Multiconfiguration Quasi-degenerate Second-order Perturbation Research Articles

Iron(II) Complexes with Porphyrin and Tetrabenzoporphyrin: CASSCF/MCQDPT2 Study of the Electronic Structures and UV-Vis Spectra by sTD-DFT.

The geometry and electronic structures of iron(II) complexes with porphyrin (FeP) and tetrabenzoporphyrin (FeTBP) in ground and low-lying excited electronic states are determined by DFT (PBE0/def2-TZVP) calculations and the complete active space self-consistent field (CASSCF) method, followed by the multiconfigurational quasi-degenerate second-order perturbation theory (MCQDPT2) approach to determine the dynamic electron correlation. The minima on the potential energy surfaces (PESs) of the ground (3A2g) and low-lying, high-spin (5A1g) electronic states correspond to the planar structures of FeP and FeTBP with D4h symmetry. According to the results of the MCQDPT2 calculations, the wave functions of the 3A2g and 5A1g electronic states are single determinant. The electronic absorption (UV-Vis) spectra of FeP and FeTBP are simulated within the framework of the simplified time-dependent density functional theory (sTDDFT) approach with the use of the long-range corrected CAM-B3LYP function. The most intensive bands of the UV-Vis spectra of FeP and FeTBP occur in the Soret near-UV region of 370-390 nm.

Molecular Structure, Thermodynamic and Spectral Characteristics of Metal-Free and Nickel Complex of Tetrakis(1,2,5-thiadiazolo)porphyrazine.

The Knudsen effusion method with mass spectrometric control of the vapor composition was used to study the possibility of a congruent transition to the gas phase and to estimate the enthalpy of sublimation of metal-free tetrakis(1,2,5-thiadiazolo)porphyrazine and its nickel complex (H2TTDPz and NiTTDPz, respectively). The geometrical and electronic structure of H2TTDPz and NiTTDPz in ground and low-lying excited electronic states were determined by DFT calculations. The electronic structure of NiTTDPz was studied by the complete active space (CASSCF) method, following accounting dynamic correlation by multiconfigurational quasi-degenerate second-order perturbation theory (MCQDPT2). A geometrical structure of D2h and D4h symmetry was obtained for H2TTDPz and NiTTDPz, respectively. According to data obtained by the MCQDPT2 method, the nickel complex possesses the ground state 1A1g, and the wave function of the ground state has the form of a single determinant. Electronic absorption and vibrational (IR and resonance Raman) spectra of H2TTDPz and NiTTDPz were studied experimentally and simulated theoretically.

Peculiarities of electronic structure and chemical bonding in iron and cobalt metal complexes of porphyrazine and tetra(1,2,5-thiadiazole)porphyrazine

The geometrical and electronic structures of iron and cobalt metal complexes of porphyrazine and tetra(1,2,5-thiadiazole)porphyrazine in ground and low-lying excited electronic states were determined by DFT calculations and the complete active space (CASSCF) method with following accounting dynamic correlation by multiconfigurational quasidegenerate second-order perturbation theory (MCQDPT2). A geometrical structure of D[Formula: see text] symmetry has been obtained for all four complexes. According to data obtained by the MCQDPT2 method, the complexes of cobalt and iron possess the ground states 2A[Formula: see text] and 3A[Formula: see text], respectively, and wave functions of the ground states have the form of a single determinant. It is shown that the crystal field theory (CFT) can be used to describe the sequence of electronic states of the investigated complexes. The nature of the bonds between metal atoms and nitrogen atoms has been described using the analysis of the electron density distribution in the frame of Bader’s quantum theory of atoms in molecule (QTAIM).

Jahn-Teller effect versus spin-orbit coupling: The structure of the free molybdenum pentafluoride molecule

The complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate second-order perturbation (MCQDPT2) calculations have been used to study the ground and low-lying excited electronic states of MoF5. Scalar-relativistic effects and spin-orbit coupling (SOC) have been taken into account employing the third-order Douglas-Kroll-Hess (DKH) Hamiltonian and full Breit-Pauli operator, respectively. The optimization and the calculation of the Hessian matrix have been performed by non-routine procedure employing the symmetry coordinates for all considered (D3h, C2v and C4v) geometrical configurations. The trigonal bipyramidal D3h structure of molybdenum pentafluoride MoF5 possesses two-fold degenerate ground first excited electronic orbital state and undergoes Jahn-Teller distortion to the C2v configurations. Accounting of SOC quenches the JT effect. As a result the D3h structure of MoF5 is no longer the conical intersection of two adiabatic potential energy surfaces (APES) but two separated APES corresponding to ground (12E1/2) and nearest excited (12E3/2) spin-orbit states are appeared instead. The D3h trigonal bipyramidal structure in the ground spin-orbit state corresponds to the 2-nd order saddle point on APES and lies above the minima (C2v structure, 12E1/2 state) by 244 cm−1. The warping barrier (another C2v structure) in the trough of APES and the Berry pseudorotation barrier (C4v structure, ground spin-orbit state 2E3/2) are 19 and 310 cm−1, respectively. Therefore MoF5 molecule is characterized in the gas phase by a large-amplitude motion of fluorine atoms in the equatorial plane and non-rigid intramolecular rearrangement by Berry mechanism. Despite such non-rigid nuclei dynamic, the calculated harmonic frequencies of MoF5 molecule do not contradict to available experimental IR spectrum. The repeat refinement of MoF5 gas electron diffraction data has been carried out using static and pseudoconformer models based on the SO-MCQDPT2 results. Corresponding to only extreme points on APES the configurations have been included into the pseudoconformer model. The both models have been fitted to the experimental data by the same quality.

Jahn-Teller effect enhanced by spin-orbit coupling. From theory to Experiment:Example of niobium tetrachloride NbCl4

The ground and lower-lying excited electronic states of NbCl4 molecule were systematically studied by the complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate second-order perturbation (MCQDPT2) methods. Scalar-relativistic effects and spin-orbit coupling (SOC) have been taken into account employing the third-order Douglas-Kroll-Hess (DKH) Hamiltonian and full Breit-Pauli operator, respectively. The high-symmetrical Td structure with spin-orbit ground 2G3/2 state undergoes Jahn-Teller distortion along all non-totally symmetric vibrational coordinates. As a result the compressed (2E1/2 state) and elongated (2E3/2 state) D2d structures correspond to minimum and first order saddle point on the lower sheet of adiabatic potential energy surface (APES). The height of the warping barrier in the trough of APES and the Jahn-Teller stabilization energy are 179 and 438 cm−1, respectively. For two-mode 2E⊗e Jahn-Teller problem the vibronic Schrödinger equation has been solved in the diabatic basis. It was shown, the topology of the lower sheet determined mainly the energies and wave functions ground and few low-lying vibronic levels. The original methodology of the simulation of gas-phase electron diffraction (GED) data was developed. This methodology is based on the analytic description of some APES numerically obtained. The computed radial distribution function and thermal average parameters were compared with the experimental GED data. A good agreement between experimental and theoretical results supports the reliability of the theoretical data.

Polarizability of electronically excited molecular oxygen: theory and experiment

Multireference ab initio calculations (by using the extended multiconfiguration quasi-degenerate second-order perturbation theory method XMCQDPT2) have been employed to determine the static polarizabilities for the ground triplet () and two excited singlet (a1Δg and ) electronic states of molecular oxygen. The corrections to polarizabilities for the nuclear zero point motion are also reported. It was revealed that the excitation of an O2 molecule into the lowest singlet electronic states leads to a decrease in its polarizability: the polarizabilities of O2(a1Δg) and O2() are smaller (by factors of 0.967 and 0.962, respectively) than that of O2(). The theoretical results for the state were quantitatively confirmed experimentally employing the diffraction at laser-induced gratings.

Quantum Chemical Study of the Redox Potential of the Co(OH2)62+/3+ Couple and the Singlet-Quintet Gibbs Energy Difference of the Co(OH2)63+ Ion.

The geometry and vibrational frequencies of Co(OH2)62+ in the quartet state and Co(OH2)63+ in the singlet and quintet states were computed with quantum mechanics/molecular mechanics (QM/MM), whereby the LC-BOP-LRD functional was used for the QM part involving the Co(OH2)6n+ ( n = 2, 3) ions. The surrounding 124 MM water molecules were treated with the MMFF94 force field. The hydration energy differences between low-spin Co(OH2)63+ and Co(OH2)62+ or Co(OH2)63+ in the quintet state were also calculated using this method. The electronic energy of the Co(OH2)6n+ ( n = 2, 3) ions was calculated with wave function theory, multistate extended general multiconfiguration quasi-degenerate second-order perturbation theory and spin-orbit configuration interaction. The redox potential of the Co(OH2)62+/3+ couple, and the singlet-quintet (adiabatic) Gibbs energy difference of Co(OH2)63+, computed based on these data, agree with the experiment.

