Jason D. Thompson

Parameterization of charge model 3 for AM1, PM3, BLYP, and B3LYP

We have recently developed a new Class IV charge model for calculating partial atomic charges in molecules. The new model, called Charge Model 3 (CM3), was parameterized for calculations on molecules containing H, Li, C, N, O, F, Si, S, P, Cl, and Br by Hartree–Fock theory and by hybrid density functional theory (DFT) based on the modified Perdew–Wang density functional with several basis sets. In the present article we extend CM3 to semiempirical molecular orbital theory, in particular Austin Model 1 (AM1) and Parameterized Model 3 (PM3), and to the popular BLYP and B3LYP DFT and hybrid DFT methods, respectively. For the BLYP extension, we consider the 6‐31G(d) basis set, and for the B3LYP extension, we consider three basis sets: 6‐31G(d), 6‐31+G(d), and MIDI!6D. We begin with the previous CM3 strategy, which involves 34 parameters for 30 pairs of elements. We then refine the model to improve the charges in compounds that contain N and O. This modification, involving two new parameters, leads to improved dipole moments for amides, bifunctional H, C, N, O compounds, aldehydes, ketones, esters, and carboxylic acids; the improvement for compounds not containing N results from obtaining more physical parameters for carbonyl groups when the OCN conjugation of amides is addressed in the parameterization. In addition, for the PM3 method, we added an additional parameter to improve dipole moments of compounds that contain bonds between C and N. This additional parameter leads to improved accuracy in the dipole moments of aromatic nitrogen heterocycles with five‐membered rings.

Predicting aqueous solubilities from aqueous free energies of solvation and experimental or calculated vapor pressures of pure substances

In this work, we explore the possibility of making predictions of solubilities from free-energy calculations by utilizing the relationship between solubility, free energy of solvation, and solute vapor pressure. Because this relationship is only strictly valid when all activity and fugacity coefficients are unity, it is not clear when it will hold and when it will break down for a given solute–solvent system. So we have tested the validity of this relationship using a variety of liquid solutes and solid solutes in liquid water solvent. In particular, we used a test set of 75 liquid solutes and 15 solid solutes composed of H, C, N, O, F, and Cl. First we compared aqueous free energies of solvation calculated from experimental solute vapor pressures and aqueous solubilities to experimental aqueous free energies of solvation for the 90 solutes in the test set and obtained a mean-unsigned error (MUE) of 0.26 kcal/mol. Second, we compared aqueous solubilities calculated from experimental solute vapor pressures...

Electronic Structure and Bonding in Hexacoordinate Silyl ± Palladium Complexes**

Trimerization of [Pd(R2PCH2CH2PR2){1,2-C6H4(SiH2)2}] (1; where R Me or Et) has been demonstrated to produce a trinuclear complex 2where two of the Pd atoms can be readily characterized as PdII centers but the nature of the third, central Pd atom is less clear (Scheme 1, by-products not shown).[1] While this Pd atom (Pd1) is drawn in Scheme 1 as bonding directly to two Si atoms and further interacting with two Si Si bonds, the interatomic distances from the X-ray crystal-structure data could also be interpreted to be consistent with an absence of Si Si bonds and instead six Pd Si bonds,[1] that is, the central metal would formally be PdVI. Both structures are without precedent in palladium coordination chemistry,[2, 3] although compounds of Pd[4±7] and Pt[8, 9] have been reported for inorganic compounds of the form [MFn] (M Pd, Pt; n 2,4,6), where the extreme electronegativity of fluorine is exploited for the generation of higher oxidation states.[10] To better understand the nature of the bonding in 2, we have carried out DFT calculations on 2 and relevant model compounds. With the exception of single-point calculations using the X-ray geometries of 2a and 2b, all the structures were fully optimized and verified as minima by analytic frequency calculations. The functional employed was of the hybrid variety[11] and combined exact Hartree ± Fock exchange with the gradient-corrected exchange and correlation functionals of Becke[12] and Lee, Yang, and Parr,[13] respecRu Ru

More reliable partial atomic charges when using diffuse basis sets

We present a method that alleviates some of the sensitivity to the inclusion of diffuse basis functions when calculating partial atomic charges from a Lowdin population analysis. This new method locally redistributes that part of the Lowdin population that comes from diffuse basis functions so that the final charges closely resemble those calculated without diffuse functions. We call this method the redistributed Lowdin population analysis (RLPA). The method contains one parameter for each atomic number, and we optimized the parameter for the 6-31+G(d) basis set. The method has been tested on compounds that contain H, Li, C, N, O, F, Si, P, S, Cl, and Br. For a test set of 398 compounds with experimental and high-level theoretical dipole moments, the dipole moments derived from the charges obtained by standard Lowdin population analysis have errors 35% larger than those obtained by the corresponding RLPA using the same basis set. In judging the quality of the RLPA with respect to the test set of dipole moments, we have also found that dipole moments derived from Mulliken population analysis have errors 120% larger than those derived from RLPA for the same basis set. The new method is particularly successful for the 207 systems containing only first row atoms (H, C, N, O, F) for which the errors in the dipole moments computed from the partial atomic charges obtained by standard Lowdin and Mulliken analysis are respectively 115 and 419% larger than those obtained by RLPA.