaction energies for trimers andhigher aggregates complicates the assignment further.A size-selective study, based on tunable VUV photo-

ionization, reports the detection of a dimer and a trimer ofacetone.17 The spectrum indicates a preference of the stacked-type (s) dimer over the planar, ring-like (r) one (A2-s and A2-rdimers shown in Figure 1), in robust agreement with the

quantum chemical calculations. Although the spectral featuresdue to a trimer were identified in the spectrum, the authors ofref 17 did not report any structural analysis. A DFT calculation,reported in ref 18, finds that the most stable dimer has stacked-type geometry and the most stable trimer is cyclic with C3hsymmetry (similar to A3-r, shown in Figure 1). A near-edge X-ray absorption fine structure spectroscopic measurementconfirms the presence of CO···H−C hydrogen bondingupon clustering, although the observed features are too subtleto unequivocally determine the cluster structure. A recent size-selective IR-VUV study19 reports the spectra from monomer totetramer in the region of the carbonyl overtone and claims thatthe dominant structure of trimers and tetramers is cyclic (C3hand C4h symmetry, respectively).Our calculations confirm that the stacked dimer A2-s has a

significant energy advantage over the ring dimer A2-r (Figure1), and there is little doubt that this dimer dominates at lowtemperature. Trimers show a competition between twostructures: stacked A3-s and cyclic A3-r (Figure 1). In agreementwith earlier theoretical results,18,19 the cyclic trimer is found tobe somewhat lower in energy by our DFT calculations.Nevertheless, the energy difference between the cyclic andstacked trimers is only 2 kJ/mol, which allows both species tobe present in an expansion.The tetramers show a much larger variety of possible

geometries with similar energies (Figure 2). Our strategy forthe search of the stable configurations was to combine twodimers, or a trimer and a monomer, so that attractiveinteractions are likely to occur. The lowest structure found(A4-rs) was constructed from a cyclic trimer and a monomerand it is effectively a distorted ring structure with some stackingelements. We were unable to locate a C4h symmetric minimum

Table 1. Electronic Binding Energies ΔEel (kJ/mol) andWavenumbers (cm−1) of the Most IR-Active OH StretchingAbsorption Bands of Methanol Clusters Mn Relative to theMonomer M1

a

−ΔEel ωOH νOH ωOD νOD

M1 3811 {25} 3686 (125) 2782 2718 (64)M2 28.6 3649 {473} 3575 (74) 2665 2638 (27)M3 85.2 3548 {828} 3474 (74) 2591 2571 (20)

3543 {886} 3469 (74) 2588 2567 (21)M4 147.9 3355 {3880} 3294 (61) 2453 2444 (9)

aThe calculated harmonic wavenumbers ωOH are unscaled. Values forCH3OD are provided as well. Band strengths for CH3OH aggregatesin the double harmonic approximation are given in braces in km/mol.The experimental anharmonic wavenumbers νOH and νOD are from ref15. The deviation of ν from the harmonic prediction is given inparentheses.

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

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structure, mentioned in ref 19 as the energetically most stable;it always collapsed to other structures during the geometryoptimization. The most persistent structure that resulted fromdifferent 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 isolationexperiments in solid argon and characterized computationally.11

To the best of our knowledge, no further studies of mixedcomplexes are available. It was reported that the binary complexexists in two forms in the cryogenic host. The most stableconfiguration has two hydrogen bonds: a strong one betweenthe carbonyl oxygen and the hydroxyl hydrogen atom and amuch weaker interaction between the alcohol oxygen and amethyl hydrogen of acetone, closing a near planar six-membered ring.11 Our calculations converged to a similarstructure (Figure 3).

