EPR spectra of deuterated methyl radicals trapped in low temperature matrices

Abstract

EPR spectra of CHD 2 , and CD 3 radicals have been investigated in low-temperature matrices of H 2 , D 2 , and Ne at temperatures 1.6-4.2 K. A method of condensation from the gas phase on a cold substrate has been used. With decreasing sample temperature, a transformation of the shape of the CD 3 spectrum in H 2 , D 2 , and Ne matrices and CHD 2 spectrum in H 2 and Ne was observed. This transformation was reversible in the above temperature range. The temperature effects are explained as reflecting a change in the populations of the lowest rotational states of the radicals. Based on the present data and known results for deuterated methyl radicals in Ar obtained in photolytic experiments, we compare the temperature behavior of the EPR spectra for the radicals trapped in various matrices. As a result an existence of a hindering barrier for the radical rotation is suggested.

Electron Paramagnetic Resonance Spectroscopic Study on Nonequilibrium Reaction Pathways in the Photolysis of Solid Nitromethane (CH3NO2) and D3-Nitromethane (CD3NO2)

“…Bielski et al reported that the total separation of the four lines of the methyl radical formed after photolysis of nitromethane in a water matrix or a tetrachloride matrix at 77 K was 65 G, in close agreement to our experimental findings of 69.45 G . The EPR parameters of the D3-methyl radical trapped in low-temperature matrices were also reported. − It shows a septet of lines with an average hfcc of about 3.59 G. In our experiments we measured an average hfcc of 3.8 G septet lines for the D3-methyl radical (CD 3 ) …”

“…Similarly, we predict septet lines for the D3-methyl radical (CD 3 ) due to interaction of the unpaired electron with the three adjacent deuterium atoms 2 H ( I = 1). The methyl radical trapped in low-temperature matrices have been studied by EPR spectroscopy previously; thus, the EPR parameters are already known. − It was shown that at 4.2 K, the methyl radical has four EPR lines with a hyperfine coupling constant (hfcc) of an average of about 23.1 G. In agreement with these studies, our experiment at 5 K yields well-resolved and separated quartet lines attributed to the methyl radical with hfcc values of 23.06, 23.12, and 23.27 G (Figure a). Bielski et al reported that the total separation of the four lines of the methyl radical formed after photolysis of nitromethane in a water matrix or a tetrachloride matrix at 77 K was 65 G, in close agreement to our experimental findings of 69.45 G .…”

Rotation Dynamics Do Not Determine the Unexpected Isotropy of Methyl Radical EPR Spectra

“…In the calibration measurements, the microwave cavity was removed from the magnet. It turned out in further experiments that the electron cyclotron resonance line positions was at almost exactly "g" = 28 The Bruker WinEPR software was applied in the simulation. a The principal axes of the A-and g-tensors are assumed to coincide.…”

“…(c) δg ⊥ = −0.018%, while other parameters still correspond to the computed values in vacuum. (d) Experimental spectrum, for CH 3 trapped in solid CO at 4.2 K, Dmitriev and Zhitnikov 28. The Bruker WinEPR software was applied in the simulation.…”