Ab initio studies on calicene

Optimum geometries of planar and 90°-twisted C_2v calicene are calculated with single-configuration STO-3G and 3-21-G wavefunctions. The barrier to ring–ring rotation is computed. Bond alternation is pronounced in the planar form and decreases in the twisted form, while dipolar character increases on twisting.     

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.     

Counterpoise corrections to the evaluation of the bimolecular interaction energy components

Counterpoise corrections to the coupling terms of the bimolecular interaction energy decomposition are introduced and examined on a set of electron donor-acceptor dimers X·BH₃ (X=H₂O, NH₃, PH₃, LiH, CO). The interaction energy decomposition of Kitaura and Morokuma, and the decoupling of E _MIXsuggested by Nagase Fueno, Yamabe and Kitaura have been employed.Counterpoise corrections have numerical influence on two terms only of the NFYK decoupling. This decoupling gives useful additional information on the nature of the chemical interaction when applied to STO-3G minimal basis set wavefunctions, but fails when applied the 4-31G wavefunctions.     

The electronic structure of small lithium clusters

Ab initio molecular orbital calculations using the STO-6G and STO6-21G basis sets have been performed for the cluster series Li _n ⁺ , Li _n , and Li _n ^– (wheren=2–7). Thirty-two optimized structures are discussed and reported, many of which (especially for the anionic structures) have not yet been considered. The calculations suggest that for all three species the optimum geometries are planar. Of the two levels of theories that were investigated, STO-6G//STO-6G and STO6-21G//STO-6G, the latter hybrid theory was found to be less reliable. In particular, for the anionic structures these calculations should provide a platform from which more sophisticated, i.e., configuration interaction, geometry optimization can be performed.     

Comparative study of the molecular electrostatic potential obtained from different wavefunctions. Reliability of the semiempirical MNDO wavefunction

A systematic analysis of the molecular electrostatic potential (MEP) is presented. This study has been performed with a twofold purpose: first, to study the MEP dependence with regard to the quality of the basis set used to compute the ab initio SCF wavefunction and second, to develop and to assess a new strategy for computing isoelectrostatic potential maps using the semiempirical MNDO wavefunction. The only differences between this procedure and the ab initio SCF MEP computation lie in the freezing of the inner electrons and in the origin of the first-order density matrix. The statistical analysis of MEPs computed for a large number of molecules from MNDO wavefunction and ab initio SCF wavefunctions obtained using STO-3G, 4-31G, 6-31G, 4-31G*, 6-31G*, and 6-31G** basis sets points out the ability of any wavefunction to reproduce the general topological characteristics of the MEP surfaces. Nevertheless, split-valence basis sets including polarization functions are necessary to obtain accurate MEP minimum energy values. MNDO wavefunction tends to overestimate the MEP minima depth by a constant factor and shows an excellent ability to reflect the relative variation of MEP minima energies derived from a rather sophisticated (6-31G*) basis set, lacking of the shortcomings detected in the semiempirical CNDO approximation.     

Geometry optimization and electronic structures of molecules H_3AXAH_3

The geometries of molecules H_3AXAH_3(X=O,S,Se and A=C,Si)have been optimizedusing STO-3G ab initio calculations and gradient method and the results are in good agreement withreported experimental values.From the STO-3G optimized geometries,we have also calculated theelectronic structures of these molecules using 4-31G and 6-31G basis sets to obtain the MO energies.atomic net charges and dipole moments.The ionization potentials calculated by 6-31G basis set are ingood agreement with experimental values.     

Integral Hellmann-Feynman investigations oftrans bent acetylene

The integral Hellmann-Feynman (IHF) theorem has been applied to various wavefunctions representing the¹ A _u state of acetylene with a view to testing traditional explanations of the excited state geometry. When LCAOSCF wavefunctions are used, the electronic energy changes associated with the individual corresponding orbitals (CMG's) are in sympathy with the trend in orbital (i.e. MO) energies suggested by Walsh. However, when LMO wavefunctions based on hybrid AO's are employed, the IHF results are against all experience; and imply that a change in hybridisation, fromsp tosp ², is not a viable model for the change in geometry.     

A conformational analysis of the six isomers of thiobispyridine

The conformational behaviour of the six isomers of thiobispyridine has been investigated using ab initio STO-3G*//rigid-roto, STO-3G*//STO-3G* and 6–31G**//STO-3G* molecular orbital models. The analysis reveals both the importance of optimising critical structure parameters and the basis set dependence of calculated rotational barrier heights. The most reliable model (6–31G**//STO-3G*) clearly indicates that the minimum energy conformers are not planar and that energy barriers between 30–100 kJ mol^?1 restrict inter-conversion to planar structures, thereby preventing conjugation between the p-electrons of the sulfur atom and the π system of both pyridine rings. From the calculated barrier heights, two mechanisms can be employed to explain conformer interconversion about the C? S bond: a disrotatory one-ring flip or a conrotatory two-ring flip mechanism. Where comparisons can be made (eg. 2,2′-thiobispyridine), dipole moment calculations are shown to be in good agreement with experiment. Finally, of the six isomers, appropriately substituted 2,2′, 2,3′- and 2,4′-thiobispyridines are most prone to a Smiles rearrangment.     

