A theoretical investigation on the proton transfer tautomerization mechanisms of 2-thioxanthine within microsolvent and long range solvent.

A relative complete study on the mechanisms of the proton transfer reactions of 2-thioxanthine was carried out with density functional theory. The models were designed with monohydrated and dihydrated microsolvent catalyses either with or without the presence of water solvent considered with the polarized continuum model (PCM). A total number of 114 complexes and 67 transition states were found with the B3LYP/6-311+G** calculations. The energies were refined with both B3LYP/aug-cc-pVTZ and PCM-B3LYP/aug-cc-pVTZ methods. The activation energies were reported with respect to the Gibbs free energies obtained in conjunction with the standard statistical thermodynamics. Possible reaction pathways were confirmed with the intrinsic reaction coordinates. Pathways via C8 atom on the imidazole ring, via the bridged C4 and C5 atoms between pyrimidine and imidazole rings and via N, O and S atom on the pyrimidine ring were examined. The results show that the most feasible pathway is the proton transfers within the long range solvent surrounding via the N, O and S atoms in the pyrimidine ring with di-hydrated catalysis: N(7)H + 2H2O → IM89 → IM90 → P13 + 2H2O → IM91 → IM92 → P6 + 2H2O → IM71 → IM72 → P7 + 2H2O → IM107 → IM108 → P18 + 2H2O → IM111 → IM112 → P19 + 2H2O → IM113 → IM114 → P17 + 2H2O → IM105 → IM106 → N(9)H + 2H2O that has the highest energy barrier of 44.0 kJ mol(-1) in the transition of IM89 to IM90 via TS54. The small energy barrier is in good agreement with the experimental observation that 2-TX tautomerizes at room temperature in water. In the aqueous phase, the most stable intermediate is found to be IM21 [N(7)H + 2H2O] and the possible co-existing species are the monohydrated IM1, IM9, IM39 and IM46, and the di-hydrated IM5, IM8, IM13, IM16, IM81, IM89, IM90, IM91 and IM106 complexes that have a relative concentration larger than 10(-6) (1 ppm) with respect to IM21.

Does the copolymer poly(vinylidene cyanide-tricyanoethylene) possess piezoelectricity?

The geometry, energy, internal rotation barrier, dipole moment, and molecular polarizability of the α- and ß-chain models of poly(vinylidene cyanide-tricyanoethylene) [P(VDCN-TrCN)] were studied with density functional theory at the B3PW91/6-31G(d) level. The effects of the chain length and the TrCN content on the copolymer chain stability, the chain conformation, and the electrical properties of P(VDCN-TrCN) were examined and compared with those of poly(vinylidene fluoride-trifluoroethylene) and PVDCN to gauge whether P(VDCN-TrCN) would be expected to possess substantial piezoelectricity. The results of this study showed that the stability of the ß conformation increases and the energy difference per monomer unit between the ß- and α-chains decreases with increasing TrCN. However, introducing TrCN into VDCN will not significantly enhance the radius of curvature of the P(VDCN-TrCN) chains. The average dipole moment per monomer unit in the ß-chain is affected by the chain curvature and the TrCN content. The amount of piezoelectricity present in P(VDCN-TrCN) is slightly smaller than that in PVDCN, and is less than that in poly(vinylidene fluoride-trifluoroethylene).