C. Van Alsenoy

Ab initio calculations on large molecules: The multiplicative integral approximation

In the multiplicative integral approximation (MIA), two‐electron integrals are evaluated using an expansion of a product of two Gaussians in terms of auxiliary functions. An estimator of the error introduced by the approximation is incorporated in the self‐consistent field (SCF) calculations and the integrals for which the error estimate is larger than a preset value are systematically corrected. In this way the results of a MIA‐assisted calculation have the same accuracy as a conventional calculation. The full exploitation of the expansion technique while constructing the Fock‐matrix allows important time savings. Results are presented for a number of test cases.

Atomic charges, dipole moments, and Fukui functions using the Hirshfeld partitioning of the electron density

In the Hirshfeld partitioning of the electron density, the molecular electron density is decomposed in atomic contributions, proportional to the weight of the isolated atom density in the promolecule density, constructed by superimposing the isolated atom electron densities placed on the positions the atoms have in the molecule. A maximal conservation of the information of the isolated atoms in the atoms‐in‐molecules is thereby secured. Atomic charges, atomic dipole moments, and Fukui functions resulting from the Hirshfeld partitioning of the electron density are computed for a large series of molecules. In a representative set of organic and hypervalent molecules, they are compared with other commonly used population analysis methods. The expected bond polarities are recovered, but the charges are much smaller compared to other methods. Condensed Fukui functions for a large number of molecules, undergoing an electrophilic or a nucleophilic attack, are computed and compared with the HOMO and LUMO densities, integrated over the Hirshfeld atoms in molecules.

Pentagon adjacency as a determinant of fullerene stability

Optimisation of geometries of all 40 fullerene isomers of C40, using methods from molecular mechanics and tight-binding to full abinitio SCF and DFT approaches, confirms minimisation of pentagon adjacency as a major factor in relative stability. The consensus predictions of 11 out of 12 methods are that the isomer of lowest total energy is the D2 cage with the smallest possible adjacency count, and that energies rise linearly with the number of adjacencies. Quantum mechanical methods predict a slope of 80–100 kJ mol-1 per adjacency. Molecular mechanics methods are outliers, with the Tersoff potential giving a different minimum and its Brenner modification a poor correlation and much smaller penalty.

Systematic study of the parameters determining stockholder charges

Abstract The stockholder recipe to calculate atomic charges requires the definition of the so-called promolecule density. This density is defined in terms of the densities of the atoms of the molecule which are determined by their spectroscopic state and the level of theory used. In this study the basis set dependence, the effect of the atomic spectroscopic state and electron correlation upon stockholder charges are investigated.

A Hirshfeld partitioning of polarizabilities of water clusters

A new Hirshfeld partitioning of cluster polarizability into intrinsic polarizabilities and charge delocalization contributions is presented. For water clusters, density-functional theory calculations demonstrate that the total polarizability of a water molecule in a cluster depends upon the number and type of hydrogen bonds the molecule makes with its neighbors. The intrinsic contribution to the molecular polarizability is transferable between water molecules displaying the same H-bond scheme in clusters of different sizes, and geometries, while the charge delocalization contribution also depends on the cluster size. These results could be used to improve the existing force fields.

Ab initio studies of structural features not easily amenable to experiment: Part 49. Conformational analysis and molecular structures of ethylenediamine and aminoethanol

