Gradient-corrected Density Functional Theory Methods Research Articles

Electronic Structure and Circular Dichroism of Natural Alboatisins Isolated from Aerial Parts of Isodon Albopilosus: DFT and TDDFT Study

The stable conformations of a series of bioactive molecules, (−)-alboatisins A–C, are identified via Monte Carlo searching with the MMFF94 molecular mechanics force field. Then, the optical rotation (OR) values, vibrational circular dichroism (VCD), and electronic circular dichroism (ECD) spectra were calculated using the gradient-corrected density functional theory method. The vibrational and transition modes of molecular chirality were explored in terms of their microscopic origin. The calculated specific rotations are in agreement with the experimental values. From the OR analysis, it was concluded that optical rotation values are regulated by hydroxyl substitution. Vibrations occurring on the chiral skeleton may cause strong absorption in VCD spectra; VCD spectra are thus the spectral response to deformation vibrations on the chiral carbon skeleton. The lowest-energy negative Cotton effect is caused by σ → π* transition. Frontier molecular orbital analysis showed that strong ECD absorptions are produced when the dominant transition on the chiral skeleton is asymmetric; ECD spectra show the result of transitions lacking asymmetry on the chiral skeleton.

Density functional study on chirospectra of bruguierols

The theoretical investigation of the chiral optics is induive to the elucidation of the optically rotational mechanism and the design of the novel chiral drugs. The optical rotation (OR), the vibrational circular dichroism (VCD),and the electronic circular dichroism (ECD) spectra of a series of newly-found bio-active molecules, bruguierols A—C, are calculated with the gradient-corrected density functional theory method. On the basis of molecular structure, normally vibrational modes and electronic structure, we explore the microscopic origin of molecular chirality and discuss the solvent effects of OR and ECD spectra. The results show that the introduction of OH modulates the molecular chirality. The methyl group and the phenyl group enhance the molecular chirality. The normal vibrations and the electronic transitions on the chiral skeleton play critical roles in producing the chiral spectra. The solvent effect decreases OR and weakens the ECD spectra.

Density functional study on chirospectra of hydrogen-bonded systems X− (H2O)3 (X = F, Cl, Br, I)

This paper calculates the molecular structures, infrared, Raman, circular dichroism spectra and optical rotatory powers of some hydrogen-bonded supramolecular systems as a cyclic water trimer, (H2O)3 and its pyramidal halide complexes, X− (H2O)3 (X = F, Cl, Br, I) with the gradient-corrected density functional theory method at the B3LYP/6–311++G(2d,2p) and B3LYP/Aug–cc–pVTZ levels. It finds that the complexation of halide anions with the water trimer can efficiently modulate the chirally optical behaviors. The calculated vibrational circular dichroism spectrum illuminates that the vibrational rotational strength of S(+)–(H2O)3 mostly originates from the O–H rocking modes, whereas chirality of S(–)–X− (H2O)3 (X = F, Cl, Br, I) has its important origin in the O–H stretching modes. The calculated optical rotatory power demonstrates that S(+)–(H2O)3 and S(+)–F−(H2O)3 are positively chiral, whereas S(–)–X−(H2O)3 (X = Cl, Br, I) are negatively chiral. With the polarizable continuum model, calculated bulk solvent effect in the solvents water and carbontetrachloride and argon shows that the positive chirality of S(+)–(H2O)3 is enhanced and the negative chirality of S(–)–X−(H2O)3 (X = Cl, Br, I) and the positive chirality of S(+)–F−(H2O)3 are reduced with an augmentation of the solvent dielectric constant.

DFT and metal-metal bonding: a dys-functional treatment for multiply charged complexes?

Density functional theory (DFT) calculations are reported for 16 binuclear transition-metal complexes. Structural motifs studied include face-shared and edge-shared bioctahedra, carboxylate-bridged "paddlewheel" complexes, and nonbridged dimers possessing direct metal-metal bonds. Most of these structure types are represented both by multiply charged (tri- and tetra-anionic, and tetracationic) and by neutral or singly charged examples. Geometry optimizations for these species, in the vacuum phase, use the "broken-symmetry" approach coupled with nine different DFT methods. We find a clear dichotomy in the performance of different DFT approaches. For the eight neutral or singly charged complexes, orthodox gradient-corrected DFT methods such as BP and PBE perform generally very well in reproducing in vacuo the complex geometries obtained from X-ray crystallographic studies. In contrast, these orthodox approaches fail to reliably mimic the crystalline geometries for more highly charged complexes such as Mo(2)Cl(9)(3-), Cr(2)(CH(3))(8)(4-), and Rh(2)(NCCH(3))(10)(4+). Much closer agreement with experimental condensed-phase structures for the multiply charged dinuclear complexes is seen for two "local-density-approximation" approaches, X alpha and VWN, and for VWN+B-LYP, an unorthodox combination of the VWN local and B-LYP nonlocal density functionals. The very good performance of the latter approaches arises from an essentially fortuitous cancellation of errors: while the generally overbinding nature of these approaches suggests that they will not reliably describe true gas-phase structures, this overbinding compensates very well for the coulombic distortion expected when complexes are removed from the charge-stabilizing environment of the crystalline or solvated state. We recommend that, as an alternative to the (computationally expensive) incorporation of solvent-field corrections, VWN+B-LYP is the preferred method for structural characterization of triply or more highly charged dinuclear complexes, while orthodox approaches such as PBE perform best for neutral or mildly charged complexes.

