Methyl Radical in Clathrate Silica Voids. The Peculiar Physisorption Features of the Guest–Host Molecular Dynamics Interaction

Abstract

EPR line shape simulations of CH3/SiO2 clathrates and comparison to CH3/N2O and CH3/SiO2 experiments reveal the motional conditions of the CH3 radical up to the unusual regime of its stability, the high-temperature diffusional regime, at 300 K. In the low-temperature region, the CH3 in clathrates is found to rotate around the in-plane axes even at as low temperatures as 3.8 K. However, nonrotating methyls performing only libration about the C2-axes as well as around the C3-axis are also found, proving the existence of special sites in the clathrate voids that begin to accumulate a significant fraction of methyl radicals at temperatures below approximately 7 K. A distinctive feature in the spectrum anisotropy and line width temperature profiles is found nearby 25 K, which is interpreted as the radical physisorption inside the voids that occurs with the sample temperature lowering. The unusual increase of the CH3/SiO2 clathrate EPR spectral width with temperature over approximately 120 K has its origin in repeated angular momentum vector alterations due to frequent collisions with the clathrate void walls between periodical free rotation periods. This relaxation mechanism resembles to spin-rotation interaction known only for small molecular species in nonviscous fluids but unknown earlier for methyl hosted in solids.

Supramolecular Organization in Confined Nanospaces

“…Indeed, clathrasils have an exceptional thermal stability at normal pressure. By this virtue, even highly reactive species – such as the methyl radical and the S 2 molecule – can be characterized spectroscopically . For example, femtosecond pump‐probe experiments captured the photodissociation and recombination reactions of iodine molecules inside the cages of decadodecasil‐3R zeosil (DDR) .…”

“…By this virtue, even highly reactive species -such as the methyl radical [321] and the S 2 molecule [322] -can be characterized spectroscopically. [323] For example, femtosecond pump-probe experiments captured the photodissociation and recombination reactions of iodine molecules inside the cages of decadodecasil-3R zeosil (DDR). [324] Such experimental analyses provide relevant insight on the dynamics and reactivity of confined molecules -which might be also potentially useful in the realization of confined quantum dots.…”

Low-temperature tunneling of CH3 quantum rotor in van der Waals solids

“…It changes from nearly symmetric for methyl radical in solid p-H 2 [12], to asymmetric spectrum with split anisotropic lines and additional satellite transitions of non-rotating methyls, in cold solids consisting of the linear CO 2 , N 2 O molecules. The above two sets of main and additional transitions are characterized by either isotropic or axial spin-Hamiltonian vs. rhombic spin-Hamiltonian, respectively [13,14], depending on the actual dispersion forces between matrix and CH 3 radicals [15]. The experimental anisotropy variation of the EPR spectra of methyl radical in the present work was related to increasing rotational rate of the radical with temperature increase.…”

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

“…However, the multiplets are superimposed in such a way that the doublet transitions nearly coincide with the two central lines of the quartet. In the last two decades, high-resolution EPR spectroscopy of methyl made possible the close inspection of the contribution of the A - and E -state radicals to the EPR spectrum, including the residual anisotropy of methyls in both states. − The matrices thus studied by high-resolution EPR include solid noble gases, Ne, Ar, Kr, molecular hydrogens, H 2 , D 2 , and matrices of linear molecules, N 2 , CO, N 2 O, CO 2 , and SiO 2 clathrate. In all these cases the matrix molecules were assumed not to perform any rotational motion.…”