Fast reactions of hydroxycarbenes: tunneling effect versus bimolecular processes

Direct Observation of Gas-Phase Hydroxymethylene: Photoionization and Kinetics Resulting from Methanol Photodissociation

Carbene species play an integral role in high-energy chemistry, transition-metal-carbene chemistry, catalysis, photolytic formation of carbohydrates, and possibly even the formation of interstellar sugars. In 1921, "reactive formaldehyde"─now known as hydroxymethylene (HCOH)─was first implicated as an intermediate in photocatalytic processes. However, due to its transient nature, direct observation of HCOH has predominantly been attained using cryogenic isolation methods. As a result, HCOH gas-phase reactivity measurements have been limited. We directly observed HCOH using photoionization spectroscopy following UV photodissociation of methanol. Our measurements show it reacts slowly with O2 at room temperature. This work provides evidence for the formation mechanism of HCOH from CH3OH and its subsequent reactivity under gas-phase reaction conditions.

Identification of Glycolaldehyde Enol (HOHC═CHOH) in Interstellar Analogue Ices

Glycolaldehyde is considered the entry point in the aqueous prebiotic formose (Butlerow) reaction although it mainly exists in its unreactive hydrated form in aqueous solution. The characterization of the more reactive nucleophilic enol form under interstellar conditions has remained elusive to date. Here we report on the identification of glycolaldehyde enol (1,2-ethenediol, HOHC═CHOH) in low temperature methanol-bearing ices at temperatures as low as 5 K. Exploiting isotope labeling and isomer-selective photoionization coupled with reflectron time-of-flight mass spectrometry, our results unravel distinct reaction pathways to 1,2-ethenediol, thus demonstrating the kinetic stability, availability for prebiotic sugar formation, and potential detectability in deep space.

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.

A neutralization-reionization and reactivity mass spectrometry study of the generation of neutral hydroxymethylene

Neutral hydroxymethylene HCOH is an important intermediate in several chemical reactions; however, it is difficult to observe due to its high reactivity. In this work, neutral hydroxymethylene and formaldehyde were generated by charge exchange neutralization of their respective ionic counterparts and then were reionized and detected as positive-ion recovery signals in neutralization-reionization mass spectrometry in a magnetic sector instrument of BEE geometry. The reionized species were characterized by their subsequent collision-induced dissociation mass spectra. The transient hydroxymethylene neutral was observed to isomerize to formaldehyde with an experimental time span exceeding 13.9 µs. The vertical neutralization energy of the HCOH(+•) ion has also been assayed using charge transfer reactions between the fast ions and stationary target gases of differing ionization energy. The measured values match the result of ab initio calculations at the QCISD/6-311 + G(d,p) and CCSD(T)/6-311 + + G(3df,2p) levels of theory. Neutral hydroxymethylene was also produced by proton transfer from CH(2) OH(+) to a strong base such as pyridine, confirmed by appropriate isotopic labeling. There is a kinetic isotope effect (KIE) for H(+) versus D(+) transfer from the C atom of the hydroxymethyl cation of ∼3, consistent with a primary KIE of a nearly thermoneutral reaction.

Intramolecular hydroxycarbene C-H-insertion: The curious case of (o-methoxyphenyl)hydroxycarbene

The first C-H insertion of a hydroxycarbene species in the gas phase has been observed experimentally by means of high vacuum flash pyrolysis (HVFP) and subsequent matrix isolation: (o-Methoxyphenyl)glyoxylic acid gives non-isolable (o-methoxyphenyl)hydroxycarbene upon pyrolysis at 600 °C, which rapidly inserts into the methyl C-H bond. The insertion product, 2,3-dihydrobenzofuran-3-ol, was trapped in an excess of Ar at 11 K and characterized by infrared spectroscopy. The insertion process kinetically outruns the alternative [1,2]H-tunneling reaction to o-anisaldehyde, a type of reaction observed for other hydroxycarbenes. Traces of the dehydration product, benzo[b]furan, were also detected. The potential energy hypersurface including the insertion and hydrogen migration processes was computed at the all-electron coupled-cluster level of theory encompassing single and double substitutions and perturbatively included triple excitations [AE-CCSD(T)] in conjunction with a correlation-consistent double-ζ basis set (cc-pVDZ) by utilizing density functional theory (DFT) optimized geometries (M06-2X/cc-pVDZ) with zero-point vibrational energy (ZPVE) corrections. Exchange of the methoxy for a trifluoromethoxy group successfully prevents insertion and (o-trifluoromethoxy)benzaldehyde is produced instead; however, the carbene cannot be observed under these conditions. Thermal decomposition of (o-methoxyphenyl)glyoxylic acid in refluxing xylenes does not give the insertion product but yields o-anisaldehyde. This unanticipated outcome can be rationalized by protonation of the hydroxycarbene intermediate leading to the tautomeric formyl group. Thermochemical computations at M06-2X/cc-pVDZ in conjunction with a self-consistent solvent reaction field model support this suggested reaction pathway.