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

The covalent linkages formed during functionalization of MCM-41 mesoporous molecular sieves with five chloroalkylsilanes ((EtO)₃Si(CH ₂Cl), (MeO)₃Si(CH₂CH₂CH ₂Cl), Cl₃Si(CH₂CH₂CH₃), Cl₂Si(CH₃)(CH₂Cl) and Cl₂Si(CH ₃)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 D ⁿ) and the location of functional groups relative to these sites. Optimal geometries of local environments representing the Qⁿ, T ⁿ 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.