Cage effect on stability and molecular dynamics of [(CH3)3N]•+ and [(CH3)3NCH2]•+ generated in γ-irradiated zeolites

Abstract

An EPR study was carried out to elucidate the zeolite cage effect on the stability and molecular dynamics of [(CH₃)₃N]^•+ and [(CH₃)₃NCH₂]^•+ radical cations generated in γ-irradiated Al-offretite, SAPO-37 and SAPO-42, which contained (CH₃)₄N⁺ ions as organic template. The [(CH₃)₃N]^•+ radical cation was stable at room temperature when it was generated in relatively small size cages such as the sodalite cages of SAPO-37 and the β-cages of SAPO-42. On the other hand, the [(CH₃)₃NCH₂]^•+ radical cation was stable not only in the sodalite cages of SAPO-37, but also in relatively large cages such as gmelinite cages or the main channels of Al-offretite and the α-cages of SAPO-42. Strongly temperature dependent EPR spectra were observed for the [(CH₃)₃NCH₂]^•+ radical cation stabilized in Al-offretite and SAPO-42. They were successfully analyzed by a spectral simulation method assuming a three-site exchange for the methyl protons. The evaluated exchange rates were in the order SAPO-37 (sodalite cages)<Al-offretite (gmelinite cages or main channels)<SAPO-42 (α-cages) in the temperature range 110–300 K.