High-Resolution 27Al NMR Shift Calculation for Aluminosilicate Species Structural Units (Qn)/Q by Gaussian, Deconvolution and Resolution Enhancement Methods

Hartree-Fock self-consistent-field (HF-SCF) theory and the Gauge-including atomic orbital (GIAO) methods are used in calculation of 27Al NMR chemical shifts for units of compounds of various aluminate species of precursors for zeolites. Calculations were performed at geometries optimized at AM1 semi-empirical method. The GIAO-HF-SCF calculations were carried out using three different basis sets: 6-31G*, 6-31+G** and 6-311+G(2d, p). The intensity of partially overlapping NMR peaks may be separated by deconvolution into individual Gaussian component peaks. The relative concentrations of aluminosilicate species present as q1, q2, q3 and q4 structural units, have been estimated from integrated intensities of the corresponding signals. It is pertinent to notice that the spectra in and the estimated data showed that the intensity of q4 (the band at lowest frequency), seems to increase as Si/Al mole ratio (at constant Si=0.875 and TMAOH=1 M) decreases, i.e. with increasing aluminates concentration. However, the bands become broader as the concentration of Al increases, so that operation of backward linear prediction (BLP) is of variable effect. From this study, one can expect that cage-like species might be dominant due to the stability of the corresponding silicate anions in TMAOH silicate solutions. The formation of three-member rings apparently causes some deshilding compared to chains or larger rings.