The most stable mixed trimer with two acetones (M1A2) maybe viewed as being formed from a stacked acetone dimer and amethanol monomer or from a mixed dimer and an acetonemonomer (Figure 3). Mixed trimers with similar bindingenergies have two methanol subunits (M2A1 and M2A1′ inFigure 3). They involve a methanol dimer terminated by anacetone subunit and differ slightly in their methyl grouporientation.Mixed tetramers offer a variety of different structures and

their diversity increases with acetone content (Figure 4). Thereis only one leading configuration for the tetramer with threemethanol subunits (M3A1). Tetrameric structures with two

methanol subunits show alternation ((MA)2) or no alternation(M2A2) in the molecular sequence. M2A2 is energeticallyfavored over (MA)2 and exists in two iso-energetic config-urations, which differ in some weak CH···O contacts.Numerous tetramers with three acetone subunits can beconstructed from acetone stack and ring dimers (Figure 4).

2.4. Segregation Process in a Nanoscopic Alcohol−Ketone Mixture. The minimum boiling point azeotrope foracetone/methanol mixtures3 indicates that the interactionbetween the two molecules is unfavorable in the liquid, despitea strong OH···OC hydrogen bond interaction. At lowertemperatures, limited miscibility in the solid state and a eutecticmelting point are thus expected. Indeed, Sapgir reported aeutectic 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 theoriginal data20 reveals that the tabulated columns for mass andmole fractions must have been interchanged. Therefore, theeutectic composition occurs at smaller concentrations ofacetone (xA = 0.50). This correction also brings the idealmelting point depression of acetone by methanol RTm

2/ΔmH =46 K (using Tm = 178.7 K23 and ΔmH = 5.72 kJ/mol24) intobetter agreement with the observed initial slope of 60 K,indicative of little solubility of methanol in solid acetone. Withthe uncorrected data, the experimental slope would be 100 K.On the methanol side, the agreement between the expectedmelting point depression of 65−80 K (with Tm = 175.6 K andΔmH = 3.22 or 3.86 kJ/mol depending on inclusion orexclusion of the latent heat of 0.64 kJ/mol for a solidcrystalline-II to crystalline-I phase transition at 157.4 K25) andthe observed one of 30 K is worse, but still better than for the20 K extracted from the nominal data of Sapgir. The residualdeviations are well within those expected from the nonidealliquid mixing behavior, so that there is no experimentalevidence for pronounced solid state mixing from the meltingdiagram.The solid−liquid phase diagrams for mixtures of the alcohols

and ketones with considerably longer hydrocarbon tails are alsoessentially eutectic.26 The demixing tendency is pronounced inthe mixture of 1-dodecanol/2-tridecanone and 1-dodecanol/2-dodecanone. Upon fast cooling, it freezes as a metastable solidand later segregates to dodecanol-rich and tridecanone-richregions 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 13kJ/mol,22 which is significantly smaller than the interactionenergy between 1-alcohols (23−28 kJ/mol) found earlier bythe same method.27 Despite the distorted methanol−acetonephase diagram used in ref 22 due to the typographical error inthe original data (see above), the difference in the interactionenergies is consistent with the demixing tendency. Additionalsupport for this theoretical finding comes from the Ramanstudies of acetone−methanol solutions. The noncoincidenceeffect for the CO stretching mode (difference between theisotropic and anisotropic Raman modes) persists longer upondilution in methanol than expected in a statistical model, inparticular at low temperature.5 This is consistent with acetoneclustering in methanol-rich solutions, indicating that themethanol−methanol interactions are strong compared withmethanol−acetone interactions.4

Figure 2. Lowest minima found on the acetone tetramer potentialenergy hypersurface at the B3LYP-D3/def2-TZVP level withcorresponding electronic binding energies.

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

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We therefore have the theoretical prediction that acetone−methanol mixing is favorable at the molecular pair level and theexperimental observation that it is not favorable at the bulksolid and liquid level. This does not reflect a theoreticaldeficiency, but rather a system-size dependent onset ofdemixing, as we will show in the following. For this purposewe analyze weighted energy differences δ(n,k), which can bedefined for a mixed complex formed by n methanol subunitsand k acetone subunits as follows:

δ = Δ −·Δ + ·Δ

++ +n k E

n E k En k

( , ) (M A )(M ) (A )

n kn 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 pureclusters of methanol and acetone, respectively. They aretabulated in the Supporting Information. δ(n,k) values reflectthe energy change upon forming a mixed cluster from pureclusters of the same size. Positive values of δ(n,k) signal ademixing 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.

