The water–methanol complexes. I. A matrix isolation study and an ab initio calculation on the 1-1 species

Abstract

The infrared spectrum of the CH3OH:H2O complex isolated in a nitrogen matrix is here reported. The complex is identified through observation of all three internal modes of the water moiety and 10 out of 12 vibrations of the methanol subunit. Several conformers of the CH3OH...OH2 are evidenced through ir photochemical and thermal conversion process, but none corresponds to the reverse CH3HO...HOH structure, with water playing the role of the proton donor. Ab initio calculations on both structures have been performed to obtain their equilibrium geometries and vibrational spectra. They allow to account for the ir spectral changes (frequencies as well as intensities) of the two submolecules interacting through hydrogen bonding within the complex.

Using the C–O stretch to unravel the nature of hydrogen bonding in low-temperature solid methanol–water condensates

“…In fact, the observed blue-shift in the O-H bending mode and a red-shift in the O-H stretching mode of H 2 O with increasing CH 3 OH concentration (Figure 4) is also consistent with this picture. It is also interesting to note that in this arrangement, with CH 3 OH acting as a PA when bonded to H 2 O acting as a PD, no significant change in the O-H bond length of CH 3 OH is reported 33,58 . Therefore with this picture in mind, that CH 3 OH molecules are most likely to act as a PA to H 2 O with increasing H 2 O concentration (decreasing R), it would stand to reason that the blue PD C-O stretch component of CH 3 OH would thus remain unaffected, as our results show.…”

“…Therefore, it would appear that the presence of H 2 O environment has little effect on the CH 3 OH PD hydrogen bonds. It has been reported in literature [33][34][35]39,58 that gas phase and matrix isolated heterodimers of CH 3 OH and H 2 O preferentially and exclusively bond with the CH 3 OH acting as a PA and the H 2 O as PD. Within an amorphous solid where a random arrangement of molecules and hydrogen bonded configurations are likely, it is still possible that during deposition there is preferential adsorption of CH 3 OH and H 2 O in the CH 3 OH(PA)· · · H 2 O(PD) arrangement.…”

“…In CH 3 OH the vibrational spectra of the O-H, C-H and the C-O stretch are sensitive to the interaction between nearest neighbours. The interactions between CH 3 OH and H 2 O have been studied in mixed CH 3 OH/H 2 O liquids [29][30][31][32] , heterodimers [33][34][35] and small clusters both in the gas phase and in matrix isolation studies 33,34 , supported with extensive computational studies [36][37][38][39][40][41][42][43][44] . The O-H stretch has been used extensively to study hydrogen bonding between H 2 O and CH 3 OH in dimers, matrix isolation and in clusters, however, in the bulk condensed and liquid phases this poses a problem as the broad absorption bands due to the O-H stretching frequencies of both CH 3 OH and H 2 O overlap.…”

Thermal desorption of circumstellar and cometary ice analogs

“…On the other hand, less volatile species like CH 3 OH can form larger groups of segregated molecules when the ice temperature is sufficiently high, despite the fact that they do not diffuse so easily through the ice structure. Once methanol molecules meet, the CH 3 OH-CH 3 OH interactions are stronger, since these molecules form hydrogen bonds with a similar strength to those formed by H 2 O molecules (Bakkas et al 1993; note that, unlike CO, CO 2 , and NH 3 , water and methanol are both liquids at standard conditions) In addition, methanol desorbs at higher temperatures compared to CO, CO 2 , and NH 3 . Therefore, segregated-CH 3 OH seeds can grow without desorbing or being disrupted, forming larger regions of segregated CH 3 OH which can be detected by the FTIR spectrometer.…”

Microwave rotation-tunneling spectroscopy of the water–methanol dimer: Direct structural proof for the strongest bound conformation

“…[26][27][28][29][30] Taken as whole, they fall 3:2 in favor of the MW global minimum, and there are results which find the minima separated by ϳ1 kcal/mol for both conclusions. In all fairness, however, these calculations were performed over the span of two decades, with increasingly sophisticated methods and computational resources, and the findings of the studies are correlated with basis set size, with the more flexible wave functions determining that the WM structure is more stable.…”