Alcohol clusters

The importance of surface characterization in molecular architecture chemistry and engineering is obvious. Solid surfaces are becoming essential building blocks for constructing molecular architectures, as demonstrated in self-assembled monolayer formation [6] and alternate layer-by-layer adsorption [7]. Surface-induced structuring of liqnids is also well-known [8,9], which has implications for micro- and nano-technologies (i.e., liqnid crystal displays and micromachines). The virtue of the force measurement has been demonstrated, for example, in our report on novel molecular architectures (alcohol clusters) at solid-liquid interfaces [10]. 

III. ALCOHOL CLUSTER FORMATION ON SILICA SURFACES IN CYCLOHEXANE... 

The contact of adsorbed ethanol layers should bring about the long-range attraction observed between glass surfaces in ethanol-cyclohexane mixtures. The attraction starts to decrease at -0.5 mol% ethanol, where ethanol starts to form clusters in the bulk phase. It is conceivable that the cluster formation in the bulk influences the structure of the adsorbed alcohol cluster layer, thus modulating the attraction. We think that the decrease in the attraction is due to the exchange of alcohol molecules between the surface and the bulk clusters. 

Figure 15. (a) Mass spectrum of alcohol clusters about Li+. Data taken at T = 150 K and (b) same as (a) but with an expanded intensity scale An = Li+(CH30H) Bn = Li+(CH30H) (H20). Taken with permission from ref. 106. 

After raising a selection of topical issues in this field and briefly introducing some spectroscopic and numerical techniques to probe the hydrogen bond dynamics, recent results for alcohol clusters are presented in order of increasing complexity. They are followed by some general conclusions and an outlook on future research goals. 

Among the wealth of issues relevant to hydrogen bonding in alcohol clusters, this review will focus on aspects related to hydrogen bond patterns and on the dynamical implications over a wide range of time scales. Some key questions connected to these aspects will be formulated. 

Figure 3. Librational OH modes in hydrogen bonded alcohol clusters may be correlated with overall rotation (bottom left) and torsion (top left) of the monomer (illustrated for methanol), but methyl rotation is actually decoupled from OH torsion by hydrogen bonding. Note that the wavenumbers of monomer rotation (fa 4 cm-1) and torsion (fa 280 cm-1) are much lower than that of the cluster libration (fa 600cm ) [93].

Anion solvation in alcohol clusters has been studied extensively (see Refs. 135 and 136 and references cited therein). Among the anions that can be solvated by alcohols, the free electron is certainly the most exotic one. It can be attached to neutral alcohol clusters [137], or a sodium atom picked up by the cluster may dissociate into a sodium cation and a more or less solvated electron [48]. Solvation of the electron by alcohols may help in understanding the classical solvent ammonia and the more related and reactive solvent water [138], By studying molecules with amine and alcohol functionalities [139] one may hope to unravel the essential differences between O- and N-solvents. One should note that dissociative electron attachment processes become more facile with an increasing number of O—H groups in the molecule [140],... 

As became obvious in the preceding section, progress in understanding alcohol clusters very much depends on the ability to generate these clusters in supersonic jet expansions or in other variants of low temperature isolation and to detect their dynamics via spectroscopic methods. Therefore, some important spectroscopic tools employed in this field shall be summarized, with focus on the alcoholic systems that have been addressed by them. Solution [22, 26, 141, 142] and supercritical [24 26] state techniques will not be covered systematically. 

Microwave spectroscopy is probably the ultimate tool to study small alcohol clusters in vacuum isolation. With the help of isotope substitution and auxiliary quantum chemical calculations, it provides structural insights and quantitative bond parameters for alcohol clusters [117, 143], The methyl rotors that are omnipresent in organic alcohols complicate the analysis, so that not many alcohol clusters have been studied with this technique and its higher-frequency variants. The studied systems include methanol dimer [143], ethanol dimer [91], butan-2-ol dimer [117], and mixed dimers such as propylene oxide with ethanol [144]. The study of alcohol monomers with intramolecular hydrogen-bond-like interactions [102, 110, 129, 145 147] must be mentioned in this context. In a broader sense, this also applies to isolated ra-alkanols, where a weak Cy H O hydrogen bond stabilizes certain conformations [69,102]. Microwave techniques can also be used to unravel the information contained in the IR spectrum of clusters with high sensitivity [148], Furthermore, high-resolution UV spectroscopy can provide accurate structural information in suitable systems [149, 150] and thus complement microwave spectroscopy. 

