Characterization of covalent linkages in organically functionalized MCM-41 mesoporous materials by solid-state NMR and theoretical calculations

Abstract

The covalent linkages formed during functionalization of MCM-41 mesoporous molecular sieves with five chloroalkylsilanes ((EtO)₃Si(CH₂Cl), (MeO)₃Si(CH₂CH₂CH₂Cl), Cl₃Si(CH₂CH₂CH₃), Cl₂Si(CH₃)(CH₂Cl) and Cl₂Si(CH₃)₂) have been investigated using high-resolution solid-state NMR spectroscopy and DFT calculations. Structural information was obtained from ¹H-¹³C and ¹H-²⁹Si heteronuclear (HETCOR) NMR spectra, in which high resolution in the ¹H dimension was obtained by using fast MAS. The ¹H-¹³C HETCOR results provided the assignments of ¹H and ¹³C resonances associated with the surface functional groups. Sensitivity-enhanced ¹H-²⁹Si HETCOR spectra, acquired using Carr-Purcell-Meiboom-Gill refocusing during data acquisition, revealed the identity of ²⁹Si sites (Qⁿ, Tⁿ, and Dⁿ) and the location of functional groups relative to these sites. Optimal geometries of local environments representing the Qⁿ, Tⁿ and Dⁿ resonances were calculated using molecular mechanics and ab initio methods. Subsequently, DFT calculations of ²⁹Si, ¹³C, and ¹H chemical shifts were performed using Gaussian 03 at the B3LYP/6-311++G(2d,2p) level. The theoretical calculations are in excellent accord with the experimental chemical shifts. This work illustrates that state-of-the-art spectroscopic and theoretical tools can be used jointly to refine the complex structures of inorganic-organic hybrid materials.