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

situation changes qualitatively for tetramers: homotetramers arepredicted to be more stable than the mixed complexes. Onlythe M2A1 cooperativity gain in M2A2 is still able to more or lesscompensate for one-half of the cooperativity gain in M4 in thissimplified picture. Though quantitative changes with the levelof theory must be expected, the qualitative trend appears to berobust, as all essential interaction mechanisms, includingdispersion interaction, are accounted for.The main reason for the change in binding preference is to

be sought in the strong cooperativity of alcoholic hydrogenbonds, which exceeds the pairwise preference for the alcohol−ketone interaction. This predicted energy trend is also reflectedin the unscaled harmonic red shifts with respect to themethanol monomer. Despite a larger bathochromic shift for theM1A1 complex (calc: 187 cm−1) than for the methanol dimerM2 (calc: 162 cm−1), none of the tetrameric OH vibrations,with bathochromic shifts from 261 to 458 cm−1, exceeds theshift of the Raman active band of methanol tetramer (calc: 567cm−1). This shows that methanol cooperativity is slowed downby addition of acetone, as expected for a hydrogen bondterminus.28 Actually, the bathochromic shift is so sensitive tocooperativity, that even the OH wavenumbers of mixed trimersare predicted to fall short of the concerted OH stretch ofmethanol trimer (Supporting Information).15 This is also aconsequence of focusing on the analysis of methanol only.Therefore, the energy analysis is more relevant for thethermodynamic behavior.In summary, the cohesion forces in the mixture do not reach

those in the pure components, rationalizing the azeotropicbehavior and the positive excess enthalpy of the liquid mixtureas well as the solid state demixing. It remains to be seenwhether increased alkyl groups and thus London dispersionforces will remove or at least delay this demixing tendency.

3. EXPERIMENTAL DETAILS

Diluted gas mixtures of methanol (CH3OH: VWR, 99.9%,CH3OD: 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 preparedin a 67 l reservoir by filling it through three individual gas linesup to a pressure of 0.6 bar. In two of these lines helium flowedthrough cooled saturators picking up the molecules. The thirdline was used for further dilution with helium. Theconcentration of the substances was controlled by changingtheir vapor pressure via the saturator temperature and by apulsed admission of He gas through the third line.

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

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

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The gas mixtures were expanded in 120 ms pulses from thereservoir through a 600 × 0.2 mm2 slit nozzle into a 23 m3

vacuum system (0.1 mbar) that was continuously evacuated at2500 m3/h. Pumping for 35 s was needed after each expansionto regenerate the vacuum. The pulses were synchronized to 2cm−1 spectral resolution scans of a Bruker IFS 66v/S FTIRinstrument with its IR beam parallel to the slit nozzle. Highlight throughput was achieved by a 4 mm aperture for thetungsten or globar source in combination with CaF2 optics andsuitable band-pass filters in front of a l-N2 cooled InSb detector.Typically the interferograms from 50 pulses were coadded toimprove the signal-to-noise ratio. More experimental details canbe found in ref 29.

4. JET SPECTRA AND EXPERIMENTAL ASSIGNMENTS

Supersonic jet spectra with a systematic variation of absoluteand relative concentrations as well as isotopic substitutionusually yield unambiguous size assignments for dimers andmost mixed trimers in the OH stretching range. The reason isthe wide spectral separation of bands as a function of hydrogenbond interaction, spanning more than 10% of the vibrationalwavenumber. Together with the expected cluster abundances,which should drop with increasing cluster size and withdecreasing content of a given component in the expansion, theonly remaining uncertainties are coincidental band overlap orsplitting due to resonances or subtle isomerism. For the mixedtetramers of methanol and acetone, this sizing approach, whichfalls short of rigorously size-resolved techniques for aromaticcompounds,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 Theobservation of two bands was attributed to two differentconformers. They are further shifted than the OH donor bandin the methanol dimer by 1−30% (the large uncertainty stemsfrom extensive matrix splitting in particular for methanoldimer11,31). A related system, in which methanol is replaced byHF, has also been measured in the room temperature gasphase32 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

