Free Radicals Formed by H Atom Addition to Allenes as Determined by Muon Spin Spectroscopy

Area of Science:

• Physical Chemistry
• Chemical Kinetics
• Spectroscopy

Background:

• Allyl and vinyl radicals are key intermediates in chemical processes like combustion and synthesis.
• The formation pathways of these radicals from allenes are not fully understood.

Purpose of the Study:

• To investigate the competitive formation of allyl and vinyl radicals from allenes via hydrogen atom addition.
• To identify and characterize the radicals formed using muonium (Mu) as a hydrogen isotope analogue.

Main Methods:

• Utilized muon spin spectroscopy (μSR), including transverse-field muon spin rotation (TF-μSR) and muon level-crossing resonance (μLCR).
• Studied muonium adducts of 1,1-dimethylallene and 1-methoxyallene in solution.
• Compared experimental hyperfine constants (hfcs) with density functional theory (DFT) calculations and electron spin resonance (ESR) data for radical identification.

Main Results:

• Detected multiple radicals formed by positive muon irradiation of 1,1-dimethylallene and 1-methoxyallene.
• Identified radicals by analyzing muon and proton hfcs, correlating findings with existing ESR data and DFT results.
• Demonstrated that muonium (and hydrogen atoms) can add to all three carbon atoms of the allene system.

Conclusions:

• Muonium addition to allenes leads to the formation of both allyl and vinyl radical species.
• The addition of muonium (and hydrogen atoms) to allenes exhibits a preference for the central carbon atom.
• μSR provides a powerful tool for characterizing transient radical intermediates in chemical reactions.