A quick solvation energy estimator based on electronegativity equalization.

ESE-EE (Easy Solvation Estimation with Electronegativity equalization) is a quick method for estimation of solvation-free energies ΔGºsolv , which uses a thoroughly fitted electronegativity equalization (EE) scheme to obtain atomic charges, which are further employed in a scaled noniterative COSMO-like calculation to evaluate the electrostatic component of ΔGºsolv . Nonelectrostatic corrections including adjustable parameters are also added. For neutral solutes, ESE-EE yields a mean absolute error (MAE) in ΔGsolv ° of 1.5 kcal/mol for aqueous solutions; 1.0 kcal/mol for nonaqueous polar protic solvents; 0.9 kcal/mol for polar aprotic solvents; and about 0.6 kcal/mol for nonpolar solvents. Since ESE-EE only requires a molecular geometry as input for a ΔGºsolv prediction, it can be utilized for a rapid screening of ΔGºsolv for large neutral molecules. However, for ionic solutes, ESE-EE yields larger errors (typically several kcal/mol) and is recommendable for preliminary estimations only. Upon a special refitting, ESE-EE is able to yield partition coefficients with a good accuracy.

Base-Stabilized Phosphinidene Oxide, Imide and Sulfide.

Oxidation of a base-stabilized phosphinidene (κ2 -NNP)P (12, NNP=phosphinoamidinate) with N2 O afforded a labile phosphinidene oxide (κ2 -NNP)P=O (16) which was characterized by NMR spectroscopy. Further oxidation of 16 by N2 O or reaction of 12 with two equivalents of pyridine oxide afforded the isolable dioxide (κ2 -NNP)PO2 which was characterized by NMR and SC XRD. Trapping of 16 with tolyl isocyanate resulted in P=O/N=C metathesis, eventually affording a urea-ligated phosphine (κ1 -NNP)P(NTol)2 C=O (17) The mechanism of this reaction was elucidated by DFT calculations. Reactions of phosphinidene 12 with azides generated transient imines (NNP)P=NR, which in the case of R=Tol underwent cycloaddition with tolyl Isocyanate to afford the urea product 17, and in the case of R=SiMe3 reacts with N3 SiMe3 via the addition of N-Si across the P=N bond affording, after the extrusion of dinitrogen, a P,N-heterocyclic compound. Both products of the reactions with azides have been fully characterized, both in solution and the solid-state. Finally, reaction of phosphinidene 12 with one equivalent of sulfur resulted in the isolation of the base-stabilized phosphinidene sulfide (κ2 -NNP)P=S that has also been fully characterized.

Dense Neural Network for Calculating Solvation Free Energies from Electronegativity-Equalization Atomic Charges.

I propose a dense Neural Network for evaluation of solvation free energies ΔG°solv for molecules and ions in water and nonaqueous solvents, Easy Solvation Energy with Electronegativity Equalization charges and Dense Neural Network (ESE-EE-DNN). As input features, it uses the Conductor-like Screening Model (COSMO) electrostatic energy, atomic cavity surface areas, total cavity volume, and induced surface charges. For the COSMO calculation, electronegativity-equalization atomic charges are employed. ESE-EE-DNN exhibits fairly high accuracy, similar or even superior to that of mainstream density functional theory-based methods. For neutral solutes in water, polar protic, polar aprotic, and nonpolar solvents, ESE-EE-DNN yields a root-mean-square error (RMSE) of 1.25, 1.36, 0.70, and 0.71 kcal/mol, respectively. ESE-EE-DNN is particularly advantageous for ionic solutes, with an RMSE of 2.82 and 1.42 kcal/mol for aqueous and nonaqueous ion solutions, correspondingly. ESE-EE-DNN is very efficient due to a fast evaluation of the electronegativity-equalization charges.

The Insertion of EII and EIV Chlorides (E=Si, Ge) into the Si-Si Bond of Disilylene.

