ROTATION AND MATRIX EFFECTS ON THE EPR SPECTRA OF METHYL RADICALS TRAPPED IN GAS SOLIDS. YURIJ A. DMITRIEV 1 and NIKOLAS-PLOUTARCH BENETIS 2 1 A. F. Ioffe Physical-Technical Institute, St. Petersburg, Russia 2 Department of Pollution Control, Technological Educational Institution, TEI, West Macedonia, Kozani , Greece.

The aim of the study: How and to which extent the rotation and the symmetry of a small radical in low temperature matrices may impact the lineshape of the EPR spectra of the radical Why methyl radicals, CH 3, and their deuteron substituted isotopomers, CD 3, CH 2 D, CHD 2 ? 1.exceptionally small momentum of inertia 2.kinetic energies of the lowest rotational states comparable to k T 3.quantum statistics instead of Boltzmann statistics could be expected at liquid He temperatures #2

The two lowest rotational levels (angular momentum J = 0, 1) for a free, light, symmetric-top, methyl-type rotor. The rotational constant B is about 7 K and the experimental temperature about 5 Kelvin. The kets | J, M, L> of the rotational states indicate the quantum projections M and L of the angular momentum J on the molecular symmetry axis and on a laboratory frame, respectively. The model of the symmetric-top free-methyl rotor, i.e. a planar, methyl radical of D 3 symmetry considered in three dimensions. The rotation C 3 and C 2 axes of symmetry are indicated in the figure while the three protons are indexed by -1, 0, and 1. #3

What was well known from the most studies about the EPR spectrum of CH 3 trapped in solid gases? 1.consisted of four nearly equidistant hyperfine transitions 2.rather broad with the linewidth of the order of 1 G 3.revealed no structure 4.were attributed to the zero rotational level of the radical species 5.suggested isotropic hyperfine interaction New findings on the EPR of methyls in solid Ar : (Yamada T, Komaguchi K, Shiotani M, Benetis N P and Sørnes A R, 1999 High Resolution EPR and Quantum Effects on CH 3, CH 2 D, CHD 2 and CD 3 Radicals under Argon-Matrix Isolation Conditions J. Phys. Chem. A 103: ) Hiroshima University, Japan, X-ray radiolysis of CH 4 in Ar 1.extremely narrow EPR lines of a few hundredths of Gauss 2.doublet of “E” symmetry resolved for the first time in solid gas matrices 3.quartet of “A” symmetry consisted of identical unstructured lines 4.difference in the splittings of the “A” lines (second order shift) 5.suggested isotropic hyperfine interaction #4

Yamada T, Komaguchi K, Shiotani M, Benetis N P and Sørnes A R, 1999 High Resolution EPR and Quantum Effects on CH 3, CH 2 D, CHD 2 and CD 3 Radicals under Argon-Matrix Isolation Conditions J. Phys. Chem. A 103: High-resolution EPR spectra of the CD 3 radical in Ar matrix at: (a) 4.1 K and (b) 25.0 K. The strong central singlet in (a), marked by star (*), has relative intensity 105:1 to the outermost left transition. (D signifies Deuterium-atom transition) in t #5 Temperature dependent EPR spectra of CH 3 radical obtained in Ar matrix containing 0.2 mol% CH 4 after X-ray- irradiation at 4.2 K. (a) 6 K; (b) 12 K; (c) 20 K; (d) 40 K. The doublet of “E” symmetry is marked as a star.

1. Yamada T, Komaguchi K, Shiotani M, Benetis N P and Sørnes A R 1999 High Resolution EPR and Quantum Effects on CH 3, CH 2 D, CHD 2 and CD 3 Radicals under Argon-matrix Isolation Conditions J. Phys. Chem. A 103: Benetis N P and Dmitriev Yu A 2009 Inertial Rotation and Matrix Interaction Effects on the EPR Spectra of Methyl Radicals Isolated in “Inert” cryogenic matrices J. of Physics: Condens. Matter The isotropic lineshape for the methyl radical is not obvious because alfa protons exhibit approximately a rhombic hf interaction to the unpaired electron. The explanation was found on the assumption of nearly free radical rotation and averaging the anisotropic interaction over the rotation. #6 The average hf Hamiltonian for the |JML rotational level:

