Rapid preparation of gaseous methanediol (CH2(OH)2)

Infrared characterization of hydrated products of glyoxal in aqueous solution

The simplest and most abundant dicarbonyl in the atmosphere, glyoxal ((CHO)2), and its geminal diols via stepwise hydration reactions, monohydrate (CHOCH(OH)2) and dihydrate ((HC(OH)2)2), are proposed to be responsible for the generation of atmospheric acid and the increase in aerosol viscosity. In this work, the hydrates of glyoxal were prepared by dissolving glyoxal trimer dihydrate (C6H10O8) in H2O and D2O and probed by infrared absorption spectrometry at varied temperatures. In glyoxal aqueous solution at a concentration of < 1 wt%, the monomeric dihydrate is predominant. Coupled with the predicted vibrational wavenumbers and the corresponding intensities using the B3LYP/aug-cc-pVTZ method, the intense IR bands at 1075 cm-1 and 1073 cm-1 are attributed to the C-O stretching modes of dihydrate and deuterium substituted dihydrate at the hydroxyl groups, denoted as d4-dihydrate ((HC(OD)2)2). Upon heating of the d4-dihydrate solution to cause dehydration, a new band generated at 1745 cm-1 was attributed to the C=O stretching mode of d2-monohydrate (CHOCH(OD)2). Comparing the predicted wavenumbers of glyoxal monohydrate and the observed vibrational wavenumbers of the glycolaldehyde (HCOCH2OH), the wavenumber of the C=O stretching mode of monohydrate is reasonably presumed to be 1745 ± 5 cm-1. These infrared characterizations of the glyoxal hydrates provide suitable detection windows for further investigating the roles of glyoxal and its hydrates in atmospheric and aerosol chemistry, as well as studying the relevant reaction kinetics.

Room-Temperature Conversion of Methane to Methanediol by [FeO2]. J Phys Chem Lett 2023;14:1633-1640. [PMID: 36752636 DOI: 10.1021/acs.jpclett.2c03786] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Inspired by the activities of P-450 enzyme and Rieske oxygenases in nature, in which the high-valent Fe-oxo complexes play a key role for oxidation of alkanes, the oxidation process of methane by the high-valent iron oxide cation [FeO₂]⁺ has been explored by using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry complemented by high-level quantum chemical calculations. In contrast to the previously reported [FeO]⁺/CH₄ and [Fe(O)OH]⁺/CH₄ systems, which afford [FeOH]⁺ as the main product, the generation of Fe⁺ dominates the reaction of [FeO₂]⁺ with CH₄. Theoretical calculations suggest a novel "oxygen rebound" pathway for the liberation of methanediol. In particular, the inevitable valence increase of Fe prior to C-H activation is similar to the cytochrome P-450 mediated processes. To our best knowledge, this study provides the first example of methane activation by the high-valent Fe(V)-oxo species in the gas phase, which may thus bridge the gas-phase model and the condensed-phase biosystems.

Rotamers of Methanediol: Composite Ab Initio Predictions of Structures, Frequencies, and Rovibrational Constants

Geminal diols are known to be important intermediates in atmospheric ozonolysis and the aerosol cycle. Recently, the simplest member of this class, methanediol, was interrogated in the gas phase with infrared spectroscopy. To aid in future spectroscopic investigations of methanediol, including in the interstellar medium, we report fundamental frequencies and rovibrational constants for the two rotamers of this molecule using ab initio composite methods along with vibrational perturbation theory. Sensitivity of the predictions to the level of theory and the treatment of anharmonic resonances are carefully assessed. The OH stretching harmonic frequencies of both rotamers are particularly sensitive to the level of theory. The CH stretches of the C_s rotamer are sensitive to the treatment of anharmonic resonances with VPT2-based effective Hamiltonian models. Equilibrium bond distances and harmonic frequencies are converged conservatively to within 0.0005 Å and 3 cm^-1, respectively. The effect of tunneling on the rotational constants is investigated with a 2D variational calculation, based on a relaxed hydroxyl torsional potential energy surface. Tunneling is found to be negligible in the lower energy C₂ rotamer but should modify the rotational constants of the C_s rotamer on the order of MHz, giving rise to rotational line splittings of the same order. The rovibrational constants of the C_s rotamer are dominated by hydroxyl torsional effects, and here we see evidence for the breakdown of vibrational perturbation theory.

Infrared Characterization of Isotopic Analogues of Methanediol in Aqueous Solution

Dissolved methanediol in aqueous solution has been treated as the precursor for the formation of atmospheric formic acid in multiphase environments. In this work, methanediol, CH₂(OH)₂, and its isotopic analogues, CH₂(OD)₂, CD₂(OH)₂, and CD₂(OD)₂, in aqueous solution were prepared by dissolving paraformaldehyde and deuterium-substituted paraformaldehyde powders in H₂O and D₂O under reflux. Their infrared absorption contours of formaldehyde solutions at concentrations of <1 wt % are not dependent on the concentration, mainly referring to the characteristics of the monomeric configuration, and can be categorized into two parts. At wavenumbers >2000 cm^-1, broad bands of moderate strengths were ascribed to the stretching modes of two OH or OD groups, observed at 3000-3700 and 2050-2750 cm^-1, respectively. At wavenumbers of 950-1200 cm^-1, the isotopic analogues of methanediols composed of CH₂ moieties are featured with a singlet strong band at ca. 1030 cm^-1, mainly attributed to the O-C-O stretching modes; the isotopic methanediols containing CD₂ moieties manifested two intense bands at ca. 1100 and 980 cm^-1, majorly enveloping the CD₂ deformation and O-C-O stretching modes. The aforementioned spectral features were assigned on the basis of density functional theory, ωB97XD, with the basis set aug-cc-pVTZ and the solvent effect using the conductor-like polarizable continuum model. In addition, the predicted energetics suggested that the trans-methanediol is more stable than the cis- conformer by ca. 0.62 kcal mol^-1 and majorly contributes to the infrared features. At higher concentrations of CH₂(OH)₂, extra bands at 920 and 1104 cm^-1 appeared and were attributed to the C-O-C stretching modes of the dimeric/polymeric methanediol; that is, HO(CH₂O)_nH, n ≥ 2. These infrared characterizations of the isotopic analogues of methanediols provided suitable detection windows in the relevant atmospheric and aerosol reactions in the laboratory studies.

Confirmation of gaseous methanediol from state-of-the-art theoretical rovibrational characterization

High-level rovibrational characterization of methanediol, the simplest geminal diol, using state-of-the-art, purely ab initio techniques unequivocally confirms previously reported gas phase preparation of this simplest geminal diol in its C₂ conformation. The F12-TZ-cCR and F12-DZ-cCR quartic force fields (QFFs) utilized in this work are among the largest coupled cluster-based anharmonic frequencies computed to date, and they match the experimental band origins of the spectral features in the 980-1100 cm^-1 range to within 3 cm^-1, representing a significant improvement over previous studies. The simulated spectrum also matches the experimental spectrum in the strong Q branch feature and qualitative shape of the 980-1100 cm^-1 region. Additionally, the full set of rotational constants, anharmonic vibrational frequencies, and quartic and sextic distortion constants are provided for both the lowest energy C₂ conformer as well as the slightly higher C_s conformer. Several vibrational modes have intensities of 60 km mol^-1 or higher, facilitating potential astronomical or atmospheric detection of methanediol or further identification in laboratory work especially now that gas phase synthesis of this molecule has been established.