Quantum Chemical Study of the Water Exchange Mechanism of the Neptunyl(VI) and -(V) Aqua Ions.

The water exchange reaction of the neptunyl(VI) and -(V) aqua ions was investigated with quantum chemical methods. Associative ( a) and dissociative ( d) exchange mechanisms were studied. The geometries and vibrational frequencies were computed with density functional theory (DFT) and multiconfiguration self-consistent field (MCSCF) methods. The Gibbs activation energies (Δ G‡) for a and d pathways were calculated with DFT and wave function theory (WFT), extended general multiconfiguration quasi-degenerate second-order perturbation theory (XGMC-QDPT2) including spin-orbit (SO) coupling at the SO configuration interaction (CI) level. The DFT-WFT agreement for Δ G‡ was poor for the investigated functionals except for LC-BOP-LRD. Due to ligand-field effects, Δ G‡ for the associatively activated exchange reaction of NpO2(OH2)52+ with a fδ1 electron configuration is higher than for the actinyl(VI) aqua ions of U, Pu, and Am exhibiting f0, fδ2, and fδ2fϕ1 electron configurations.

Reaction of H2 with O2 in Excited Electronic States: Reaction Pathways and Rate Constants.

Comprehensive quantum chemical analysis with the use of the multireference state-averaged complete active space self-consistent field approach was carried out to study the reactions of H2 with O2 in a1Δg, b1Σg+, c1Σu-, and A'3Δu electronically excited states. The energetically favorable reaction pathways and possible intersystem crossings have been revealed. The energy barriers were refined employing the extended multiconfiguration quasi-degenerate second-order perturbation theory. It has been shown that the interaction of O2(a1Δg) and O2(A'3Δu) with H2 occurs through the H-abstraction process with relatively low activation barriers that resulted in the formation of the HO2 molecule in A″ and A' electronic states, respectively. Meanwhile, molecular oxygen in singlet sigma states (b1Σg+ and c1Σu-) was proved to be nonreactive with respect to the molecular hydrogen. Appropriate rate constants for revealed reaction and quenching channels have been estimated using variational transition-state theory including corrections for the tunneling effect, possible nonadiabatic transitions, and anharmonicity of vibrations for transition states and reactants. It was demonstrated that the calculated reaction rate constant for the H2 + O2(a1Δg) process is in reasonable agreement with known experimental data. The Arrhenius approximations for these processes have been proposed for the temperature range T = 300-3000 K.

Boron Dipyridylmethene (DIPYR) Dyes: Shedding Light on Pyridine-Based Chromophores

Boron dipyrromethene (BODIPY) derivatives have found widespread utility as chromophores in fluorescent applications, but little is known about the photophysical properties of pyridine-based BODIPY analogues, dipyridylmethene dyes. Indeed, it has been reported that boron difluoride dipyridylmethene (DIPYR) is nonemissive, and that derivatives of DIPYR have modest, if any, luminescence. In this report, we explore this little-touched area of chemical space and investigate the photophysical properties of three simple DIPYR dyes: boron dipyridylmethene, boron diquinolylmethene, and boron diisoquinolylmethene. The three dyes strongly absorb in the blue-green part of the spectrum (λem = 450-520 nm, ε= 2.9-11 × 104 M-1 cm-1) and display green fluorescence with high quantum yields (ΦPL = 0.2, 0.8, and 0.8, respectively). Key photophysical properties in these systems were evaluated using a combination of TD-DFT and extended multiconfigurational quasidegenerate second-order perturbation theory (XMCQDPT2) methods and compared to experimental results, revealing that high quantum yields of the quinoline and isoquinoline derivatives are a result of the relative reordering of S1 and T2 state energies upon benzannulation of the parent structure. The intense absorption and high emission efficiency of the benzannulated derivatives make these compounds an intriguing class of dyes for further derivatization.

