Ab Initio Study of Proton Chemical Shift in Supercritical Methanol Using Gas-Phase Approximation

Abstract

Chemical shifts of the OH proton in supercritical methanol referenced to the methyl proton of methanol monomer have been estimated theoretically using the ab initio molecular orbital (MO) method. The degree of dissociation from hydrogen-bonded methanol clusters to monomers calculated using the CCSD(T)/6-31+G(d)//MP2(frozen-core)/6-31+G(d) level of theory indicates that supercritical methanol is comprised of 89% monomer and 10% cyclic tetramer plus ∼1% dimer at the critical point (T c = 512.6 K; P c = 8.09 MPa). The predominant existence of the cyclic tetramers rather than the dimers in supercritical methanol is in contrast to previous theoretical results for supercritical water that indicate the composition of, except for 80% monomer, 20% dimer with little existence of a larger size of clusters at the critical point (T c = 647.1 K; P c = 22.06 MPa). It is also found that a significant fluctuation of the composition of methanol should be caused by a greater change in the degree of dissociation of the cyclic tetramer near the critical point. On the basis of the above supercritical methanol composition, the chemical shift of the OH proton is determined to be −2.00 ppm at the MP2(frozen-core)/6-31+G(d)//MP2(frozen-core)/6-31+G(d) level of theory, which excellently reproduces the recent NMR experimental results.

Elucidating the Vibrational Spectra of Hydrogen-Bonded Aggregates in Solution: Electronic Structure Calculations with Implicit Solvent and First-Principles Molecular Dynamics Simulations with Explicit Solvent for 1-Hexanol in n-Hexane

“…In the latter case, a hydroxyl group participating as the donor in a hydrogen bond has a shifted vibrational frequency appearing as a broad band centered at 3350 cm -1 as compared to a non-hydrogen-bonding hydrogen with a vibrational frequency of ∼3650 cm -1 . With the fraction of donating hydroxyl groups known through these quantitative spectral measures, various models can be employed to analyze the results, the most prevalent being an assumption of an equilibrium between monomers and cyclic tetramer structures. − ,, ,, It is known, however, that a situation where a given hydroxyl group is acting as a hydrogen-bond acceptor but not donor will result in this hydroxyl group appearing as isolated with respect to spectroscopic probing. ,− This can lead to an overestimation of the concentration of “isolated” molecules. In the extreme case, a dimeric structure of two alcohols is thought to only form a single hydrogen bond, so that every dimer spuriously contributes one count to the isolated molecule measure.…”

Calculation of the Solvation Free Energy of the Proton in Methanol

“…DFT calculations on (CH 3 OH) n clusters in the gas phase were reported by Hagemeister and co-workers. 33 MP2 34,35 and CCSD (T) 36 calculations for the small clusters were also reported.…”

Rapid One-Pot Solvothermal Batch Synthesis of Porous Nanocrystal Assemblies Composed of Multiple Transition-Metal Elements

“…As a result, heterogeneous aggregates form (Figures b and S4b). In contrast, the present approach heated the whole solution rapidly to reach the supercritical state of MeOH. − ,− This environment would allow quick nucleation of nascent particles of mixed-metal oxides to give a narrow size distribution, as illustrated in Figure (ii). Judging from the product yield (>90%) based on the initial molar concentration of the metal ions, the metal ions in the precursor solution were immediately consumed within the temperature rising.…”