Nonlocal Density Functional Theory Method Research Articles

Relations between Structural Parameters and Adsorption Characterization of Templated Nanoporous Materials with Cubic Symmetry

A systematic approach is proposed to structural characterization of templated nanoporous materials with cubic symmetry by gas adsorption. We hypothesize that regular structures of these materials can be described in terms of triply periodic minimal surfaces (TPMS), similarly to bicontinuous mesophases observed in oil−water, lipid, block copolymer, and other amphiphilic systems. We relate topological characteristics of TPMS to the pore structure parameters evaluated from adsorption measurements, such as the specific surface area, pore volume, mean pore size, and also pore wall thickness. The relations obtained can be used for discrimination of possible TPMS morphologies. The method developed is used for characterization of newly synthesized MCM-48 mesoporous materials by low-temperature nitrogen adsorption. We show that adsorption data fully support the minimal gyroid model of MCM-48 structure (Ia3d space group) established earlier by the X-ray diffraction (XRD) and transmission electron microscopy studies. The mean pore size of MCM-48 can be accurately described by the hydraulic diameter calculated from the capillary condensation region of nitrogen adsorption isotherms by the nonlocal density functional theory method. Moreover, the adsorption method allows one to estimate the pore wall thickness, which cannot be done by XRD. For a series of high-quality MCM-48 materials reported recently in the literature, the calculated mean wall thickness varied from 0.8 to 1.2 nm. The adsorption method developed is recommended as a complement to X-ray diffraction and electron microscopy techniques for characterization of nanoporous materials.

Unified Approach to Pore Size Characterization of Microporous Carbonaceous Materials from N2, Ar, and CO2 Adsorption Isotherms

We present a unified approach to pore size characterization of microporous carbonaceous materials such as activated carbon and carbon fibers by nitrogen, argon, and carbon dioxide adsorption at standard temperatures, 77 K for N2 and Ar and 273 K for CO2. Reference isotherms of N2, Ar, and CO2 in a series of model slit-shaped carbon pores in the range from 0.3 to 36 nm have been calculated from the nonlocal density functional theory (NLDFT) using validated parameters of intermolecular interactions. Carbon dioxide isotherms have also been generated by the grand canonical Monte Carlo (GCMC) method based on the 3-center model of Harris and Yung. The validation of model parameters includes three steps: (1) prediction of vapor−liquid equilibrium data in the bulk system, (2) prediction of adsorption isotherm on graphite surface, (3) comparison of the NLDFT adsorption isotherms in pores to those of GCMC simulations, performed with the parameters of fluid-fluid interactions, which accurately reproduce vapor−liquid equilibrium data of the bulk fluid. Pore size distributions are calculated by an adaptable procedure of deconvolution of the integral adsorption equation using regularization methods. The deconvolution procedure implies the same grid of pore sizes and relative pressures for all adsorbates and the intelligent choice of regularization parameters. We demonstrate the consistency of our approach on examples of pore structure characterization of activated carbons from adsorption isotherms of different gases and from different models (NLDFT and GCMC). Since the CO2 isotherms measured up to 1 atm are not sensitive to pores wider then 1 nm, the NLDFT method for CO2 has been extended to high-pressure CO2 adsorption up to 34 atm. The methods developed are suggested as a practical alternative to traditional phenomenological approaches such as DR, HK, and BJH methods.

Electron affinities of metals computed by density functional theory and ab initio methods

An extensive computational study of the meal electron affinity was performed using the ab initio and density functional theory (DFT) methods. HF, MP2, MP3, MP4, QCISD, and QCISD(T) was used as computational methods, while the hybrid, local, and nonlocal DFT methods with the LYP, P86, PW91, and VWN correlation functionals were used. Two basis sets, one small and applicable to almost all metals (LanL2DZ) and one large [6-311 + + G(3df, 3 pd)] used only for small metals, were employed. The computed results were compared with the experimental data and the capabilities of the DFT methods to perform this study were discussed. © 1997 John Wiley & Sons, Inc.

The determination of the equilibrium structures of oxygen, ozone, and hydrogen peroxide using the ab initio and density functional theory methods

The geometries and energies of small oxygen containing molecules are studied by both the ab initio and density functional theory (DFT) methods. The RHF, MP2, and QCISD(T) ab initio methods, BHandH, BHandHLYP, BeckeSLYP, Becke3P86 DFT hybrid methods, BLYP, and the BP86 non-local DFT methods with the 3-21G ∗, 6-31G(d,p), 6-311 + G(2d,2p) and 6-311 + + G(3df,3pd) basis sets were used for the computational study. The obtained results from the different methods were compared to the experimental values. The suitability of the DFT methods for reproducing experimental data were discussed.

