Acidities of arsenic (III) and arsenic (V) thio- and oxyacids in aqueous solution using the CBS-QB3/CPCM method.

A number of calculations of acidic As(III) and As(V) species formed with sulfur and oxygen (H(3)AsS(3), H(2)AsS(3)(-), HAsS(3)(2-), AsS(3)(3-), H(3)AsS(4), H(2)AsS(4)(-), HAsS(4)(2-), AsS(4)(3-), H(3)AsO(3), H(2)AsO(3)(-), HAsO(3)(2-), AsO(3)(3-), H(3)AsO(4), H(2)AsO(4)(-), HAsO(4)(2-), and AsO(4)(3-)) are presented. pK(a)s for successive deprotonations in both the gas phase and aqueous solution (using both explicit water molecules and a self-consistent reactive field conductive polarizable continuum model (SCRF CPCM) for solvation) are fitted to known experimental values for the H(3)PO(4) and H(3)AsO(4) series' of deprotonations with a linear extrapolation showing r(2) values of 0.97 for the CBS-QB3 method with a single explicit water molecule in the CPCM.

Explaining the effects of T-O-T bond angles on NMR chemical shifts in aluminosilicates: A natural bonding orbital (NBO) and natural chemical shielding (NCS) analysis.

It has long been recognized that the 29Si and 27Al NMR chemical shifts for aluminosilicate crystals and glasses correlate to some extent with the T-O-T bond angle (where T is the tetrahedral atom Si or Al). With increasing T-O-T bond angle, the 29Si and 27Al NMR shieldings increase and the shifts thus become more negative. This result has been demonstrated both experimentally and through quantum computations.

(29)Si NMR shifts and relative stabilities calculated for hypercoordinated silicon-polyalcohol complexes: role in sol-gel and biogenic silica synthesis.

Penta- and hexa-coordinated silicon is rare, occurring as a transient species in some glasses, nonaqueous organosilicon solutions and organosilicon gels such as silicone, and is stable at high pressures within the earth in dense phases such as stishovite. The stable form expected in aqueous solution is quadra-coordinated silicon. A recent study proposed the existence of hypercoordinated silicon-polyalcohol complexes in aqueous solution, based on (29)Si NMR shifts at -102 to -103 ppm and -145 to -147 ppm.