Ab Initio SCF MO Calculations Research Articles

Aryl vinyl sulfides as probes for electrophilic versus electron transfer mechanisms

The distinction between electrophilic and electron transfer mechanisms of addition reactions to the vinyl double bond of aryl vinyl sulfides, ethers and selenides has been achieved by studying substituent effects. Specifically, the effects of meta and para substituents on the rates of electrophilic addition reactions correlate with Hammett σ values, while ionization of the substrates to the corresponding radical cations correlates with σb+. The significance of the respective correlations are confirmed by statistical tests. The σ (electrophilic)/σb+ (ET) dichotomy is in accord with the conventional paradigm for σ/σb+ correlations and is further supported by ab initio SCF MO calculations. The application of this criterion to the reactions of aryl vinyl sulfides and ethers with tetracyanoethylene indicates that formation of cyclobutanes occur via direct electrophilic addition to the electron-rich alkene and not via an electron transfer mechanism. This conclusion is further supported by the results of previous radical cation probe studies.

A linear relation between the ionization potentials of HeI photoelectron spectroscopy and NMR chemical shifts. A PES study of the electronic structure of alkyl isocyanides

HeI photoelectron spectra (PES) of alkyl isocyanides RNC, where R is -CH 3, -CH 2CH 3, -CH 2CH 2CH 3, -CH(CH 3) 2, -C(CH 3) 3 and -cyclo-C 6H 11, are reported. The assignment of the bands is made with the aid of bandshapes, fine vibrational structure and ab initio SCF MO calculations with STO-6G basis sets for each of the molecules studied. The first sharp peak with the lowest ionization potential and with different vibrational spacings is assigned to electron ionization of the HOMO which is composed of a dominant contribution of the carbon atom, similar to the ‘lone-pair’ orbital of the carbon atom in the isocyano group. A linear relationship between the lowest PES experimental ionization potentials and the 13C NMR chemical shifts of the carbon in the isocyano group is found.

HeI photoelectron spectroscopic (PES) studies of the electronic structure of Al(CH 3) 3, In(C 2H 5) 3 and Cd(CH 3) 2 compounds

HeI photoelectron spectra (PES) of Al(CH 3) 3, In(C 2H 5) 3 and Cd(CH 3) 2 compounds are recorded. The assignment of the PES bands is made on the basis of bandshape and ab initio SCF MO calculations using the 3-21G basis sets. Both PES experiment and theoretical calculation show that the characters of the HOMO [5e′ for Al(CH 3) 3, 12e′ for In(C 2H 5) 3 and 3a 2u for Cd(CH 3) 2] are mostly metal-carbon antibonding. The bond orders between the metal and carbon atoms for Cd(CH 3) 2, Al(CH 3) 3 and In(C 2H 5) 3 are 0.9251, 0.7166 and 0.6957 respectively. The strength of metal-carbon bonding is Cd(CH 3) 2 > Al(CH 3) 3 > In(C 2H 5) 3.

The study of HeI photoelectron spectroscopy (PES) of the electronic structure for CIONO 2

The HeI photoelectron spectroscopy of chlorine nitrate (CIONO 2) is reported. The assignment of the HeI spectrum has been made using bandshape and vibrational structures of the bands, as well as GAUSSIAN 86 ab initio SCF MO calculations. The vibrational spacings of three bands in the low ionization potential region (< 13.50 eV) are respectively: 1246 ± 60 cm −1 for the first band, 770 ± 50 cm −1 for the second band and 545 ± 50 cm −1 for the third band.

Effects of Aqueous and Organic Solvents on the Conformational Properties of the Helix-Forming alpha-Methyl-beta-L-aspartamyl Residue.

The conformational preferences of N-acetyl-N'-methyl-alpha-methyl-beta-L-aspartamide, which is the model compound for helical poly(beta-L-aspartate)s, have been determined by ab initio SCF-MO computations. Two driving patterns have been found for the existing 13 minimum energy conformations: (i) intramolecular hydrogen bonding interactions of both amide-amide and amide-ester type; and (ii) repulsive interactions between the four oxygen atoms contained in the molecule. Self-consistent reaction-field (SCRF) calculations based on the method proposed by Miertus, Scrocco, and Tomasi have been performed in order to evaluate the effect of the solvent on the conformational preferences of the compound subject of study. Water and carbon tetrachloride were the solvents chosen for this purpose, and results have been discussed and interpreted on the basis of their electronic structures. The conclusions drawn from this study are of assistance to understand some features of the conformational transitions experimentally found in poly(beta-L-aspartate)s.

Relationships between the HOMO energies and pKa values in monocyclic and bicyclic azines

Ab initio SCF-MO calculations have been used to interpret the observed basicities of certain monocyclic and bicyclic azines. We studied (A) pyridine and the single ring diazines pyridazine, pyrimidine and pyrazine; (B) the benzodiazines cinnoline, quinazoline, quinoxaline and phthalazine, and (C) the naphthyridines substituted at positions 1,5, 1,6,1,7, 1,8 and 2,7, in addition to quinoline and isoquinoline. We report that a good linear relationship exists between the experimental pK values and the HOMO energy for each of the separate series (A), (B) and (C). There is also a good linear relation between the HOMO energy and the first π -electron ionization energy. The basicities of these azines may therefore be directly interpreted in terms of the HOMO energies, as predicted by frontier orbital theory.