Geometrical and electronic structure of the molybdenum and tungsten halides MX3 and MX4 (M = Mo, W; X = F, Cl): Jahn-Teller effect and spin-orbit coupling

The ground and lower-lying excited electronic states of MX3 and MX4 (M = Mo, W; X = F, Cl) molecules were systematically studied by the complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate second-order perturbation (XMCQDPT2) methods. Scalar-relativistic effects and spin-orbit coupling were taken into account employing the third-order Douglas-Kroll-Hess (DKH) Hamiltonian and full Breit-Pauli operator. The ground orbital state of MoF3 and MoCl3 complexes is quadruplet 4A2′ state in the equilibrium configuration with D3h symmetry. If the spin-orbit interaction is taken into account the calculations result in two 4E1/2 and 4E3/2 spin-orbit states with 11–14 cm−1 energy differences instead of one 4A2′ state. Spin-orbit interaction quenches the Jahn-Teller effect in 2E″ ground orbital state of WF3 complex in D3h configuration, and C2v configuration with (4E1/2 + 4E3/2) spin-mixed state corresponds to the minimum on the PES. The WCl3 complex reveals strong interaction of the spin-orbit states of different multiplicity. For WCl3 complex the configuration with D3h symmetry corresponds to the minimum on the PES for four lowest spin-mixed electronic states. The MoF4, MoCl4 and WCl4 complexes possess the tetrahedral equilibrium configuration with 3A2 orbital electronic state. If the spin-orbit interaction is taken into account the ground state changes to the threefold degenerated 3T2 spin-orbit state with weak Jahn-Teller effect. In the case of WF4 complex the strong Jahn-Teller effect has been discovered in the first excited 1E singlet electronic state of the tetrahedral structure, and D2d configuration with 1A1 electronic state possesses the lowest energy. The comparison with available experimental data has been performed. The tendencies in molecular parameters of complexes studied were analyzed.

Structure and Properties of Precursor/Successor Complex and Transition State of the FeCl2+/Fe2+ Electron Self-Exchange Reaction via the Inner-Sphere Pathway

The electron self-exchange reaction FeCl(OH2)5(2+) + Fe(OH2)6(2+) → Fe(OH2)6(2+) + FeCl(OH2)5(2+), proceeding via the inner-sphere pathway, was investigated with quantum chemical methods. Geometry and vibrational frequencies of the precursor/successor complex, (H2O)5Fe(III)ClFe(II)(OH2)5(4+)/(H2O)5Fe(II)ClFe(III)(OH2)5(4+) (P/S), and the transition state, (H2O)5FeClFe(OH2)5(4+⧧) (TS), were computed with the LC-BOP functional and CPCM hydration. Bent and linear structures were computed for the TS and P/S. The electronic coupling matrix element (Hab) and the electronic energies were calculated with multistate extended general multiconfiguration quasi-degenerate second-order perturbation theory (XGMC-QDPT2) and spin-orbit configuration interaction (SO-CI). Since the Fe···Fe distance changes considerably along the electron transfer step, the transformation P → TS → S, equations based on the hypothesis of a fixed donor-acceptor distance cannot be applied. Hence, the rate constant for the electron transfer step (ket) was calculated as described previously (Rotzinger, F. P. Inorg. Chem. 2014, 53, 9923). ket is very fast, ∼9.4 × 10(8)-6.6 × 10(9) s(-1) at 0 °C. The experimental rate constant of the title reaction (k) is much slower and controlled by the formation of the precursor complex. The substitution of a water ligand by FeCl(OH2)5(2+) at Fe(OH2)6(2+) is rate-determining.

Investigation of the water exchange mechanism of the Plutonyl(VI) and Uranyl(VI) ions with quantum chemical methods

The water exchange reactions of [PuO2(OH2)5]2+ and [UO2(OH2)5]2+ were investigated with density functional theory (DFT) and wave function theory (WFT). Geometries and vibrational frequencies were calculated with DFT and CPCM hydration. The electronic energies were evaluated with general multiconfiguration quasi-degenerate second-order perturbation theory (GMC-QDPT2). Spin-orbit (SO) effects, computed with SO configuration interaction (SO–CI), are negligible. Both Actinyl(VI) ions react via an associative exchange mechanism, most likely Ia. The Gibbs activation energies (ΔG‡) at 25 °C are 33–34 and 30–37 kJ mol−1 for [PuO2(OH2)5]2+ and [UO2(OH2)5]2+, respectively. ΔG‡ for dissociative mechanisms (D, Id) is higher by more than 15 kJ mol−1.

Structure and properties of the precursor/successor complex and transition state of the CrCl²⁺/Cr²⁺ electron self-exchange reaction via the inner-sphere pathway.