Theoretical studies of thermal syn elimination reaction of organic amine oxide, sulfoxide and phosphoxide by ab initio and density functional methods

The geometries of the reactants and transition structures for the Ei elimination of organic oxides (CO, PO, NO and SO) were generated with the RHF/6-31G(d), MP2/6-31G(d) ab initio, SVWN/6-31G(d) and BLYP/6-31G(d) density functional theory (DFT) models. The energies were evaluated at the same level of theory. The planarity of the transition structures was confirmed for all of the applied theory models. It was estimated that 2–7 kcal mol −1 is the necessary energy to change the reactants into a conformer which is suitable for elimination. Aldehydes and phosphoxides are not suitable starting materials for the preparation of olefins. The most reactive species are the nitroxides. The order of reactivity is in general agreement with experimental knowledge. The local and non-local DFT methods predict very similar activation energies. These activation energies are closer to the expected values than the energies predicted with the MP2/6-31G(d) theoretical model.

Density functional theory and ab initio study of bond dissociation energy for peroxonitrous acid and peroxyacetyl nitrate

Geometries and energies of two nitro peroxides, HOONO2 and CH3COOONO2 are calculated by two common ab initio (ROHF and MP2), local (SVWN), four hybrid (BHandH, BHandHLYP, Becke3LYP, and Becke3P86) and two non-local (BLYP and BP86) density functional theory (DFT) methods. In all calculations standard GAUSSIAN-type basis sets [6-31+G(d) and 6-311++G(3df,3pd)] are used. Based on the comparison of computed and experimental values both Becke3LYP hybrid and BLYP non-local DFT methods generate energies that are in excellent agreement with experimental values. The suitability of other methods for computing these and similar molecules are discussed.

A study of nitrogen oxides by using density functional theory and their comparison with ab initio and experimental data

Ab initio and density functional theory (DFT) methods have been applied to generated geometries of chemical systems built only from nitrogen and oxygen. The results of hybrid, local, and nonlocal DFT methods were compared with ab initio HF and MP2 methods. The suitability of DFT methods for modeling nitrogen-oxygen chemical systems is discussed. © 1996 John Wiley & Sons, Inc.

Direct ab initio dynamics studies of proton transfer in hydrogen-bond systems

We present systematic direct ab initio dynamics studies of proton transfer in hydrogen-bond systems using the tautomerization in gas phase formamidine and its monohydrated complex as model reactions. The thermal rate constants were calculated using a canonical variational transition state theory (CVT) with multidimensional semiclassical tunneling corrections within a small-curvature ground-state approximation. The reaction valleys were calculated at the second-order Mo/ller–Plesset (MP2) perturbation theory, Hartree–Fock (HF) and nonlocal Becke’s half-and-half exchange and Lee–Yang–Parr correlation (BH&H–LYP) density functional theory (DFT) levels of theory using the 6-31 G(d,p) basis set. For accurate rate constants, the potential energy along the minimum energy path was scaled to match the single-point coupled cluster calculations including single and double excitations plus correction for triple excitation [CCSD(T)] at the MP2/6-31 G(d,p) classical barrier for each reaction. In the HF rate calculations, the HF frequencies were also scaled by a factor of 0.9. We found that adding a water to assist the proton transfer significantly enhances the tautomerization rate. Tunneling contributions in both systems are quite substantial and cannot be corrected by the Wigner approximation. We found that vibrational excitation of the solvent symmetriclike stretching mode would significantly enhance the rate of proton transfer in the formamidine–water complex. We also found that nonlocal DFT methods, particular the BH&H–LYP functionals studied here, can provide accurate potential energy information for dynamical calculations. Due to the computational advantage of DFT methods, prospects for dynamical studies of large polyatomic chemical reactions are quite encouraging.

Density functional theory applied to proton-transfer systems. A numerical test

The structure, interaction energy and proton-transfer features of the (H 5O 2 + complex are determined by using a nonlocal density functional theory method developed recently by Salahub and co-workers. The results are compared to those of a high- order Møller-Plesset electron-correlation calculation. The DFT calculation predicts correctly a symmetrical configuration of the complex at the equilibrium O—O distance, and yields a correct value of the H-bonding interaction energy. For a larger O—O separation, the electronic correlation effects on the double-well proton potential curve are overestimated in the DFT approach, whereas they are well reproduced by the second-order Møller-Plesset expansion.