The original force field for clay materials (ClayFF) developed by Cygan et al. (J. Phys. Chem. B 2004, 108, 1255 is modified to describe negative charging of the (101) quartz surface above its point of zero charge (pH ≈ 2.0−4.5). The modified force field adopts the scaled natural bond orbital charges derived by the quantum mechanical calculations which are used to obtain the desired surface charge density and to determine the delocalization of the charge after deprotonation of surface silanol groups. Classical molecular dynamics simulations (CMD) of the ( 101) surface of α-quartz with different surface charge densities (0, −0.03, −0.06, and −0.12 C m −2 ) are performed to evaluate the influence of the negative surface charge on interfacial water and adsorption of Na + , Rb + , and Sr 2+ ions. The CMD results are compared with ab initio calculations, X-ray experiment, and the triple-layer model. The modified force field can be easily implemented in common molecular dynamics packages and used for simulations of interactions between quartz surfaces and various (bio)molecules over a wide range of pH values.

The mechanism of substitution water exchange reactions in square planar trans-Pt[(NH(3))(2)T(H(2)O)](n+) complexes is studied (T = H(2)O, NH(3), OH(-), F(-), Cl(-), Br(-), H(2)S, CH(3)S(-), SCN(-), CN(-), PH(3), CO, CH(3)(-), H(-), C(2)H(4)). The trans effect is explained in terms of sigma-donation and pi-back-donation whose relative strengths are quantified by the changes of electron occupations of 5d platinum atomic orbitals. The sigma-donation strength is linearly correlated with the Pt-H(2)O (leaving ligand) bond length (trans influence). The kinetic trans effect strength correlates proportionally with the sigma-donation ability of the trans-ligand except the ligands with strong pi-back-donation ability that stabilizes transition state structure. The sigma-donation ability of the ligand is dependent on the sigma-donation strength of the ligand in the trans position. Therefore the trans effect caused by sigma-donation can be understood as a competition between the trans-ligands for the opportunity to donate electron density to the central Pt(II) atom. The influence of the trans effect on the reaction mechanism is also shown. For ligands with a very strong sigma-donation (e.g. CH(3)(-) and H(-)), the substitution proceeds by a dissociative interchange (I(d)) mechanism. Ligands with strong pi-back donation ability (e.g. C(2)H(4)) stabilize the pentacoordinated intermediate and the substitution proceeds by a two step associative mechanism. For ligands with weak sigma-donation and pi-back-donation abilities, the highest activation barriers have to be overcome and substitutions can be described by an associative interchange (I(a)) mechanism. The results are supported by the energy decomposition and the natural orbital analysis.

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In this paper we explore cisplatin interactions with sulfur-containing amino acids in a polarizable continuum model. Two cisplatin hydrated complexes were considered as reactants (chloro complex, cis-[Pt(NH3)2Cl(H2O)]+; hydroxo complex, cis-[Pt(NH3)2(OH)(H2O)]+). We considered the following reaction mechanism: first step, substitution of the aqua ligand by amino acid; second step, dissociative chelate formation. For the optimized complex (at the B3LYP/6-31+G(d)/COSMO level), the energy profile was determined using the B3LYP/6-311++G(2df,2pd) level and two different PCM models-COSMO and UAKS/DPCM methods which were adapted for use on transition metal complexes. The results show thermodynamic preference for bonding by cysteine sulfur followed by the amino group nitrogen, methionine thioether sulfur, and carboxyl-group oxygen. Methionine slightly prefers the Pt-N(Met) coordination in the chloro complex, but in the hydroxo complex it prefers the Pt-S(Met) coordination. A similar trend follows from the bonding energies: BE(Pt-S(Cys)) = 80.8 kcal/mol and BE(Pt-N(Met)) = 76 kcal/mol. According to the experimental observations, the most stable structures found are kappa2(S,N) chelates. In the case of methionine, the same thermodynamic stability is predicted also for the kappa2(N,O) chelate. This differs from the gas-phase results, where kappa2(S,N) and even kappa2(S,O) were found to be more stable than kappa2(N,O) complex.