Anomalous EPR Intensity Distribution of the Methyl Radical Quartet Adsorbed on the Surface of Porous Materials. Comparison with Solid Gas Matrix Isolation
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- 作者：Nikolas P. Benetis ; Yurij Dmitriev
- 刊名：The Journal of Physical Chemistry A
- 出版年：2013
- 出版时间：May 23, 2013
- 年：2013
- 卷：117
- 期：20
- 页码：4233-4250
- 全文大小：570K
- 年卷期：v.117,no.20(May 23, 2013)
- ISSN：1520-5215
文摘
The two inner lines of the EPR quartet of methyl radicals trapped in cryogenic gas matrices are superpositions of the inner transitions of an A-proton-spin quartet and an E-proton-spin doublet. Their intensity relative to the outer lines provides information on the population of the methyl-rotation quantum states. The above intensity ratio for the CH3 in solids is a challenging problem of the quantum dynamics and statistical thermodynamics. The influence of the quantum-mechanical/inertial rotation on the intensity distribution of the hf components of methyl radical on the surface of porous materials, e.g., silica gel, is investigated by EPR line shape simulations and compared with spectra of the radical isolated in the bulk of solid gas samples. The experimental part of this study includes the first in literature EPR observation of methyl radical in the bulk of N2O solid and provides new essential information on CH3 in CO2 and Ar matrices, thus, covering both strongly hindered and almost free rotation of the radical. We verify the observation of nonrotating methyl radicals in a N2O matrix, discovered earlier in cold CO2, give a thorough account of their EPR characteristics, and explore their formation at the inner surface of porous materials. Combination of a classical spin-Hamiltonian with employment of quantum effects due to nuclear spin-rotation coupling and the radical symmetry were used to interpret the experimental spectral observations. The cause of experimentally found unexpected contribution of the excited degenerate E-doublets to the EPR spectrum down to 4.2 K and A/E transition amplitude ratios sometimes as high as ca. 1:8 at liquid-N2 temperature is sought. The validity of Bose-Einstein quantum (BEq-) statistics of the spin rotation states in addition to the classical Maxwell-Boltzmann (Boltzmann) statistics was also assessed against experimental population A/E-ratio data. The BEq-statistics were not previously applied to similar systems. Furthermore, detailed consideration of the laboratory/free space rotational degeneracy and the planarity of methyl radical was also included in this work.