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Infrared Spectroscopy of Methanol and Methanol/Water Clusters in Helium Nanodroplets: The OH Stretching Region Media I. Sulaiman, Shengfu Yang, and Andrew Michael Ellis J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b11170 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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The Journal of Physical Chemistry

Infrared Spectroscopy of Methanol and Methanol/Water Clusters in Helium Nanodroplets: the OH Stretching Region

Media I. Sulaiman, Shengfu Yang* and Andrew M. Ellis*

Department of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, UK

Email: [email protected]; [email protected]

Manuscript submitted to Journal of Physical Chemistry A

1 ACS Paragon Plus Environment


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Abstract Infrared (IR) spectra of methanol clusters in helium nanodroplets are reported in the OH stretching region for the first time. A simple series of intense bands are seen which almost perfectly match previous gas phase studies of these clusters and which are consistent with cyclic structures for the trimer and larger clusters. This finding differs from an earlier report of (CH3OH)n clusters in helium nanodroplets, which focused on the CO stretching region and concluded that while the trimer was cyclic, the tetramer and pentamer adopted branched structures based on a cyclic trimer core. We also present preliminary data for small (CH3OH)n(H2O) clusters, and in particular we report the first IR spectra for (CH3OH)2(H2O) and (CH3OH)3(H2O). Supporting ab initio calculations suggest that, like the pure methanol clusters, cyclic structures are adopted by these mixed solvent clusters in helium droplets.


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Introduction The molecules in liquid or solid methanol are held together by a mixture of hydrogen bonding and van der Waals interactions. A detailed exploration of the nature of these intermolecular forces and how they evolve as molecules are progressively added is possible through the study of clusters of methanol. There have been many spectroscopic studies of methanol clusters, particularly in the gas phase1-14 but also in inert matrices at low temperature.15-19 The experimental work, supported by theoretical predictions,20-26 shows that the smallest cluster, the dimer, exists as an open structure with a single hydrogen bond holding the two molecules together: one methanol molecule acts as a proton donor and the other as an acceptor. For the trimer and larger clusters the adoption of cyclic structures maximizes the number of hydrogen bonds and is energetically preferred over chain-like structures. IR and Raman spectra of these clusters in supersonically-cooled gas expansions and molecular beams have found clear signals for the cyclic trimer and tetramer in the OH stretching region, while the signals for the pentamer and larger clusters seem to merge into a single broad OH stretching band.2,3,5-14 No evidence has been found for non-cyclic methanol clusters larger than the dimer in the gas phase. Preferential production of cyclic isomers for the trimer and larger clusters has also been noted in experiments using solid nitrogen and argon matrices.15,19 Liquid helium nanodroplets provide an alternative matrix environment in which to study molecules and their clusters. Characteristics of helium nanodroplets as a matrix in which to embed molecules include a low steady-state temperature (0.4 K), rapid cooling of dopants by evaporative loss of helium atoms from the surface of the superfluid helium, and very small perturbations to the vibrational spectra of the dopants by the surrounding helium.27-29 In essence, the helium droplets provide a cold, gas-like, environment. Methanol and its clusters were among the earliest of molecules studied in helium nanodroplets. IR



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spectra of small methanol clusters were recorded by Behrens et al. in the CO stretching region.30 The spectra were recorded using a depletion method, in which IR absorption was registered by a subsequent decline in ion signal produced by electron ionization of the helium droplets (see also below). This approach confers an element of mass selectivity and can be used to selectively remove contributions of neutral clusters to an IR spectrum smaller than a given size. A strong band was identified which is in good agreement with the absorption band known for the cyclic trimer, (CH3OH)3, in the gas phase.2 However, the bands assigned to the tetramer and pentamer were spread over a wider range than predicted in density functional theory (DFT) calculations for the cyclic isomers. It was concluded that the methanol trimer adopts a cyclic structure in superfluid helium, just like the gas phase, whereas the tetramer and pentamer have structures based upon a cyclic trimer core with one external methanol molecule (tetramer) or a dangling chain of two methanol molecules (pentamer). The possibility of different structures being found in gas phase and helium nanodroplet experiments should come as no surprise. The rapid cooling in helium droplets makes it possible to trap clusters in potential energy wells lying above the global potential energy minimum. A classic example of this is the case of water clusters in helium nanodroplets. For water the cyclic trimer, tetramer and pentamer represent the lowest energy structures.31 However, the situation changes for the hexamer. Here more three-dimensional structures, the prism and cage structures, have a lower energy than the cyclic isomer.31-33 Nevertheless, Nauta and Miller were able to show that the cyclic version of the water hexamer is the primary product in helium nanodroplets.34 The work by Behrens et al. seems to suggest an almost inverse situation for methanol clusters where, instead of forming fully cyclic isomers, branched structures are adopted in liquid helium which are metastable.30 Since liquid helium nanodroplets provide a useful environment in which to study hydrogenbonded clusters, and can sometimes allow access to parts of the intermolecular potential


