Maria Jaworska

Electronically excited states of vitamin B12: benchmark calculations including time-dependent density functional theory and correlated ab initio methods.

Time-dependent density functional theory (TD-DFT) and correlated ab initio methods have been applied to explore the electronically excited states of vitamin B(12) (cyanocobalamin or CNCbl). Different experimental techniques have been used to probe the excited states of CNCbl, revealing many issues that remain poorly understood from an electronic structure point of view. Due to its efficient scaling with size, TD-DFT emerges as one of the most practical tools that can be used to study the electronic properties of these fairly complex molecules. However, the description of excited states is strongly dependent on the type of functional used in the calculations. In the present contribution, the choice of a proper functional for vitamin B(12) was evaluated in terms of its agreement with both experimental results and correlated ab initio calculations. Three different functionals, i.e., B3LYP, BP86, and LC-BLYP, were tested. In addition, the effect of the relative contributions of DFT and HF to the exchange-correlation functional was investigated as a function of the range-separation parameter, μ. The issues related to the underestimation of charge-transfer excitation energies by TD-DFT were validated by the Λ diagnostic, which measures the spatial overlap between occupied and virtual orbitals involved in the particular excitation. The nature of the low-lying excited states was also analyzed based on a comparison of TD-DFT and ab initio results. Based on an extensive comparison with experimental results and ab initio benchmark calculations, the BP86 functional was found to be the most appropriate in describing the electronic properties of CNCbl. Finally, an analysis of electronic transitions and reassignment of some excitations were discussed.

The cobalt-methyl bond dissociation in methylcobalamin: New benchmark analysis based on density functional theory and completely renormalized coupled-cluster calculations

The Co-CMe bond dissociation in methylcobalamin (MeCbl), modeled by the Im-[Co(III)corrin]-Me(+) system consisting of 58 atoms, is examined using the coupled-cluster (CC), density-functional theory (DFT), complete-active-space self-consistent-field (CASSCF), and CASSCF-based second-order perturbation theory (CASPT2) approaches. The multilevel variant of the local cluster-in-molecule framework, employing the completely renormalized (CR) CC method with singles, doubles, and noniterative triples, termed CR-CC(2,3), to describe higher-order electron correlation effects in the region where the Co-CMe bond breaking takes place, and the canonical CC approach with singles and doubles (CCSD) to capture the remaining correlation effects, abbreviated as CR-CC(2,3)/CCSD, is used to obtain the benchmark potential energy curve characterizing the Co-CMe dissociation in the MeCbl cofactor. The Co-CMe bond dissociation energy (BDE) resulting from the CR-CC(2,3)/CCSD calculations for the Im-[Co(III)corrin]-Me(+) system using the 6-31G* basis set, corrected for the zero-point energies (ZPEs) and the effect of replacing the 6-31G* basis by 6-311++G**, is about 38 kcal/mol, in excellent agreement with the experimental values characterizing MeCbl of 37 ± 3 and 36 ± 4 kcal/mol. Of all DFT functionals examined, the best dissociation energies and the most accurate description of the Co-CMe bond breaking in the Im-[Co(III)corrin]-Me(+) system are provided by B97-D and BP86 corrected for dispersion using the D3 correction of Grimme et al., which give 35 and 40 kcal/mol, respectively, when the 6-311++G** basis set is employed and when the results are corrected for ZPEs and basis set superposition error. None of the other DFT approaches examined provide results that fall into the experimental range of the Co-CMe dissociation energies in MeCbl of 32-40 kcal/mol. The hybrid DFT functionals with a substantial amount of the Hartree-Fock (HF) exchange, such as B3LYP, considerably underestimate the calculated dissociation energies, with the magnitude of the error being proportional to the percentage of the HF exchange in the functional. It is argued that the overstabilization of diradical structures that emerge as the Co-CMe bond is broken and, to some extent, the neglect of dispersion interactions at shorter Co-CMe distances, postulated in previous studies, are the main factors that explain the substantial underestimation of the Co-CMe BDE by B3LYP and other hybrid functionals. Our calculations suggest that CASSCF and CASPT2 may have difficulties with providing a reliable description of the Co-CMe bond breaking in MeCbl, since using adequate active spaces is prohibitively expensive.

