| Abstract: 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=1, 2, 3 and 4 respectively. Superdelocalizability of oxygen atom in hydroxyl group is defined as,where ni 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 S0E is represented as:We may calculate S0E 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 S0E are 8.9197 and 8.9719 respectively. For the ring type, S0E 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 S0E by CNDO/2, and then pKa values are readily obtained. The three kinds of ring molecular configurations are suggested. |
