29Si NMR Chemical Shift Calculation for Silicate Species by Gaussian Software

Hartree–Fock self-consistent-field (HF-SCF) theory and the Gauge-including atomic orbital (GIAO) methods are used in the calculation of 29 Si NMR chemical shifts for ABOUT 90 units of 19 compounds of various silicate species of precursors for zeolites. Calculations have been performed at geometries optimized at the AM 1 semi-empirical method. The GIAO-HF-SCF calculations were carried out with using three different basis sets: 6-31G * , 6-31+G ** and 6-311+G(2d,p). To demonstrate the quality of the calculations the calculated chemical shifts, δ, were compared with the corresponding experimental values for the compounds in study. The results, especially with 6-31+g ** are in excellent agreement with experimental values. The calculated chemical shifts, in practical point of view, appear to be accurate enough to aid in experimental peak assignments. The difference between the experimental and calculated 29 Si chemical shift values not only depends on the Q n units but also it seems that basis set effects and the level of theory is more important. For the series of molecules studied here, the standard deviations and mean absolute errors for 29 Si chemical shifts relative to TMS determined using Hartree–Fock 6-31+G ** basis is nearly in all cases smaller than the errors for shifts determined using HF/6-311+G(2d,p).