hydrogen bond acceptor (CD3)2CO), must be also attributedto the M1A1 dimer. It is further shifted than the methanol dimer(3575 cm−1) by 40%. Any remaining doubt can be removed bydeuteration of the methanol OH group. An analogoustransition is observed at 2608 cm−1 (showing virtually noshift with (CD3)2CO as a binding partner), which correspondsto a dimerization shift of −110 cm−1, again almost 40% largerthan 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) andeven for the M1A1 shift from M1 (1.42), which is more sensitiveto anharmonic effects. In the matrix, there is more scatteringdue to site effects (1.36 for M1, 1.35 for M2, 1.36−1.37 forM1A1) and hence no stable ratio for the monomer−mixeddimer shift from M1 (1.12−1.20). This shows that matrix siteeffects complicate the analysis. One possible interpretation ofthe unstable isotope ratio in matrix measurements is ananharmonic resonance in one of the two isotopomers, which isintensified by matrix embedding due to nonuniform shifts of alllevels. Indeed, we will see such a resonance for the M1A2 trimerlater on.

4.2. Mixed Trimers. M1A2 leaves a distinct trace inexpansions with high acetone and low methanol content(solid line in Figure 6): a single band appears at 3462 cm−1. It isfurther red-shifted than M1A1 and shifts to 3460 cm−1 for thebetter acceptor (CD3)2CO. The enhanced red shifts caused bythe second acetone unit are remarkable and believed to be dueto further polarization in the stacked antiparallel acetone dimer.The mixed trimer is close to M3 transitions at 3469 and 3474cm−1. The same pattern is found for deuterated methanol,where the mixed trimer transition appears at 2563 cm−1 (2561cm−1 for (CD3)2CO), whereas the deuterated methanol trimerabsorbs at 2567 and 2571 cm−1.For the other mixed trimer, M2A1, one expects two OH

stretching bands that scale more strongly with methanolcontent than M1A2 and are further red-shifted than M1A1.Inspection of the methanol-rich trace (dotted) in Figure 6suggests the presence of three such bands, a single peak at 3456cm−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 wavenumberaxis to match the position of the methanol monomer and M1A1cluster bands, one can see that the doublet merges into onemore intense band (Figure 7). This spectral behavior ischaracteristic for a Fermi resonance, possibly with the COstretching overtone of acetone through the hydrogen bond,which is lost for the deuterated complex.The CO stretch fundamental of acetone is itself affected

by (weak) Fermi resonances. An early study traces theresonance to the combination band of C−C−C symmetricstretch and CO bending modes,33 but recent jet experimentssuggest that there are more energy levels involved [to bepublished]. These resonances are also sensitive to theenvironment.34 One might expect that the CO overtone isprone to similar anharmonic resonances that are sensitive to thecomplexation partner. In our measurements of pure acetoneexpansions two weak peaks at 3433 and 3467 cm−1 are visiblein the 2ν(CO) region (lower trace in Figure 8). A relativelysmall difference between the average wavenumber of the M2A1doublet and the 2ν(CO) bands of acetone (23 and 57 cm−1)is favorable for a Fermi resonance between 2ν(CO) andν(OH) modes.

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

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This is as far as robust assignment entirely based on isotopeeffects and a comparison of cluster band intensities reaches.The resulting ν(OH) band positions of the heterodimer andthe heterotrimers are collected in Table 2. The close agreementand systematic trend of residual deviations from the methanol-scaled 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 themethanol trimer and tetramer OH stretching bands (Figure 8and also corresponding spectra of deuterated methanol inFigure S4 in the Supporting Information). The spectra in thefigures are ordered such that the methanol content increasesfrom bottom to top, whereas the acetone content decreases.The limiting spectra correspond to pure methanol and pureacetone expansions, respectively. It is likely that many of the