Abstract The structures of eleven conformations of aminoethanol and of ten conformations of ethylenediamine were determined by ab initio gradient geometry optimization on the 4–21G level. The calculations show that many energetically different conformers exist for the gauche and trans forms (NCCO or NCCN torsions) of both systems. The results indicate that structural effects on NH bond distances associated with aliphatic N⋯N or NH⋯O type hydrogen bonding are less noticeable than those obtained previously for other XH⋯Y interactions. In the same way as CH and CC bonds in aliphatic compounds studied previously, NH bonds in the same NH2-group anti-periplanar to CH are found here to be slightly longer (∼0.001 A) than NH bonds antiperiplanar to CC. The most stable 4–21G conformations of ethylenediamine (NCCN ≅ 58° and 62°) are identical with the main conformer identified by others in the gas electron diffraction data of this compound (NCCN = 64° ± 4), but the calculations afford a more detailed description of the NH2 arrangements (NC torsions) than that obtained by the gas phase experimental data. Structural effects of electron lone-pair orientation are discussed. A small but potentially significant discrepancy exists between the electron diffraction rg average CC bond distance (1.545 A ± 0.008) and the calculated rg value (1.528 A ± 0.003; 4–21G average value empirically corrected as described previously). Experimental and calculated average rg CN bond distances are 1.469 A ± 0.004 and 1.463 A ± 0.006, respectively. Some aspects of the CC bond discrepancy are discussed in detail.

Ab initio studies of structural features not easily amenable to experiment: Part 31. Conformational analysis and molecular structures of ethylene glycol

Abstract The geometries of ten conformations of ethylene glycol have been refined without any geometrical constraints by the ab initio gradient method on the 4-21G level. The two most stable forms found are tGg′ and gGg′ , both of which are stabilized by internal hydrogen bonding. A number of conformationally dependent structural trends are apparent in the optimized local geometries. Bond distances and bond angles in different conformations can vary by up to 0.01–0.02 A and 5°, respectively. The results are found to be consistent with most of the previous investigations of the system.

The electronegativity equalization method II: Applicability of different atomic charge schemes

The amenability of different schemes for the calculation of atomic charges in the electronegativity equalization method (EEM) is investigated. To that end, a large training set of molecules was composed, comprising H, C, N, O, and F, covering a wide range of medicinal chemistry. Geometries are calculated at the B3LYP/6-31G* level. Atomic charges are calculated using five different methods, belonging to different types of population analysis. Effective electronegativities and hardness values are calibrated from the different quantum chemically calculated atomic charges. The resulting quality of EEM charges is investigated for the different types of atomic charge calculation methods. EEM-derived Mulliken and NPA charges are in good agreement with the ab initio values, electrostatic potential derived, and Hirshfeld charges show no good agreement.

Structure of gaseous methyl acetate as determined by joint analysis of electron diffraction, microwave and infrared spectroscopy, supplemented by a valence force field and constraints from geometry relaxed ab initio calculations

Abstract The structure of methyl acetate was studied by joint analysis of gas phase electron diffraction, microwave and IR data, using constraints taken from relaxed 4-21G gradient geometry and force field calculations. All data are in accord with a planar heavy-atom skeleton in the syn conformation. The geometry of methyl acetate in the gas phase is essentially equal to that in the crystal. Some r g — r e corrections have been evaluated. Subject to the ab initio constraints the following internal coordinates ( r 0 α structure) have been found: CO = 1.206 A, H 3 CO = 1.438 A, CO = 1.357 A, CC = 1.496 A, 〈CH〉 = 1.078 A, ∠COC = 116.4°, ∠;OCO = 123.0°.

The molecular structure of pyridine in the gas phase determined from electron diffraction, microwave and infrared data and ab-initio force-field calculations

Abstract The gas-phase molecular structure of pyridine was studied by joint analysis of electron diffraction, microwave and infrared data, augmented by vibrational constraints taken from force-field calculations at the 4-21G ab-initio level. Geometrical constraints arising from 4-21G, 4-31G, 4-21GN* and microwave results were tested. The 4-21GN* constraints were significantly better than the others. The range of models that fit all available experimental data was then investigated with respect to the difference between the CNC and NCC valence angles. This resulted in the following best-fitting model ( r g distances, r 0 α angles): NC = 1.344 A; C 2 C 3 = 1.399 A; C 3 C 4 = 1.398 A; CH (average) = 1.094 A; CNC = 116.1°; NCC = 124.6°; C 2 C 3 C 4 = 117.8°; C 3 C 4 C 5 = 119.1°; NCH = 115.2°. The data suggest that the perturbation resulting from the N atom is primarily in the CNC part of the ring.