Photodissociation of Co-C bond in methyl- and ethylcobalamin: an insight from TD-DFT calculations.

The mechanism of Co-C bond photodissociation in methylcobalamin (MeCbl) and ethylcobalamin (EtCbl) has been examined by means of time-dependent density functional theory (TD-DFT). The present contribution extends our recent study (J. Phys. Chem. B 2007, 111, 2419-2422) where relevant excited states involved in the photolysis of MeCbl have been identified. To obtain reliable structural models, the high-resolution crystal structure of MeCbl was used as the source of initial coordinates. The full MeCbl was simplified by replacing the corrin side chains by H atoms and the resulting geometry was optimized. The model of EtCbl was generated from the simplified structure of MeCbl by replacing methyl group with ethyl. For both models, the low-lying singlet and triplet excited states have been computed along the Co-C coordinate at TD-DFT/BP86/6-31G(d) level of theory. These calculations reveal that the photodissociation process is mediated by the repulsive 3(sigmaCo-C-->sigma*Co-C) triplet state. The overall mechanism of photodissociation for both systems is similar but energetic details are different, reflecting the difference in Co-C bond strength in MeCbl and EtCbl. In both cases the key intermediate involved in Co-C bond photodissociation is identified as first excited state (S1). The S1 intermediate has mixed character: it can be described as predominantly dCo-->pi*corrin metal-to-ligand charge transfer (MLCT) state with contribution from sigma bond to corrin charge transfer (SBLCT) where upon electronic excitation the electron density shifts from the axial NIm-Co-C bonding to corrin ligand. The optimized geometry of the S1 indicates that the structure of the corrin remains essentially unchanged in comparison to ground state (S0). The major structural change occurs in the NIm-Co-C moiety, which becomes bent with elongated Co-C bond in S1 state. Finally, it is proposed that the photolysis of Co-C bond is in line with the mechanism of heme-CO photolysis, where participation of the dFe-->pi*porphyrin has been suggested.

Electronic structure of the S1 state in methylcobalamin: Insight from CASSCF/MC-XQDPT2, EOM-CCSD, and TD-DFT calculations

The methylcobalamin cofactor (MeCbl), which is one of the biologically active forms of vitamin B12, has been the subject of many spectroscopic and theoretical investigations. Traditionally, the lowest‐energy part of the photoabsorption spectrum of MeCbl (the so‐called α/β band) has been interpreted as an S0→S1 electronic transition dominated by π→π* excitations associated with the CC stretching of the corrin ring. However, a more quantitative band‐shape analysis of the α/β spectral region, along with circular dichroism (CD), magnetic CD, and resonance Raman data, has revealed the presence of a second electronic transition that involves the CoCMe bond weakening. Conversely, the lowest‐energy excitations based on transient absorption spectroscopy measurements have been interpreted as metal‐to‐ligand charge transfer (MLCT) transitions. To resolve the existing controversy about the interpretation of the S1 state of MeCbl, calculations have been performed using two independent ab initio wavefunction‐based methods. These include the modified variant of the second‐order multiconfigurational quasi‐degenerate perturbation theory (MC‐XQDPT2), using complete active space self‐consistent field orbitals, and the equation‐of‐motion coupled‐cluster singles and doubles (EOM‐CCSD) approach using restricted Hartree–Fock orbitals. It is shown that both ab initio methods provide a consistent description of the S1 state as having an MLCT character. In addition, the performance of different types of functionals, including hybrid (B3LYP, MPW1PW91, TPSSh), generalized‐gradient‐approximation‐type (GGA‐type) (BP86, BLYP, MPWPW91), meta‐GGA (TPSS), and range‐separated (CAM‐B3LYP, LC‐BLYP) approaches, has been examined and the results of the corresponding time‐dependent density functional theory calculations have been benchmarked against the MC‐XQDPT2 and EOM‐CCSD data. The hybrid functionals support the interpretation in which the S1 state represents a π→π* transition localized on corrin, while pure GGA, meta‐GGA, and LC‐BLYP functionals produce results consistent with the MLCT assignment.

Electronic and structural properties of low-lying excited states of vitamin B12.

