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

Abstract

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.