K. V. Bozhenko

A density functional theory study of the zero-field splitting in high-spin nitrenes

This work presents a detailed evaluation of the performance of density functional theory (DFT) for the prediction of zero-field splittings (ZFSs) in high-spin nitrenes. A number of well experimentally characterized triplet mononitrenes, quartet nitrenoradicals, quintet dinitrenes, and septet trinitrenes have been considered. Several DFT-based approaches for the prediction of ZFSs have been compared. It is shown that the unrestricted Kohn-Sham and the Pederson-Khanna approaches are the most successful for the estimation of the direct spin-spin (SS) interaction and the spin-orbit coupling (SOC) parts, respectively, to the final ZFS parameters. The most accurate theoretical predictions (within 10%) are achieved by using the PBE density functional in combination with the DZ, EPR-II, and TZV basis sets. For high-spin nitrenes constituted from light atoms, the contribution of the SOC part to ZFS parameters is quite small (7%-12%). By contrast, for chlorine-substituted septet trinitrenes, the contribution of the SOC part is small only to D value but, in the case of E value, it is as large as the SS part and has opposite sign. Due to this partial cancellation of two different contributions, SS and SOC, the resulting values of E in heavy molecules are almost two times smaller than those predicted by analysis of the widely used semiempirical one-center spin-spin interaction model. The decomposition of D(SS) into n-center (n=1-4) interactions shows that the major contribution to D(SS) results from the one-center spin-spin interactions. This fact indicates that the semiempirical SS interaction model accurately predicts the ZFS parameters for all types of high-spin nitrenes with total spin S=2 and 3, if their molecules are constructed from the first-row atoms.

A comparative study of small 3d-metal oxide (FeO)n, (CoO)n, and (NiO)n clusters

Geometrical and electronic structures of the 3d-metal oxide clusters (FeO)n, (CoO)n, and (NiO)n are computed using density functional theory with the generalized gradient approximation in the range of 1 ≤ n ≤ 10. It is found that the cluster geometries are similar in the (FeO)n and (CoO)n series but noticeably different in the (NiO)n series for several values of n. All of the lowest total energy states are found to possess relatively small spin multiplicities and are either antiferromagnetic or ferrimagnetic except for the states of (NiO)3, (NiO)4, (NiO)9, and (NiO)10, which are ferromagnetic. The computed polarizabilities per atom undergo a steep decrease when compared to the atomic values of the MO monomers (M = Fe, Co, and Ni). Surprisingly, the polarizability does not strongly depend on either M or n in all the considered series when n varies from 3 to 10. The binding energies per atom are the largest in the (FeO)n series, followed by the binding energies of (CoO)n and (NiO)n.

Synthesis, structure, and properties of a new representative of the family of calix[4]arene-containing [MnII2MnIII2]-clusters

A new representative of calix[4]arene-containing tetranuclear manganese complexes of the [MnII2MnIII2] type was obtained. According to the data of magnetoochemical studies, the complex exhibits properties of molecular magnet at the temperature below 5 K. Parameters of the exchange interaction and the activation energy were determined. The influence of the peripheral environment on the magnetic properties of the tetranuclear manganese framework in the structure of the complex was revealed.

Redox properties of [Fe2(SC6H5)2(NO)4]: an experimental study and quantum chemical modeling

Reduction of the complex [Fe2(SC6H5)2(NO)4] in an aprotic solvent was studied by cyclic voltammetry in a wide range of potential scan rates. It was established that transfer of the first electron is reversible and the redox potential of this reaction was determined. Further reduction of the complex is irreversible because the product of attachment of the second electron is unstable and partially decomposes during the characteristic time of potential scan. The molecular and electronic structures of mono- and dianion of the complex as well as its theoretical redox potential value were calculated using the density functional theory methods with the local (BP86) and hybrid (B3LYP) functionals. The former functional better describes the geometry of the complex while the latter gives a better insight into its electronic structure. The extra negative charge is delocalized over NO groups, phenyl ligands, and iron atoms. The calculated redox potentials of one-electron reduction of the complexes are close to the experimental values obtained by analyzing cyclic voltammograms. Attachment of the second electron opens the decomposition channel of the complex, which is also consistent with experimental data.