new peaks are due to mixed tetramers, but larger clusterscannot be ruled out, in particular for high acetone content.Although several of the mixed peaks show a systematicevolution as a function of methanol and acetone concentration,first rising and then leveling off or dropping from top tobottom, a firm assignment without size-selective measurementsis difficult due to a number of reasons. The probability of bandoverlap among methanol oligomers and mixed clusters issubstantial, the competition from pure methanol clusters isstrong, the expected cluster isomerism is large, the difference inconcentration scaling between neighboring cluster composi-tions is not as large as for dimers and trimers, and the regioncontains acetone modes such as the complex Fermi resonancepattern related to the overtone of the CO stretching modenear 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 ofacetone-rich mixed clusters and complicate the assignment. Inparticular the two bands marked with a dagger (Figure 8) looklike blue-shifted M1An counterparts of the corresponding pureacetone CO overtone modes in the lowest trace. The blueshift may even reflect the limited miscibility of the twocompounds.The experimental observations allow us to determine the

tetramer stoichiometry, but the success of the uniform linearscaling of predicted harmonic wavenumbers for mixed dimersand trimers in the preceding sections (Table 2) encourages aquantum chemistry-driven tentative assignment. For thispurpose, we use the scaled harmonic OH and OD wave-numbers listed in the Supporting Information. In Table 3, theresulting list of best predictions based on pure methanololigomers (scaled) is mapped on the observed additional peaksin the spectra, also considering relative intensities in thepresence of multiple bands.Five things may be noted. Although the difference of the

M2A2 and M2A2′ binding energies is vanishingly small, thepredicted band positions differ noticeably (Table S1 in theSupporting Information). Significantly better agreementbetween predicted and observed wavenumbers for the M2A2structure rules out M2A2′ as a probable candidate for theassignment. Strongly shifted bands corresponding to concertedOH (OD) stretching at 3302 cm−1 (2450 cm−1) seem weak orbroad, which may be related to their partial proton-transfercharacter. Raman spectroscopy would be better suited to

Figure 7. Influence of methanol deuteration on the coexpansionspectra of methanol and acetone. The spectrum of the deuteratedcompound (dotted line) was superimposed on the nondeuterated one(solid line) so that the methanol monomer and heterodimer bandsmatch.

Figure 8. Overview of the spectral changes in the ν(OH) region uponvariation of methanol and acetone concentrations. The assignment ofthe observed tetrameric peaks is tentative. The weak bands markedwith an asterisk do not find straightforward assignment. The daggersmark two features that may have contributions from the 2ν(CO)vibrational mode. See text for details.

Table 2. Experimental νOH/νOD Wavenumbers for the MixedDimer and Trimers of Acetone with Methanol and TheirDeuterated Analoguesa

νOH/cm−1 νOD/cm

−1

CH3OH/(CH3)2CO

CH3OH/(CD3)2CO

CH3OD/(CH3)2CO

CH3OD/(CD3)2CO

M1A1 3530(−9) 3529 2608(−17) 2608M2A1 3494(−0)b c 2580(−13) 2579

3485(−9)b

3456(+5) c 2558 (−7) 2558M1A2 3462(+5) 3460 2563(−6) 2561

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

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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 concentrationevolution pointing to band overlap. Some stronger bands thatfind no assignment (3438, 3475/78 cm−1) fall in the region ofpure acetone bands. Other bands that find no assignment(3351, 3373, 3384 cm−1) continue to gain intensity withincreasing acetone concentration, indicating a size larger thantetramers. The resulting assignments of M3A1 and M2A2 arethus plausible, but tentative (Table 3), whereas an assignmentof M1A3 appears difficult due to the multitude of structures andoverlap with the CO overtone. It would be desirable to havemore accurate spectral predictions35 to verify the tetramerassignment. In summary, the combination of theory and firmexperimental mixed trimer assignments allows for sometentative proposals for mixed tetramer bands building on twoor three methanol units, consistent with qualitative concen-tration scaling.