High level of ab initio and density functional theory computational study of structural and energetic properties of tetrafluorhydrazine rotamers

The HF, MP2, MP3, MP4, and QCISD ab initio methods were compared with local, hybrid, and gradient-corrected density functional theory (DFT) methods for computing structures and energies of N2F4 rotamers. In all DFT calculations 6-311+G(2d) basis set was used. The generated structures energies of trans- and gauche-N 2F 4 rotamers, and their dissociation energies to nitrogen difluoride were compared with experimental data. Suitable hybrid and gradient-corrected DFT methods for determining structures and energies for these and similar molecular systems were discussed.

Ab initio and density function theory computational studies of the CH 4 + H → CH 3 + H 2 reaction

The activation barrier for the CH 4 + H → CH 3 + H 2 reaction was evaluated with traditional ab initio and Density Functional Theory (DFT) methods. None of the applied ab initio and DFT methods was able to reproduce the experimental activation barrier of 11.0–12.0 kcal/mol. All ab initio methods (HF, MP2, MP3, MP4, QCISD, QCISD(T), G1, G2, and G2MP2) overestimated the activation energy. The best results were obtained with the G2 and G2MP2 ab initio computational approaches. The zero-point corrected energy was 14.4 kcal mol −1. Some of the exchange DFT methods (HFB) computed energies which were similar to the highly accurate ab initio methods, while the B3LYP hybrid DFT methods underestimated the activation barrier by ~3 kcal mol −1. Gradient-corrected DFT methods underestimated the barrier even more. The gradient-corrected DFT method that incorporated the PW91 correlational functional even generated a negative reaction barrier. The suitability of some computational methods for accurately predicting the potential energy surface for this hydrogen radical abstraction reaction was discussed.

A density functional theory study of the proton abstraction from methane with triplet oxygen — an example of a very difficult computational problem

The computational study of the hydrogen abstraction with triplet oxygen from methane was performed with several ab initio and density functional theory (DFT) methods. The computed values were compared with experimental values, and the accuracy of some computational methods for computing this and similar radical systems is discussed. It was demonstrated that open-shell chemical reactions are very difficult computational problems and Becke's three functional hybrid DFT methods produce the most accurate activation barriers for these reactions which are 4–8 kcal mol −1 below the experimental values. In addition, there are some gradient-corrected DFT methods that compute negative activation barriers.

Can CH cations and nobel gases form stable chemical systems? The density functional theory and ab initio studies of the stability of CHHe + and CHHe 2+

The singlet and triplet structures ofCHHe + and doublet structure CHHe 2+ were investigated with hybrid, local and gradient-corrected density functional theory (DFT) methods, as well as HF, MP2, G1, G2 and CBS-Q ab initio methods. In all calculations, a 6–311++G(3df,3pd) basis set was used. High level ab initio calculations predict that singlet CHHe + was ∼26 kcal mol −1 more stable than the corresponding triplet CHHe +, while hybrid and gradient-corrected DFT methods estimate that the energy preference is around 16 kcal mol −1. Nevertheless, CHHe 2+ should be more stable than singlet CHHe + because the computed CHe bond dissociation energy for singlet CHHe 2+ is ~14 kcal mol −1 in comparison with ~1 kcal mol −1 for CHHe +. These results indicate that CHHe 2+ should be detectable in mass spectroscopic studies.

Structural and vibrational analysis of indole by density functional and hybrid Hartree–Fock/density functional methods

Accurately assigned vibrational frequencies for indole and its derivatives, particularly tryptophan, are critical for analyses of vibronic coupling, excited state fluorescence spectra and resonance Raman spectra. Examination of published, assigned, experimental vibrational frequencies for indole reveals that while most observed bands assigned to fundamentals are in agreement among the different reports, there are notable exceptions in six different spectral regions. We have calculated the structure and assigned the harmonic vibrational spectrum of indole by application of local and gradient-corrected density functional theory methods (DFT) and a hybrid Hartree–Fock/density functional (HF/DFT) method. The hybrid method B3LYP predicts the structure of indole within 0.005 A and 1.1° of a recently published CASSCF structure. Unscaled harmonic frequencies calculated with the gradient-corrected method BLYP with the 6-31G(d) basis set exhibit the best overall agreement with experiment with an RMSD of 31 cm–1 for all modes including hydrogen stretching and bending. The hybrid B3LYP method gives frequencies in closest agreement for the out-of-plane modes, except for the NH wagging mode. Our calculations, which include electron correlation effects for approximately the same computational resources as a Hartree–Fock calculation, predict two fundamentals in the 1330–1360 cm–1 region, where a Fermi resonance pair has previously been hypothesized. This report demonstrates the utility of DFT and HF/DFT methods for heteroaromatic molecules such as indole. We provide the first complete assignment of vibrational modes for indole based on unscaled calculated frequencies which have not been empirically fit to experiment.

The performance of a family of density functional methods

The results of a systematic study of molecular properties by density functional theory (DFT) are presented and discussed. Equilibrium geometries, dipole moments, harmonic vibrational frequencies, and atomization energies were calculated for a set of 32 small neutral molecules by six different local and gradient-corrected DFT methods, and also by the ab initio methods Hartree–Fock, second-order Mo/ller–Plesset, and quadratic configuration interaction with single and double substitutions (QCISD). The standard 6-31G* basis set was used for orbital expansion, and self-consistent Kohn–Sham orbitals were obtained by all DFT methods, without employing any auxiliary fitting techniques. Comparison with experimental results shows the density functional geometries and dipole moments to be generally no better than or inferior to those predicted by the conventional ab initio methods with this particular basis set. The density functional vibrational frequencies compare favorably with the ab initio results, while for atomization energies, two of the DFT methods give excellent agreement with experiment and are clearly superior to all other methods considered.