Methanol dimer

One after the other, examine methanol dimer and acetic acid dimer. Do the hydrogen-bond lengths in these systems differ significantly from the optimum distance in water dimer Are the hydrogen-bond angles in these compounds significantly different from those in water dimer Rationalize your results. 

Jursic, B. S., 1999, Study of the Water-Methanol Dimer With Gaussian and Complete Basis Set Ah Initio, and Density Functional Theory Methods , J. Mol. Struct. (Theochem), 466, 203. 

Mass losses of more than one monomer unit appear as unresolved shoulders. The loss of up to five methanol monomers from the protonated octamer is observed. Another peak was found to correspond to loss of water from the protonated methanol dimer ion via the dehydration reaction,... 

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. 

The preparation of single isomers for methanol dimer, trimer, and presumably tetramer [16] in a supersonic jet expansion contrasts the structural diversity that can be prepared and manipulated in cryogenic matrices [34]. It underscores the ability of supersonic jet expansions to funnel all intermolecular isomers down to the global minimum, if there are no major barriers to overcome on the way. 

Infrared, Raman, microwave, and double resonance techniques turn out to offer nicely complementary tools, which usually can and have to be complemented by quantum chemical calculations. In both experiment and theory, progress over the last 10 years has been enormous. The relationship between theory and experiment is symbiotic, as the elementary systems represent benchmarks for rigorous quantum treatments of clear-cut observables. Even the simplest cases such as methanol dimer still present challenges, which can only be met by high-level electron correlation and nuclear motion approaches in many dimensions. On the experimental side, infrared spectroscopy is most powerful for the O—H stretching dynamics, whereas double resonance techniques offer selectivity and Raman scattering profits from other selection rules. A few challenges for accurate theoretical treatments in this field are listed in Table I. 

Figure 3 Pyridine-methanol dimer model used to evaluate the effect of the hydrogen bonding on the chemical shift principal components of aromatic nitrogens.

The assigment of the parent ion [FB(CD3OD)2] as the precursor of the fluoroanisole product has been clearly demonstrated by an infrared UV doubleresonance experiment. Here, the product ion signal is monitored as the infrared excitation is scanned. When the IR laser is in resonance with the C-O stretching of the methanol dimer within the 1-2 cluster, a decrease of the product signal is observed since the population of the parent ion decreases by the loss of one methanol molecule through the IR predissociation process (Brutschy et al. 1992). 

Jorgensen WL (1979) An intermolecular potential function for the methanol dimer from ab-initio calculations. J Chem Phys 71 5034-5038... 

Whereas experimental assessments of the frequency of the OH stretch in the donor molecule of the water dimer cover a range between 3500 and 3600 cm in the gas phase , the assignment is clearer in the methanol dimer, at 3574 cm F A recent work has optimized the geometries of the dimers of water, methanol, and silanol at the MP2 level . The vibrational frequencies include correlation by this approach, and are then corrected for BSSE and anharmonicity. The basis sets applied were DZP, as well as a triple- set, and is polarized under the rubric VTZ(2df,2p). 

The authors conclude with their contention that a correlated treatment of the OH stretch, in isolation from other motions, and corrected by a one-dimensional anharmonic approach, can produce frequency shifts within 10 cm" of experiment. Comparison of the three systems indicates the OH stretching frequency suffers somewhat of a larger red shift in the methanol dimer than in the water dimer however this difference might not be observed at the SCF level. The shifts in the silanol dimer are quite a bit larger in magnitude. 

For the adsorption of two methanol molecules per bridging hydroxy groups (2 1 loading) studies on all zeolites, SOD [29], CHA [22, 30], FER [22] agree that a pro-tonated methanol dimer is formed (Fig. 22.3). The obvious reason is the high PA of the methanol dimer that exceeds even the PA of ammonia (Table 22.1). 

Figure 22.3 Protonated methanol dimer in zeolite chabasite (CHA) as predicted by CPMD simulations [30].

Mooij W, Duijneveldt Fv, Rijdt Jv-D-vd, Eijck Bv (103) Transferable ab initio intermolecuular potentials. 1. Derivation from methanol dimer and trimer calculations. J Phys Chem A 103 9872... 

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