The oxirene radical cation and its neutralization in the gas phase

Gas-phase detection of oxirene

Oxirenes-highly strained 4π Hückel antiaromatic organics-have been recognized as key reactive intermediates in the Wolff rearrangement and in interstellar environments. Predicting short lifetimes and tendency toward ring opening, oxirenes are one of the most mysterious classes of organic transients, with the isolation of oxirene (c-C₂H₂O) having remained elusive. Here, we report on the preparation of oxirene in low-temperature methanol-acetaldehyde matrices upon energetic processing through isomerization of ketene (H₂CCO) followed by resonant energy transfer of the internal energy of oxirene to the vibrational modes (hydroxyl stretching and bending, methyl deformation) of methanol. Oxirene was detected upon sublimation in the gas phase exploiting soft photoionization coupled with a reflectron time-of-flight mass spectrometry. These findings advance our fundamental understanding of the chemical bonding and stability of cyclic, strained molecules and afford a versatile strategy for the synthesis of highly ring-strained transients in extreme environments.

On the generation of oxirene and dimethyloxirene by retro-Diels–Alder reactions, and reactions of dimethyloxirene: a computational study

The isomerization of oxirene (oxacyclopropene) (1) to ketene, dimethyloxirene (7) to dimethylketene via the oxo carbene ("ketocarbene"), and the retro-Diels–Alder extrusion of oxirene and dimethyloxirene from their formal adducts (9 and 24, respectively) with benzene were studied computationally. All species were optimized at the MP2(fc)/6–31G(df,p) level; the species involving 1 were also subjected to MP2(fc)/6–31G(df,p) frequency and single-point CCSD(T)/6–31G(df,p) calculations. At the CCSD(T)/6–31G(df,p)//MP2(fc)/6–31G(df,p) level 1 isomerized to ketene in one step with a barrier of 2.8 kJ mol–1 and a reaction energy of –320.6 kJ mol–1. The extrusion of 1 from 9 had a late transition state and activation and reaction energies of 264.2 and 214.2 kJ mol–1, respectively, cf. cyclopropene extrusion from its adduct (192.3 and 95.9 kJ mol–1), indicating an antiaromatic destabilization energy of 214.2 – 95.9 = 118 kJ mol–1 for 1. The carbene 8 from ring-opening of 7 lay 10.9 kJ mol–1 above 7 (CCSD(T)/6–31G(df,p)//MP2(fc)/6–31G(df,p)), but the transition state could not be found; 8 isomerized to dimethylketene (252.7 kJ mol–1 below 7) with a barrier of 16.4 kJ mol–1, and to s-(Z)- and s-(E)-butenone with barriers of 28.5 and 35.4 kJ mol–1, respectively. The UV (TDDFT, B3P86/6–311++G**//MP2(fc)/6–31G(df,p)) spectra of 1 and 7 were calculated. Discrepancies were seen between the calculated IR spectra of 7 (bis(trifluoromethyl)oxirene) and perfluoro ethyl methyloxirene, and those attributed to these species in earlier matrix-isolation work. Key words: oxirene, dimethyloxirene, ab initio, retro-Diels–Alder, Diels–Alder.

Studies of unusual simple molecules by neutralization-reionization mass spectrometry

Reactive or unstable molecules are key intermediates in many important reactions, but can be difficult to prepare for experimental studies. Species with missing (:CH-OH) or extra (H3) substituents can often be formed conveniently in the gas phase by neutralizing a beam of a more stable ionic counterpart (CH = O+H, H3+). Reionization of the neutral after approximately 10(-6) seconds tests its stability, whereas its unimolecular chemistry can be probed by preparing it with different amounts of internal energy. The resulting neutral products are reionized and mass analyzed. Isomers are then characterized by ion dissociation and a third mass-analysis step. Many unusual molecules have been characterized with this technique, which can also be used to probe complex unimolecular chemistry, such as that of cyclobutadiene and ethylene oxide.