Even more striking observation concerns the EPR spectrum of the CD 3 radical in solid Ar which collapsed to a mere singlet from the commonly observed septet of the ground rotational level with intensity distribution 1:1:2:3:2:1:1. (Yamada T, Komaguchi K, Shiotani M, Benetis N P and Sørnes A R, 1999 High Resolution EPR and Quantum Effects on CH 3, CH 2 D, CHD 2 and CD 3 Radicals under Argon-Matrix Isolation Conditions J. Phys. Chem. A 103: ) Hiroshima University, Japan, X-ray radiolysis of CH 4 in Ar #7 The finding was verified by the group of Ioffe Institute, condensation from the gas phase Thorough account for the rotation, nuclear and electron degrees of freedom in constructing the wave function of the zero rotational level according to the Pauli principle led to the exclusion of certain transitions from the septet which, thus, collapsed to the totally antisymmetric singlet with respect to the D-atom exchange. 1. Yamada T, Komaguchi K, Shiotani M, Benetis N P and Sørnes A R 1999 High Resolution EPR and Quantum Effects on CH 3, CH 2 D, CHD 2 and CD 3 Radicals under Argon-matrix Isolation Conditions J. Phys. Chem. A 103: Benetis N P and Dmitriev Yu A 2009 Inertial Rotation and Matrix Interaction Effects on the EPR Spectra of Methyl Radicals Isolated in “Inert” cryogenic matrices J. of Physics: Condens. Matter

Temperature study of CD 3 radicals matrix isolated in H 2, D 2 and Ne matrices by the deposition technique. The temperature dependence of the relative intensity of the CD 3 central component (m F = 0) to the neighbor transitions (m F = ± 1) for the CD 3 radical trapped in low-temperature solids: H 2 (filled squares), D 2 (open circles), and Ne (triangles). CD 3 radical in solid D 2. Amplitudes of the CD 3 central line (triangles) and its nearest high-field neighbor (filled circles) versus temperature. The D-atom transition is used as a reference signal. Dmitriev Yu A, EPR spectra of deuterated methyl radicals trapped in low temperature matrices Low Temp. Phys. 31:5 423 – 428 #8

EPR spectrum of CH 3 radicals trapped in solid Ar. 1.Dmitriev Yu A and Zhitnikov R A 2001 EPR Study of Methyl Radicals. Anisotropy and Tumbling Motion in Low-Temperature Matrices J. Low Temp. Phys Dmitriev Yu A High Resolution EPR and the origin of the spectrum anisotropy of CH 3 radicals in Ar, Kr, and CO matrices at liquid helium temperatures Physica B: Condensed Matter Popov E, Kiljunen T, Kunttu H and Eloranta J 2007 Rotation of methyl radicals in solid argon matrix J. Chem. Phys Jyväskylä, Finland, deposition experiments EPR spectrum anisotropy for CH 3 trapped in low temperature matrices: (a) CO matrix (simulated spectrum); (b) Ar matrix (simulated); (c) Ar matrix (experimental); (d) Kr matrix (experimental). Deposition experiments which revealed the EPR anisotropy of CH 3 in solid gases #9

Dmitriev Yu A 2008 Peculiarities of EPR spectra of methyl radicals in quench-condensed krypton films Low Temp. Phys –77 The 4.2 K EPR spectrum of methyl radicals trapped in solid krypton. Resonance frequency: f res = MHz. The attenuation of the microwave power is 26 dB. Narrow lines of the doublet of “E” symmetry are marked as stars. The projection m F corresponds to the z component of the total nuclear spin. Microwave power dependent EPR spectrum of CH 3 radicals in solid Kr. The numbers in figure show the power attenuation in the cavity. E-lines both broad and narrow become prominent as the power increases. The sample temperature is 4.2 K. (m F = -1/2) (-3/2) #10

Common to all observations: 1.the anisotropy turned out to be very weak as compared to the expected powder samples of the CH 3 radical at low temperatures 2.exclusion of certain EPR transitions for the deuterated methyl isotopomers Difference: remnants of the spectrum anisotropy were found in deposition experiments and are absent in the radiolysis study even for the narrower lines. The rotation of radicals in samples deposited from vacuum, thus, shows more classical behavior than rotation of radicals obtained by radiolysis. Tentative explanation: An electronically excited molecule (radiolysis) is able to distort the matrix cage pushing outwards atoms. If the distortion is partly inelastic, the impurity molecules become “more free” to rotate. #11

Sincere gratitude to Professor Benetis for the fruitful collaboration the Russian Foundation for Basic Research for a partial financial support of the study #12