The electron self-exchange reaction CrCl(OH2)5(2+) + Cr(OH2)6(2+) → Cr(OH2)6(2+) + CrCl(OH2)5(2+), proceeding via the inner-sphere pathway, was investigated with quantum-chemical methods. Geometry and vibrational frequencies of the precursor/successor (P/S) complex, (H2O)5Cr(III)ClCr(II)(OH2)5(4+)/(H2O)5Cr(II)ClCr(III)(OH2)5(4+), and the transition state (TS), (H2O)5CrClCr(OH2)5(4+‡), were computed with density functional theory (DFT) and conductor polarizable continuum model hydration. Consistent data were obtained solely with long-range-corrected functionals, whereby in this study, LC-BOP was used. Bent and linear structures were computed for the TS and P/S. The electronic coupling matrix element (H(ab)) and the reorganizational energy (λ) were calculated with multistate extended general multiconfiguration quasi-degenerate second-order perturbation theory. The nuclear tunneling factor (Γ(n)), the nuclear frequency factor (ν(n)), the electronic frequency factor (ν(el)), the electron transmission coefficient (κ(el)), and the first-order rate constant (k(et)) for the electron-transfer step (the conversion of the precursor complex into the successor complex) were calculated based on the imaginary frequency (ν(‡)) of the TS, the Gibbs activation energy (ΔG(‡)), H(ab), and λ. The formation of the precursor complex via water substitution at Cr(OH2)6(2+) was also investigated with DFT and found to be very fast. Thus, the electron-transfer step is rate-determining. For the substitution reaction, only a bent TS structure could be obtained. The overall rate constant (k) was estimated as the product K(A)k(et), whereby K(A) is the equilibrium constant for the formation of the ion aggregate of the reactants Cr(OH2)6(2+) and CrCl(OH2)5(2+), Cr(H2O)6·CrCl(OH2)5(4+) (IAR). k calculated for the bent and linear isomers agrees with the experimental value.

Quasi-degenerate second-order perturbation theory for occupation restricted multiple active space self-consistent field reference functions

A multi-configuration quasi-degenerate second-order perturbation method based on the occupation restricted multiple active space (ORMAS-PT/ORMAS) reference wavefunction is presented. ORMAS gives one the ability to approximate a complete active space self-consistent field (CASSCF) wavefunction using only a subset of the configurations from the CASSCF space. The essential idea behind ORMAS-PT is to use the multi-reference Møller-Plesset formalism to correct the ORMAS reference energy. A computational scheme employing direct CI methodology is presented. Several tests are presented to demonstrate the performance of the ORMAS-PT method.

Investigation of the Ligand-Field States of the Hexaammine Cobalt(III) Ion with Quantum Chemical Methods

The ligand-field (LF) transition energies of the Co(NH3)6(3+) ion have been computed with multiconfiguration quasidegenerate second-order perturbation theory (MCQDPT2). The water solvent was treated with the polarizable continuum model (PCM), and the environment in crystals was modeled by the Co(NH3)6·Cl4(-) complex. The Co-N bond lengths, calculated for the hydrated cation and the Co(NH3)6·Cl4(-) model compound, agree with those in the crystal structures. The vertical transition energies agree with experiment, whereby those based on Co(NH3)6·Cl4(-) are more accurate than those for the hydrated ion. The 0-0 transitions were based on the OPBE geometries of ground and excited (1)T1g, (3)T1g, (5)T2g states of the hydrated ion. The (3)T1g state is the lowest excited state; the (5)T2g state lies higher by >0.6 eV.

Mechanism for the Substitution of an Aqua Ligand of UO2(OH2)5(2+) by Chloride.

Geometry and energy of the reactant (UO2(OH2)5·Cl(+)), the transition state (UO2(OH2)5···Cl(+) (‡)), and the product (UO2Cl(OH2)4·OH2(+)) of the title reaction have been computed with complete active space SCF (geometries and vibrational frequencies) and multiconfiguration quasi-degenerate second-order perturbation theory (total energies). Hydration was treated using the polarizable continuum model. The two investigated active spaces, (12/11) and (12/12), produce the same results. In contrast to the water exchange reaction on UO2(OH2)5(2+), which proceeds via the associative (A) mechanism (which is a two step reaction involving an intermediate with an increased coordination number, UO2(OH2)6(2+)), water substitution by chloride follows the associative interchange (Ia) mechanism (which does not proceed via any intermediate). In this case, structure and imaginary mode of the transition state are not straightforward criteria for the attribution of the substitution mechanism, since they are both typical for the A pathway. The Ia mechanism was derived from the computed intrinsic reaction coordinate, which showed that no intermediate (for example UO2Cl(OH2)5(+)) exists as a local minimum on the potential energy surface. The activation free enthalpy is 31 kJ mol(-1). As for the water exchange reaction, the dissociative mechanism is unlikely to operate because of its higher free activation enthalpy (by ≈25 kJ mol(-1)).

