Experimental and theoretical investigations into the counter-intuitive shift in the antisymmetric ν(Si–O) vibrational modes upon deuteration of solvated silicic acid (H4SiO4)

The IR and Raman spectra of fully deuterated silicic acid (D4SiO4) have been obtained for the first time in solution and contrasted with the analogous spectra of H4SiO4. The IR spectra feature antisymmetric ν(SiO) stretching modes at 939 and 951 cm−1 for H4SiO4 and D4SiO4 respectively. The observed increase in frequency of the ν(SiO) modes upon deuteration is contrary to the expected effect of increasing the reduced mass. Broader and weaker bands due to δ(SiOX), X = H or D, deformations occur in the IR spectra at ∼1100 and 800 cm−1, respectively. The symmetric ν(SiO) modes in the Raman occur at 787 and 764 cm−1 for H4SiO4 and D4SiO4, respectively, and exhibit the expected decrease in frequency upon deuteration. To analyse these phenomena, RB3LYP/6-31+G(d) calculations were initially performed on gaseous H4SiO4 and D4SiO4. Significant coupling between antisymmetric ν(SiO) and δ(SiOH) deformations, calculated to be of comparable frequency, is noted in gaseous H4SiO4. The calculated frequencies of the δ(SiOD) modes in gaseous D4SiO4 occur ∼200 cm−1 lower than those of ν(SiO) vibrations and the modes are not coupled. However, the predicted gas phase frequencies of δ(SiOX) modes are ∼200 cm−1 lower than those observed for the solvated H(D)4SiO4, and the observed reverse isotope shift of the antisymmetric ν(SiO) is not reproduced. Inclusion of a highly disordered 28 water solvation shell is found to significantly stiffen δ(SiOX) modes, leading to a significant blue-shift of 200–300 cm−1 relative to analogous gas-phase frequencies and providing an accurate description of the bending modes. This frequency shift decouples the ν(SiO) and δ(SiOH) modes in H4SiO4 but leads to coupling in solvated D4SiO4 which is responsible for the observed reverse isotope shift of the antisymmetric ν(SiO) modes.