PW91 Methods Research Articles

Theoretical study of the catalytic effect of TM-C4H4 and TM-C5H5 (TM = Cr, Ti, V, Sc) on the activation of O2 at the cathode and CH3OH at the anode in “CH3OH-O2″ fuel cell via DFT computational method

The study of the interaction of oxygen molecule with organometallic compounds containing transition metals has always been an interesting subject for research, as the O2 activation by them is a measure of the efficiency of organometallic catalysts. This study examined the catalytic effect of some organometallic compounds containing transition metals with the general formula of TM-CmHm on increasing the reaction rate of “methanol-oxygen” fuel cell using the method of density functional theory (DFT) and PW91 method as well as the basis set of 6-31G(d) (TM represents each of Sc, Ti, V and Cr and m is equal to 4 or 5). The structures of the mentioned compounds were optimized by the natural bonding orbital calculations (NBO). Then, the diagrams of the density state (DOS) were plotted for them and afterward, gap energy (Eg), chemical hardness (η) ، chemical potential (µ) and electrophilicity (ω) were calculated in each case. The Eg and η of ScC5H5 were smaller than that of the others, indicating greater reactivity. Theoretical calculations showed that each of the TMs in TM-CmHm had a partial negative electric charge, which was higher in ScC5H5 than in others causing higher chemical activity. The adsorption energies of O2 and CH3OH were calculated and reported on each of the TM-CmHm. The adsorption energy of O2 and CH3OH on ScC5H5 was more negative than that of the other TM-CmHm used. This confirms that the compound had a greater efficiency and ability to play a catalytic role in the “oxygen - methanol” fuel cell. The results showed that the bond length of O=O increased by about 24% on average due to its adsorption on ScC5H5. The O-H bond length was significantly increased in response to the methanol adsorption process on ScC5H5. Increasing the bond length caused instability of the bond and raising its chemical activity and reactivity. The adsorption kinetics of each of O2 and CH3OH on ScC5H5 were also studied by the above-computational method. The potential energy variation of the “adsorbate/adsorbent” system in terms of reaction coordinate (bond length of O=O or O-H) in each case was evaluated with a one transition state observed in each case.

The adsorption mechanism and electronic properties of FeO–HNCO clusters

The configurations, electronic and spin of the FeO–HNCO clusters are investigated at PW91 method. The calculated results show that the Fe–O–C–N four-member ring preferred to form the FeO–HNCO cluster and it has higher kinetic stability. The isomer which possesses an Fe–O–C triangle ring has higher kinetic activity. The hybridization of sp orbital of C and N atoms of the FeO–HNCO clusters is stronger. For the lowest-energy FeO–HNCO cluster, the Fe and O atoms have the opposite spin direction.

Theoretical studies of structure and racemization mechanism of aspartate-intercalated hydrotalcite

The structure and racemization mechanism of l-aspartic acid (l-ASP) and l-ASP-intercalated layered double hydroxide (l-ASP-LDH, with intercalated magnesium–aluminum hydrotalcite as the LDH), were theoretically studied using the B3LYP and PW91 method of density functional theory at 6-31G(d,p) and Lanl2dz level, respectively. The structural parameters obtained for l-ASP-LDH are in agreement with experimental data for materials, indicating that this theoretical method is appropriate for the hydrotalcite system. The computational results show that enantiomerism of l-ASP is difficult in the ground state due to high energy barriers, while in the excited state l-ASP can easily transform to d-ASP via proton transfer from the chiral carbon atom to the carbonyl oxygen atom, where the carbonyl oxygen atom plays a role as a medium for proton migration. In addition, hydrotalcite could inhibit racemization of l-ASP because the combination of the oxygen atom and hydrotalcite layers hindered proton movement.

The study of SCN− adsorption on B12N12 and B16N16 nano-cages

DFT calculations were prepared to study the adsorption behavior and electronic properties of the pure and doping metals BN nano-cages on SCN− molecule at the B3LYP, PBE, and PW91 methods. The results express that SCN− adsorption upon the B atom of B12N12 is stronger than that of isolated B16N16 nano-cage by considering various basis sets. Our calculations reveal that the aluminum doping can significantly improve both the adsorption energy and electronic properties of B11N12 nano-cage to SCN−. Hence, we concluded that the Al-doped B11N12 nano-cage can be served as a reliable material for SCN− adsorption.

