A computational investigation on the geometries,stabilities, antioxidant activity,and the substituent effects of the L‐ascorbic acid and their derivatives

The geometries, stabilities, and antioxidant activities of L‐Ascorbic acid (1a), D‐erythroascorbate (2a), and D‐erythroascorbate glucoside (3a) as well as their sulfur and selenium derivatives are systematically investigated by using density functional theory. Emphasis is placed on studies of the two main mechanisms, that is, hydrogen atom donation and single‐electron transfer, and the O—H bond dissociation enthalpy and the ionization potential are computed in the gas phase and water solution. The calculated results indicate that the 2‐OH group in the five‐membered ring acts as an important H atom donor to free radicals. The 2‐OH radical spin density distribution shows that the unpaired electron is mostly located at the C3 atom of the five‐membered ring and partially at the vicinal O atoms, proving that a certain delocalization of the odd electron is effective in the five‐membered ring. In water aqueous solution, the antioxidant capacity and the electron donating ability are increased as the O atom in the five‐membered ring of 1a, 2a, and 3a is replaced by S and Se, respectively, in good agreement with experimental measurements; Furthermore, their antioxidant capacities are enhanced as compared with the standard antioxidant (resveratrol). © 2013 Wiley Periodicals, Inc.     

Intermolecular interactions of a size‐expanded guanine analogue with gold nanoclusters

The interactions between a size‐expanded Guanine analogue x‐Guanine (xG) and gold nanoclusters, Au_n (n = 2, 4, 6, and 8), were studied theoretically using density functional theory. Geometries of neutral complexes were optimized using the B3LYP functional with the 6‐31+G(d,p) basis set for xG and the LANL2DZ basis set for gold clusters. The binding modes, interaction strength, and the charge‐transfer properties of different Au_n‐xG complexes were investigated. Natural population analysis was performed for natural bond order charges. It was found that gold nanoclusters form stable complexes with xG and these binding results in a substantial amount of electronic charge being transferred from xG to the gold clusters. The vertical first ionization potential, electron affinity, Fermi Level, and the HOMO–LUMO gap of xG and its complexes with gold nanoclusters were also analyzed. © 2013 Wiley Periodicals, Inc.     

DFT in a nutshell

The purpose of this short essay is to introduce students and other newcomers to the basic ideas and uses of modern electronic density functional theory, including what kinds of approximations are in current use, and how well they work (or not). The complete newcomer should find it orients them well, while even longtime users and aficionados might find something new outside their area. Important questions varying in difficulty and effort are posed in the text, and are answered in the Supporting Information. © 2012 Wiley Periodicals, Inc.     

Mechanism of aziridination of styrene catalyzed by copper(I) bis(oxazoline)

Experimental studies show that copper complexes can be effectively anchored onto the pores of mesoporous solids, having a good catalytic performance in several reactions, among them the aziridination of olefins and in particular, styrene. In this work, the mechanism of the aziridination of styrene catalyzed by a bis(oxazoline) copper(I) complex was studied in detail by means of density functional theory (DFT) calculations. For such reactions in the homogeneous phase, our calculations revealed a wide diversity of reaction‐pathways, which have not been considered in previous studies, and should be taken into account due to the small energy differences between them. What is more, our results show that there is a strong dependence on the chosen DFT functional. This has profound implications on the way the heterogeneous reaction is studied. © 2013 Wiley Periodicals, Inc.     

Exact relations between the electron density and external potential for systems of interacting and noninteracting electrons

The differential virial theorem (DVT) is an explicit relation between the electron density ρ( r ), the external potential, kinetic energy density tensor, and (for interacting electrons) the pair function. The time‐dependent generalization of this relation also involves the paramagnetic current density. We present a detailed unified derivation of all known variants of the DVT starting from a modified equation of motion for the current density. To emphasize the practical significance of the theorem for noninteracting electrons, we cast it in a form best suited for recovering the Kohn–Sham effective potential v_s( r ) from a given electron density. The resulting expression contains only ρ( r ), v_s( r ), kinetic energy density, and a new orbital‐dependent ingredient containing only occupied Kohn–Sham orbitals. Other possible applications of the theorem are also briefly discussed. © 2012 Wiley Periodicals, Inc.     

Performance of density functional theory in computing nonresonant vibrational (hyper)polarizabilities

A set of exchange‐correlation functionals, including BLYP, PBE0, B3LYP, BHandHLYP, CAM‐B3LYP, LC‐BLYP, and HSE, has been used to determine static and dynamic nonresonant (nuclear relaxation) vibrational (hyper)polarizabilities for a series of all‐trans polymethineimine (PMI) oligomers containing up to eight monomer units. These functionals are assessed against reference values obtained using the Møller–Plesset second‐order perturbation theory (MP2) and CCSD methods. For the smallest oligomer, CCSD(T) calculations confirm the choice of MP2 and CCSD as appropriate for assessing the density functionals. By and large, CAM‐B3LYP is the most successful, because it is best for the nuclear relaxation contribution to the static linear polarizability, intensity‐dependent refractive index second hyperpolarizability, static second hyperpolarizability, and is close to the best for the electro‐optical Pockels effect first hyperpolarizability. However, none of the functionals perform satisfactorily for all the vibrational (hyper)polarizabilities studied. In fact, in the case of electric field‐induced second harmonic generation all of them, as well as the Hartree–Fock approximation, yield the wrong sign. We have also found that the Pople 6–31+G(d) basis set is unreliable for computing nuclear relaxation (hyper)polarizabilities of PMI oligomers due to the spurious prediction of a nonplanar equilibrium geometry. © 2013 Wiley Periodicals, Inc.     