Reactions of silicon and germanium dichlorides L⋅ECl2 (E=Si, L=IPr; E=Ge, L=dioxane) with the phosphinoamidinato-supported disilylene ({κ2 (N,P)-NNP}Si)2 resulted in formal tetrylene insertions into the Si-Si bond. In the case of the reaction with silylene, two products were isolated. The first product ({κ2 (N,P)-NNP}Si)2 SiCl2 , is the formal product of direct SiCl2 insertion into the Si-Si bond of ({κ2 (N,P)-NNP}Si)2 and thus features two separated silylamido silylene centers. Over time, migration of the SiCl2 group to a lateral position afforded the second product, the disilylene {κ2 (Si,P)-SiCl2 NNP}Si-Si{κ2 (N,P)-NNP}. In contrast, insertion of GeCl2 resulted only in the isolation of the germanium analogue of {κ2 (Si,P)-SiCl2 NNP}Si-Si{κ2 (N,P)-NNP}, containing a Ge atom in the central position namely, compound {κ2 (Si,P)-SiCl2 NNP}Ge-Si{κ2 (N,P)-NNP}, which is a rare example of a silylene-germylene. Finally, reaction of disilylene ({κ2 (N,P)-NNP}Si)2 with SiCl4 and SiHCl3 led to the formation of the new bis(silyl)silylene, ({NNP}SiCl2 )2 Si:. All four new products from these insertion reactions have been characterized by multinuclear NMR and single-crystal X-ray diffraction studies.

Fast non-iterative calculation of solvation energies for water and non-aqueous solvents.

We propose an efficient and accurate non-iterative method, dubbed uESE, for calculating solvation free energies. Apart from a COSMO-like electrostatic term, the model takes into account non-electrostatic contributions, which depend on atomic surfaces, induced surface charge densities, and the molecular volume. uESE is tested on 35 polar and 57 non-polar solvents. The calculated and experimental solvation free energies are compared for 2892 systems. The method exhibits an excellent performance, which is superior to major solvation methods. The mean absolute error of predicted solvation energies is found below 1 kcal/mol for neutral solutes and below 3 kcal/mol for ions. The calculated data are almost independent of the quantum-chemical method or/and basis sets employed.

Solvation Free Energies for Aqueous and Nonaqueous Solutions Computed Using PM7 Atomic Charges.

We describe a simple and accurate method, ESE-PM7, for calculating solvation free energies ΔGsolv° in aqueous and nonaqueous solutions. The method is based on a noniterative COSMO algorithm. Molecular geometries and atomic charges calculated using the semiempirical method PM7 are used to calculate ΔGsolv°. The method has been tested on 92 different solvents and 988 solutes. The mean absolute errors (MAEs) in ΔGsolv° in aqueous solutions estimated by the ESE-PM7 approach are found to be 1.62 kcal/mol for 389 neutral solutes and 3.06 kcal/mol for 139 ions. The MAEs for neutral molecules in organic solvents are 0.97, 0.74, and 0.51 kcal/mol in polar protic, polar aprotic, and nonpolar solvents, respectively. The developed method can be employed to quickly screen ΔGsolv° values of extended molecular systems including pharmaceutical and biological molecules.

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.

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 cA r2n-2 exp(-αA r), 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.

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.

Sequential Oxidation and C-H Bond Activation at a Gallium(I) Center.

In situ oxidation of the GaI compound NacNacGa by either N2 O or pyridine oxide results in the generation of a labile monomeric oxide, NacNacGa(O), which can easily cleave the C-H bonds of aliphatic and aromatic substrates featuring good donor sites. The products of this reaction are gallium organyl hydroxides. DFT calculations show that these reactions start with the formation of NacNac-Ga(O)(L) adducts, the oxo ligand of which can easily abstract protons from nearby C-H bonds, even for sp2 -hybridized carbon centers. Aliphatic amines do not enter this reaction for kinetic reasons, presumably because of the unfavorable sterics.

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.

Oxidative Cleavage of the C═N Bond on Al(I).

