Ab initio calculations on the isomerization of the 1,5-hexadiyne radical cation

Ab initio calculations at the MRCI//ROHF/6-31G** level suggest that the 1,5-hexadiyne radical cation is not stable and isomerizes to the 1,2,4,5-hexatetraene radical cation without a barrier. After this rearrangement, the internal energy of the ions will be sufficient for a subsequent isomerization to the benzene structure. At energies of about 1 eV above the vertical ionization energy, the 1,5-hexadiyne radical cation may dissociate to linear neutral and ionic C₃H₃ fragments. These results are in excellent agreement with previous experiments.     

Theoretical study for the HNSi–HSiN isomerization: ab initio and DFT calculations

Ab initio and density functional theory (DFT) calculations using the GAUSSIAN 94 program have been performed to investigate the molecular structures of HNSi and HSiN in the ground state as well as the transition state for the HNSi–HSiN isomerization reaction at the 6-311G(d,p), 6-311+G(2d,p) and 6-311+G(2df,p) basis sets. The results show that DFT calculations at higher levels of theory reproduce experimental vibrational frequencies of both HNSi and HSiN better than ab initio methods including electron correlation effects. Those calculated geometries are accurate enough to predict the rotational constant of HNSi. The barrier height for the isomerization reaction is found to be about 10 kcal/mol.     

Propionaldehyde: ab initio and semi-empirical calculations

The internal rotation of propionaldehyde about the 1–2 bond has been studied by means of ab initio calculations. The most stable conformer has methyl and carbonyl eclipsed. Increasing the 1–2 dihedral angle to 60°, 120°, and 180° gives energies of 1.7, 0.4, and 0.7 Kcal/mol, respectively. The agreement with force field calculations and with experiment is reasonable.     

The place of sioh groups in the absolute acidity scale from ab initio calculations

The deprotonation energy of a (H₃Si)O-H group as a simplified structural element of silica gel and aluminosilicates is estimated from ab initio calculations on H₂O, CH₃0H and SiH₃0H to be near 1475 kJ/mol. Results of the effect of Lewis acids on the Brønsted acidity of SiOH are given.     

Large-scale ab initio calculations of archetypical ionic liquids

Fully ab initio large-scale calculations of archetypical ionic liquids consisting of up to eight ion pairs are presented for the first time. These are used to validate the computationally efficient Fragment Molecular Orbital approach applied to these semi-Coulombic systems, paving the way towards accurate prediction of their transport properties.     

Simultaneous ab initio calculations of isoelectronic diatomic molecules

Large gaussian basis sets are employed in simultaneous configuration interaction calculations for the ground states of isoelectronic diatomic molecules. The resulting potential energy curves for three members respectively of four different isoelectronic molecule sequences show the applicability of the method. Comparisons with available results of standard configuration interaction calculations for selected molecules are given. Using our method we often get lower upper bounds for the electronic energy, save computer time and treat physically totally different molecules simultaneously.     

Classical chemical concepts from ab initio SCF calculations

Alternative definitions of bond order, valency, gross orbital populations and total atomic charge for SCF wavefunctions are compared. It is found that there are sound theoretical and numerical reasons for preferring definitions based on the Löwdin density matrix.     

Convergence problem in ab initio crystal orbital calculations

The various computational procedures for treating the problem of lattice summations in ab initio crystal orbital calculations are briefly reviewed and the cutoff problem is discussed. Several numerical examples are presented for demonstrating the fast convergence of the total energy per unit cell with respect to the inclusion of further neighbors' interactions, in case reasonable cutoff procedures are applied. The importance of electrostatic considerations for the interpretation of long range effects is stressed.     

A charge-conserving approximation method for ab initio calculations

A new method is presented for approximate ab initio calculations in quantum chemistry. It is called CCAM (charge conserving approximation method). The calculation method does not include the use of empirical parameters. We use Slater type orbitals as basis set, replacing STO's by STO-2G functions to evaluate three- and four-center integrals and making the STO-2G two-orbital charge distributions have the same total charge as STO. The results are presented for test calculations on five molecules. In view of these results, CCAM is better than ab initio calculations over STO-6G in the results on total energies, kinetic energies and occupied orbital energies. In atomic populations, dipole moments and unoccupied orbital energies, CCAM is also satisfactory. We estimate that CCAM would be as fast as ab initio calculations over STO-2G in evaluating molecular integrals.     

An ab initio method for performing valence-electron-only calculations

We present a new method for performing valence-electron-only calculations on systems containing heavy atoms. The method contains no adjustable parameters and when used with a large orbital basis set yields exactly the same valence electron energy as an all-electron ab initio calculation. Good results are also obtained with truncated orbital basis sets.     