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energy surface that are not sampled in gas phase experiments, it is important to understand how this environment can affect the structures of molecular clusters. Although IR spectra of methanol clusters in liquid helium nanodroplets have been reported in the CO stretching region,30 the only previous study in the OH stretching region focused on the methanol monomer.35 Since the OH stretching region is highly sensitive to the nature of the hydrogen bonding, and therefore the structures adopted, we present here IR spectra of methanol clusters in this spectral region. Contrary to the findings by Behrens et al., our data are entirely consistent with gas phase studies and show cyclic structures for the tetramer and pentamer. Furthermore, we present preliminary IR data for clusters of methanol with a single added water molecule, and in particular make the first observation of IR bands of (CH3OH)2H2O and (CH3OH)3H2O. These mixed clusters are also found to adopt cyclic structures.

Experimental Full details of the apparatus can be found in previous publications.36,37 In brief, helium nanodroplets with a mean size close to 5000 helium atoms were formed by expanding precooled gaseous helium into a vacuum through a 5 µm pinhole. The droplets then passed through a pick-up cell, where dopants were added. Experiments were carried out both on pure methanol clusters and for mixed methanol/water clusters. For the latter, water and methanol were pre-mixed in the liquid state and the resulting vapor was then added to the pick-up cell. The flow of vapor into the pick-up cell was controlled by a needle valve. Infrared spectra were recorded via a depletion method which exploits the evaporative loss of helium atoms as the absorbed energy becomes dispersed into the droplet.27-29 This effect causes a decrease in the droplet size and therefore its geometric cross section, which can be registered by a drop in the signal identified via electron ionization mass spectrometry



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downstream of the IR excitation zone. In the current experiments mass-selective ion detection was used in order to provide a degree of mass selectivity: this aids spectral assignment, as mentioned earlier and also detailed later.

Computational details Ab initio calculations were performed in support of the experiments. The calculations used MP2 methodology combined with aug-cc-pVDZ basis sets centred on the various atoms. The only exception was for the (3)+2 structure of the methanol pentamer (see later), which would only converge to a stable structure using the DFT/B3LYP method (also with an aug-cc-pVDZ basis set). The principal aim here was to predict the IR spectra for small methanol clusters and mixed methanol/water clusters in order to confirm or eliminate specific IR band assignments. Plausible equilibrium structures were first calculated using geometry optimization and then IR absorption spectra were calculated using the double harmonic approximation, which means that the vibrational frequencies need to be scaled for any meaningful comparison with experiment. Since our focus later is on the stretching bands from OH groups, and especially those involved in hydrogen bonds (bonded OH stretches), we have used a comparison of the well-known bonded OH band of methanol dimer with the calculated equivalent to determine a suitable scaling factor (0.972). This scaling factor was then applied to the calculated harmonic vibrational frequencies of all other clusters, including the mixed methanol-water clusters. The corresponding scaling factor for the DFT calculation on the (3)+2 structure of the methanol pentamer was 0.976. All calculations were performed using Gaussian 0938 on the Slater cluster at the UK National Service for Computational Chemistry Software (NSCCS). Images of the equilibrium structures of all of the clusters calculated in this work are provided in the Supporting Information.


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Results and Discussion

1.