Reaction of the cyclic guanidine TolN═SIMe with the aluminum(I) compound NacNacAl (1) results in the unprecedented cleavage of the C-N multiple bond to give, after rearrangement, the carbene-ligated Al(III) amide, NacNac'Al(NHTol)(SIMe) (6). DFT calculations revealed that these reactions proceed via a bimolecular mechanism in which either the basic Al(I) center or the transient Al═NTol species deprotonates the methyl group of the NacNac ligand.

A Simple Local Correlation Energy Functional for Spherically Confined Atoms from ab Initio Correlation Energy Density.

We propose a simple method of calculating the electron correlation energy density ec (r) and the correlation potential Vc (r) from second-order Møller-Plesset amplitudes and its generalization for the case of a configuration interaction wavefunction, based on Nesbet's theorem. The correlation energy density obtained by this method for free and spherically confined Be and He atoms was employed to fit a local analytical density functional based on Wigner's functional. The functional is capable of producing a strong increase in the correlation energy with decreasing confined radius for the Be atom.

Unusual Reactions of NacNacAl with Urea and Phosphine Oxides.

The reaction of cyclic urea 1,3-dimethyl-2-imidazolidinone with the aluminum(I) compound NacNacAl (1) gives an unexpected adduct of urea with the isomerized aluminum(III) hydride NacNac'AlH(O═SIMe) (3). A related reaction of 1 with phosphine oxides results in cleavage of the P═O bond and formation of hydroxyl derivatives NacNac'Al(OH)(O═PR3) [R = Ph (5) and Et (6)]. Density functional theory calculations revealed that these reactions proceed via a bimolecular mechanism in which either the basic aluminum(I) center or the transient Al═O species deprotonate the methyl group of the NacNac ligand.

Correlation energy, correlated electron density, and exchange-correlation potential in some spherically confined atoms.

We report correlation energies, electron densities, and exchange-correlation potentials obtained from configuration interaction and density functional calculations on spherically confined He, Be, Be2+ , and Ne atoms. The variation of the correlation energy with the confinement radius Rc is relatively small for the He, Be2+ , and Ne systems. Curiously, the Lee-Yang-Parr (LYP) functional works well for weak confinements but fails completely for small Rc . However, in the neutral beryllium atom the CI correlation energy increases markedly with decreasing Rc . This effect is less pronounced at the density-functional theory level. The LYP functional performs very well for the unconfined Be atom, but fails badly for small Rc . The standard exchange-correlation potentials exhibit significant deviation from the "exact" potential obtained by inversion of Kohn-Sham equation. The LYP correlation potential behaves erratically at strong confinements. © 2016 Wiley Periodicals, Inc.

Oxidative Cleavage of C=S and P=S Bonds at an AlI Center: Preparation of Terminally Bound Aluminum Sulfides.

The treatment of cyclic thioureas with the aluminum(I) compound NacNacAl (1; NacNac=[ArNC(Me)CHC(Me)NAr]- , Ar=2,6-Pri2 C6 H3 ) resulted in oxidative cleavage of the C=S bond and the formation of 3 and 5, the first monomeric aluminum complexes with an Al=S double bond stabilized by N-heterocyclic carbenes. Compound 1 also reacted with triphenylphosphine sulfide in a similar manner, which resulted in cleavage of the P=S bond and production of the adduct [NacNacAl=S(S=PPh3 )] (8). The Al=S double bond in 3 can react with phenyl isothiocyanate to furnish the cycloaddition product 9 and zwitterion 10 as a result of coupling between the liberated carbene and PhN=C=S. All novel complexes were characterized by multinuclear NMR spectroscopy, and the structures of 5, 9, and 10 were confirmed by X-ray diffraction analysis. The nature of the Al=S bond in 5 was also probed by DFT calculations.

Modeling exact exchange potential in spherically confined atoms.

