Molecular precursors of mesostructured silica materials in the atrane route:: A DFT/GIAO/NBO theoretical study

Quantum chemical calculations using density functional theory have been carried out to investigate two assumed molecular precursors and identified as silatranes (N[OCH2CH2](3)Si-OCH2CH2N-(CH2CH2OH)(2) and N[OCH2CH2](3)Si-OCH2CH2N-(CH2CH2OH)(2)Na+) which are present in the synthesis of mesoporous silica based material namely "the atrane route". One of the ways in this synthesis leads to the well-known MCM-41. Additionally, in this work has been also investigated two others molecules such as triethanolamine (TEAH3) and sodatrane which are present in the medium. Gas phase and solution equilibrium geometries of the previous molecules were fully optimized at B3LYP level, modeling solvent effects using a self-consistent reaction field (SCRF) Onsager model, a discrete electrostatic representation of triethanolamine molecules in the first solvation shell. Calculations of the H-1 and (13) C NMR chemical shifts at GIAO/B3LYP/6-31G(d,p) levels of theory are also presented. The importance of basis set is previously discussed in the GIAO calculations. Finally, a natural bond orbital (NBO) analysis has been performed to provide insight into the role of electronic delocalization in the chemical shielding. This work has shown that the assumed geometries of the molecular precursors that have been deduced from a previous work on a FAB-MASS analysis (Fast Atomic Bombardment Mass Spectroscopy) are confirmed by DFT calculations. The theoretical study on the chemical shifts of nucleus as (13) C and H-1 from full geometry optimizations of the molecular precursors and sodatrane describes hydrogens bonded to each CH2O and CH2N groups as not equivalent; in the same way, it appears that some carbons are not magnetically equivalent. GIAO calculations from a full geometric optimization of TEAH3 overestimate the experimental chemical shifts; however, a threefold-symmetry (C3) of optimized geometry yields an improvement of the theoretical chemical shifts. The discrepancies in the GIAO calculations might be related to the hyperconjugative interactions such as LPO1 -> (sigma*(C-X), LPO2 -> sigma*(C-X) and LPN -> sigma*(C-X) with X = C, H and also sigma -> sigma*(C-X) with X = O; N, H. These interactions are characterized by delocalization energies which are generally important and peculiarly for LP -> (sigma*(C-X). A reasonable level of agreement between GIAO calculations and experimental NMR studies is obtained which confirms the presence of the molecular precursors in the "atrane route" and taking into account the hyperconjugative interactions might explain some discrepancies in GIAO calculations. (C) 2007 Published by Elsevier B.V.