Methanol clusters

Figure 1 shows several IR spectra recorded for pure methanol clusters in the OH stretching region. These spectra differ in the mass channel used for detection, the size of the helium droplets used, and the quantity of added methanol. The focus in this paper is on the OH stretching region above 3000 cm-1, although the spectra in Figure 1(a) and 1(b) extend to lower wavenumbers and so also show CH stretching structure. The lowest mass channel used for detection was m/z 33 and corresponds to detection of (CH3OH)H+ ions. The IR spectrum recorded in this mass channel is shown in trace (a) and consists of three strong OH stretching bands at 3295, 3477 and 3571 cm-1, along with a much weaker peak at 3686 cm-1. The assignment of these bands is straightforward based on a comparison with previous IR work and is summarized in Figure 1 and in Table 1. The two highest frequency and sharpest peaks derive from methanol dimer. This cluster is held together by a single hydrogen bond and so there is a dangling OH, which is the source of the weak band at 3686 cm-1. The other OH group is involved in the hydrogen bond, which weakens the OH bond and induces a significant red-shift of the corresponding IR band. Hydrogen bond formation also strongly increases the transition dipole moment. The two other, broader, peaks seen in Figure 1 near 3480 and 3300 cm-1 match the expected positions for the cyclic trimer and tetramer, respectively, of methanol (see Table 1). In these cyclic structures all of the OH groups are involved in hydrogen bonds and so there is no free OH stretch at higher frequency.



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Fig. 1. Infrared spectra of methanol clusters recorded by detection of (a) (CH3OH)H+ at m/z 33, (b) (CH3OH)3H+ at m/z 97 and (c) (CH3OH)3H+ at m/z 97 but at a higher methanol pick-up pressure than in (b).

Behrens et al. have previously reported that the fragmentation of methanol cluster ions in helium nanodroplets following electron ionization is dominated by the following processes,30

(CH3OH)n+

→

(CH3OH)n-1H+ + CH3O

(1a)

→

(CH3OH)n-2H+ + CH3O + CH3OH

(1b)


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According to this scheme, if (CH3OH)H+ is detected any depletion induced by IR absorption should derive from either neutral (CH3OH)2 or (CH3OH)3. In Figure 1(a) we see bands from both of these clusters, but also a substantial contribution from (CH3OH)4. There is also some weak absorption at the expected position for the pentamer (see below), and so the fragmentation process is more extensive than implied in the scheme above. Nevertheless, a marked change is seen when detection switches to (CH3OH)3H+ at m/z 97. In this IR spectrum, shown in blue in Figure 1(b), the IR bands attributed to methanol dimer and trimer are now absent, as would be expected since the smallest neutral cluster that can contribute to the ion signal at m/z 97 is the tetramer. At this higher mass the tetramer absorption is clearly seen along with another equally intense band at a lower frequency, which can be assigned to the pentamer. At higher methanol pick-up pressures, as shown in Figure 1(c), the peak assigned to the pentamer is significantly broadened. This suggests underlying and unresolved contributions from larger clusters, such as the hexamer.

Positions (cm-1) and assignments of OH stretching bands of methanol clusters.

Table 1.

This work

CRDS7

FTIR8

3686

3683.8

3571

3574.4

3575

(CH3OH)2

3477

3473.2

3474

(CH3OH)3

3295

3278.5a)

3293

(CH3OH)4

(CH3OH)2

3239 a)

Assigned carrier

(CH3OH)5

Thought to be a typographical error in the original reference.7

There can be no doubt that we are seeing the cyclic versions of the methanol trimer, tetramer and pentamer. The band positions are in excellent agreement with previous work in the gas phase, where the assignment to cyclic clusters has been firmly established.7,8



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However, in their helium droplet experiment Behrens et al. interpreted their IR data in the CO stretching region rather differently.30 For the trimer the conclusion was the same, namely a cyclic structure is formed. However, absorption features assigned to the tetramer were attributed to a (3)+1 structure, which consists of a cyclic trimer core with one additional methanol molecule hydrogen bonded to the ring. Likewise, the pentamer signal was assigned to a (3)+2 structure, where a chain of two methanol molecules is attached to the trimer ring. We can eliminate these isomers under our experimental conditions by using ab initio calculations, whose findings are shown in Figure 2. The predicted IR spectra for the fully cyclic clusters are simple and are in good agreement with the observed IR spectra. On the other hand, the (3)+1 and (3)+2 branched structures are predicted to have intense bands at positions where no absorption is seen in the IR spectra. The predicted spectra of the branched structures are more complicated than for the fully cyclic structures, as expected since there are several quite distinct OH environments which will deliver substantially different stretching frequencies. This is not consistent with our spectroscopic observations and so we conclude that homodromic cyclic methanol clusters dominate in our experiments.