In this work, local exchange potentials corresponding to the Hartree-Fock (HF) electron density have been obtained using the Zhao-Morrison-Parr method for a number of closed-shell confined atoms and ions. The exchange potentials obtained and the resulting density were compared with those given by the Becke-Johnson (BJ) model potential. It is demonstrated that introducing a scaling factor to the BJ potential allows improving the quality of the resulting density. The optimum scaling factor increases with decreasing confinement radius. The performance of Karasiev and Ludeña's SCα-LDA method as well as of the Becke-88 exchange potential for reproducing the HF electron densities in confined atoms has been also examined.

Predicting Solvation Free Energies Using Electronegativity-Equalization Atomic Charges and a Dense Neural Network: A Generalized-Born Approach.

I propose a dense Neural Network, ESE-GB-DNN, for evaluation of solvation free energies ΔG°solv for molecules and ions in water and nonaqueous solvents. As input features, it employs generalized-Born monatomic and diatomic terms, as well as atomic surface areas and the molecular volume. The electrostatics calculation is based on a specially modified version of electronegativity-equalization atomic charges. ESE-GB-DNN evaluates ΔG°solv in a simple and highly efficient way, yet it offers a high accuracy, often challenging that of standard DFT-based methods. For neutral solutes, ESE-GB-DNN yields an RMSE between 0.7 and 1.3 kcal/mol, depending on the solvent class. ESE-GB-DNN performs particularly well for nonaqueous solutions of ions, with an RMSE of about 0.7 kcal/mol. For ions in water, the RMSE is larger (2.9 kcal/mol).

Facile activation of H-H and Si-H bonds by boranes.

The borane B(C(6)F(5))(3) is a precatalyst for H/Dexchange between H(2) and deuterium-labeled silanes (D(3)SiPh, D(2)SiMePh, DSiMe(2)Ph, DSiEt(3)). Experimental and DFT studies reveal that B(C(6)F(5))(3) itself cannot activate dihydrogen but converts to HB(C(6)F(5))(2) under the action of hydrosilane. The latter species easily activates H-H and Si-H bonds by a σ-bond metathesis mechanism, which was further confirmed by the reactions of BD(3)·THF with H(2).

Hydrogen motion in proton sponge cations: a theoretical study.

This work presents a study of intramolecular NHN hydrogen bonds in cations of the following proton sponges: 2,7-bis(trimethylsilyl)-1,8-bis(dimethylamino)naphthalene (1), 1,6-diazabicyclo[4.4.4.]tetradecane (2), 1,9-bis(dimethylamino)dibenzoselenophene (3), 1,9-bis(dimethylamino)dibenzothiophene (4), 4,5-bis(dimethylamino)fluorene (5), quino[7,8-h]quinoline (6) 1,2-bis(dimethylamino)benzene (7), and 1,12-bis(dimethylamino)benzo[c]phenantrene (8). Three different patterns were found for proton motion: systems with a single-well potential (cations 1-2), systems with a double-well potential and low proton transfer barrier, ΔEe (cations 3-5), and those with a double-well potential and a high barrier (cations 6-8). Tests of several density functionals indicate that the PBEPBE functional reproduces the potential-energy surface (PES) obtained at the MP2 level well, whereas the B3LYP, MPWB1K, and MPW1B95 functionals overestimate the barrier. Three-dimensional PESs were constructed and the vibrational Schrödinger equation was solved for selected cases of cation 1 (with a single-well potential), cation 4 (with a ΔEe value of 0.1 kcal mol(-1) at the MP2 level), and cations 6 (ΔEe = 2.4 kcal mol(-1)) and 7 (ΔEe=3.4 kcal mol(-1)). The PES is highly anharmonic in all of these cases. The analysis of the three-dimensional ground-state vibrational wave function shows that the proton is delocalized in cations 1 and 4, but is rather localized around the energy minima for cation 7. Cation 6 is an intermediate case, with two weakly pronounced maxima and substantial tunneling. This allows for classification of proton sponge cations into those with localized and those with delocalized proton behavior, with the borderline between them at ΔEe values of about 1.5 kcal mol(-1). The excited vibrational states of proton sponge cations with a low barrier can be described within the framework of a simple particle-in-a-box model. Each cation can be assigned an effective box width.