Determination and characterization of a transition structure for water–formaldehyde addition

Quantum mechanical calculations of the geometric, energetic, electronic, and vibrational features of a transition structure for gas-phase water–formaldehyde addition (FW1?) are described, and a new transition-structure search algorithm is presented. Basis-set-dependent effects are assessed by comparisons of computed properties obtained from self-consistent field (SCF) molecular orbital (MO) calculations with STO-3G, 4-31G, and 6-31G^** basis sets in the absence of electron correlation. The results obtained suggest that STO-3G-level calculations may be sufficiently reliable for the prediction of the transition structure of FW1? and for the transition structures of related carbonyl addition reactions. Moreover, the calculated activation energy for formation of FW1? from water and formaldehyde (?44 kcal mol^?1) is very similar in all three basis sets. However, the energy of formaldehyde hydration predicted by STO-3G (? ?45 kcal mol^?1) is about three times larger than that predicted by the other two basis sets, with the activation energy for dihydroxymethane dehydration also being too large in STO-3G. Calculated force constants in all three basis sets are generally too large, leading to vibrational frequencies that are also too large. However, uniformly scaled force constants (in internal coordinates) give much better agreement with experimental frequencies, scaled 4-31G force constants being slightly superior to scaled STO-3G force constants.     

Electronic structure and molecular conformation of furan and pyrrole azomethines. An ab initio molecular orbital study

STO-3G minimal basis set ab initio molecular orbital calculations were employed to study the electronic structure and conformational preferences in furan-2-N-methylmethyleneimide ( 1 ) and pyrrole-2-N-methylmethyleneimide ( 2 ). The theoretical results were examined by comparison with the parent molecular systems through a population analysis and molecular orbital interactions considerations. The OCCN-trans and the NCCN-cis forms were found to be the most stable structures in 1 and 2 , respectively. Comparisons were made with available experimental data. The theoretical results indicate thatπ-electron interactions and molecular orbital interactions are not significant factors in determining the conformational preferences which most likely depend on dipole-dipole interactions.     

Transferable group contributions for a variety of chemical phenomena and compounds

An approximate method for calculating molecular electrostatic potential (MEP) maps and atomic point charge models for large molecules in a reduced computational time is proposed and tested for two widely used basis sets (STO-3G and 6–31G*). The method avoids the molecular orbital calculation of the whole system by expressing its first order electronic density matrix in terms of transferable localized orbitals (TLO), previously determined on model molecules, via a localization process followed by the cutting of the tails, and stored in two databases (one for each basis set). For systems with a canonic electronic structure TLO are made of a single vector, involving either two nuclei (to describe the covalent bond between those atoms) or one nucleus (to describe lone pairs and inner shells). Conversely, delocalized systems require many-center TLO, formed by a suitable number of vectors. Density functions of large chemical compounds can thus be built up automatically from a code that recognizes which fragments are contained in the system of interest, extracts them from the chosen database, reorders the atoms consistently with the pertinent TLO and places them in the correct position and orientation on the relevant atoms. A great number of chemical groups were parameterized and the efficiency of the method was evaluated on different systems, including aliphatic hydrocarbons. Numerical calculations on several molecules revealed that this approximation brought no significant loss of accuracy with respect to the corresponding Hartree-Fock (HF) values for the examined properties. Although the method is specifically designed to produce approximate wavefunctions, the point charge models obtained by fitting the corresponding MEP represent a viable alternative when ab initio HF calculations are not affordable, and can be used in connection with any popular force field.From the Proceedings of the 28th Congreso de Químicos Teóricos de Expresión Latina (QUITEL 2002)     

Semiempirical AM1 electrostatic potentials and AM1 electrostatic potential derived charges: A comparison with ab initio values

Electrostatic potentials calculated from AM1 wave functions have been compared with ab initio STO-3G values and qualitative agreement has been found. Atomic charges derived from AM1 electrostatic potentials for both experimental and AM1 optimized geometries are of comparable quality with STO-3G potential derived charges. These results suggest that the AM1 electrostatic potential may be useful both in its own right and also for deriving atomic charges for use in molecular dynamics studies.     

Efficient determination and characterization of transition states using ab-initio methods

The gradient of the potential energy with respect to nuclear coordinates has been calculated using ab-initio single determinant molecular orbital methods. The calculated gradient is used together with very efficient minimization methods to locate and characterize transition states on many-dimensional potential energy surfaces. Previously such methods have only been applied to semi-empirical potential functions. Although the calculation of the gradient in addition to the energy increases the computational time by about a factor of four, we have demonstrated the feasibility of these calculations by locating the transition state for the model rearrangement of HNC to HCN using both minimal (STO-3G) and split valence shell (4-31G) basis sets. Further use of such methods in the direct application of ab-initio wavefunctions to dynamical investigations is discussed.     