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Fig. 2. Comparison of the infrared spectrum of methanol clusters in the OH stretching region recorded by detection of (CH3OH)H+ at m/z 33 (upper trace) with ab initio predictions (lower trace) for the fully cyclic trimer and tetramer along with the (3)+1 tetramer and (3)+2 pentamer. The predicted band positions for fully cyclic trimer and tetramer are in excellent agreement with experimental band positions, unlike the predictions for the (3)+1 and (3)+2 isomers.

The reason for the different conclusions reached in this study versus that by Behrens et al.30 is unclear. We broadly agree with their theoretical predictions of the IR absorption behaviour in the CO stretching region and their conclusions about the cluster structures on that basis alone do not seem unreasonable. We note that Behrens et al.30 claimed to be using helium droplets with a mean size of 2700 helium atoms, which is somewhat smaller than the mean droplet size used here (roughly 5000 helium atoms). Nevertheless, we would be surprised if this led to different cluster structures. Another possibility is the neglect of 11 ACS Paragon Plus Environment


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anharmonicity in the calculated CO stretching structure by Behrens et al. Their harmonic oscillator calculations predict a number of CO stretching bands within a relatively narrow 10 cm-1 range, and our own calculations confirm this. However, a slightly wider spread of these features over 20 cm-1 range would be sufficient to account for the band structure seen by Behrens et al. and anharmonic coupling might deliver this. This is clearly a matter worthy of further investigation.

2.

Methanol + water

We have also performed some limited experiments with a mixture of methanol and water. The aim here, rather than being an exhaustive investigation of mixed methanol/water clusters, was to add a small quantity of water to form methanol clusters with one water molecule (primarily) and to see if the resulting mixed clusters remain cyclic. To do, this, an 80:20 (by volume) liquid methanol/water mixture maintained at 2 °C was used for doping the pick-up cell. Given that methanol has a much higher vapor pressure than water, this gave a methanol/water ratio of ∼ 30:1 in the vapor and therefore ensured that the pick-up of two or more water molecules had negligible probability. Figure 3 shows IR depletion spectra recorded at two different masses, m/z 33 and m/z 51. The spectrum at m/z 33 contains contributions from several different pure methanol clusters, as illustrated in the figure, with the dimer and trimer being particularly prominent. However, it also contains two additional peaks marked by arrows, a stronger one at 3525 cm-1 and a weaker one at 3380 cm-1. Both of these peaks become more intense when recorded at m/z 51. Since the ion detected at this mass is (CH3OH)(H2O)H+, pure methanol clusters are eliminated from the m/z 51 spectrum, as seen by the disappearance of the methanol dimer peak. The remaining signals must come from one or more neutral clusters containing at least one water molecule. Given the deliberately low partial pressure of added water, we expect


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that the dominant mixed clusters will contain only one water molecule. Note also that the signal/noise ratio is compromised for the IR spectrum recorded at m/z 51 because only a small subset of the doped helium droplets will contain a single water molecule.

Fig. 3. Infrared spectra obtained for a methanol/water vapour mixture (∼30:1) with detection at m/z 33 ((CH3OH)H+) and m/z 51 ((CH3OH)(H2O)H+). The arrows in the m/z 33 spectrum refer to peaks discussed in the main text. In the lowest trace theoretical predictions of the IR band structure for cyclic (CH3OH)2(H2O) (red lines) and (CH3OH)3(H2O) (blue lines) are shown.

We can assign the absorption bands in the m/z 51 spectrum in Figure 3 with the aid of ab initio calculations and we find that all of the observed features can be explained rather well assuming cyclic structures. The two most intense bands, at ca. 3300 and 3480 cm-1, coincide with bands seen in the pure methanol spectrum for the cyclic tetramer and trimer, respectively. According to the ab initio predictions we also expect strong bands at roughly 13 ACS Paragon Plus Environment