Infrared absorption spectroscopy is also a powerful tool for matrix isolation studies, which have been carried out extensively for alcohol clusters [34, 88, 103]. Recently, the gap between vacuum and matrix isolation techniques for direct absorption spectroscopy has been closed by the study of nano matrices that is, Ar-coated clusters of alcohols [80]. Furthermore, alcohol clusters can be isolated in liquid He nanodroplets, where metastable conformations may be trapped [160]. 

In condensed phases, the noncoincidence effect between IR and Raman spectra provides insights into the intermolecular coupling [170, 171]. The combination of IR and Raman spectroscopy is also useful in the study of alcohol clusters in the supercritical state [25]. 

Once the alcohol or at least the cluster contains a soft ionization or fluorescence chromophore, a wide range of experimental tools opens up. Experimental methods for hydrogen-bonded aromatic clusters have been reviewed before [3, 19, 175]. Fluorescence can sometimes behave erratically with cluster size [176], and short lifetimes may require ultrafast detection techniques [177]. However, the techniques are very powerful and versatile in the study of alcohol clusters. Aromatic homologs of ethanol and propanol have been studied in this way [35, 120, 121, 178, 179]. By comparison to the corresponding nonaromatic systems [69], the O—H - n interaction can be unraveled and contrasted to that of O—H F contacts [30]. Attachment of nonfunctional aromatic molecules to nonaromatic alcohols and their clusters can induce characteristic switches in hydrogen bond topology [180], like aromatic side chains [36]. Nevertheless, it is a powerful tool for the size-selected study of alcohol clusters. 

Computational methods that assist the characterization of alcohol cluster dynamics are essential, numerous, and diverse Here, we can only briefly mention some of them that turn out to be particularly useful for the analysis presented below. Where available, we refer to authoritative reviews on these subjects. 

If affordable, there is a range of very accurate coupled-cluster and symmetry-adapted perturbation theories available which can approach spectroscopic accuracy [57, 200, 201]. However, these are only applicable to the smallest alcohol cluster systems using currently available computational resources. Near-linear scaling algorithms [192] and explicit correlation methods [57] promise to extend the applicability range considerably. Furthermore, benchmark results for small systems can guide both experimentalists and theoreticians in the characterization of larger molecular assemblies. 

Aromatic alcohol clusters have been well-studied, also for methodical reasons. The UV chromophore can be exploited for sensitive detection of the IR spectrum [35, 36, 120, 179]. Time-domain experiments become possible [21], which show that the initial energy flow out of the O—H stretching mode occurs primarily via C—H stretching and bending doorway states. Like in the case of carboxylic acid dimers [245], the role of the hydrogen bond is to shift the O—H stretching mode closer to these doorway states and thus to accelerate the initial energy flow. 

The hydrogen bonds in aliphatic alcohol clusters can be modified in a systematic, yet subtle, way by replacing hydrogen atoms of the alkyl group by fluorine atoms [248, 249]. This leads to only modest changes in spatial extension, but it introduces polarity into the hydrophobic alkyl chains. Despite their polarity, the fluorine atoms are not considered to be attractive hydrogen bond acceptors [250]. Huorinated alkanes have quite remarkable properties that can be related to this combination of polarity and weak hydrogen bond propensity. Alcohols with... 

Although most of the reported gas-phase experiments do not investigate the temporal evolution of alcohol clusters explicitly, the frequency-domain spectral information can nevertheless be translated into the time domain, making use of some elementary and robust relationships between spectral and dynamical features [289]. According to this, the 10-fs period of the hydrogen-bonded O—H oscillator is modulated and damped by a series of other phenomena. Energy flow into doorway states is certainly slower than for aliphatic C—H bonds [290] but on a time scale of a few picoseconds, energy will nevertheless have... 

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