Marcin Palusiak

Aromaticity from the viewpoint of molecular geometry: application to planar systems.

to Planar Systems Tadeusz M. Krygowski,*,† Halina Szatylowicz,*,‡ Olga A. Stasyuk,‡ Justyna Dominikowska, and Marcin Palusiak †Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland ‡Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Department of Theoretical and Structural Chemistry, Faculty of Chemistry, University of Łod́z,́ Pomorska 163/165, 90-236 Łod́z,́ Poland

Nature of a Hydride–Halogen Bond. A SAPT-, QTAIM-, and NBO-Based Study

The nature of a hydride-halogen bond is investigated by means of high-level quantum mechanical calculations expended with symmetry-adapted perturbation theory (SAPT), quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO) methods. As model hydride-halogen bonded systems complexes between either LiH or HBeH and either XCF(3) or XCCH (X = F, Cl, Br, I) are used. It is shown that the formation of a hydride-halogen bond leads to the elongation of the R(δ+)-H(δ-) hydride bond, which is accompanied by the blue shift of the ν(R-H) stretching vibration frequency and the increase of the IR intensity of this mode. All these effects, although untypical in the case of, e.g., hydrogen bonds, can be considered as rather typical for hydride-halogen bonded systems. The decomposition of the interaction energy based on the SAPT method clearly indicates the dominant role of the induction term, thus the inductive nature of a hydride-halogen bond in opposition to previous findings. NBO-based analysis indicates the charge transfer from the hydride molecule to the more remote parts of the halogen donor and that the elongation of the R-H bond is caused by the charge outflow from the σ(RH) bonding orbital.

Basis Set and Method Dependence in Atoms in Molecules Calculations

The influence of various basis sets used in HF and DFT/B3LYP calculations to the values of atoms in molecules (AIM) parameters derived from the electron density distibution for weak hydrogen-bonded systems is investigated. Using three model complexes, F(3)CH...NH(3), F(3)CH...NCH, and FCCH...NH(3), we show that values of the most important AIM parameters calculated in the bond critical point of the H...N hydrogen bond are almost independent of both the method and the basis set. Only the smallest Dunning-type cc-pVDZ or aug-cc-pVDZ basis sets may lead to poor results, whereas even medium-sized Pople-type basis sets can give reasonable results converting to those obtained from the use of large Dunning-type basis sets.

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.

Interplay between Intramolecular Resonance-Assisted Hydrogen Bonding and Local Aromaticity. II. 1,3-Dihydroxyaryl-2-aldehydes

The interplay between aromaticity and hydrogen bonding in 1,3-dihydroxyaryl-2-aldehydes is investigated by means of quantum-chemical calculations. The position of the extra ring formed by substituents interacting through the hydrogen bond (HB) is found to influence both the strength of the HB and the local aromaticity of the polycyclic aromatic hydrocarbon (PAH) skeleton. The HBs are stronger and the entire system is energetically more stable when a kinked-like structure is generated by formation of the quasi-ring. Relatively greater loss of aromaticity of the ipso-ring can be observed for these kinked-like structures because of the larger participation of pi-electrons coming from the ipso-ring in the formation of the quasi-ring. We conclude that the quasi-ring partially adopts the role of a typical aromatic ring, the position of which has a meaningful influence on the aromaticity of the rest of the rings. This makes it possible to explain and modify the properties of 1,3-dihydroxyaryl-2-aldehydes by the planned substitution to the appropriate position of the given PAH.

Basis Set and Method Dependence in Quantum Theory of Atoms in Molecules Calculations for Covalent Bonds

The influence of various small- and medium-size basis sets used in Hartree-Fock (HF) and density functional theory (DFT)/B3LYP calculations on results of quantum theory of atoms in molecules based (QTAIM-based) analysis of bond parameters is investigated for several single, double, and triple covalent bonds. It is shown that, in general, HF and DFT/B3LYP methods give very similar QTAIM results with respect to the basis set. The smallest 6-31G basis set and DZ-quality basis sets of Dunning type lead to poor results in comparison to those obtained by the most reliable aug-cc-pVTZ. On the contrary, 6-311++G(2df,2pd) and in a somewhat lesser extent 6-311++G(3df,3pd) basis sets give satisfactory values of QTAIM parameters. It is also demonstrated that QTAIM calculations may be sensitive for the method and basis set in the case of multiple and more polarized bonds.

