A theoretical study of the conversion of gas phase methanediol to formaldehyde

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

The simplest geminal diol CH₂(OH)₂ serves as an important precursor to form atmospheric formic acid. CH₂(OH)₂ vapour can be generated by the evaporation of an aqueous formaldehyde solution, prepared by dissolving paraformaldehyde under reflux. Its rovibrational feature at 980-1100 cm^-1 is consistent with the simulation and free of the intense interferences of H₂O and CH₂O.

Synthesis of methanediol [CH2(OH)2]: The simplest geminal diol

Methanediol [CH₂(OH)₂] represents a pivotal atmospheric volatile organic compound and plays a fundamental role in aerosol growth. Although sought for decades, methanediol has never been identified due to the inherent dehydration tendency of two adjacent hydroxyl groups (OH) at the same carbon atom. Here, we prepare and identify methanediol via processing of low-temperature ices followed by sublimation into the gas phase. These findings open up a concept to synthesize and characterize unstable geminal diols—critical organic transients in Earth’s atmosphere. The excited state dynamics of oxygen may also lead to methanediol in methanol-rich interstellar ices in cold molecular clouds, followed by sublimation in star-forming regions and prospective detection of these reactive intermediates in the gas phase by radiotelescopes.

Geminal diols—organic molecules carrying two hydroxyl groups at the same carbon atom—have been recognized as key reactive intermediates by the physical (organic) chemistry and atmospheric science communities as fundamental transients in the aerosol cycle and in the atmospheric ozonolysis reaction sequence. Anticipating short lifetimes and their tendency to fragment to water plus the aldehyde or ketone, free geminal diols represent one of the most elusive classes of organic reactive intermediates. Here, we afford an exceptional glance into the preparation of the previously elusive methanediol [CH₂(OH)₂] transient—the simplest geminal diol—via energetic processing of low-temperature methanol–oxygen ices. Methanediol was identified in the gas phase upon sublimation via isomer-selective photoionization reflectron time-of-flight mass spectrometry combined with isotopic substitution studies. Electronic structure calculations reveal that methanediol is formed via excited state dynamics through insertion of electronically excited atomic oxygen into a carbon–hydrogen bond of the methyl group of methanol followed by stabilization in the icy matrix. The first preparation and detection of methanediol demonstrates its gas-phase stability as supported by a significant barrier hindering unimolecular decomposition to formaldehyde and water. These findings advance our perception of the fundamental chemistry and chemical bonding of geminal diols and signify their role as an efficient sink of aldehydes and ketones in atmospheric environments eventually coupling the atmospheric chemistry of geminal diols and Criegee intermediates.

Gaseous infrared spectra of the simplest geminal diol CH2(OH)2 and the isotopic analogues in the hydration of formaldehyde

The infrared spectrum of the simplest geminal diol, methanediol or methylene glycol (CH2(OH)2), was successfully probed in the gaseous hydration of formaldehyde. The observed absorption bands coincided with the anharmonic vibrational wavenumbers predicted by B3LYP/aug-cc-pVTZ calculation. Based on the predicted rotational parameters and dipole derivatives, the simulated rovibrational contours of CH2(OH)2 agreed with the experimental spectrum, and the band origins of the OCO symmetric stretching mode (b-type) and the OCO asymmetric stretching mode (a-type) were determined to be 1027 and 1058 cm-1, respectively. In addition, the isotopic analogues, CD2(OH)2, CH2(OD)2, and CD2(OD)2, were also investigated. The band origins of the CD2 wagging mode (a-type) and the COH bending mode (a, c-type) of CD2(OH)2 were also determined to be 1121 and 1301 cm-1, respectively. The successful infrared characterization of gaseous methanediol makes it possible to directly investigate the relevant chemical reactions of geminal diols in atmosphere, astrophysics, and water-mediated reactions.

