ELECTRONEGATIVITY MODELS FOR THE INFRARED VIBRATIONAL INTENSITIES OF THE HALOMETHANES

Ab initio molecular orbital calculations and empirical electronegativity models are used to understand the linear electronegativity relationships observed for the carbon mean dipole moment derivatives and atomic effective charges calculated from the experimental infrared vibrational intensities of the halomethanes. The charge-charge flux-overlap interpretation of the molecular orbital results shows that only the charge contribution is important in explaining the variations in these parameters for the fluoromethanes. For this reason a simple electrostatic model is sufficient to explain their fundamental infrared intensity sums. The mean dipole moment derivative values determined from the experimental intensities suggest the absence of a saturation effect on the ability of substituted fluorine atoms to drain electron density from the carbon atoms. A similar model has been used by others to explain the increasing thermodynamic stabilities of the fluoromethanes with increasing fluorine substitution. In contrast intramolecular charge transfer is predominant in determining the chloromethane intensities. The fluorochloromethane intensities can only be explained using models combining characteristics of the fluoro- and chloromethane models. The charge equilibration procedure introduced recently in the literature is found to be significantly superior to the simpler electronegativity equalization method for calculating atomic charges for the prediction of the infrared intensity sums of the halomethanes.117144144415

A simple potential model criterion for the quality of atomic charges

The simple potential model has been shown to be useful in relating core electron binding energies measured in the X-ray region with mean dipole moment derivatives obtained from experimental infrared vibrational intensities. The importance of including relaxation corrections to the experimental Is ionization energies of sp, sp(2), and sp(3) hybridized carbon atoms are investigated here. Although relaxation energies obtained from 6-31G(d,p) and 6-311++G(3df,3p) basis sets using Delta SCF calculations show differences of about 1 eV for most molecules studied, relative differences are of the order of 0.1 eV. Exceptions are the CO, CO2, COS, and CS2 molecules where discrepancies are larger. Relaxation energy corrections improve simple potential model fits with mean dipole moment derivatives for all carbon atom models but is most pronounced for the sp hybridized atoms. The simple potential model corrected for relaxation energies is investigated as a criterion for testing the quality of Mulliken, CHELPG, Bader and GAPT carbon atomic charges calculated from MP2/6-311++G(3d,3p) wave functions. The GAPT charges are in excellent agreement with the experimental mean dipole moment derivatives (within 0.067e) and provide superior statistical fits to the simple potential model when compared with those obtained for the ether charges.103254918492

INFRARED VIBRATIONAL INTENSITIES AND POLAR TENSORS OF THE FLUOROCHLOROMETHANES

The polar tensors of CFCl3, CF2Cl2, and CF3Cl determined from experimental infrared intensities are reported. The sign ambiguities in the dipole moment derivatives are resolved by comparing alternative polar tenser solutions with the results of MP2/6-311+G(3d) molecular orbital calculations using bidimensional principal component projections of the polar tenser spaces. The carbon mean dipole moment derivatives of-the fluorochloromethanes are in good agreement with the values predicted by an electronegativity model equation obtained from both fluoro- and chloromethane polar tenser data. These values are shown to be intermediate to those expected on the basis of separate fluoro- and chloromethane models. The mean dipole moment derivatives of the fluorine and chlorine atoms vary with the degree of fluorine-chlorine substitution. This contrasts with the almost constant values of these quantities already reported for the fluoro- and chloromethanes.9929113571136

Infrared vibrational intensities, polar tensors, and core electron energies of the group IV hydrides and the fluorosilanes

