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

Abstract

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.

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

“…22 CH 2 (OH) 2 can also be generated via energetic processing of low-temperature methanol–oxygen ices and characterized with mass spectrometry. 23 In this work, gaseous CH 2 (OH) 2 was simply generated via the volatilization of a 10% (w/w) aqueous formaldehyde solution, which was prepared by dissolving paraformaldehyde in pure water under reflux at 85 °C for 300 minutes and filtering it through a 0.22 μm syringe filter. The infrared spectrum of the vapour from the methanediol solution is shown in Fig.…”

Mechanistical study on the formation of hydroxyacetone (CH3COCH2OH), methyl acetate (CH3COOCH3), and 3-hydroxypropanal (HCOCH2CH2OH) along with their enol tautomers (prop-1-ene-1,2-diol (CH3C(OH)CHOH), prop-2-ene-1,2-diol (CH2C(OH)CH2OH), 1-methoxyethen-1-ol (CH3OC(OH)CH2) and prop-1-ene-1,3-diol (HOCH2CHCHOH)) in interstellar ice analogs

“…The spectra of the unprocessed ices revealed prominent absorptions (Fig. 5a and Table 3) of methanol such as the broad O–H stretching mode (3020–3600 cm −1 ; v 1 ), C–H stretching fundamental (2993 cm −1 ; v 2 ), the symmetric stretching modes of the methyl group (2956 cm −1 , v 9 ; 2828 cm −1 , v 3 ), the C–H bending mode (1455 cm −1 ; v 5 ), and the C–O stretching mode (1030 cm −1 ; v 8 ); 42,45,46 intense absorptions of acetaldehyde such as the overtone mode (2865 cm −1 ; 2 v 6 ), stretching mode of the carbonyl moiety (1718 cm −1 ; v 4 ), and γ r (CH 3 ) (1123 cm −1 ; 2 v 8 ) 16,47 are also prominent.…”

“…2a and Table S1, ESI†)), we pursue the following strategy to investigate their formation in methanol–acetaldehyde ices. Upon interaction with energetic electrons, the methanol (CH 3 OH) molecule can decompose via atomic hydrogen loss to the hydroxymethyl radical (ĊH 2 OH, A ) and methoxy radical (CH 3 Ȯ, B ); 40,43,44,46 likewise, the acetaldehyde (CH 3 CHO) molecule can decompose via atomic hydrogen loss to the acetyl radical (CH 3 ĊO, C ) 36 and – in principle – to the vinoxy radical (ĊH 2 CHO, D ). Species A–D correspond to radicals containing distinct functional groups which are incorporated into products by recombination.…”

“…On the other hand, bulk-ice chemistry has been demonstrated to form COMs carrying carbonyl (CO) functional groups via non-equilibrium chemistry. 2,16,20,36–39 In particular, Bennet et al and Jones et al 11,14,15,40–46 revealed that methanol (CH 3 OH) is initially radiolyzed to methoxy radicals (CH 3 Ȯ) plus atomic hydrogen and hydroxymethyl radical (ĊH 2 OH) plus atomic hydrogen by proxies of galactic cosmic rays. Likewise, Kleimeier et al 16,36,37,47 exposed that interstellar ices containing acetaldehyde (CH 3 CHO) can store at least acetyl radicals (CH 3 ĊO).…”

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

“…Zhu et al then reported successful synthesis and detection of gaseous methanediol in 2021. 25 Gaseous methanediol is prepared by exposing methanol-oxygen ices to high energy electrons followed by sublimation. Analysis with photoionization-reflectron time-of-flight mass spectrometry subsequently identifies the species together with IR spectroscopy.…”