An assessment of pure,hybrid, meta,and hybrid‐meta GGA density functional theory methods for open‐shell systems: The case of the nonheme iron enzyme 8R–LOX

The performance of a range density functional theory functionals combined in a quantum mechanical (QM)/molecular mechanical (MM) approach was investigated in their ability to reliably provide geometries, electronic distributions, and relative energies of a multicentered open‐shell mechanistic intermediate in the mechanism 8R–Lipoxygenase. With the use of large QM/MM active site chemical models, the smallest average differences in geometries between the catalytically relevant quartet and sextet complexes were obtained with the B3LYP^* functional. Moreover, in the case of the relative energies between ⁴II and ⁶II , the use of the B3LYP^* functional provided a difference of 0.0 kcal mol^–1. However, B3LYP^± and B3LYP also predicted differences in energies of less than 1 kcal mol^–1. In the case of describing the electronic distribution (i.e., spin density), the B3LYP^*, B3LYP, or M06‐L functionals appeared to be the most suitable. Overall, the results obtained suggest that for systems with multiple centers having unpaired electrons, the B3LYP^* appears most well rounded to provide reliable geometries, electronic structures, and relative energies. © 2012 Wiley Periodicals, Inc.     

New basis sets for the evaluation of interaction‐induced electric properties in hydrogen‐bonded complexes

Interaction‐induced static electric properties, that is, dipole moment, polarizability, and first hyperpolarizability, of the CO? (HF)_n and N₂? (HF)_n, n = 1–9 hydrogen‐bonded complexes are evaluated within the finite field approach using the Hartree–Fock, density functional theory, Møller–Plesset second‐order perturbation theory, and coupled cluster methods, and the LPol‐n (n = ds, dl, fs, fl) basis sets. To compare the performance of the different methods with respect to the increase of the complex size, we consider as model systems linear chains of the complexes. We analyze the results in terms of the many‐body and cooperative effects. © 2012 Wiley Periodicals, Inc.     

Understanding the electronic energy transfer pathways in the trimeric and hexameric aggregation state of cyanobacteria phycocyanin within the framework of Förster theory

In the present study, the electronic energy transfer pathways in trimeric and hexameric aggregation state of cyanobacteria C‐phycocyanin (C‐PC) were investigated in term of the Förster theory. The corresponding excited states and transition dipole moments of phycocyanobilins (PCBs) located into C‐PC were examined by model chemistry in gas phase at time‐dependent density functional theory (TDDFT), configuration interaction‐singles (CIS), and Zerner's intermediate neglect of differential overlap (ZINDO) levels, respectively. Then, the long‐range pigment‐protein interactions were approximately taken into account by using polarizable continuum model (PCM) at TDDFT level to estimate the influence of protein environment on the preceding calculated physical quantities. The influence of the short‐range interaction caused by aspartate residue nearby PCBs was examined as well. Only when the protonation of PCBs and its long‐ and short‐range interactions were properly taken into account, the calculated energy transfer rates (1/K) in the framework of Förster model at TDDFT/B3LYP/6‐31+G* level were in good agreement with the experimental results of C‐PC monomer and trimer. Furthermore, the present calculated results suggested that the energy transfer pathway in C‐PC monomer is predominant from β‐155 to β‐84 (1/K = 13.4 ps), however, from α‐84 of one monomer to β‐84 (1/K = 0.3–0.4 ps) in a neighbor monomer in C‐PC trimer. In C‐PC hexamer, an additional energy flow was predicted to be from β‐155 (or α‐84) in top trimer to adjacent β‐155 (or α‐84) (1/K = 0.5–2.7 ps) in bottom trimer. © 2013 Wiley Periodicals, Inc.     

NMR shielding constants of CuX,AgX, and AuX (X = F,Cl, Br,and I) investigated by density functional theory based on the Douglas–Kroll–Hess Hamiltonian

Two‐component relativistic density functional theory (DFT) with the second‐order Douglas–Kroll–Hess (DKH2) one‐electron Hamiltonian was applied to the calculation of nuclear magnetic resonance (NMR) shielding constant. Large basis set dependence was observed in the shielding constant of Xe atom. The DKH2‐DFT‐calculated shielding constants of I and Xe in HI, I₂, CuI, AgI, and XeF₂ agree well with those obtained by the four‐component relativistic theory and experiments. The Au NMR shielding constant in AuF is extremely more positive than in AuCl, AuBr, and AuI, as reported recently. This extremely positive shielding constant arises from the much larger Fermi contact (FC) term of AuF than in others. Interestingly, the absolute values of the paramagnetic and the FC terms are considerably larger in CuF and AuF than in others. The large paramagnetic term of AuF arises from the large d‐components in the Au d_π –F p_π and Au sd_σ–F p_σ molecular orbitals (MOs). The large FC term in AuF arises from the small energy difference between the Au sd_σ + F p_σ and Au sd_σ–F p_σ MOs. The second‐order magnetically relativistic effect, which is the effect of DKH2 magnetic operator, is important even in CuF. This effect considerably improves the overestimation of the spin‐orbit effect calculated by the Breit–Pauli magnetic operator. © 2013 Wiley Periodicals, Inc.