Investigation of the molecular and crystalline structure of 2,4,6-triazido-3-chloro-5-trifluoromethylpyridine and rotation barrier of the γ-azidogroup around a C-N bond

The molecular and crystalline structure of 2,4,6-triazido-3-chloro-5-trifluoromethylpyridine has been investigated by X-ray diffraction at temperatures of 200 and 293 K. The triazide molecule is nonplanar, and the γ-azidogroup significantly deviates from the aromatic ring plane. Nonempirical quantum-chemical calculations of the internal rotation barrier of the γ-azidogroup around the C-N bond have been performed in the B3LYP/6-311G** approximation. It is shown that the minimum of the total energy of this system corresponds to the dihedral angle between the γ-azidogroup and pyridine cycle planes (∼34.5°), whereas the structures with the γ-azidogroup, oriented in the pyridine cycle plane or perpendicularly to it, are characterized by very low barriers: ∼0.26 and 1.74 kcal mol−1, respectively.

Thiacalix[4]arene-containing M2Ln2 complexes (M = MnII, CoII; Ln = EuIII, PrIII): synthesis, structure, and magnetic properties

Three new representatives of thiacalix[4]arene-containing tetranuclear 3d-4f metal complexes with the M2Ln2 fragment (M = MnII, CoII; Ln = EuIII, PrIII) were synthesized. Their molecular and crystal structures were studied by X-ray diffraction analysis. According to the magnetochemical measurements, the complexes exhibit the paramagnetic properties with weak antiferromagnetic interactions at low temperatures (T < 30–50 K). The influence of the environment of the rare-earth elements on the spin-orbital coupling and magnetic properties of the complexes was found.

Dependence of Properties and Exchange Coupling Constants on the Charge in the Mn2On and Fe2On Series

The geometrical structure and properties of the neutral and singly charged Mn2O nq and Fe2O nq clusters ( q = 0, ±1) are computed using density functional theory with the generalized gradient approximation in the range 1 ≤ n ≤ 7. The geometrical structures and spin multiplicities of the corresponding species in all six series are similar except for a few exceptions. Antiferromagnetic coupling of total spin magnetic moments of the metal atoms in the lowest total energy states is observed for the majority of species in all six series when n = 1-5; correspondingly, the computed magnetic exchange coupling constants are mostly negative. The states of Mn2O nq and Fe2O nq are nonmagnetic or weakly ferromagnetic when n > 5 except for Mn2O7+ where the ground state is antiferromagnetic. The computed adiabatic electron affinities and ionization energies of the neutral species in both series are quite close to one another and increase as n increases. However, the binding energies of a single oxygen atom and of an O2 dimer decrease as n increases and the Mn2O7+ and Fe2O7+ cations are barely stable with respect to the O2 abstraction. The most stable and least stable species at a given n are the anions and the cations, respectively. The electric dipole polarizability per atom decreases sharply when n moves from 1 to 4 and then remains nearly constant for larger n values in the anion series, whereas it is close to the asymptotic value already at n = 2 in the neutral series.

Quantum chemical study of the Cspiro-O bond dissociation in spiropyran molecules

To study the possibility of photochromic transformations in the crystals of bifunctional compounds, quantum chemical B3LYP/6-31G(d) calculations of the Cspiro-O bond dissociation energies were carried out with the Gaussian-03 program for two dissimilar neutral spiropyrans (Sp) and three cations of their salts in the singlet ground state. For Cspiro-O bond dissociation in the cations Sp+ and the neutral systems SpI consisting of Sp+ and I−, the energy barriers do not exceed 10 kcal mol−1, increasing when moving from the cations Sp+ to the neutral systems SpI. The changes in the HOMO and LUMO energies when going from the closed to open form of the cations Sp+ correlate with the energy barriers to the corresponding Cspiro-O bond dissociations.

A quantum-chemical study on the selectivity of photolysis of azido groups in 2,4,6-triazido-3,5-dichloropyridine

The total energies and geometric parameters of three alternative excited singlet spin states S1(α1), S1(α2), and S1(γ), which result from local photochemical excitation of nonequivalent α- and γ-azido groups in the 2,4,6-triazido-3,5-dichloropyridine (TACP) molecule were determined by quantum-chemical calculations using the basis sets B3LYP/6-311G* and CIS/6-311G*, as well as the activation energies of N-N2 bond dissociation in these locally excited groups. It was found that the photolysis selectivity for the α-azido groups in TACP is due to their stronger π-π conjugation with the pyridine ring in the ground state S0 and a lower energy of their local excitation to the S1(α1) and S1(α2) states.