A simple model for calculating atomic charges in molecules

We propose a new atomic-charge analysis, termed adjusted charge partitioning (ACP) scheme. To partition the molecular electronic density into atomic components, weighting factors cAr2n-2exp(-αAr) with atomic parameters cA and αA are used. Extensive numerical tests were performed for 540 molecules containing 17 main-group elements H, Li to F, Na to Cl, Br, and I. The estimated dipole moments and atomic charges are compared with the data provided by a large number of alternative atomic-charge schemes including the Mulliken, Löwdin, Hirshfeld, Hirshfeld Iterative, CM5, ESP, NPA, and QTAIM population analyses. These tests show that the resulting atomic charges are insensitive to basis sets used, chemically consistent and accurately reproduce experimental dipole moments. © 2018 the Owner Societies

Iterative Atomic Charge Partitioning of Valence Electron Density

We propose an atomic charge partitioning scheme, iterative adjusted charge partitioning (I-ACP), belonging to the stockholder family and based on partitioning of the valence molecular electron density. The method uses a Slater-type weighting factor cAr2n–2exp(–αAr), where αA is a fixed parameter and cA is determined iteratively. The parameters αA were fitted for 17 main-group elements. The I-ACP scheme is shown to produce consistent, chemically meaningful atomic charges. Several stockholder-type charge-partitioning are compared. Extensive numerical tests demonstrate that in most cases, I-ACP surpasses most other methods by reproducing more accurately molecular dipole moments. © 2018 Wiley Periodicals, Inc. © 2019 Wiley Periodicals, Inc

Fast and accurate calculation of hydration energies of molecules and ions

We present an efficient method with adjustable parameters for calculating the hydration free energy of molecules and ions using the gas-phase geometry and atomic charges. In most cases, the method yields accurate results, with a mean absolute error for neutral molecules below 1 kcal mol-1 and for ions about 3 kcal mol-1. Overall, despite its simplicity, the proposed method shows the best performance among major computational approaches applied to estimate hydration free energies. The method requires as input Cartesian cordinates and CM5 atomic charges only, which are easily available from any DFT calculation, and yields the hydration energy in a matter of seconds for a medium-size molecule or ion. © the Owner Societies

A simple COSMO-based method for calculation of hydration energies of neutral molecules

A simple, non-iterative method to estimate hydration free energies of neutral molecules, ESE, is developed. It requires only atomic charges computed for isolated species. To obtain the solvation free energy, the COSMO electrostatic term is supplemented by an extra correction that describes the cavitation energy, van der Waals and specific interactions. This term depends on atomic parameters that are adjusted using a reference dataset. Despite its simplicity, the ESE method provides accurate hydration energies with a mean absolute error below 1 kcal mol-1, superseding most accurate existing polarization continuum methods. We show that the proposed scheme can be directly extended to non-aqueous solutions. © 2019 the Owner Societies

Mechanism of olefin metathesis with catalysis by ruthenium carbene complexes: Density functional studies on model systems

Gradient-corrected (BP86) density functional calculations were used to study alternative mechanisms of the metathesis reactions between ethene and model catalysts [(PH3)(L)- Cl2Ru=CH2] with L=PH3 (I) and L = C3N2H4=imidazol-2-ylidene (II). On the associative pathway, the initial addition of ethene is calculated to be rate-determining for both catalysts (DeltaG(298)(not equal) approximate to 22 - 25 kcal mol(-1)). The dissociative pathway starts with the dissociation of phosphane, which is rather facile (DeltaG(298)(not equal) approximate to5-10 kcal mol(-1)). The resulting active species (L)Cl2Ru=CH2 can coordinate ethene cis or trans to L. The cis addition is unfavorable and mechanistically irrelevant (AG(298)(not equal) approximate to 21-25 kcal mol(-1)). The trans coordination is barrierless, and the rate-determining step in the subsequent catalytic cycle is either ring closure of the a complex to yield the ruthenacyclobutane (catalyst I, DeltaG(298)(not equal) = 12 kcal mol(-1)), or the reverse reaction (catalyst II, ring opening, DeltaG(298)(not equal) = 10 kcal mol(-1)), that is, II is slightly more active than I. For both catalysts, the dissociative mechanism with trans olefin coordination is favored. The relative energies of the species on this pathway can be tuned by ligand variation, as seen in (PMe3)(2)Cl2Ru=CH2 (III), in which phosphane dissociation is impeded and olefin insertion is facilitated relative to I. The differences in calculated relative energies for the model catalysts I-III can be rationalized in terms of electronic effects. Comparisons with experiment indicate that steric effects must also be considered for real catalysts containing bulky substituents