Muonium Addition to a peri‐Trifluoromethylated 9‐Phosphaanthracene Producing a High‐Energy Paramagnetic π‐Conjugated Fused Heterocycle

Abstract

In this communication, we report muon spin rotation/resonance (μSR) studies for understanding radical reactivity of 10‐mesityl‐1,8‐bis(trifluoromethyl)‐9‐phosphaanthracene. Transverse‐field muon spin rotation (TF‐μSR) and muon avoided level‐crossing resonance (μLCR) measurements successfully visualized a paramagnetic species produced by regioselective addition of muonium (Mu) to the skeletal phosphorus atom. Density functional theory (DFT) calculations for the P‐muoniation product suggested two possible isomers. Whereas the most stable isomer including the envelope‐type phosphorus heterocycle shows considerably different hyperfine coupling constants (hfcs) from those of the TF‐μSR and μLCR, the metastable structure accompanying the almost planar tricyclic π‐conjugated skeleton could simulate the experimentally determined hfcs. The metastable planar π‐conjugated paramagnetic tricyclic‐fused skeleton is promoted by the larger zero‐point energy due to the light muon (μ+), one ninth of the proton mass.

Muonium Addition to the Radicalic Carbon in 1,3-Diphosphacyclobutane-2,4-diyl

“…The muon beamline at the TRIUMF cyclotron (M20 beamline) was used, and the LAMPF spectrometer was employed. In this study, we conducted transverse-field muon spin rotation (TF-¯SR) measurements (see Supporting Information and our previous papers 19,20 ). Gaussian 09 and ADF2021 software packages were employed for density-functional theory (DFT) calculations.…”

Muon Spin Rotation/Resonance (μSR) for Studying Radical Reactivity of Unsaturated Organophosphorus Compounds

“…Furthermore, TF-μSR measurements using a powder sample of 6 were attempted, and resulted in determining the absolute A μ of 254.7 � 0.6 MHz. [18] The substantially different A μ parame-ters between the solution and the powder samples (ΔA μ = 33 MHz) would not be comparable with the A μ data of the exohedral muonium adduct on C 60 (ΔA μ = 6.5 MHz). [21] However, the deviation of ΔA μ = 33 MHz is comparable with the temperature A μ dependence of such as cyclohexadienyl radicals on SiO 2 surface [22] and the paramagnetic species in clathrate hydrate of 2,5-dihydrofuran.…”

“…Furthermore, TF‐μSR measurements using a powder sample of 6 were attempted, and resulted in determining the absolute A μ of 254.7±0.6 MHz [18] . The substantially different A μ parameters between the solution and the powder samples (Δ A μ =33 MHz) would not be comparable with the A μ data of the exohedral muonium adduct on C 60 (Δ A μ =6.5 MHz) [21] .…”

“…[17] We employed 10-mesityl-1,8-bis(trifluoromethyl)-9-phosphaanthracene (6) for the μSR study (Figure 12). [18] Figure 12a displays a TF-μSR spectrum of a THF solution (0.16 M) of 6 at 298 K. A transverse magnetic field of 1.45 T corresponds to the diamagnetic signal (ν D ) of 196 MHz, and the difference between the radical precession frequencies (ν R2 -ν R1 ) is equal to the absolute muon hfc (A μ ) of 221.83 � 0.04 MHz. This TF-μSR spectrum is compatible with the regioselective addition of muonium to 6 affording single observable radical species, which is in sharp contrast to the muoniation of anthracene affording three (or four) paramagnetic species.…”

“…In the DFT calculations, the H atom was employed in the place of Mu, and the muon hfc was calculated by multiplying the relative gyromagnetic ratio to proton ( γ μ / γ p =3.1833). The ordinary DFT structure optimization at the UωB97XD/6‐311G(d,p) level, where Mu was calculated as H atom in optimizing the structures, provided the structure showing remarkable envelope‐type conformation of the 6‐membered phosphorus heterocycle (Figure 13b) [18] . The notable butterfly‐like tricyclic skeleton is promoted by both the P−Mu unit and the flanking CF 3 group.…”