Time-dependent density functional theory (TD-DFT) has been applied to explore electronically excited states of vitamin B(12) (cyanocobalamin or CNCbl). To explain why the Co-C bond in CNCbl does not undergo photodissociation under conditions of simple photon excitation, electronically excited states have been computed along the Co-C(CN) stretched coordinate. It was found that the repulsive (3)(σ(Co-C) → σ*(Co-C)) triplet state drops in energy as the Co-C(CN) bond lengthens, but it does not become dissociative. Low-lying excited states were also computed as function of two axial bond lengths. Two energy minima have been located on the S(1)/CNCbl, as well as T(1)/CNCbl, surfaces. The full geometry optimization was carried out for each minimum and electronic properties associated with each optimized structure were analyzed in details. One minimum was described as excitation having mixed ππ*/MLCT (metal-to-ligand charge transfer) character, while the second as ligand-to-metal charge transfer (LMCT) transition. Neither of them, however, can be viewed as pure MLCT or LMCT transitions since additional excitation to or from σ-bonds (SB) of N-Co-C unit have also noticeable contributions. Inclusion of solvent altered the character of one of the excitations from ππ*/MLCT/SBLCT to ππ*/LMCT/LSBCT-type, and therefore, both of them gained significant contribution from LMCT/LSBCT transition. Finally, the nature of S(1) electronic state has been comparatively analyzed in CNCbl and MeCbl cobalamins.

Structure and UV-Vis spectroscopy of the iron-sulfur dinuclear nitrosyl complexes [Fe2S2(NO)4]2− and [Fe2(SR)2(NO)4]

Calculations of the electronic structure, geometry and electronic spectra of Roussin’s red salt dianion (RRS) and Roussin’s red diester (RRE) were carried out with the RB3LYP and UB3LYP methods. The electronic structure emerging from these calculations may be described as composed of two {Fe(NO)2}9 units, in which ferric ion (S = 5/2) is antiferromagnetically coupled to two NO− ligands (each with S = 1), giving S = 1/2; the units are antiferromagnetically coupled to each other yielding a total S = 0. The S2− bridges (in RRS) or SR− bridges (RRE) mediate the antiferromagnetic coupling. The character of the frontier orbitals controls the dinuclear species’ reactivity, which is initiated by electrophilic attack on S-localized HOMO orbitals (RRS) or nucleophilic attack on the Fe–S antibonding LUMO orbital (RRE). The contrasting susceptibility to electrophilic/nucleophilic attack is also assisted by the sulfur charge, which is negative in RRS and positive in RRE. The calculated spectra of RRS and RRE show substantial resemblance to the experimental spectra. The calculated transitions are mainly of charge transfer character: At long wavelengths they are described as π*NO → d (LMCT), at short wavelengths (below 250 nm) the most intense transitions are d → π*NO (MLCT). In the middle part of the spectra both types of transitions are present. Some contribution of sulfur to the transitions throughout the whole spectrum is observed. The π*NO → d transitions are assumed to be responsible for the photochemical reactivity of both compounds, which is initiated by photodissociation of the NO group.

GIAO NMR calculations for carbazole and its N-methyl and N-ethyl derivatives. Comparison of theoretical and experimental 13C chemical shifts

High‐level ab initio calculations were performed at the restricted Hartree–Fock (RHF) level of theory on carbazole and its N‐methyl and N‐ethyl derivatives. Single‐point gauge‐invariant atomic orbitals (SP GIAO) RHF NMR calculations on ab initio RHF optimized geometries were performed. The 6–31G* and 6–311++G** basis sets were used and some calculations were performed within a density functional theory using a recent B3PW91 hybrid functional. The theoretically predicted multinuclear magnetic resonance chemical shifts of carbazole and its N‐methyl and N‐ethyl derivatives in the gas phase are compared with experimental NMR data in CDCl3 solutions. A revised assignment of 13C NMR spectra of simple carbazoles is proposed. Copyright

Role of the Axial Base in the Modulation of the Cob(I)alamin Electronic Properties: Insight from QM/MM, DFT, and CASSCF Calculations.

