Theoretical Chemistry Accounts

Discrepancies found for some oxygen-containing compounds in a recent VESCF study of dipole moments have been further investigated by studying in detail furan, oxazole, isoxazole, and three isomeric oxadiazoles. In a preliminary geometrical study it was found that the method described previously for estimating ring angles and bond lengths satisfactorily accounted for observed geometries. However in those compounds where more than one ring atom carries lone pair σ-electrons, the electrostatic forces originating in these lone-pair dipoles have a considerable influence on ring geometry. When this is incorporated in the geometrical treatment, good agreement with known geometries is obtained. The effect of using Burns' rules for orbital exponents rather than Slater's rules to derive lone pair moments and the effect of including lone pair potentials in the hamiltonian have been studied. The best agreement between calculated and observed dipole moments (within 0.2 D) is obtained by using the BJ method that was used in the previous study but with revised values for some of the basic resonance integrals. The revised values are shown to be more in keeping with the relevant bond lengths and atomic numbers than the values previously used.

The red phosphorescent osmium(II) complexes [Os(LR)2(PH3)2] (L = 2-pyridyltriazole (ptz): R = H (1a), CF3 (1b), t-Bu (1c)); L = 2-pyridylpyrazole (ppz): R = H (2a), CF3 (2b), t-Bu (2c)); L = 2-phenylpyridine (ppy): R = H (3a)) were explored using density functional theory (DFT) methods. The ground- and excited-state geometries of the complexes were optimized at the B3LYP/LANL2DZ and UB3LYP/LANL2DZ levels, respectively. The absorption and phosphorescence of the complexes in CH2Cl2 media were calculated based on the optimized ground- and excited-state geometries using time-dependent density functional theory method with the polarized continuum model. The optimized geometry structural parameters of the complexes in the ground state agree well with the corresponding experimental values. The lower-lying unoccupied molecular orbitals of the complexes are dominantly localized on the L ligand, while the higher-lying occupied ones are composed of Os(II) atom and L ligand. The low-lying metal-to-ligand and intraligand charge transfer (MLCT/ILCT) transitions and high-lying ILCT transitions are red-shifted with the increase in the π-donating ability of the L ligand and the π electron-donating ability of R substituent. The calculation revealed that the phosphorescence originated from 3MLCT/3ILCT excited state. However, the complex 3a displayed different types of MLCT/ILCT excited state compared with that of 1a–2c, and the different types of transition were also found in the absorption. In addition, we found that the phosphorescence quantum efficiency of Os(II) complexes is related to the metal composition in the high-energy occupied molecular orbitals, it will be helpful to designing highly efficient phosphorescent materials.

An applicable formulation of ab initio crystal structure determination based on the Dirac equation is presented. For this purpose, Dirac equation without regard to electron correlation effects is reduced to its spin-free one-component form by means of regular approximations, and then, connected to crystallographic notions. Thus, a relativistically valid structural description of a crystal structure is made possible by using single crystal X-ray diffraction data. The relativistic scheme was tested with a previously reported crystal structure containing heavy elements, and the results show that accuracy of the phase assignment process increases as the order of regular approximation is raised.

We recently demonstrated that dispersion-correcting potentials (DCPs), which are atom-centered Gaussian-type functions that were developed for use with B3LYP (Torres and DiLabio in J Phys Chem Lett 3:1738–1744, 2012), greatly improved the ability of the underlying functional to predict non-covalent interactions. However, the recent application of the B3LYP–DCP approach to study the β-scission of the cumyloxyl radical led to a calculated barrier height that was over-estimated by ca. 8 kcal/mol. We demonstrate in the present work that the source of this error arises from the previously developed carbon atom DCPs, which erroneously alters the electron density in the C–C covalent-bonding region. In this work, we developed a new C-DCP with a form that was expected to less strongly influence the electron density in the covalent bonding region. Tests of the new C-DCP, in conjunction with previously published H-, N-, and O-DCPs, with B3LYP–DCP/6-31+G(2d,2p) on the S66, S22B, HSG-A, and HC12 databases of non-covalently interacting dimers showed that it is one of the most accurate methods available for treating intermolecular interactions, giving mean absolute errors (MAEs) of 0.19, 0.27, 0.16, and 0.18 kcal/mol, respectively. Additional testing on the S12L database of very large complexation systems gave an MAE of 2.6 kcal/mol, demonstrating that the B3LYP–DCP/6-31+G(2d,2p) approach to be one of the best-performing and most feasible methods for treating large systems containing significant non-covalent interactions. Finally, we showed that the modeling of C–C-making/C–C-breaking chemistry is well predicted using the newly developed DCPs. In addition to predicting a barrier height for the β-scission of the cumyloxyl radical, that is, within 1.7 kcal/mol of the high-level value, application of B3LYP–DCP/6-31+G(2d,2p) to 10 databases that include reaction barrier heights and energies, isomerization energies, and relative conformation energies gives performance that is among the best of all available dispersion-corrected density-functional theory approaches.

Bài báo mô tả các phương pháp cho phép đánh giá hiệu quả các tích phân hai điện tử trên các hàm lobe Gaussian đã được cô đặc. Sự cải thiện về tốc độ tính toán được thực hiện bằng cách tránh tính toán các tích phân mà: 1. đủ nhỏ theo lý do số học, 2. bằng không do tính đối xứng, 3. giống nhau với các tích phân khác do tính đối xứng. Các ví dụ về tính hiệu quả của những kỹ thuật này được đưa ra. Chúng tôi cũng báo cáo thời gian cho một xử lý bổ sung các tích phân hai điện tử trong một phép tính năng lượng Hartree Fock và năng lượng tương quan.

Results from anab initio MO-SCF-LCAO study of the thiophene molecule, using an extended set of contracted Gaussian basis functions, are presented. The ordering of the molecular orbitals and the ionization energies are discussed in relation to experimental data from electron spectroscopy. A number of molecular properties have been computed in good agreement with available experimental information. The effect of 3d functions on sulphur for the description of the chemical bond and for the physical and chemical properties of thiophene is elucidated. Molecular potential energy maps are used in a discussion of the mechanism for electrophilic substitution reactions.