Orbital deletion procedure and its applications

The orbital deletion procedure is introduced, which is suited to quantitatively investigating the electronic delocalization effiect in earboeations and boranes. While the routine, ab initio molecular orbital methods can generate wavefunetions for real systems where all electrons are delocalized, the present orbital deletion procedure can generate wavefunctions for hypothetical reference molecules where electronic delocalization effect is deactivated. The latter wavefunetion normlly corresponds In the most stable resonance structure in terms of the resonance theory. By comparing and analyzing the delocalized and the localized wavefunetions, one can obtain a quantitative and instinct pieture to show how electronic deloealizalion inside a molecule affects the molecular structure, energy as well as other physical properties. Two examples are detailedly discussed. The first is related to the hypercoujugation of alkyl groups in carbocations and a comparison of the order of stability of carbocations is made, T     

Integrated spatial electron populations in molecules: Application to simple molecules

The electron projection function P(x, z) = ∫ ρ(x, y, z) dy is used to evaluate charge transfer and covalency in two series of molecules, LiX and CH₃X (X = Li, BeH, BH₂, CH₃, NH₂, OH, and F), with wavefunctions derived from STO-3G, 4-31G, and, in some cases, 6-31* ab initio calculations. The precision of the method and comparison with Mulliken populations analysis are described. Particular attention is given to CH₃Li which by our criteria is wholly ionic.     

The electronic structure of peracids. Functional models for cytochrome p-450

An extensive set of ground state ab initio and semiempirical molecular orbital calculations has been performed on both peroxytrifluoroacetic and peroxyacetic acids. The equilibrium geometry of peroxyacetic acid was calculated with the STO-3G and MINDO/3 methods, and peroxide rotational barriers for both peracids were obtained with STO-3G and PCILO. Extended basis set calculations with the 6-31G^** basis were performed for both peracids to compare the electronic structures of these two compounds. Electrostatic potential maps in the region of the peroxide bonds of both peracids were also calculated using INDO wavefunetions. These results are discussed with respect to the enhanced reactivity of peroxytrifluoracetic acid relative to peroxyacetic acid and the nature of the oxygen electrophilicity in these compounds and by analogy in cytochrome P-450, for which peroxytrifluoroacetic acid is considered to be an effective chemical model.     

A study of the electronic structure of the hexafluorobenzene and pentafluorobenzene molecules by ultrasoft X-ray emission spectroscopy

The electronic structure of the hexafluorobenzene and pentafluorobenzene molecules was studied by ultrasoft X-ray emission spectroscopy. The FK _α and CK _α spectra of these compounds in the gas phase were obtained. The results of quantum-chemical calculations performed at the RHF/STO-6G//6-31G level were used to construct the theoretical spectra. The highest occupied molecular orbitals were found to consist largely of the 2p _π carbon atomic orbitals. The contribution of fluorine orbitals was small. π-Type interactions mainly involved deeper valence orbitals.     

Polymerization of coordinated monomers. XXI. Ab initio molecular orbital study on the binary molecular complex composed of methyl methacrylate and boron trifluoride

The stereo- and electronic structures of the binary molecular complex composed of methyl methacrylate and boron trifluoride are obtained by using an ab initio molecular orbital method with an STO-3G basis set. The total energy change on the binary molecular complex formation is ?1.3 X 10^?2 Hartree (?8.2 kcal/mol). The electron transfer from methyl methacrylate to boron trifluoride and the change in the energy level of the lowest unoccupied molecular orbital of methyl methacrylate on the complex formation with boron trifluoride are much smaller than those on the complex formation with boron trichloride. A twisted form in which the dihedral angle between the vinyl plane and the ester plane is 16.9° is the most stable structure of the binary molecular complex composed of methyl methacrylate and boron trifluoride. A strong bonding overlap population between a β-hydrogen of methyl methacrylate and a fluorine of boron trifluoride is found in this conformation. © 1992 John Wiley & Sons, Inc.     

The Electronic Structure of Cubane (C8H8) as Revealed by Photoelectron Spectroscopy

High resolution He (Iα) and He (IIα) photoelectron spectra of cubane are reported. The assignments of the bands to different states of the cubane radical cation are made on the basis of ab initio STO-3G and MINDO/3 calculations, using geometries optimized within each treatment. The vibrational fine-structure observed supports the proposed assignment. An open shell MINDO/3 model for ground state cubane radical cation suggests that the Jahn-Teller distorted system fluctuates between twelve equivalent structures of C_2v-symmetry. Localized molecular orbitals derived from the STO-3G model of cubane indicate that the major feature which discriminates this molecule with respect to other hydrocarbons is the large interaction matrix element between the opposed CC-σ-orbitals of each face.     

Potential-derived point-charge model study of electrostatic interactions in DNA base components

Ab initio electrostatic potentials obtained using STO-3G wavefunctions for guanine, cytosine, adenine, and thymine are used to calculate potential-derived (PD) point charges for these base components. Calculated PD point charges are used to estimate the electrostatic contributions to hydrogen-bonding and stacking interaction energies of ten sequence isomers of B-DNA. These estimates are in excellent agreement with the results of the more elaborate segmental multipole moment expansion technique.