Isolation of the CH₃˙ rotor in a thermally stable inert matrix: first characterization of the gradual transition from classical to quantum behaviour at low temperatures

Abstract

Matrix isolation is a method which plays a key role in isolating and characterizing highly reactive molecular radicals. However, the isolation matrices, usually composed of noble gases or small diamagnetic molecules, are stable only at very low temperatures, as they begin to desegregate even above a few tens of Kelvin. Here we report on the successful isolation of CH3˙ radicals in the cages of a nearly inert clathrate-SiO2 matrix. This host is found to exhibit a comparable inertness with respect to that of most conventional noble gas matrices but it is characterized by a peculiar thermal stability. The latter property is related to the covalent nature of the host material and gives the opportunity to study the confined radicals from a few degrees of Kelvin up to at least room temperature. Thanks to this advantage we were able to explore with continuity for the first time the CH3˙ rotor properties by electron paramagnetic resonance spectroscopy, starting from the quantum rotations which are observable only at the lowest temperatures (T ≈ 4 K), going through the gradual transition to the classical motion (4 K < T < 30 K), and ending with the properties of the fully classical rotor (T > 30 K). The method of isolation presented here is found to be very effective and promising, as it is expected to be applicable to a large variety of different molecular radicals.

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) .…”

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

“…The EPR measurements were carried out with a Bruker EMX spectrometer working at a frequency of about 9.8 GHz (X-band) and with a magnetic-field modulation frequency of 100 kHz. The spectra were recorded in the temperature range from 4 to 300 K using a standard Oxford He cryostat …”

“…The Raman spectrum obtained by Buscarino et al for a melanophlogite sample from Fortullino (Italy) shows two peaks at 2902 and 2911 cm –1 , thus suggesting CH 4 molecules enclathrated in the two types of cages.…”

“…Our present study is based on the analysis of the EPR line shape peculiarities. It turned out that the temperature dependence of the broadening of the CH 3 EPR line shapes in the SiO 2 clathrate melanophlogite is opposite compared to the SG isolation. In particular, at the highest experimental temperatures where the anisotropy of the EPR spectra of the CH 3 in SG isolation decreases by regular motional narrowing, the broadening of the CH 3 in melanophlogite increases.…”

“…It is very important to notice that no spectroscopic study is available in the literature, except for the EPR investigation by Buscarino et al . revealing the temperature transformations of the guest-molecule spectrum in the range 4–300 K. The method employed by Kolesov and Geiger turned out to be insensitive to the temperature changes.…”