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these positions for cyclic (CH3OH)3H2O and (CH3OH)2H2O clusters. However, theory also predicts additional bands near 3400 cm-1 (for both (CH3OH)2H2O and (CH3OH)3H2O) and 3530 cm-1 ((CH3OH)2H2O only). These predictions fit with the observation of new bands in the IR spectrum at 3380 and 3525 cm-1. Furthermore, the relative band intensities predicted by the ab initio calculations are in good agreement with the experimental relative band intensities. We therefore conclude that we are observing IR bands of the cyclic mixed methanol/water clusters (CH3OH)2H2O and (CH3OH)3H2O. Although mixed methanol/water clusters have been studied previously, most of the focus has been on the heterodimer, (CH3OH)H2O,39-47 including its observation in helium nanodroplets during a study of the OH(CH3OH) radical-molecule complex.48 In contrast, experimental and theoretical studies of larger (CH3OH)nH2O clusters are relatively few.43,44,49-53 The only spectroscopic work was a combined IR and Raman study by Nedić et al. in which a single Raman band was assigned to (CH3OH)2H2O in the OH stretching region and tentative assignments were made of Raman bands to (CH3OH)3H2O and (CH3OH)4H2O.51 However, no IR bands were assigned to these clusters and so the present study represents, as far as we are aware, the first spectroscopic observation of IR bands of (CH3OH)2H2O and (CH3OH)3H2O. The Raman band reported by Nedić et al. for (CH3OH)2H2O was observed at 3425 cm-1. This band almost certainly corresponds to the lowest frequency OH stretching band in the simulation for (CH3OH)2H2O in Figure 3 (the lowest frequency line in red), which the ab initio calculations predict to be at 3410 cm-1. The vibration responsible is essentially a symmetrical ring breathing vibration and is the weakest of the four IR OH stretching bands but is the most intense (by a factor of more than three) of the Raman active bands of (CH3OH)2H2O, according to the ab initio calculations. This presumably explains why Nedić et al. were able to see the 3425 cm-1 band in their Raman spectrum but we see nothing above the noise level at this position in our IR spectrum. For the


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other bands there is good agreement between theory and experiment, which shows that in liquid helium droplets the clusters resulting from a homogeneous mixture of methanol and water vapor derive from self-assembly into fully cyclic clusters. Calculations suggest that these

cyclic

structures

are

the

lowest

energy

isomers

of

(CH3OH)2H2O

and

(CH3OH)3H2O.43,44,49 Finally, we note that there has been much interest over many decades in trying to understand why liquid methanol-water mixtures show a negative entropy of mixing. The prevailing view for several decades was the so-called “iceberg model”, in which water molecules were assumed to cluster together on the hydrophobic methyl groups.54 However, more recent experimental and theoretical work tends to favour an explanation in terms of incomplete mixing of the water and the methanol,55-59 although the debate continues right up to the present.60 The study in this paper cannot shed any light on this issue, except to say that there is no evidence of segregation of the methanol and water in the very limited experiments performed here, i.e. no structure based on a water molecule dangling from a cyclic methanol ring. It appears that the water molecule simply slots into the cyclic structure adopted by pure methanol clusters. However, it would be interesting to determine if segregation occurs in mixed methanol/water clusters with more equal quantities of both solvents, such as (CH3OH)3(H2O)3. Several possible structures are conceivable for this particular mixed cluster and experiments on such clusters are viable with the approach described in this paper.

Conclusions Infrared spectra in the OH stretching region have been recorded for pure methanol clusters trapped in liquid helium nanodroplets. The data are in excellent agreement with previous gas phase studies and show that the trimer, tetramer and pentamer adopt fully cyclic structures in helium nanodroplets. This conclusion is in marked contrast to an earlier study of these



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clusters in helium nanodroplets, which focused on the CO stretching region and came to the conclusion that while the trimer is cyclic, the tetramer and pentamer adopt structures based upon a cyclic trimer, in which additional methanol molecules dangle from this ring. We have also reported the first IR spectra of (CH3OH)2H2O and (CH3OH)3H2O. In this preliminary work there is good agreement between the observed spectra and the predicted spectra from ab initio calculations when fully cyclic structures are assumed. This work could be readily extended to other combinations of methanol and water and therefore to more fully explore the structural landscape and hydrogen bonding in these mixed solvent clusters.

Supporting information Images of the equilibrium structures of the various clusters discussed in this paper, which were derived from ab initio calculations, are contained within the Supporting Information along with the Cartesian coordinates of the atoms.

Acknowledgements The authors wish to thank the UK Engineering and Physical Sciences Research Council (EPSRC, grant EP/J021342/1) and the Leverhulme Trust (grant numbers RPG-2012-552 and RPG-2012-740) for grants in support of this work. The authors are also grateful for the scholarship provided to MIS by the Human Capacity Development Program from the Kurdistan Government.

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ToC Graphic



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Figure 1 272x208mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2 289x202mm (300 x 300 DPI)



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Figure 3 289x202mm (300 x 300 DPI)


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Water Clusters in

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