Comparativeab initiostudies on the molecular structure and spectroscopic properties of FeF2: Single reference versus multireference methods

The electronic excitation energies, molecular geometry, quadratic force fields, and vibrational frequencies in the ground (5)Delta(g) and low-lying excited (5)Sigma(g) (+) and (5)Pi(g) electronic states of iron difluoride are studied at sophisticated levels of theory. Two families of basis sets, nonrelativistic and Douglas-Kroll-Hess relativistic, are used that range in quality from triple-zeta to quintuple-zeta. These are augmented by additional diffuse functions (on fluorine atoms) and tight functions (on all atoms) for the description of core-valence correlation and utilized to determine complete basis set molecular properties. The quality of electron correlation treatment using conventional single reference coupled cluster methods CCSD and CCSD(T) is compared to that attained at the multiconfigurational quasidegenerate second-order perturbation theory (CASSCF+MCQDPT2) and the electron attachment equation-of-motion coupled cluster (EOMEA-CCSD) levels. Spin-orbit coupling effects are studied by the SO-MCQDPT2 method using the full Breit-Pauli spin-orbit operator. Effects of spin contamination in the coupled cluster molecular calculations are carefully analyzed. Results of the single reference CCSD(T) and multireference calculations are found to be in a remarkable agreement. The calculations indicate that the EOMEA-CC approach provides a suitable tool for an accurate treatment of FeF(2) and other systems where delicate electron correlation effects have to be carefully dealt with. The inclusion of relativistic effects is shown to be necessary for an accurate description of the molecular geometry and excitation energies of FeF(2). The results of calculations are in good agreement with the experimental data available. The predicted FeF(2) molecular properties are compared to those of the related FeF(3).

The Water‐Exchange Mechanism of the [UO2(OH2)5]2+ Ion Revisited: The Importance of a Proper Treatment of Electron Correlation

The water-exchange mechanism of [UO(2)(OH(2))(5)](2+) has been reinvestigated by using ab initio molecular orbital (MO) methods. The geometries and the vibrational frequencies were computed with CAS-SCF(12/11)-SCRF and CAS-SCF(12/11)-PCM methods, which take into account static electron correlation (using the complete active space self-consistent field (CAS-SCF) technique, based on an active space of 12 electrons in 11 orbitals) and hydration (using the self-consistent reaction field (SCRF) and polarizable continuum model (PCM) techniques). The total energies were computed with multiconfiguration quasi-degenerate second-order perturbation theory, the MCQDPT2(12/11)-PCM method, which treats static and dynamic electron correlation as well as hydration. The adequacies of other currently used quantum chemical methods, MP2, CCSD(T), B3 LYP, and BLYP, are discussed. For the associative and dissociative pathways, thermodynamic activation parameters (DeltaH( not equal), DeltaS( not equal), and DeltaG( not equal)) were computed. For the associative mechanism, the calculated DeltaH( not equal) and DeltaG( not equal) values agree with experiment, whereas for the dissociative mechanism, they are higher by approximately 20 kJ mol(-1). The dissociative mechanism is preferred for substitution reactions of uranyl(VI) complexes with ligands that are stronger electron donors than water. The question of whether a concerted (I(a) or I(d)) or a stepwise (A or D) mechanism operates is discussed on the basis of the computed lifetime of the respective intermediate, and the duration of the vibration with which the intermediate is transformed into the product.

Ab initio study of the ground and lower-lying excited electronic states of NiX 2 and FeX 2 (X=F, Cl, Br, I) molecules

The ground and lower-lying excited electronic states of FeX 2 and NiX 2 (X=F, Cl, Br, I) molecules are systematically investigated by ab initio method at the complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate second-order perturbation (MCQDPT2) levels of theory. It is concluded that the dynamic electron correlation has to be taken into account in the prediction of the properties for such kind of molecules. The equilibrium bond lengths r e(M–X), force constants and harmonic vibrational frequencies are calculated for the ground and lower-lying excited electronic states. The spin-orbit coupling (SOC) effects are analysed.