Quantum chemical computations are used to study the electronic and structural properties of the cob(I)alamin intermediate of the cobalamin-dependent methionine synthase (MetH). QM(DFT)/MM calculations on the methylcobalamin (MeCbl) binding domain of MetH reveal that the transfer of the methyl group to the substrate is associated with the displacement of the histidine axial base (His759). The axial base oscillates between a His-on form in the Me-cob(III)lamin:MetH resting state, where the Co-N(His759) distance is 2.27 Å, and a His-off form in the cob(I)alamin:MetH intermediate (2.78 Å). Furthermore, QM/MM and gas phase DFT calculations based on an unrestricted formalism show that the cob(I)alamin intermediate exhibits a complex electronic structure, intermediate between the Co(I) and Co(II)-radical corrin states. To understand this complexity, the electronic structure of Im···[Cob(I)alamin] is investigated using multireference CASSCF/QDPT2 calculations on gas phase models where the axial histidine is modeled by imidazole (Im). It is found that the correlated ground state wave function consists of a closed-shell Co(I) (d(8)) configuration and a diradical contribution, which can be described as a Co(II) (d(7))-radical corrin (π*)(1) configuration. Moreover, the contribution of these two configurations depends on the Co-NIm distance. At short Co-NIm distances (<2.5 Å), the dominant electronic configuration is the diradical state, while for longer distances it is the closed-shell state. The implications of this finding are discussed in the context of the methyl transfer reaction between the Me-H4folate substrate and cob(I)alamin.

Time-dependent density functional theory study of cobalt corrinoids: Electronically excited states of methylcobalamin

Time-dependent density functional theory (TDDFT) has been applied to the analysis of the electronic spectra of methylcobalamin (MeCbl) and its derivative in which the trans axial base was replaced by a water molecule (MeCbi[Single Bond]H(2)O). The latter corresponds to the situation encountered in strongly acidic solutions. The study primarily focuses on the accuracy of two functionals, the hybrid B3LYP and the gradient corrected BP86, in dealing with the electronic excitations. The high resolution crystal structure of MeCbl was the source of the initial coordinates. To generate the initial structures, the full MeCbl was simplified by replacing the corrin side chains by H atoms. The vertical excitation energies, together with the corresponding oscillator strengths, were calculated at the optimized BP86 and B3LYP structures of the ground electronic state of the complexes. The NBO analysis shows that the B3LYP functional gives a bonding description of the ground state as a more polarized covalent bond compared to that given by BP86. The latter functional has more covalent bonding and is thus more appropriate for modeling the axial bonding properties. To validate the accuracy of the present TDDFT analysis, the computed excitations were directly compared to the absorption spectra of MeCbl. In order to obtain a reliable agreement between experiment and theory, the two-parameter scaling technique was introduced, which compensates differently the low-energy and high-energy excitations. Electronic excitations strongly depend on the choice of the functional. Transitions involving corrin pi-->pi(*) excitations are better described by the B3LYP functional while transitions associated with metal-to-ligand (dpi-->pi(*)d) excitations are better described by BP86. These differences can be associated with the different bonding descriptions obtained by B3LYP and BP86.

Mechanism of Co-C bond photolysis in the base-on form of methylcobalamin.

A mechanism of Co-C bond photodissociation in the base-on form of the methylcobalamin cofactor (MeCbl) has been investigated employing time-dependent density functional theory (TD-DFT), in which the key step involves singlet radical pair generation from the first electronically excited state (S1). The corresponding potential energy surface of the S1 state was constructed as a function of Co-C and Co-Naxial bond distances, and two possible photodissociation pathways were identified on the basis of energetic grounds. These pathways are distinguished by whether the Co-C bond (path A) or Co-Naxial bond (path B) elongates first. Although the final intermediate of both pathways is the same (namely a ligand field (LF) state responsible for Co-C dissociation), the reaction coordinates associated with paths A and B are different. The photolysis of MeCbl is wavelength-dependent, and present TD-DFT analysis indicates that excitation in the visible α/β band (520 nm) can be associated with path A, whereas excitation in the near-UV region (400 nm) is associated with path B. The possibility of intersystem crossing, and internal conversion to the ground state along path B are also discussed. The mechanism proposed in this study reconciles existing experimental data with previous theoretical calculations addressing the possible involvement of a repulsive triplet state.