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)(3)Si(CH2Cl), (MeO)(3)Si(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 H-1-C-13 and H-1-Si-29 heteronuclear (HETCOR) NMR spectra, in which high resolution in the H-1 dimension was obtained by using fast MAS. The H-1-C-13 HETCOR results provided the assignments of H-1 and C-13 resonances associated with the surface functional groups. Sensitivity-enhanced H-1-Si-29 HETCOR spectra, acquired using Carr-Purcell-Meiboom-Gill refocusing during data acquisition, revealed the identity of Si-29 sites (Q(n), T-n, and D-n) and the location of functional groups relative to these sites. Optimal geometries of local environments representing the Q(n), T-n and D-n resonances were calculated using molecular mechanics and ab initio methods. Subsequently, DFT calculations of Si-29, C-13, and H-1 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