Quantum Impurity Rotator in a Matrix of Quantum Rotors: Electron Paramagnetic Resonance Dynamics of CH₃ in Solid CD₄ Matrix

Abstract

The rotational dynamics and the geometry of a light and flexible impurity molecule like methyl, matrix isolated in van der Waals solid, are supposed to be sensitive to the host molecule dynamics and order alterations of the matrix. In addition, the location of the impurity and its interaction with the matrix molecules is of prime importance. Large energy gaps between rotation levels of quantum rotators allow precise investigation of temperature-assisted quantum tunneling effects. The molecular rotation of methyl radicals isolated in the deuterated solid methane isotopomer, CD4, was investigated both by experimental and theoretical electron paramagnetic resonance (EPR) methods. The reduction of the quantum rotation frequency evident from the EPR spectrum of methyl radical at liquid-He temperatures was explained and connected to the irregular ratio of the central doublet to the outer quartet hf transitions. The involvement of temperature in the alteration of methyl symmetry between the C3 and D3 groups and the molecular host–host and guest–host interaction strengths were also examined by constructing temperature profiles of the rotation correlation times in the three phases of solid methane. The present study proves the deep impact that a van der Waals matrix may have on the geometry and the rotation levels of a substitutionally trapped quantum impurity rotor, effects that are yet very little known. This close correlation between dynamics of an impurity particle and the matrix molecules has great potential in developing sensitive physicochemical probes for van der Waals solids.

Anisotropic saturation of the EPR spectrum of an DCO radical stabilized in solid CO at 4.2 K and its orientation motion

“…The theory of orientation motion of both matrix and impurity molecules in solids obviously needs further development. The most interesting is the case of van der Waals solids at cryogenic temperatures, when correlation of angular motion and, in particular, quantum tunneling of matrix molecules and impurity particle [27,28] is possible. It should be assumed that when an impurity particle is embedded in the substitutional position of the van der Waals crystal lattice, " adaptation " (relaxation) of the matrix and the impurity particle itself occurs, which for an impurity particle can manifest itself, for example, in a change in its symmetry [27][28][29].…”

Анизотропное Насыщение Спектра ЭПР Радикала DCO, Стабилизированного В Твердом CO При 4.2 K, И Его Ориентационное Движение

“…Теория ориентационного движения как матричных, так и примесных молекул в твердых телах, очевидно, нуждается в дальнейшем развитии. Наиболее интересен случай ван-дер-ваальсовых твердых тел при криогенных температурах, когда возможна корреляция углового движения и, в частности, квантового туннелирования матричных молекул и примесной частицы [27,28]. Следует предположить, что при встраивании примесной частицы в замещающее положение кристаллической решетки ван-дер-ваальсового кристалла происходит " адаптация" (релаксация) матрицы и самой примесной частицы, что для примесной частицы может проявиться, например, в изменении ее симметрии [27][28][29].…”