Comparative Analysis of the Ni (II) Complex of the N, N'-Bis-(4-Hydroxysalicylidene)-1, 2-Diaminoethane: Combined Experimental and Theoretical Study (DFT/ PW91)

This study deals with the identification of the title compound, Ni (II) complex, by means of quantum chemical calculations. The optimized molecular structure, vibrational frequencies and corresponding vibrational assignments, ultraviolet-visible (UV-vis) spectrum of the title molecule in the ground state were evaluated using density functional theory (DFT) with the standard PW91 method using the ADF package. The results show that the obtained optimized geometric parameters (bond lengths, bond angles and dihedral angles) and vibrational frequencies were observed to be in good agreement with the available experimental results. Moreover, the calculation of the electronic spectrum was compared with the experimental ones.

Structural Model Studies for the Peroxo Intermediate P and the Reaction Pathway from P → Q of Methane Monooxygenase Using Broken-Symmetry Density Functional Calculations

Several structural models for the active site of the peroxo intermediate state "P" of the hydroxylase component of soluble methane monooxygenase (MMOH) have been studied, using two DFT functionals OPBE and PW91 with broken-symmetry methodology and the conductor-like screening (COSMO) solvation model. These active site models have different O2 binding modes to the diiron center, such as the mu-eta2,eta2, trans-mu-1,2 and cis-mu-1,2 conformations. The calculated properties, including optimized geometries, electronic energies, Fe net spin populations, and Mössbauer isomer shift and quadrupole splitting values, have been reported and compared with available experimental results. The high-spin antiferromagnetically (AF) coupled Fe3+ sites are correctly predicted by both OPBE and PW91 methods for all active site models. Our data analysis and comparisons favor a cis-mu-1,2 structure (model cis-mu-1,2a shown in Figure 9) likely to represent the active site of MMOH-P. Feasible structural changes from MMOH-P to another intermediate state MMOH-Q are also proposed, where the carboxylate group of Glu243 side chain has to open up from the mono-oxygen bridging position, and the dissociations of the terminal H2O ligand from Fe1 and of the oxygen atom in the carboxylate group of Glu144 from Fe2 are also necessary for the O2 binding mode changes from cis to trans. The O-O bond is proposed to break in the trans-conformation and forms two mu-oxo bridges in MMOH-Q. The terminal H2O molecule and the Glu144 side chain then rebind with Fe1 and Fe2, respectively, in Q.

Feasibility of density functional methods to predict dielectric properties of polymers

Feasibility of density functional theory (DFT) to predict dielectric properties such as polarizability of saturated polymers is investigated. Small saturated molecules, methane and propane, which is a monomer of polypropylene chain, are used in testing the methods. Results for polarizabilities based on several density functionals together with different basis sets are compared and contrasted with each other, with results by Hartree-Fock and second-order Moller-Plesset perturbation theory, as well as experimental data. The generalized gradient approximation PW91 method together with the 6-311++G(**) basis set is found to be the most suitable method, in terms of sufficient accuracy and computational efficiency, to calculate polarizabilities for large oligomers of polypropylene. The dielectric constant is then determined using the calculated polarizabilities and the Clausius-Mossotti equation. The molecular DFT methods at the PW916-311++G(**) level together with the Clausius-Mossotti equation give dielectric constants for saturated polymers such as polypropylene in good accordance with the experimental values.

Density functional theory characterisation of 4-hydroxyazobenzene

We report the structural properties, infrared (IR) and Raman spectra, dipole moment, polarisability, hardness and chemical potential of the trans and cis configurations of 4-hydroxyazobenzene calculated using the B3LYP functionals. All calculations were performed with the following basis sets: 6-31G, 6-31++G, 6-31G(d,p), 6-31++G(d,p), 6-31G(2d,2p), 6-31++G(2d,2p) and 6-311++G(2d,2p). We observed that 6-31++G(d,p) gives similar results to 6-311++G(2d,2p). Consequently, SVWN and PW91 methods were also used in association with 6-31++G(d,p) to test the influence of the different models of exchange and correlation functionals. A planar structure was obtained for all the optimised trans configuration structures. In both isomers, the presence of the hydroxyl group leads to an asymmetry in certain structural parameters. From these results, two IR or Raman active frequencies can be used to easily distinguish trans and cis configurations. The trans configuration was found to be more stable than the cis configuration by 67 +/- 2 kJ mol(-1) at 0 K. The difference of the dipole moment between trans and cis for 4-hydroxyazobenzene was found to be lower than for trans and cis azobenzene.

