Matrix Isolation of Radicals

Abstract

Matrix isolation spectroscopy is a viable tool for the isolation and characterization of free radicals. In inert media (in particular, rare gases such as argon or neon) at cryogenic temperatures (below 30 K), radicals are immobilized and kinetically stabilized. In some cases, it was also possible to investigate bimolecular reactions of matrix‐isolated radicals with small molecules. The spectroscopic characterization of simple alkyl radicals such as methyl and ethyl, of unsaturated radicals such as allyl and vinyl, and of aromatic radicals is described here.

2H-Imidazol-2-oneO-Oxide: A Criegee Intermediate from a σ0π2Singlet Ground-State Carbene

“…Molecular dioxygen, 3 O 2 , despite being a “high-energy” paramagnetic diradical, is commonly reluctant to undergo low barrier reactions with closed-shell molecules. , Therefore, exceptions to this conventional wisdom attract our attention and make us ponder. − Being a diradical, dioxygen’s reactivity is rather different when it comes to binding with other open-shell species such as mono- and diradicals. In this regard, hydrocarbon radicals often recombine barrierlessly with 3 O 2 under the formation of peroxy radicals in combustion and atmospheric environments. − In a similar vein, triplet carbenes react readily with triplet O 2 in an overall spin-allowed reaction at a rate near the diffusion limit. − The products of these reactions, namely, carbonyl O -oxide Criegee intermediates (CI, R 2 CO + –O – ), are also the central intermediates in the (atmospheric) alkene ozonolysis and are involved in autoxidation chemistry. ,− In contrast to the high reactivity of triplet carbenes, singlet carbenes often do not react with 3 O 2 or at a much lower rate. − Hence, CIs formally arising from singlet carbenes are frequently poorly characterized, which is unfortunate because they may enjoy significant stereoelectronic stabilization . In this paper, we demonstrate that the spin selectivity of the carbenic O 2 addition can be circumvented in the cryogenic reaction of 2 H -imidazol-2-ylidene 1 (Figure A), which is a singlet ground-state carbene with an uncommon σ 0 π 2 electron configuration. − …”

On-Surface Synthesis and Real-Space Visualization of Aromatic P3N3

“…Matrix isolation spectroscopy is a practical approach for the characterization (e.g., infrared and UVvisible spectroscopy) of highly reactive molecules via their isolation in noble gas matrices at low temperatures. [45][46] These conditions help suppress isomerization to thermodynamically more stable isomers and aggregation.…”

Gas-Phase Optical Spectra of the Indenyl Radical and Its Quantitative Detection in a Jet-Stirred Reactor

“…Although the stable indene molecule has been detected in combustion and pyrolysis processes, − there have been only a few reports on the detection of the indenyl radical by vacuum ultraviolet (VUV) photoionization mass spectrometry. ,− However, a quantitative determination of the abundance of the reactive indenyl radical is still limited by the lack of an accurate photoionization cross section . Optical diagnostics based on molecular spectroscopy are well-suited for fundamental and application studies of radicals because they are nonintrusive and molecule-specific and allow for good temporal and spatial resolution. − However, for the indenyl radical, the only available optical absorption data were obtained in a 4 K neon matrix, which because of the effect of the matrix on the molecular geometry of the chromophore cannot be directly used for in situ diagnostics of gas-phase processes. On the other side, the two lowest-lying electronic states, X̃ 2 A 2 and à 2 B 1 , have been characterized by slow electron velocity-map imaging (SEVI) in the gas phase .…”

“…12 Optical diagnostics based on molecular spectroscopy are well-suited for fundamental and application studies of radicals because they are nonintrusive and molecule-specific and allow for good temporal and spatial resolution. 15−17 However, for the indenyl radical, the only available optical absorption data were obtained in a 4 K neon matrix, 18 which because of the effect of the matrix on the molecular geometry of the chromophore 19 cannot be directly used for in situ diagnostics of gas-phase processes. On the other side, the two lowest-lying electronic states, X ̃2A 2 and A ̃2B 1 , have been characterized by slow electron velocity-map imaging (SEVI) in the gas phase.…”