Simple ab initio calculations using a floating basis

Calculations are described on three rotamers of hydrogen disulphide (transgauche- and cis-HSSH) using an ab initio Floating Gaussian Orbital model. The optimised geometrical and electronic structures of each rotamer are discussed in terms of several electronic properties, a population and orbital analysis and an extensive partitioning of the electronic energy amongst the orbitals. The so-called Gauche Effect in HSSH is discussed in connexion with the various models proposed to account for this particular structural feature.     

Stereoelectronic structure of methoxyphenyltrichlorostannanes by the results of ab initio calculations

The RHF/3-21G* and MP2/3-21G* levels were implemented for the study of the stereoelectronic structure of 2- and 4-methoxyphenyl- and 2,6-dimethoxyphenyltrichlorostannanes. In the 4-methoxyphenyltrichlorostannane the Sn atom is tetracoordinated, while in 2-methoxy- and 2,6-dimethoxyphenyltrichlorostannanes it is pentacoordinated due to the intramolecular interaction of the O atom with it. In this case a closure occurs of a 4-membered ring. In the last molecule, the Sn atom interacts almost exclusively with only one of the O atoms. The intramolecular interaction between the Sn and O atoms leads to the electron density transfer from C and H atoms of the methoxy groups as well as from the Sn atom to the atoms of its coordination polyhedron. As a result the electron density increases also on the O atom involved in this interaction.     

Proton affinity of proline and modified prolines using the kinetic method: role of the conformation investigated by ab initio calculations

The proton affinities of proline, cis-3-methylproline and cis-3-ethylproline have been measured by the kinetic method using an ion trap instrument; the values obtained are 936, 940.5, and 943 kJ mol(-1), respectively. The experimental values are consistent with those obtained by high-level ab initio calculations (B3LYP/6-31+G*//B3LYP/6-31G* and B3P86/6-31+G*//B3LYP/6-31G*). Several conformations of neutral and protonated proline were considered, in particular the endo and exo ring structure and the position of the carboxyl group. These results show the importance of the position of the hydrogen atom of the carboxyl group in determining the most stable protonated proline structure.     

Exploitation of symmetry in periodic Self-Consistent-Field ab initio calculations: application to large three-dimensional compounds

Symmetry can dramatically reduce the computational cost (running time and memory allocation) of Self-Consistent-Field ab initio calculations for crystalline systems. Crucial for running time is use of symmetry in the evaluation of one- and two-electron integrals, diagonalization of the Fock matrix at selected points in reciprocal space, reconstruction of the density matrix. As regards memory allocation, full square matrices (overlap, Fock and density) in the Atomic Orbital (AO) basis are avoided and a direct transformation from the packed AO to the SACO (Symmetry Adapted Crystalline Orbital) basis is performed, so that the largest matrix to be handled has the size of the largest sub-block in the latter basis. We here illustrate the effectiveness of this scheme, following recent advancements in the CRYSTAL code, concerning memory allocation and direct basis set transformation. Quantitative examples are given for large unit cell systems, such as zeolites (all-silica faujasite and silicalite MFI) and garnets (pyrope). It is shown that the full SCF of 3D systems containing up to 576 atoms and 11136 Atomic Orbitals in the cell can be run with a hybrid functional on a single core PC with 500 MB RAM in about 8 h.     

Geometry optimised ab initio calculations on cyclopropenone and dimethylcyclopropenone

Geometry optimised ab initio calculations are reported for cyclopropenone (I) and its dimethyl derivative (II). The effect of the methyl substituents in this strained ring system with an exocyclic heteroatom is to increase the bond lengths and the electronic delocalisation. For cyclopropenone the results are compared, firstly, with previous ab initio calculations, without geometry optimisation, secondly, with a semi-empirical calculation with partial optimisation, and finally, with experimental results. The calculated geometry for this molecule agrees with the experimental results.     

FT-IR spectral data and ab initio calculations for haloketenes

Photolysis of haloacetyl chlorides isolated in cryogenic xenon matrix generates hydrochloric acid which is growing at the same rate as a ketene-like moiety. This last species is found to be different in the experiment involving fluoroacetyl and chloroacetyl chloride, from Fourier transform infrared (FT-IR) spectral results. The frequencies and intensities of the corresponding FT-IR spectral features are compared with the theoretical values obtained from ab initio calculations for fluoroketene HFCCO, chloroketene HClCCO and dichloroketene ClClCCO. The theoretical results give a satisfactory account of the haloketene HXCCO (X=F or Cl) assumption.