High pressure electron spin resonance of the endohedral fullerene $^{15}\mathrm{N@C}_{60}$

TL;DR

This study investigates how applying pressure affects the electron spin resonance of $^{15} ext{N@C}_{60}$ fullerenes, revealing a linear increase in hyperfine coupling and pressure-dependent linewidth changes explained by a van der Waals interaction model.

Contribution

The paper introduces a pressure-dependent ESR measurement of $^{15} ext{N@C}_{60}$ and a model linking hyperfine coupling increase to cage compression and orbital admixture.

Findings

Hyperfine coupling increases linearly with pressure.

The model accurately predicts the pressure shift based on van der Waals interactions.

Linewidth depends on solvent viscosity and dipolar interactions.

Abstract

We measure the electron spin resonance spectrum of the endohedral fullerene molecule $^{15}\mathrm{N@C}_{60}$ at pressures ranging from atmospheric pressure to 0.25 GPa, and find that the hyperfine coupling increases linearly with pressure. We present a model based on van der Waals interactions, which accounts for this increase via compression of the fullerene cage and consequent admixture of orbitals with a larger hyperfine coupling. Combining this model with theoretical estimates of the bulk modulus, we predict the pressure shift and compare it to our experimental results, finding fair agreement given the spread in estimates of the bulk modulus. The spin resonance linewidth is also found to depend on pressure. This is explained by considering the pressure-dependent viscosity of the solvent, which modifies the effect of dipolar coupling between spins within fullerene clusters.