Quantum DFT and DF–DFT study of vibrational spectra of sulfuric acid, sulfuric acid monohydrate, formic acid and its cyclic dimer

Rigorous theoretical treatment of vibrational frequencies is critically important for the interpretation of unassigned experimental vibrational spectra and accurate determination of thermodynamic properties of molecular clusters. IR spectra of trans monomers of sulfuric and acetic acids, sulfuric acid monohydrate and cyclic dimer of the formic acid have been studied using DFT and DF–DFT methods using BLYP, B3LYP and PW91 with 12 different Pople and Dunning basis sets. New data for above-mentioned structures have been reported, scaling factors have been calculated and a comprehensive analysis of the performance of BLYP, B3LYP and PW91 methods has been performed. Comparison of the obtained results with experiments shows that results of pure PW91 and BLYP are better than predictions of well-established hybrid B3LYP method. Our analysis shows on the existence of the considerable difference in scaling factors weighted to high and low frequencies. In the case of formic acid dimer, the deviation the predicted low frequencies from the experimental data is considerable that leads to quite large (∼6–7 kcal mol −1) uncertainties in the absolute values of the cluster Gibbs free energy. In order to determine an efficient computational strategy that comprises accuracy and reasonable computational costs, the effect of density fitting (DF) and basis set superposition error (BSSE) on the vibration frequencies has been investigated. We found that application of the DF that substantially (2.5–3.5 times) increases the performance of pure PW91 and BLYP methods gives excellent results, which are very close to those of conventional DFT. This suggests that DF–DFT is a viable low-cost alternative to conventional DFT in predicting vibrational spectra. It has been found that while vibrational spectra obtained using the counterpoise correction (CP) for the BSSE do not deviate much from uncorrected ones, the difference in absorption intensities between corrected and uncorrected spectra obtained using small and medium-sized basis sets is considerable. This suggests that application of DF–DFT uncorrected for the BSSE with large basis sets is a more efficient strategy of predicting vibrational spectra than the application of conventional DFT with small basis sets.

Vibrational analysis of the inelastic neutron ascattering spectra of electron donor–acceptor complexes. II. Tetracyanoethylene–perylene by electronic structure calculations

The vibrational spectra of the electron donor–acceptor complex, tetracyanoethylene–perylene were measured at 4 K by inelastic neutron scattering (INS) using the filter analyzer neutron spectrometer (FANS) and at 20 K using the TOSCA instrument. A direct comparison of the measured spectrum was made to density functional calculations using the B3LYP/6-31G** treatment for the isolated complex, the PW91 pseudopotential plus plane-wave basis and the BLYP/dnd atom-centered basis solid-state methods. In general, comparisons of the observed and simulated spectra revealed that the PW91 method is slightly superior to the B3LYP/6-31G** and BLYP/dnd methods in predicting both vibrational frequency and intensity above 200 cm −1. Below 200 cm −1, the BLYP/dnd calculations gave slightly better intensity agreement. Six intermolecular vibrations were theoretically predicted with the three calculation methods and were experimentally confirmed. In addition, the molecular motions of the vibrations observed in the INS spectrum were tentatively assigned. Our calculations show that solid-state calculations are not required, at least in the case of the TCNE–perylene complex, to obtain reasonably accurate vibrational frequency information of electron donor–acceptor complexes.

Sulfuric Acid and Sulfuric Acid Hydrates in the Gas Phase: A DFT Investigation

The gaseous sulfuric acid and its hydrates play an essential role in the formation and evolution of atmospheric aerosols via nucleation of binary sulfuric acid−water vapors. Recently, it has been pointed out that the dipole moment of vapor molecules and small preexisting clusters is a new parameter controlling the nucleation rates. In this paper, the dipole moments of the mono-, di-, and trihydrates of the sulfuric acid are calculated for the first time. We also report on the molecular structures, energies, vibrational frequencies, absorption intensities and dipole moments of the hydrates and compare our model predictions with the results of other studies. The density functional theory (DFT) calculations have been carried out using the PW91 method and TZP basis set. We have determined the optimized conformations of gas-phase sulfuric acid and mono-, di-, and trihydrates of sulfuric acid using different starting scenarios and computed their dipole moments. The obtained results can be utilized directly in the modeling of the atmospheric aerosol formation and they are applied for the analysis of the hydration thermodynamics.