Torsional States and Tunneling Probability in HOSOH, DOSOD, and DOSOH Molecules Analyzed at the CBS Limit

Torsional vibrations of a sulfoxylic acid molecule (HOSOH) and its two deuterated isotopologues were analyzed for the first time. Harmonic and anharmonic calculations of the vibrational frequencies of the trans- and cis-conformers were performed. More rigorous consideration of the torsional vibrations was made based on 2D potential energy and kinematic coefficient surface calculations. These calculations were made at the MP2/cc-pVTZ and MP2/cc-pVQZ levels of theory, and then the results were extrapolated to the complete basis set limit. The 2D surface of the zero-point vibrational energy of a sulfoxylic acid molecule was calculated at the MP2/cc-pVTZ level of theory in anharmonic approximation and taken into account. The energies of the torsional states were found by numerical solution of the vibrational Schrödinger equation of the restricted dimensionality using the Fourier method. 2D surfaces of the dipole moment components were calculated too. Using all these data, the torsional IR spectra of the trans- and cis-conformers of the HOSOH, DOSOD, and DOSOH molecules were also modeled at different temperatures.

Mechanistic role of water in HSSZ-13 catalyzed methanol-to-olefins conversion

Attenuation of chain initiation and termination rates and higher turnover capacities in methanol-to-olefins (MTO) conversion observed with water co-feeds are plausibly due to formaldehyde hydrolysis.

Theoretical Study on the Dynamics and Kinetics of the Reaction of CH2OH with OH

The stochastic one-dimensional chemical master equation (CME) simulation method was used to investigate the dynamics of the reaction of CH₂OH with OH. A multiwell multichannel potential energy surface (PES) was constructed at the CCSD(T)/Aug-cc-pVTZ//CBS-QB3 and QCISD(T)/Aug-cc-pVTZ//CBS-QB3 levels of theory. The constructed PES consisted of three chemically activated intermediates and two van der Waals complexes. The fractional population analysis unraveled the role of the energized intermediates and van der Waals complexes in the early stages of this complex reaction. The CME calculations provided the phenomenological rate constants through analysis of the eigenvalues and eigenvectors of collision matrices while Leonard-Jones potential was used to model the collisions. The CME results indicated that CH₂O and H₂O were the major products, in accordance with the literature. Also, the findings declared the temperature and pressure independence of the reaction over a wide range of temperature (250 to 2400 K) and pressure (0.1 to 7 atm). Furthermore, the efficiency of tunneling on the hydrogen transfer isomerization reaction of trans-HCOH to CH₂O was confirmed over the temperature range of 250 to 3000 K. The rate constants for different reaction channels are reported.

Stereoisomers of hydroxymethanes: Probing structural and spectroscopic features upon substitution

Ab initio studies on CH_x(OH)_4-x (x = 0-3) polyols are carried out to derive their structural and spectroscopic features. Several stereoisomers (both equilibrium structures and transition states) are found. Some are predicted here for the first time. We determined hence their geometrical parameters, vibrational frequencies, electronic excitation energies for the singlet manifold, and IR spectra. While the IR spectra for all polyols present similar shapes, their UV spectra exhibit however distinct band origin that are specific to each polyol and more interestingly to each diasteroisomer. Stereoelectronic effects are also noticed and discussed. It is suggested that UV spectroscopy is an efficient probe to experimentally identify polyols in mixtures involving polyols.

Photoelectron spectroscopy of the hydroxymethoxide anion, H2C(OH)O. J Chem Phys 2016;145:124317. [PMID: 27782682 DOI: 10.1063/1.4963225]

We report the negative ion photoelectron spectroscopy of the hydroxymethoxide anion, H₂C(OH)O^-. The photoelectron spectra show that 3.49 eV photodetachment produces two distinct electronic states of the neutral hydroxymethoxy radical (H₂C(OH)O^⋅). The H₂C(OH)O^⋅ ground state (X̃ ²A) photoelectron spectrum exhibits a vibrational progression consisting primarily of the OCO symmetric and asymmetric stretches, the OCO bend, as well as combination bands involving these modes with other, lower frequency modes. A high-resolution photoelectron spectrum aids in the assignment of several vibrational frequencies of the neutral H₂C(OH)O^⋅ radical, including an experimental determination of the H₂C(OH)O^⋅ 2ν₁₂ overtone of the H-OCO torsional vibration as 220(10) cm^-1. The electron affinity of H₂C(OH)O^⋅ is determined to be 2.220(2) eV. The low-lying à ²A excited state is also observed, with a spectrum that peaks ∼0.8 eV above the X̃ ²A state origin. The à ²A state photoelectron spectrum is a broad, partially resolved band. Quantum chemical calculations and photoelectron simulations aid in the interpretation of the photoelectron spectra. In addition, the gas phase acidity of methanediol is calculated to be 366(2) kcal mol^-1, which results in an OH bond dissociation energy, D₀(H₂C(OH)O-H), of 104(2) kcal mol^-1, using the experimentally determined electron affinity of the hydroxymethoxy radical.

