Chengteh Lee

Local softness and chemical reactivity in the molecules CO, SCN− and H2CO

Abstract Fukui functions (softnesses) are calculated for three species - formaldehyde, the thiocyanate ion and carbon monoxide. The fukui function for a molecule has been defined as the derivative of electron density with respect to the change of number of electrons, keeping the positions of nuclei unchanged; this differentiation is performed by finite difference. Local softness and fukui function are proportional. The calculated results, expressed in terms of contour maps and condensed values of fukui functions, substantiate the previous argument that fukui functions serve as reactivity indices for chemical reactions. Particularly, it is confirmed that: (1) a nucleophilic reagent approaches the carbon atom in formaldehyde from the direction perpendicular to the molecular plane, while an electrophilic reagent approaches the oxygen atom in the molecular plane; (2) the sulphur end is softer than the nitrogen end in the thiocyanate ion; and (3) carbon monoxide behaves like a Lewis acid in bonding with transition metals.

A theoretical study of the ionic dissociation of HF, HCl, and H2S in water clusters

The ionic dissociation of HF, HCl, and H2S in water is examined using density functional theory (DFT), Hartree–Fock (HF), and Mo/ller–Plesset theory to second order (MP2). The calculations show that HF, HCl, and H2S form fully dissociated stable clusters with four water molecules. Each cluster appears to be stabilized by the formation of six hydrogen bonds. These calculations also indicate that a minimum of four water molecules are required to form stable structures in which positive and negative ions coexist in the cluster. The hydrogen transfer between the acid and water molecules is very similar to the mechanism proposed for hydrogen transfer in water solutions. The binding energies of the hydrated hydrofluoric acid, hydrated hydrochloric acid, and hydrated hydrogen disulfide estimated with B‐LYP are 37.51, 41.17, and 20.68 kcal/mol, respectively.

Chemical bonding in water clusters

Different sizes of water clusters from a dimer to twenty water molecules are studied using density functional theory. The binding energies of water clusters are calculated, and a relationship in terms of a simple function has been found between binding energy and the size of the water clusters. The interpolation of this correlation function reproduces the binding energies for the other water clusters to an accuracy within 1 kcal/mol. The extrapolation of the function gives the binding energy, −11.38 kcal/mol, which agrees very well with the experimental binding energy of ice, −11.35 kcal/mol. We also find small water clusters composed of mainly planar four membered rings to be more stable, implying the existence of magic numbers for water clusters with sizes of 4, 8, and 12.

Density functional description of transition structures using nonlocal corrections. Silylene insertion reactions into the hydrogen molecule

The insertion reactions of SiH2 , SiHF, and SiF2 into the hydrogen molecule have been investigated using density functional methods. Local spin density (LSD) calculations were performed with the LSD exchange functional and with the Vosko, Wilk, and Nusair correlation energy functional (VWN). Nonlocal spin density corrections (NLSD) were estimated with the exchange functional of Becke and the correlation energy functional of Perdew (B–P); Becke and the correlation energy functional of Lee, Yang, and Parr (B–LYP); and Perdew and Wang (PW) generalized gradient exchange‐correlation energy functional. Reactants, transition structures, and products were fully optimized at the LSD and NLSD levels. For each of these reactions, relative energies have been calculated with density functional methods and also at the quadratic configuration interaction with single, double, and triple excitations [QCISD(T)]/6‐31G(2df,pd) level. Vibrational frequencies were also computed with local and nonlocal approximations as well as...

Evidence of the existence of dissociated water molecules in water clusters

The dissociation of water molecules in the water cluster was investigated using ab initio methods and density functional theory. A stable minimum energy configuration of a cluster containing H3O+ and OH− ions was located for a water cluster with five water molecules, (H2O)5. There are six hydrogen bonds in the dissociated water cluster to form the minimum energy structure. A similar structure with H3O+ and OH− ions was also found for a (H2O)8 cluster.

The divide‐and‐conquer density‐functional approach: Molecular internal rotation and density of states

The divide‐and‐conquer density‐functional method recently developed by Yang is applied to the calculations of internal rotation energies and density of electronic states of a tetrapeptide. The method, on comparison with the conventional Kohn–Sham method, is found to be capable of accurately describing the density of states and the small electronic energy changes in the internal rotation. The calculations further demonstrate the promise of the method for calculations of large systems beyond the reach of conventional methods.

Local density component of the Lee–Yang–Parr correlation energy functional

A systematic study of the local density component (LDC) of the Lee–Yang–Parr (LYP) correlation energy functional on several chemical systems is presented. A total of 22 equilibrium geometries, 28 reaction energies, and 22 atomization energies were calculated using the local density component of the Lee–Yang–Parr correlation energy functional (LDC‐LYP). The LDC‐LYP results were compared with the correlation energy functional of Vosko, Wilk, and Nusair (VWN), that was parametrized using the exact results of the uniform electron gas approximation. The calculations were performed with local density approximation (LDA) optimized Gaussian basis sets of the double‐zeta‐type plus polarization functions (DZVP2) and the A2 auxiliary basis sets for the density fitting. Comparison with experimental results indicates the geometries and energetics predicted with the LDC‐LYP component are in reasonable agreement with those predicted with the VWN approximation for the systems considered. Furthermore, the LDC‐LYP+BLYP per...

Hardnesses from electrostatic potentials

By generalization of a method due to Politzer et al. [J. Chem. Phys. 79, 3859 (1983)], it is demonstrated how the absolute hardness of an electronic system can be determined from the electrostatic potential, as a function of position of the system and its positive and negative ions. It is shown that to good accuracy the hardness is one‐half the electrostatic potential at the covalent radius due to the Fukui function.

Chemical applications of density functional theory: F−3 anion dissociation (F−3 → F2 + F−)

Abstract The molecular structure and binding energies have been computed for F 2 and F − 3 . Local spin density (LSD) calculations were performed with the LSD Dirac exchange functional and with the Vosko, Wilk and Nusair correlation energy functional (VWN). Non-local corrections were estimated with different exchange-correlation energy functionals. The equilibrium bond lengths are within 0.01 to 0.03 A of wave function-based ab initio results. Dissociation energies appear to be overestimated within the local density approximation. Density functional methods predict the F − 3 anion to be bound by about 50.0 ± 5 kcal/mol. Vibrational frequencies were computed with VWN, LYP, BP, BLYP and PW energy functionals.

Nonlocal density functional calculations: Comparison of two implementation schemes

We have carried out nonlocal density‐functional calculations of bond dissociation and isomerization energies of several polyatomic molecules in two schemes. In the first scheme, the nonlocal energy functional is incorporated into the optimization of both the electronic and nuclear degrees of freedom. In the second scheme, the nonlocal energy functional is only included in a non‐self‐consistent fashion in which we just use the molecular geometry and electron density determined by the corresponding local density calculations. Our study reveals that the differences of the energies are very small between these two schemes.