5. CONCLUDING REMARKSThe intermolecular interaction of two important molecularfunctionalities, hydroxyl and carbonyl groups, is ambiguous.Intrinsically, their combination in an OH···OC hydrogenbond is preferred over self-interactions, but the polarization ofalcoholic OH groups in cooperative hydrogen bond chains isstopped by CO groups, therefore building up a demixingtendency between alcohols and ketones with increasing systemsize. For methanol and acetone, we have presented combinedquantitative experimental and theoretical evidence for thisphenomenon as a function of the number of interactingmolecules.Theoretically, we have shown that up to a cluster size of

three, it is energetically more favorable to have mixedcomplexes, whereas starting at a cluster size of four apartitioning into pure clusters is lower in energy. To backthis quantum chemical prediction, we have for the first timestudied vacuum-isolated mixed methanol−acetone clusters byinfrared spectroscopy and assigned them on the basis of theirOH stretching signature, confirming the predicted harmonicfundamental wavenumbers in combination with a smoothscaling based only on pure methanol clusters. This underscoresa fully self-consistent description of energetical and spectro-scopic aspects of this important model system. We emphasizethat the jet expansions themselves do not show clusterdemixing tendencies due to kinetic barriers for significantcomposition exchange in cold cluster collisions. The exper-

imental spectra only confirm the essential correctness of thecomputational model, which in turn shows demixing to beenergetically favorable.On the way toward this goal, a number of thermodynamic

and spectroscopic findings for methanol/acetone are worthbeing mentioned:

I. A previously overlooked error in the tabulated solid−liquid phase diagram for this model system has beenuncovered.

II. A unique structure for the mixed dimer has beenidentified.

III. A Fermi resonance, presumably transmitted through theOH···OC hydrogen bond, was found for the trimericcomplex built from two methanol and one acetone unit.

IV. Some cooperativity was also evidenced by attaching asecond acetone unit to the mixed dimer, possibly due tothe polarization of the CO group in the stackedacetone dimer.

V. Some mixed tetramers, although thermodynamicallydisfavored relative to pure clusters, were tentativelyassigned despite the lack of rigorous size selection in thelinear FTIR spectroscopy.

Whenever more than one methanol is present in the clusters,OH···OH-based cooperativity is preferred. The switch fromisolated OH···OC to cooperative OH···OH···OH patterns isalso responsible for one of the most unusual supramolecularrecognition processes found for neutral molecules in the gasphase: depending on the relative chirality of the building blocks,the tetramer of methyl lactate occurs in one or the otherform.36 The quantitative success of the present work impliesthat B3LYP-D3 should be a suitable method to study thischirality recognition phenomenon in more detail.Future work will have to address the effect of alkyl chain

length and thus London dispersion forces on the demixingtendency of ketones and alcohols and may also want to have alook at the even more elementary water−acetone system. Asthe mixed dimer with methanol can be produced in highabundance in a supersonic jet expansion, it would be interestingto study its OH stretching overtone29 and to reveal the changeof OH bond anharmonicity and transition dipole moment uponacetone binding. Finally, it is worth addressing the rather poorlystudied acetone dimer by vibrational spectroscopy. The absenceof a dipole moment and of an interaction-sensitive IR-chromophore has turned this system into one of the leaststudied basic supramolecular complexes, but a combination ofIR and Raman spectroscopy promises to unravel some of itsdynamical details.

■ ASSOCIATED CONTENT*S Supporting InformationCalculated wavenumbers of the ν(OH) mode and bindingenergies of homo- and heteroclusters, quality of the scalingfunction (Figure S1), change of binding preference based onbinding 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 athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*M. A. Suhm. Electronic mail: [email protected]. Phone: +49551 39-33112. Fax: +49 551 39-33117.

Table 3. Tentative Assignment of the Spectral Features inthe ν(OH)/ν(OD) Region of the Heterotetramers ofCH3OH/CH3OD with (CH3)2CO

a

νOH/νOD

tentative assignment scaled experiment

M3A1 3407/2529 3409/25273366/2501 3370/25023285/2445 3302/2451

M2A2 3428/2538 3423/25333366/2496 3370/2502

M2A2′ 3394/25143333/2473

M1A3-s(′) 3397/2517 3409/2527

3439/2548 2ν(CO)/CH3 def + ν(CC)aExperimental and calculated wavenumbers are in cm−1.

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Anja Poblotzki for complementary studies on longchain alcohol/ketone mixtures. Funding is through Grant Su121/4, DFG (Deutsche Forschungsgemeinschaft).

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