New insights in atmospheric acid-catalyzed gas phase hydrolysis of formaldehyde: a theoretical study

A two-step mechanism of the gas phase hydrolysis of formaldehyde catalyzed by nitric acid.

A kinetic and thermochemical database for organic sulfur and oxygen compounds

Potential energy surfaces and reaction kinetics were calculated for reactions involving sulfur and oxygen, which are potentially relevant in combustion and desulfurization chemistry.

Amended Safety Assessment of Formaldehyde and Methylene Glycol as Used in Cosmetics

Formaldehyde and methylene glycol may be used safely in cosmetics if established limits are not exceeded and are safe for use in nail hardeners in the present practices of use and concentration, which include instructions to avoid skin contact. In hair-smoothing products, however, in the present practices of use and concentration, formaldehyde and methylene glycol are unsafe. Methylene glycol is continuously converted to formaldehyde, and vice versa, even at equilibrium, which can be easily shifted by heating, drying, and other conditions to increase the amount of formaldehyde. This rapid, reversible formaldehyde/methylene glycol equilibrium is distinguished from the slow, irreversible release of formaldehyde resulting from the so-called formaldehyde releaser preservatives, which are not addressed in this safety assessment (formaldehyde releasers may continue to be safely used in cosmetics at the levels established in their individual Cosmetic Ingredient Review safety assessments).

Atoms in molecules interpretation of the anomeric effect in the OCO unit

The conformational preferences of two model compounds for the O--CH2--O anomeric unit: methanediol and dimethoxymethane analyzed within the framework of the QTAIM theory provide a new interpretation of the anomeric effect. The characteristic stabilization of the gauche conformers of these compounds is accompanied by a progressive reduction of the electron population of the hydrogens of the central methylene as the number of their gauche interactions to lone pairs rises. The electron population removed from these atoms during the conformational change is gained in the gauche conformers by atoms of larger atomic number, which results in a more negative molecular energy. Also, the variations displayed by atomic populations and the QTAIM delocalization indexes are not keeping in line with the hyperconjugative model of the anomeric effect.

Influence of H2 on the gas-phase decomposition of formic acid: a theoretical study

Gas-phase decomposition of formic acid results in final products CO + H2O and CO2 + H2. Experimentally, the CO/CO2 ratio tends to be large, in contradiction with mechanism studies, which show almost equal activation energies for dehydration and decarboxylation. In this work, the influence of H2 on the decomposition mechanism of HCOOH was explored using ab initio calculations at the CCSD(T)/6-311++G**//MP2/6-311++G** level. It was found that, in the presence of H2, the reaction channels leading to CO + H2O are more than those leading to CO2 + H2. With competitive energy, H2 addition to HCOOH can reduce the latter into HCHO, which then dissociates into CO + H2 catalyzed by H2O. Compared to trans-HCOOH, cis-HCOOH and cis-C(OH)2, conformers required for decarboxylation, are less populated due to interactions with H2.

Solvent effects on the n→π[sup ∗] electronic transition in formaldehyde: A combined coupled cluster/molecular dynamics study

We present a study of the blueshift of the n-->pi* electronic transition in formaldehyde in aqueous solution using a combined coupled cluster/molecular mechanics model including mutual polarization effects in the Hamiltonian. In addition, we report ground and excited state dipole moments. Configurations are generated from molecular dynamics simulations with two different force fields, one with and one without an explicit polarization contribution. A statistical analysis using 1200 configurations is presented. Effects of explicit polarization contributions are found to be significant. It is found that the main difference in the effects on the excitation energies arises from the fact that the two force fields result in different liquid structures, and thus a different set of configurations is generated for the coupled cluster/molecular mechanics calculations.