Electron Paramagnetic Resonance Study of the Dynamics of H and D Atoms Trapped in Substituted Silasesquioxane Cages

Abstract

The hyperfine coupling (hfc) constants, A, and the g-factors of hydrogen isotopes confined in silasesquioxane (R 8 Si 8 O 12 ) cages show significant deviations from the free-particle vacuum value thus revealing the influence of the environment on the wave function of the trapped atom. Accurate measurements of the hfc constants in the temperature range of 5-300 K show a strong isotope effect, different substituents on the cage corners having a clear influence on both static and dynamic cage effects. Deviations of g-factors from the freeelectron value are independent of temperature, but they depend on substituents. To gain information about the factors influencing the trapped atoms' dynamic behavior inside the cage environments, a phenomenological model developed previously for the description of the dynamics of hydrogen isotopes in liquid water and ice (J. Chem. Phys. 1995, 102, 5989), which is based on the spherical harmonic oscillator, is applied to the more rigid silasesquioxane cage systems. The present study aims at a better understanding of matrix effects on the dynamics of particles in a constraining environment, a problem which represents a challenge for both phenomenological modeling and quantum chemical calculations.

Atomic hydrogen as high-precision field standard for high-field EPR

“…To reduce the impact of the error, we added the deviation g Li,tm – g Li to g tm and averaged over several measurements. The final analysis gave g = 2.00294(3) and A = 1413.7(1) MHz, practically identical to the literature values for H in other octaalkylsilsesquioxane hosts [39]. The largest source of uncertainty are the determination of the line centers, the positioning of the teslameter probe in the cavity, and instrumental drifts in the time between spectral acquisitions and teslameter calibrations.…”

Encapsulation of atomic hydrogen into silsesquioxane cages and ESR of encapsulated hydrogen atoms

“…Moreover, Roduner et al [14] studied the effects of substitution on the hfs of hydrogen and deuterium in D4R. The temperature dependence has been determined for hydrogen and deuterium in four different cages, HT 8, MeT s, EtT s, and QsMs and the ratios of the hfs constants in the cage to that in vacuum at 0 K were estimated (Table 1) [14]. These ratios appear to correlate with the group electronegativities, which suggests spin delocalization to cage wave functions.…”

“…Roduner and co-workers [14] studied the dependence of the g-values on the temperature and terminal substituents and found that the g-value does not depend on the temperature, but does vary from 2.00273 to 2.00306 when changing the terminal substituents.…”

“…Dilger and coworkers [16] have studied the temperature dependence of the hfs constant for hydrogen, deuterium, and muonium encapsulated in EtT s and analyzed the temperature dependence with an empirical model [17] on the basis of spin delocalization in a 3-D harmonic potential. Moreover, Roduner et al [14] studied the effects of substitution on the hfs of hydrogen and deuterium in D4R. The temperature dependence has been determined for hydrogen and deuterium in four different cages, HT 8, MeT s, EtT s, and QsMs and the ratios of the hfs constants in the cage to that in vacuum at 0 K were estimated (Table 1) [14].…”

“…Deformations of ENDOR spectra were observed at approximately 130 K, which were attributed to a phase transition of the cage from cubic to axially symmetric as the temperature decreased. They have also shown that different processes affect the spin-lattice relaxation when the temperature is above or below 100 K. The temperature-dependent hfs constants were analyzed assuming vibrations in a harmonic potentiat by Roduner et al [14], as mentioned above. The deviation of hfs from monotonous change with varying temperatures detected by ENDOR is too small to be detected by ESR.…”