On the basis of the principle of electronegativity equalization and density-functional theory, an atom−bond electronegativity equalization method has been developed for the calculation of the charge distribution in large molecules that are connectivity and geometry dependent. The effective electronegativities of an atom and a bond in a molecule are derived and given by equations that contain respective electronegativities, hardness terms, and contributions that come from other atomic and bond charges. The parameters A, B, C, and D are the valence-state electronegativity and the valence-state hardnesses. They are obtained by calibrating through calculations of more than 100 model molecules and are transferable and consistently usable. The atom−bond electronegativity equalization method is tested through calculation of the charge distributions in several large organic molecules. It has been shown that there is a very good agreement between the results obtained by the atom−bond electronegativity equalization method and those obtained by the corresponding ab initio quantum chemical calculations.
Recently, experimental and theoretical studies on the water system are very active and noticeable. A transferable intermolecular potential seven points approach including fluctuation charges and flexible body (ABEEM-7P) based on a combination of the atom-bond electronegativity equalization and molecular mechanics (ABEEM/MM), and its application to small water clusters are explored and tested in this paper. The consistent combination of ABEEM and molecular mechanics (MM) is to take the ABEEM charges of atoms, bonds, and lone-pair electrons into the intermolecular electrostatic interaction term in molecular mechanics. To examine the charge transfer we have used two models coming from the charge constraint types: one is a charge neutrality constraint on whole water system and the other is on each water molecule. Compared with previous water force fields, the ABEEM-7P model has two characters: (1) the ABEEM-7P model not only presents the electrostatic interaction of atoms, bonds and lone-pair electrons and their changing in respond to different ambient environment but also introduces "the hydrogen bond interaction region" in which a new parameter k(lp,H)(R(lp,H)) is used to describe the electrostatic interaction of the lone-pair electron and the hydrogen atom which can form the hydrogen bond; (2) nonrigid but flexible water body permitting the vibration of the bond length and angle is allowed due to the combination of ABEEM and molecular mechanics, and for van der Waals interaction the ABEEM-7P model takes an all atom-atom interaction, i.e., oxygen-oxygen, hydrogen-hydrogen, oxygen-hydrogen interaction into account. The ABEEM-7P model based on ABEEM/MM gives quite accurate predictions for gas-phase state properties of the small water clusters (H(2)O)(n) (n=2-6), such as optimized geometries, monomer dipole moments, vibrational frequencies, and cluster interaction energies. Due to its explicit description of charges and the hydrogen bond, the ABEEM-7P model will be applied to discuss properties of liquid water, ice, aqueous solutions, and biological systems.
The ABEEM-7P model, which is a transferable, intermolecular-potential seven-points approach including fluctuating charges and flexible body, is based on the combination of the atom-bond electronegativity equalization (ABEEM) and molecular mechanics (MM). This model has been successfully explored in regard to the properties of gas-phase small water clusters in reasonable agreement with available experiments and other water models. This model is further tested by comparing the calculated energetic, structural, and dynamic properties of liquid water over a range of temperatures (260−348 K) with available experimental results and those from other water models. Molecular dynamics simulations of liquid water with ABEEM-7P were performed using the Tinker MM program. All simulations were conducted in the microcanonical NVE ensemble or canonical NVT ensemble, using 216 water molecules in a cubic simulation cell furnished with periodic boundary and minimum image conditions, and the density of the solvent was set to the experimental value for the temperature of interest. The ABEEM-7P potential gives a reasonable experimental reproduction of the intramolecular O−H bond length and H−O−H bond angle in the liquid at room temperature. The ABEEM-7P model presents the quantitative charges of O atoms, H atoms, O−H bonds, and lone-pair electrons per monomer water in the liquid and their changing in response to different ambient environment from 260 K to 348 K. Especially, ABEEM-7P applies the parameter k lp ,H(R lp ,H) to explicitly describe short-range interaction of the hydrogen bond in the hydrogen-bond interaction region. The computed ABEEM-7P properties of the liquid-phase water at room temperature, such as average dipole moment, static dielectric constant, heats of vaporization, radial distribution function, and diffusion constant, are fairly consistent with the available experimental results. The ABEEM-7P model also performs well for the temperature dependence of liquid properties: the static dielectric constant and the heats of vaporization by ABEEM-7P decrease as the temperature increases, in good agreement with the experimental values.
Previous ab initio studies on reactions involving radical addition to alkenes showed that such reactions are very sensitive to theoretical levels, and thus are difficult to deal with. This motivates us to theoretically reexamine the title reaction thoroughly, which has been studied only at several low levels of theory. In the present work, the geometry optimizations and energy calculations for all species involved in the title reaction were performed at several high levels of theory. The reaction mechanism of the title reaction is discussed at the CCSD(T)/aug-cc-pVDZ//CCSD/6-31G(d,p) theoretical level. According to our study, the fluorine addition to ethylene occurs via the formation of a prereaction complex with C2v symmetry, which is pointed out for the first time. The prereaction complex evolves into a fluoroethyl radical almost without a barrier, with an exothermicity of 41.49 kcal/mol. The fluoroethyl radical can further decompose into a hydrogen atom and fluoroethylene, with an energy release of 10.33 kcal/mol. Besides the direct departure of the hydrogen atom from the fluoroethyl radical, an indirect decomposition pathway may also be open, which has not been reported before. In addition, the formation of a fluoroethyl radical from a separate fluorine atom and ethylene is described pictorially via the molecular intrinsic characteristic contour (MICC) and the electron density mapped on it. Thereby, strong interpolarization and evident electron transfer between the fluorine atom and ethylene are observed as they approach each other. The transition structure for the fluorine addition to ethylene is clearly shown to be reactant-like. This provides new and intuitional insight into the title reaction.
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