硅酸及其盐的研究 XX聚硅酸溶液中的酸强度与分子结构的关系

本文利用无机酸系列的酸强度(pKa值)与羟基上氧原子的亲电子超离域度S(E)成线性关系的规律性,找出聚硅酸系列的相应线性方程为:并由此推测低聚硅酸溶液中可能存在的硅酸结构:在三聚硅酸溶液中主要存在着六元平面环的三聚体;在四聚硅酸中则主要存在着三种环状的四聚体。

STUDIES ON SILICIC ACID AND ITS SALTS XX. RELATIONSHIP BETWEEN ACID STRENGTH AND MOLECULAR STRUCTURES OF POIYSILICIC ACLD LN SOLUTION

Brown et al. has calculated the electronic superdelocalizabilities of a series compounds of substituted methylbenzoic acids by CNDO/2 method, and found a linear relationship with acid strength. From thd curve representing the variation of dissociation constants, pKa, of the acids with the degree of polymerization, n, it is found that pKa = 9.8, 10.7, 10.2 and 9.8, for n=l, 2, 3 and 4 respectively. Sllperdelocalizability of oxygen atom in hydroxyl group is defined as,where (?) is occupation number of ith molecular orbital, C0(i) is the LCAO coefficient of oxygen atomic orbital in ith MO, mth MO is HOMO. When the linear relationship between pKa and S_0~(E) is represented as: We may calculate S_0~(E) of monomeric and dimeric silicic acid by CN DO/2 method giving 8.8180 and 8.8973 respectively, and, in consequence, a = -90.28, b= 11.33. For trimeric silicic acid, the molecular configuration may be either of linear type or of ring type. For the linear type, there are two kinds of hydroxyl group: -Si(OH)3 and Si(OH)2. Their S_0~(E) are 8.9197 and 8.9719 respectively. For the ring type, S_0~(E) is 8.8123. According to the above equation, their acid strenth(pKa) must be 10.96, 11.55, and 9.74. If 10.96 is taken as the first acid dissociation constant of the linear type trimeric acid, then we find the ring type configuration is of about 62%. The molecular configuration of tetrameric silicic acid may be either of linear types ( with and without side chain ) or of ring type. Again we choose the same bond lengths R(Si-O) = 1.6123A, R(O-H) = 1 .04A as previously, to calculate S_0~(E) by CNDO/2, and then pKa values are readily obtained. The three kinds of ring molecular configurations are suggested.