H-Bonding-Assisted Substituent Effect

In this paper we investigate the influence of intramolecular noncovalent interaction, i.e., H-bonding and Li-bonding, on the properties of substituents communicating through the resonance (mesomeric) effect in such molecular systems as salicylaldehyde, o-hydroxy Schiff base, o-nitrosophenol, and their lithium analogues. The investigated systems are usually considered as molecular patterns of intramolecular resonance-assisted hydrogen bonds (or its analogues in the case of Li-bonded systems). We show that the relation between intramolecular noncovalent interactions, A-H...B and A-Li...B, and the pi-electron delocalization in the sequence of pi-conjugated covalent bonds linking A and B can be considered in terms of the Hammett-like substituent effect in which electron-donating and electron-withdrawing properties of substituents are affected by the noncovalent interaction.

Methoxy group as an acceptor of proton in hydrogen bonds

Abstract Methoxybenzene and its complexes with HF, H 2 O, C 2 H 2 , C 2 H 4 , CH 4 , CH 3 F and NH 2 CH 3 are investigated using results of DFT calculations at B3LYP/6-311+G ∗ level of theory. These results show that oxygen atom from methoxy group is an acceptor of proton for different types of H-bonds: F–H⋯O, O–H⋯O, C–H⋯O, N–H⋯O. The analysis of the bond critical points is used to characterise those interactions. The bond valence model applied to study the accepting abilities of the methoxy group is in a very good agreement with DFT results. The Cambridge Structural Database was searched for the methoxy group abilities to act as a proton acceptor within H-bonds.

Relation between π-Electron Localization/Delocalization and H-Bond Strength in Derivatives of o-Hydroxy-Schiff Bases

Detailed investigations of electronic effects taking place within the molecular system of o-hydroxy Schiff bases have been performed. The analysis of geometry, local and global aromaticity, selected AIM-based parameters, and finally, pi-electron currents induced in the systems under consideration have been performed on the basis of quantum chemical calculations at the B3LYP/6-311+G** level of theory. The relation between localization/delocalization of pi-electrons within the whole system has been described. It has been shown that the character of the bond which is common to the phenylic ring and the quasi-ring formed as a result of H-bond formation has a crucial impact on the strength of H-bonding. The strongest H-bonds can be observed for the systems in which the sequence of formally single and double bonds within the H-bridged quasi-ring enable a pi-electronic coupling. These observations indicate that pi-electron effects play a fundamental role in the stabilization of the hydrogen bridge within omicron-hydroxy Schiff bases. Analysis of pi-ring currents induced by a magnetic field perpendicular to the molecular plane of selected analyzed systems confirms these conclusions.

Divalent carbon atom as the proton acceptor in hydrogen bonding

Proton-accepting properties of the divalent carbon atom in carbodiphosphoranes and their simple derivatives as well as in carbenes have been investigated. Both these groups of chemical compounds may be characterized by the formula CL2, where L is a sigma electron donor. Therefore, the carbon atom within both these systems, being in its atomic state, can have one or two lone electron pairs and, as a result, it may form hydrogen bonds of the type D-H...CL2, where C acts as a proton acceptor. Complexes of C(NH3)2, C(PH3)2, C[P(CH3)3]2, CF2, CCl2, and imidazol-2-ylidene with such proton donors as H2O, HCF3, HCN and HCCH have been analyzed by means of high-level quantum chemical methods. Density functional theory (DFT) and second-order Møller-Plesset (MP2) approaches have been applied in conjunction with the aug-cc-pVTZ basis set. The electron density distribution calculated by means of the atoms in the molecules procedure has also been analyzed. Proton-accepting properties of the carbon atom are discussed in detail. It is shown that the divalent carbon atom in the group of chemical systems investigated should be treated as a normal proton acceptor, similar to the much more electronegative O or N atoms. Moreover, hydrogen bonds of the type D-H...CL2 within the complexes investigated have been found to be rather strong. The highest proton accepting ability of the carbon(0) atom found for the (NH3)2C derivative of carbodiphosphorane is explained on the basis of the Leffler-Hammond postulate. Within the group of carbenes, the strongest hydrogen bonds are formed by imidazol-2-ylidene. This is attributed to the significant aromatic character of the imidazol-2-ylidene ring that increases the proton-accepting properties of the carbene carbon atom.