Principal component analysis is used to compare polar tensors of CH4, SiH4, GeH4, and SnH4 and their completely deuterated analogues determined from infrared fundamental gas-phase intensities measured in different laboratories. This analysis also includes theoretical polar tenser values obtained from effective core potential universal basis set calculations as well as from MP2/6-311++G(3d,3p) functions for CH4, SiH4, and SiF4. Theoretical values are also used to resolve sign ambiguities in the dipole moment derivatives of SiF4. Preferred polar tenser values are proposed for all these molecules. Mean dipole moment derivatives for SiH4 and SiF4 are related to the 2p core ionization energies using the simple potential model proposed by Siegbahn and collaborators. These results are confirmed by MP2/6-311++G(3d,3p) calculations for these molecules and for SiH3F, SiH2F2, and SiHF3. This study is extended to the fluorogermanes using experimental 3p and 3d core electron ionization energies and mean dipole moment derivatives calculated from MP2/A-VDZ/6-311++G(3d,3p) wave functions. The simple potential model interpretation of mean dipole moment derivatives as atomic charges implies that the silicon charge, +/-0.904e, is slightly higher than the germanium charge of +/-0.862e.102244615462

The correlation of proton affinities with atomic charges and electronegativities for the group 14 to 17 hydrides

Proton affinities for hydrides of formula AH(n-1)(-) containing the elements A from the second to the fifth period of the periodic table and groups 14 to 17 are predicted at the Hartree-Fock, MP2 and B3LYP levels of theory employing both core potential basis sets and the 3-21G basis set. The core potential methods perform well when compared with all electron calculations using the 3-21++G** basis set. The proton affinities of the hydrides containing elements from groups 15 and 16 of the periodic table are more accurate than those with elements from groups 14 and 17. A cancellation of errors appears to occur more completely if the protonated and nonprotonated molecules contain both bond and lone pairs before and after the protonation reaction. Proton affinities correlate nearly linearly with the atomic charges on the hydrogen atoms when these charges are determined by the generalized atomic polar tensor (GAPT) method. This tendency can be associated, in principle, with the group electronegativities as introduced by Iczkowski and Margrave. (C) 2000 John Wiley & Sons, Inc.21131119113

Direct numerical simulation of fully saturated flow in natural porous media at the pore scale: a comparison of three computational systems

Direct numerical simulations of flow through two millimeter-scale rock samples of limestone and sandstone are performed using three diverse fluid dynamic simulators. The resulting steady-state velocity fields are compared in terms of the associated empirical probability density functions (PDFs) and key statistics of the velocity fields. The pore space geometry of each sample is imaged at 5.06−μm voxel size resolution using X-ray microtomography. The samples offer contrasting characteristics in terms of total connected porosity (about 0.31 for the limestone and 0.07 for the sandstone) and are typical of several applications in hydrogeology and petroleum engineering. The three-dimensional fluid velocity fields within the explicit pore spaces are simulated using ANSYS® FLUENT® ANSYS Inc. (2009), EULAG Prusa et al. (Comput. Fluids 37, 1193–1207 2008), and SSTOKES Sarkar et al. (2002). These computational approaches are highly disperse in terms of algorithmic complexity, differ in terms of their governing equations, the adopted numerical methodologies, the enforcement of internal no-slip boundary conditions at the fluid-solid interface, and the computational mesh structure. As metrics of comparison to probe in a statistical sense the internal similarities/differences across sample populations of velocities obtained through the computational systems, we consider (i) integral quantities, such as the Darcy flux and (ii) main statistical moments of local velocity distributions including local correlations between velocity fields. Comparison of simulation results indicates that mutually consistent estimates of the state of flow are obtained in the analyzed samples of natural pore spaces despite the considerable differences associated with the three computational approaches. We note that in the higher porosity limestone sample, the structures of the velocity fields obtained using ANSYS FLUENT and EULAG are more alike than either compared against the results obtained using SSTOKES. In the low-porosity sample, the structures of the velocity fields obtained by EULAG and SSTOKES are more similar than either is to the fields obtained using ANSYS FLUENT. With respect to macroscopic quantities, ANSYS FLUENT and SSTOKES provide similar results in terms of the average vertical velocity for both of the complex microscale geometries considered, while EULAG tends to render the largest velocity values. The influence of the pore space structure on fluid velocity field characteristics is also discussed