EPR spectra and rotation of CH3, CH2D, CHD2, and CD3 radicals in solid H2

Abstract

EPR spectra of CH 3 , CH 2 D, CHD 2 , and CD 3 radicals have been observed in H 2 matrix in the temperature range 1.6-4.2 K. The radicals were obtained by condensation on a cold substrate of two gas flows: deuterium mixed with 2 mol % methane passed through a discharge and pure hydrogen avoiding the discharge. The CD 3 and CHD 2 spectra were found to be a superposition of two spectra: high-temperature and low-temperature. A transformation of the shape of CD 3 and CHD 2 spectrum with decreasing sample temperature was observed. This is attributed to a change in the populations of the lowest rotational states of the radicals. Compared to known results for deuterated methyl radicals in Ar, the present observations suggest an existence of a hindering barrier for the radical rotation in solid H 2 .

EPR spectra of deuterated methyl radicals trapped in low temperature matrices

“…Now, let the EPR lines be of the same width, i.e., broadened due to certain mechanisms: superhyperfine interaction in the H 2 and D 2 matrices and, possibly, structural defects in Ne. Then a relative amplitude ratio of the singlet to its nearest neighbor transition would be about 13:6 at 4.2 K, which is comparable with our results: 12:6.5 in H 2 [11], 8:6 in D 2 , and 7.5:5.4 in Ne.…”

“…Such a transformation of the shape of the spectrum suggests the transition from a mixture of the high-and low-temperature spectrum to the low-temperature double triplet. This change from high-temperature to the low-temperature spectrum for CHD 2 corresponds very well to the case of the CD 3 radical in Ne and H 2 [11]. We could draw no conclusion about the CHD 2 temperature behavior in solid D 2 because of the poorly resolved spectrum.…”

“…11, results were presented on deuterated methyl radicals trapped from the gas phase in solid H 2 . The temperatures at which the CHD 2 and CD 3 spectrum changed to the low-temperature shape turned out to be surprisingly low in H 2 matrix as compared to solid Ar [5].…”

Hyperfine Coupling in Methyl Radical Isotopomers

“…One of the most useful and versatile techniques for studying free radicals is electron paramagnetic resonance (EPR), which can measure the hfcs and thus provide information about the distribution of unpaired spin density in the radical. − The configuration and conformation of a radical can be inferred from measurements of the magnitude, sign, and temperature dependence of the hfcs. There have been many EPR experiments on the methyl radical in different matrices and solvents; − as evident from Table , the proton hfc varies with environment and generally decreases with increasing temperature in the liquid phase.…”