Barbara Bankiewicz

Electron Density Characteristics in Bond Critical Point (QTAIM) versus Interaction Energy Components (SAPT): The Case of Charge-Assisted Hydrogen Bonding

Charge-assisted hydrogen bonds (CAHBs) of N-H···Cl, N-H···Br, and P-H···Cl type were investigated using advanced computational approach (MP2/aug-cc-pVTZ level of theory). The properties of electron density function defined in the framework of Quantum Theory of Atoms in Molecules (QTAIM) were estimated as a function of distance in H-bridges. Additionally, the interaction energy decomposition was performed for H-bonded complexes with different H-bond lengths using the Symmetry-Adapted Perturbation Theory (SAPT). In this way both QTAIM parameters and SAPT energy components could be expressed as a function of the same variable, that is, the distance in H-bridge. A detailed analysis of the changes in QTAIM and SAPT parameters due to the changes in H···A distance revealed that, over some ranges of H···A distances, electrostatic, inductive and dispersive components of the SAPT interaction energy show a linear correlation with the value of the electron density at H-BCP ρ(BCP). The linear relation between the induction component, E(ind), and ρ(BCP) confirms numerically the intuitive expectation that the ρ(BCP) reflects directly the effects connected with the sharing of electron density between interacting centers. These conclusions are important in view of charge density studies performed for crystals in which the distance between atoms results not only from effects connected with the interaction between atomic centers directly involved in bonding, but also from packing effects which may strongly influence the length of the H-bond.

Kinetics of 1,4-Hydrogen Migration in the Alkyl Radical Reaction Class

The kinetics of the 1,4-intramolecular hydrogen migration in the alkyl radicals reaction class has been studied using reaction class transition-state theory combined with the linear energy relationship (LER) and barrier height grouping (BHG) approach. The rate constants for the reference reaction of n-C(4)H(9) were obtained by canonical variational transition-state theory (CVT) with the small curvature tunnelling (SCT) correction in the temperature range 300-3000 K with potential-energy surface information computed at the CCSD(T)/cc-pVDZ//BH&HLYP/cc-pVDZ level of theory. Error analyses indicate that RC-TST/LER, where only reaction energy is needed, and RC-TST/BHG, where no other information is needed, can predict rate constants for any reaction in this reaction class with excellent accuracy. Specifically, for this reaction class the RC-TST/LER method has less than 65% systematic errors in the predicted rate constants, while the RC-TST/BHG method has less than 80% error when compared to explicit rate calculations.

The shape of the halogen atom—anisotropy of electron distribution and its dependence on basis set and method used

A search through Crystal Structure Database was performed and the distances in contacts of X···N,O, X···H(N,O), and X···C type were collected together with the information on spatial arrangement of the interacting fragments. A detailed statistical analysis showed that the shape of the halogen atom cannot be simply concluded on the basis of interatomic distances in crystal state although originally the concept of anisotropic charge distribution around halogen nuclei was postulated on the basis of such an analysis. It was proven that the conclusions in that case strongly depend on the type of center interacting with the halogen atom. Therefore, it was postulated that the shape of the halogen atom can be estimated for the unperturbed (due to intermolecular interactions) halogen atom. For this purpose, a method was provided to make possible a numerical quantification of the anisotropy of the halogen atom on the basis of electron density measurements performed within the framework of Atoms in Molecules Quantum Theory. The anisotropy of Cl and Br atoms in H3C–X and F3C–X (X=Cl, Br) was estimated for MP2 and DFT-B3LYP methods and several different basis sets. The influence of the method and the basis set on the degree of anisotropic distribution of electron density around halogen nuclei was discussed.

Kinetics of 1,5-hydrogen migration in alkyl radical reaction class.

Kinetics of the 1,5-intramolecular hydrogen migration in the alkyl radicals reaction class has been studied using the reaction class transition state theory combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approach. The high pressure limits of the rate constants for the reference reaction of 1-pentyl → 1-pentyl, calculated by the Canonical Variational Transition State Theory (CVT) with the Small Curvature Tunneling (SCT), are taken from the literature. Direct comparison with available experimental data indicates that the RC-TST/LER, where only reaction energy is needed, can predict rate constants for any reaction in this reaction class with excellent accuracy. Specifically for this reaction class, the RC-TST/LER method has less than 65% systematic errors in the predicted rate constants when compared to explicit rate calculations.

Interactions of polar hydrogen bond donor solvents with ions: a theoretical study

Intermolecular interactions between molecules of protic solvents (water, methanol, formic acid, formamide, methylamine and ammonia) and monatomic ions (Li+, Na+, K+, F−, Cl− and Br−) were characterized by the total energy of interaction (Etotal) and the amount of charge which is transferred between the solvent molecule and the ion (CT). For the studied systems, linear relationships between Etotal and ln(CT) were observed and explained. In the case of complexes with metal cations, a good linear correlation between Etotal and the molar enthalpy of ion solvation (

Does electron density in bond critical point reflect the formal charge distribution in H-bridges? The case of charge-assisted hydrogen bonds (CAHBs)

\Delta H_{{{\text{i}},{\text{solv}}}}^{\text{o}}

Quasi-aromaticity—what does it mean?

ΔHi,solvo), obtained from experimental data for water, methanol, formamide and ammonia, was found. On the other hand, for complexes with anions, a planar regression between Etotal and two explanatory parameters: