The nature of multiple boron-nitrogen bonds studied using electron localization function (ELF), electron density (AIM), and natural bond orbital (NBO) methods.

Local nature of the boron-nitrogen (BN) bonding with different formal multiplicities (B≡N, B=N, B-N) have been investigated for 25 experimentally established organoboron molecules in both real and the Hilbert space, using topological analysis of electron localization function (ELF), electron density (AIM), and natural bond orbital (NBO) method. Each BN bond has been represented (ELF) by the bonding disynaptic attractor V(B,N), with the basin electron population between 5.72e and 1.83e, confirming possible existence of all the three bond types. A covalent character of bonding can be associated with the dative mechanism due to the V(B,N) bonding basin formed mainly (91-96%) by the N electron density. Similarly, the NBO method shows 2-center natural orbitals, consisting largely of the hybrids from the N atom. The AIM analysis yields the features typical for shared (H(3,-1)(r) < 0) and closed-shell (∇2ρ(3,-1)(r) > 0) interactions. The delocalization indices, describing electron exchanges between B and N quantum atoms, are smaller than 1.5, even for formally very short triple B≡N bonds. Graphical abstract .

The nature of the T=T double bond (T = B, Al, Ga, In) in dialumene and its derivatives: topological study of the electron localization function (ELF).

The local electronic structure of the Al=Al bond was studied in dialumene and derivatives of dialumene in which the Al atoms were substituted by B, Ga, or In atoms. DFT calculations were performed using the B3LYP, B3PW91, PBE0, M06-L, and M06-2X functionals. Topological analysis of the electron localization function described the covalent bonds mentioned above using the disynaptic basins Vi=1,2(B,B), Vi=1,2(Al,Al), V(Ga,Ga), and Vi=1,2(In,In). The basin populations were smaller than 4 e, as expected for a double bond: B=B 2.97 e, Al=Al 3.44-3.5 e, Ga=Ga 3.58 e, and In=In 3.86 e. The Al=Al, Ga=Ga, and In=In bonds were found to be intermediate in character between single and double bonds. Topological analysis of the ρ(r) field for dialumene showed a non-nuclear attractor along the Al=Al bond, with a pseudoatom basin population of 0.937 e. NBO analysis suggested that a double bond occurred only in the molecules containing Al, Ga, or In atoms. The character of the Ga=Ga bond was observed to be strongly dependent on the effective core potential used in the calculations.

On the nature of interactions in the F2 OXe( ) NCCH3 complex: Is there the Xe(IV)N bond?

Nature of the bonding in isolated XeOF2 molecule and F2 OXe(…) NCCH3 complexes have been studied in the gas phase (0 K) using Quantum Chemical Topology methods. The wave functions have been approximated at the MP2 and DFT levels of calculations, using the APFD, B3LYP, M062X, and B2PLYP functionals with the GD3 dispersion correction. The nature of the formal XeO bond in the XeOF2 monomer depends on the basis set used (all-electron vs. the ecp-28 approximation for Xe). Within the all-electron basis set approach the bond is represented by two bonding attractors, Vi = 1,2 (Xe,O), with total population of about 1.06e and highly delocalized electron density in both bonding basins. No bonding basins are observed using the ecp-28 approximation. These results shows that the nature of xenon-oxygen is complicated and may be described with mesomeric equilibrium of the Lewis representations: Xe((+)) O((-)) and Xe((-)) O((+)) . For both the xenon-oxygen and xenon-fluorine interactions the charge-shift model can be applied. The F2 OXe(…) NCCH3 complex exists in two structures: "parallel," stabilized by non-covalent C(…) O and Xe(…) N interactions and "linear" stabilized by the Xe(…) N interaction. Topological analysis of ELF shows that the F2 OXe(…) NCCH3 molecule appears as a weakly bound intermolecular complex. Intermolecular interaction energy components have also been studied using Symmetry Adapted Perturbation Theory. © 2016 Wiley Periodicals, Inc.

The nature of inter- and intramolecular interactions in F2OXe( )HX (X= F, Cl, Br, I) complexes.

Electronic structure of the XeOF2 molecule and its two complexes with HX (X= F, Cl, Br, I) molecules have been studied in the gas phase using quantum chemical topology methods: topological analysis of electron localization function (ELF), electron density, ρ(r), reduced gradient of electron density |RDG(r)| in real space, and symmetry adapted perturbation theory (SAPT) in the Hilbert space. The wave function has been approximated by the MP2 and DFT methods, using APF-D, B3LYP, M062X, and B2PLYP functionals, with the dispersion correction as proposed by Grimme (GD3). For the Xe-F and Xe=O bonds in the isolated XeOF2 molecule, the bonding ELF-localization basins have not been observed. According to the ELF results, these interactions are not of covalent nature with shared electron density. There are two stable F2OXe(…)HF complexes. The first one is stabilized by the F-H(…)F and Xe(…)F interactions (type I) and the second by the F-H(…)O hydrogen bond (type II). The SAPT analysis confirms the electrostatic term, Eelst ((1)) and the induction energy, Eind ((2)) to be the major contributors to stabilizing both types of complexes.

The DFT study on the reaction between benzaldehyde and 4-amine-4H-1,2,4-triazole and their derivatives as a source of stable hemiaminals and Schiff bases. Effect of substitution and solvation on the reaction mechanism.

Reaction mechanism for the benzaldehyde (ald) and 4-amine-4H-1,2,4-triazole (4at) has been investigated at the DFT (B3LYP)/6-31+G(d) computational level. Three transition states (TS) have been identified. The TS1 corresponds to hydrogen transfer from the NH2 group to the C = O bond and nucleophillic attack of the carbon atom from the aldehyde group on the nitrogen atom from the NH2 group in 4at. The result of this reaction is the hemiaminal molecule. The TS2 characterises an internal rearrangement of the benzene and triazole rings in the hemiaminal molecule. The TS3 leads to breaking of the O-H bond, the elimination reaction of the H2O molecule, and formation of the C=N bond. The final product of this reaction is a Schiff base. In order to determine the most favorable conditions for hemiaminal formation, the influence of electronic structure modification on the energetic properties during the reaction of benzaldehyde and 4-amine-4H-1,2,4-triazole has been studied. Thirteen substituents: NH2, OH, OCH3, CH3, F, I, Cl, Br, COH, COOH, CF3, CN, NO2, with different Hammett's constant values (σ = -0.66-+0.78) have been considered. Finally, the reaction mechanism has been investigated in the presence of 1 to 5 water molecules.

Nature of the bonding in the AuNgX (Ng = Ar, Kr, Xe; X = F, Cl, Br, I) molecules. Topological study on electron density and the electron localization function (ELF).

Topological analysis of the electron localization function (ELF) has been carried out for the AuNgX (Ng = Ar, Kr, Xe; X = F, Cl, Br, I) molecules using the wave function approximated by the CCSD, MP2, and DFT(B3LYP, M062X) methods including zero-order regular approximation (ZORA). In the Ng-F bond, the bonding disynaptic attractor V(Ng,F) is missing; therefore, there are no signs of the covalent binding. The nature of the Au-Ng bond depends on the computational method used. Analysis of the ELF carried out for the AuArF and AuXeF molecules, with the wave function approximated by the CCSD and MP2 methods, shows the V(Au,Ng) attractor possibly corresponding to a partially covalent binding between the gold and noble gas atom. However, its very small basin population (<1e) and a very large value of the variance of the basin population suggest that the Au-Ng bond has a very delocalized character. Such bond nature may be related to the charge shift concept with a resonance of the Au(-+)NgX, Au(+-)NgX hybrids. The weakest Au-Ng bond, in terms of the smallest amount of electron density for the V(Au,Ng) basin, is found for the AuKrF molecule with the CCSD method (0.13e). The MP2 method, however, does not yield any V(Au, Ng) population; hence, the covalent Au-Kr bond is not confirmed. Because the V(Au,Ng) attractor is also not observed with the DFT method, the proper characterization of the Au-Ng bond requires proper description of correlation effects. Additional studies on the Au2 and [AuXe](+) molecules, performed at the CCSD and B3LYP levels, exhibit no V(Au,Au) and V(Au,Xe) bonding basins either.

Electron localization function study on the chemical bonding in a real space for tetrahedrane, cubane, adamantane, and dodecahedrane and their perfluorinated derivatives and radical anions.

The nature of chemical bonding in caged cycloalkanes CnXn, CnFn(-•), (n = 4, 8, 20; X = H, F), and C10X16, C10F16(-•), (X = H, F) has been investigated using topological analysis of the ELF function, electron density, and the Laplacian of electron density at density functional theory (DFT) level. The bonding analysis performed for the perfluorinated radical anion of dodecahedrane (C20F20(-•)), bestowing an additional electron, shows an unexpected local maximum of the ELF inside the carbon cage. The presence of such an attractor confirms the sigma stellation concept presented by Irikura (J. Phys. Chem. A 2008, 112, 983) and essential change of the electron localization inside the cage. The basin belongs to the rare asynaptic type, V(asyn), and its mean electron population is 0.26 (0.36e). The value of the integrated spin density, 0.13e, shows that both spin-up and spin-down electrons reside in the vicinity of the cage center. A similar attractor has been found for perfluorinated radical anion of adamantane (C10F16(-•)). However, the saturation of the basis set suggests that such an attractor may be an artifact. For both caged perfluorinated tetrahedrane and cubane (CnFn (-•), n = 4, 8), no valence attractors are present inside the cage. Unpaired electron density is concentrated mainly on the C-C bonding basins. The results obtained in this study are complementary to those based on the molecular orbital theory presented by Irikura.

Effects of xenon insertion into hydrogen bromide. Comparison of the electronic structure of the HBr···CO2 and HXeBr···CO2 complexes using quantum chemical topology methods: electron localization function, atoms in molecules and symmetry adapted perturbation theory.

Quantum chemistry methods have been applied to study the influence of the Xe atom inserted into the hydrogen-bromine bond (HBr → HXeBr), particularly on the nature of atomic interactions in the HBr···CO2 and HXeBr···CO2 complexes. Detailed analysis of the nature of chemical bonds has been carried out using topological analysis of the electron localization function, while topological analysis of electron density was used to gain insight into the nature of weak nonbonding interactions. Symmetry-adapted perturbation theory within the orbital approach was applied for greater understanding of the physical contributions to the total interaction energy.

FONO: a difficult case for theory. The ELF and ELI-D topological studies on the chemical bonding using correlated wavefunctions.

The complicated nature of the chemical bonding in cis and trans isomers of F-O-N=O is discussed based on the results obtained from the topological analysis of electron localization function (η) (ELF), electron localizability index (Y(D)(σ)), and electron density (ρ). The calculations have been performed for correlated wavefunctions using the CCSD and CASSCF methods. The F-O1 bond with non-bonding basins, V(F) and V(')(O1), belongs to the protocovalent type (η,Y(D)(σ)) and its total population ranges between 0.2 and 0.4e. The central N-O1 bond in the cis form is protocovalent (η, Y(D)(σ)) with two basins, V(N) and V(O1). The total population oscillates between 0.7 and 0.9e. In the trans isomer, topology of ELF depends on used method. At the CCSD level only one non-bonding basin, V(N), is observed (η). Its population is about 0.5e. According to the definition of a heteronuclear charge-shift (CS) bond, only N-O1 bond in trans-FONO belongs to the CS class. A relation between η- and ρ-topology and N-O1 bond length is discussed.

Electron localization function study on intramolecular electron transfer in the QTTFQ and DBTTFI radical anions.

The unsymmetrical distribution of the unpaired electron in the ground state of the DBTTFI(•-) radical anion (bi(6-n-butyl-5,7-dioxo-6,7-dihydro-5H-[1,3]dithiolo[4,5-f]isoindole-2-ylidene) is theoretically predicted using the M06-2X/6-31+G(d,p) level of calculations. The results are additionally confirmed by single point calculations at B3LYP/aug-cc-pVTZ, LC-ωPBE/aug-cc-pVTZ, and M06-2X/aug-cc-pVTZ levels. DBTTFI, containing the TTF (tetrathiafulvalene) fragment, may be used in the construction of organic microelectronic devices, similarly to the radical anion of QTTFQ. The unsymmetrical distribution of spin density in (QTTFQ)(•-) has been confirmed using M06-2X/aug-cc-pVTZ calculations, with subsequent study using topological analysis of electron localization function (ELF). The reorganization of the chemical bonds during intramolecular electron transfer in (QTTFQ)(•-) and (DBTTFI)(•-) has been analyzed using bonding evolution theory (BET). The reaction path has been simulated by the IRC procedure, and the evolution of valence basins has been described using catastrophe theory. The simple mechanisms: (QTTFQ)(•-): η-1-3-CC(+)-0: (-•)(QTTFQ) and (DBTTFI)(•-): η-1-3-[F](4)[F(+)](4)-0: (-•)(DBTTFI), each consisting of three steps, have been observed. Two cusp or 4-fold catastrophes occur immediately after the TS. Our study shows that potential future microelectronic devices, constructed on the basis of the (QTTFQ)(•-) and (DBTTFI)(•-) systems, should exploit the properties of the C═C bond.

Comparative density functional theory and post-Hartree-Fock (CCSD, CASSCF) studies on the electronic structure of halogen nitrites ClONO and BrONO using quantum chemical topology.

In this paper, the electronic structures of cis- and trans-ClONO and BrONO are studied at the CCSD∕aug-cc-pVTZ, CASSCF(14,12)/aug-cc-pVTZ, and B3LYP/aug-cc-pVTZ computational levels. For the Cl-O bond, topological analysis of the electron density field, ρ(r), shows the prevalence of the shared-electron type bond (∇(2)ρ((3,-1)) < 0). The Br-O bond, however, represents the closed-shell interaction (∇(2)ρ((3,-1)) > 0). Topological analysis of the electron localization function, η(r), and electron localizability indicator (ELI-D), (D) (σ)(r), shows that the electronic structure of the central N-O bond is very sensitive to both electron correlation improvements (coupled-cluster single double (CCSD), CASSCF, density functional theory (DFT)) and bond length alteration. Depending on the method used, the N-O bond can be characterized as a "normal" N-O bond with a disynaptic V(N,O) basin (DFT); a protocovalent N-O bond with two monosynaptic, V(N) and V(O), basins (CCSD, CASSCF); or a new type, first discovered for FONO, characterized by a single monosynaptic, V(N) basin (CCSD, DFT). The total basin population oscillates between 0.46-0.96 e (CCSD) and 0.86-1.02 e (CASSCF). The X-O bond is described by the single disynaptic basin, V(X,O), with a basin population between 0.76 and 0.81 e (CCSD) or 0.77 and 0.85 e (CASSCF). Analysis of the localized electron detector distribution for the cis-Cl-O1-N=O2 shows a manifold in the Cl···O2 region, associated with decreased electron density.

Electron localization function and electron localizability indicator applied to study the bonding in the peroxynitrous acid HOONO.

The ground-state electronic structure of peroxynitrous acid (HOONO) and its singlet biradicaloid form (HO···ONO) have been studied using topological analysis of the electron localization function (ELF), together with the electron localizability indicator (ELI-D), at the DFT (B3LYP, M05, M052X, and M06), CCSD, and CASSCF levels. Three isomers of HOONO (cis-cis, cis-perp, and trans-perp) have been considered. The results show that from all functionals applied, only B3LYP yields the correct geometrical structure. The ELF and ELI-D-topology of the O-O and central N-O bonds strongly depends on the wave function used for analysis. Calculations carried out at CAS (14,12)/aug-cc-pVTZ//CCSD(T)/aug-cc-pVTZ level reveal two bonds of the charge-shift type: a protocovalent NO bond with a basin population of 0.82-1.08e, and a more electron depleted O-O bond with a population of 0.66-0.71e. The most favorable dissociation channel (HOONO → HO + ONO) corresponds to breaking of the most electron-deficient bond (OO). In the case of cis- and trans-HO···ONO, the ELF, ELI-D, and electron density fields results demonstrate a closed-shell O···O interaction. The α-spin electrons are found mainly (0.64e) in the lone pairs of oxygen V(i) (= 1,2)(O) from the OH group. The ß-spin electrons are delocalized over the ONO group, with the largest concentration (0.34e) on the lone pair of nitrogen V(N).

Ab initio and quantum chemical topology studies on the isomerization of HONO to HNO2. Effect of the basis set in QCT.

The article focus on the isomerization of nitrous acid HONO to hydrogen nitryl HNO(2). Density functional (B3LYP) and MP2 methods, and a wide variety of basis sets, have been chosen to investigate the mechanism of this reaction. The results clearly show that there are two possible paths: 1) Uncatalysed isomerisation, trans-HONO --> HNO(2), involving 1,2-hydrogen shift and characterized by a large energetic barrier 49.7 divided by 58.9 kcal/mol, 2) Catalysed double hydrogen transfer process, trans-HONO + cis-HONO --> HNO(2) + cis-HONO, which displays a significantly lower energetic barrier in a range of 11.6 divided by 18.9 kcal/mol. Topological analysis of the Electron Localization Function (ELF) shows that the hydrogen transfer for both studied reactions takes place through the formation of a 'dressed' proton along the reaction path. Use of a wide variety of basis sets demonstrates a clear basis set dependence on the ELF topology of HNO(2). Less saturated basis sets yield two lone pair basins, V(1)(N), V(2)(N), whereas more saturated ones (for example aug-cc-pVTZ and aug-cc-pVQZ) do not indicate a lone pair on the nitrogen atom. Topological analysis of the Electron Localizability Indication (ELI-D) at the CASSCF (12,10) confirms these findings, showing the existence of the lone pair basins but with decreasing populations as the basis set becomes more saturated (0.35e for the cc-pVDZ basis set to 0.06e for the aug-cc-pVTZ). This confirms that the choice of basis set not only can influence the value of the electron population at the particular atom, but can also lead to different ELF topology.

Quantum chemical topology study on the electronic structure of cis- and trans-FONO.

The electronic structure of cis- and trans-FONO has been studied using topological analysis of the electron localization function at the B3LYP/aug-cc-pVTZ computational level. In cis-FONO with "normal" F-O bond length (1.428-1.441 A), a protocovalent F-O bonding has been found. The central N-O bond is "drained off" with the electron density (0.40e and 0.42e) and the terminal N-O bond resembles an electron-rich single bond (2.13e-2.14e). The F...ONO form with a long F...O bond (1.719 and 1.696 A) has a diradical character and consists of F and NO(2) subunits without clear indications of the covalent bond in the F...O region.