JPCB, van Eck, 27Al-13C NMR of alkoxides

J. Phys. Chem. B, ASAP Article 10.1021/jp055058k S1520-6106(05)05058-3
Web Release Date: March 15, 2006

Multinuclear Solid-State High-Resolution and¹³C -{²⁷Al} Double-Resonance Magic-Angle Spinning NMR Studies on Aluminum Alkoxides

Anuji Abraham, Roel Prins, and Jeroen A. van Bokhoven*

Institute for Chemical and Bioengineering, Swiss Federal Institute of Technology (ETH), Zurich 8093, Switzerland

Ernst R. H. van Eck* and Arno P. M. Kentgens

Department of Physical Chemistry - Solid-State NMR, Institute for Molecules and Materials, Radboud University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, Netherlands

Abstract:

A combination of ²⁷Al magic-angle spinning (MAS)/multiple quantum (MQ)-MAS, ¹³C-¹H CPMAS, and ¹³C-{²⁷Al} transfer of population in double-resonance (TRAPDOR) nuclear magnetic resonance (NMR) were used for the structural elucidation of the aluminum alkoxides aluminum ethoxide, aluminum isopropoxide, and aluminum tertiarybutoxide. Aluminum alkoxides exist as oligomers with aluminum in different coordinations. High-resolution ²⁷Al MAS NMR experiments with high-spinning speed distinguished the aluminum atoms in different environments. The ²⁷Al MAS NMR spectrum gave well-resolved powder patterns with different coordinations. Z-filter MQ-MAS was performed to obtain the number and types of aluminum environments in the oligomeric structure. ¹³C-¹H CPMAS chemical shifts resolved the different carbon species (-CH₃, =CH₂, =CH-, and =C=) in the structures. ¹³C-{²⁷Al} TRAPDOR experiments were employed to obtain relative Al-C dipolar interactions and to distinguish between terminal and bridging alkoxides in the crystallographic structures. The complete characterization of selected aluminum alkoxides using advanced NMR methods has evidenced the tetrameric structure for aluminum isopropoxide and the dimeric structure for aluminum tertiary-butoxide, as reported in the literature, and proposed a polymeric structure for aluminum ethoxide.