High-Resolution EPR and Quantum Effects on CH₃, CH₂D, CHD₂, and CD₃ Radicals under Argon Matrix Isolation Conditions

Abstract

High-resolution EPR spectra of CH 3 , 13 CH 3 , and even CH 2 D radicals with their natural abundances have been observed in Ar matrix in the temperature range 4.2-40 K. This was achieved by X-ray radiolysis of Ar matrix containing 0.2 mol % CH 4 . The high-resolution EPR spectra of CD 3 and CHD 2 radicals were also obtained under similar conditions using CD 4 and CH 2 D 2 instead of CH 4 , respectively. At the lower temperatures, the EPR line shapes of these radicals are dominated by hyperfine (hf) patterns with anomalous intensity, attributed to quantum effects. The application of the Pauli principle in combination with D 3 point-group symmetry results in interesting exclusion of EPR transitions for both the R-proton-and the R-deuteron-rotor spectra. In contrast to the -proton methyls > • C-CH 3 , the hf coupling is anisotropic and no rotation-hindering barrier is present here. The "E" lines of the corresponding isotropic -proton methyl rotor Benetis, N. P. Chem. Phys. 1998, 226, 151] 1 are absent from their regular positions. The deuteron rotor is giving a peculiar spectrum at the lowest experimental temperature, i.e., an extremely strong central singlet superimposed on a much weaker fast motional spectrum. The quantum effects are attributed to spin-rotation coupling through the anisotropic part of the hf interaction and exchange symmetry of at least two identical fermions or bosons of the studied radicals. The experimental findings are consistent with a three-dimensional, free quantum-rotor motional model.

EPR spectra of deuterated methyl radicals trapped in low temperature matrices

“…The septet has been actually registered in the CD 4 matrix [10] at 4.2 K and solid Ar at 13 K [4]. Though these experimental results are consistent with the above theoretical scheme, another observation has been published [5] for CD 3 in Ar at 4.2 K showing a strong singlet superimposed on a weak septet. The authors explained their results with a new model of a three-dimensional, free quantum rotor with no hindering barrier present.…”

“…It has been found previously [5] that in the argon matrix the quintet transforms into a triplet with decreasing temperature from 10 K to 4.2 K. In our experiments, the spectrum of CHD 2 in D 2 and Ne was still a double quintet at 4.2 K. In Ne particularly, this spectrum showed, though, an intensity distribution different from the binomial one, which was evident from the fact that the ratio of the line amplitude at m D F ( ) = -1 to the outermost one at m D F ( ) = -2 was about 3. This deviation from the binomial distribution is particularly evident in the H 2 matrix.…”

“…For reference, a scheme of EPR transitions for CH 2 D, CHD 2 , and CD 3 radicals at high (all rotation levels are populated) and low (only the lowest rotational level is populated) temperatures is shown in Fig. 2, according to [5].…”

“…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]. The present study is aimed at obtaining new information about deuterated methyl radicals in H 2 and studying these radicals in solid D 2 and Ne in order to clarify whether the effect found in [5] is common to other matrices and to correlate new results with each other, which would help to obtain new data about the matrix effect on the radical rotation.…”

“…Methyl radical (CH 3 ) trapped in low-temperature matrices has been extensively studied by EPR [1][2][3][4][5][6][7][8]. It was shown that at low temperatures near 4.2 K, the EPR spectrum of the radical consists of four lines with equal intensity 1:1:1:1, reaching the high-temperature binomial intensity distribution 1:3:3:1 with increasing temperature.…”