High field 31P ENDOR of MnAlPO4-20:: Direct evidence for framework substitution

The incorporation of Mn(II) into framework sites in the aluminophosphate zeotype AlPO4-20, an analog of sodalite, has been investigated using pulsed electron nuclear double resonance (ENDOR) spectroscopy at 95 GHz. The field sweep echo-detected EPR spectrum showed the presence of a single Mn(II) site with a Mn-55 hyperfine coupling of 8.7 mT. ENDOR spectra, recorded using the Mims and Davies sequences, consist of an Al-27 signal at the Larmor frequency and a P-31 doublet corresponding to a hyperfine coupling of 8 MHz. The symmetry of the doublet about the P-31 Larmor frequency indicates that it originates from the M-s = +/-1/2 manifolds and that the interaction is primarily isotropic. The relatively large P-31 hyperfine interaction and the weak interaction with Al-27 provide unique and direct evidence for Mn(II) substitution of framework Al. X-band electron spin echo envelope modulation (ESEEM) measurements showed only signals at the H-1, N-14, Al-27, and P-31 Larmor frequencies. The first two are due to weak dipolar interaction with the template molecules, while the others reflect interactions with the framework.