Structure models of metallic polyacetylene

The metallic state of doped polyacetylene is studied from a structural chemical point of view by finding the optimized structure of the doped complex with the ab initio SCF MO calculation. The polson structure for the unit of metalic state is supported by comparing the calculated vibrational frequencies of the polson unit, C 13H 15Na 2, with the Raman spectra of heavily alkali metal doped polyacetylene. The coordination and the crystal structures are investigated on the potassium and iodine doped polyacetylene with the polson model.

An electronic and vibrational study of the cyclopropenyl ion and its fluoroderivatives

This article presents the results of ab initio SCF-MO calculations on both the electronic structure and vibrational spectra of the cyclopropenyl cation (C 3H 3 +) and its fluoroderivatives, C 3H 2F +, C 3HF 2 + and C 3F 3 +. A very simple and unambiguous criterion for choosing the combination of diffuse and polarization functions which, together with the 6311G basis set, best describes the electron distribution in these ions is presented. The electronic structures of the cations are analysed in detail; particular emphasis is given to the analysis of the electronic effects due to successive hydrogen-by-fluorine replacements. The results of vibrational normal mode analysis carried out for all hydrogen-deuterium isotopomers of the studied ions are presented and compared with the available experimental data. The theoretical results are used both to review some band assignments previously proposed for the fluorosubstituted molecules and to give a stronger theoretical foundation to the general interpretation of the vibrational spectra of these compounds.

UV-induced isomerization of (E)-crotonic acid. Combined matrix-isolated IR and ab initio MO study

The results of a combined vibrational and structural study of the (E)-crotonic acid monomer undertaken by matrix-isolated low-temperature IR spectroscopy and ab initio SCF-MO calculations are presented. It is shown that in both argon and krypton matrices monomeric (E)-crotonic acid exists as a mixture of two conformers of similar energies, differing by the relative orientation of the CC—CO axis (the s-cis and s-trans forms, having a CC—CO dihedral angle equal to 0° and 180°, respectively). Upon UV-irradiation in the 240–250 nm region by a xenon lamp, photoisomerization reactions about both Cα—C and CC bonds occur leading, respectively, to s-cis→ s-trans(E)-crotonic acid rotamerization and (E)-crotonic acid →(Z)-crotonic acid conversion. Results of ab initio SCF-MO calculations, in particular optimized geometries, relative stabilities, dipole moments and harmonic force fields, for the relevant conformational states of both (E) and (Z)-crotonic acids are also presented and the conformational dependence of some relevant structural parameters is used to characterize the most important intramolecular interactions present in the studied forms. Finally, results of a normal mode analysis based on the ab initio calculated vibrational spectra are used to help interpret the experimenal vibrational data, enabling a detailed assignment of the matrix-isolated spectra and the characterization of the observed photoinduced isomerization reactions.

He I photoelectron spectroscopic studies of the electronic structure of 2(3H) furanone and 2(5H) furanone

He I photoelectron spectra of 2(3H) furanone and 2(5H) furanone are recorded. The expanded bands with vibrational spacings of 1514 ± 40 cm −1 and 1250 ± 60 cm −1 in the low ionization potential (IP) region (< 12.00 eV) have been measured. To assign the HeI photoelectron spectra of the molecules studied, ab initio SCF MO calculations at the Gaussian 86 6–31∗G level are also performed for both molecules. From the comparison of HOMO ionization potentials for both molecules, it is suggested that the biological activity of 2(3H) furanone is stronger than that of 2(5H) furanone.

Framework silica structures generated using simulated annealing with a potential energy function based on an H6Si2O7 molecule

A simulated annealing technique was used to search for global and local minimum energy structures of a potential energy model for silica. The model is based on ab initio SCF MO calculations on the disilicic acid molecule, H6Si2O7. Starting with 4 SiO2 units, with the atoms randomly distributed in the unit cell, 23 distinct silica tetrahedral framework structures were found, with a variety of space group symmetries and cell dimensions. Despite the assumption of P 1 space group symmetry for the starting structure, only 7 of the local minimum energy structures were found to possess triclinic symmetry with the remainder exhibiting symmetries ranging from P c to $$I\bar 42d$$ to within 0.001 A. Although the interaction potential for the disilicic acid molecule has a single minimum energy SiO bond length and SiOSi angle, the local minimum energy structures exhibit angles that range between 105° and 180° and bond lengths that range between 1.55 and 1.68 A. The correlation observed for coesite and the other silica polymorphs between SiO bond length and fs(O) is reproduced. The generated structures show a wide variety of coordination sequences, ring sizes and framework densities, the later ranging from 19.8 to 35.5 Si/1000 A3. The energies of these structures correlate with their framework densities, particularly for higher energy structures.

The lowest energy unoccupied molecular orbitals of the cyclopolysilanes: experimental and theoretical studies

Inner-shell electron energy loss spectra at the Si2p edge have been measured for [Si(CH 3 2] n, n=5 and 6, and have been assigned using ab initio SCF MO calculations for the related molecules (SiH 2) n , n=2–6, and [Si(CH 3) 2] inn, n=3 and 4. Ou calculations indicate that the lowest energy ISEELS feature, which lies about 2 eV below the Si2p ionization threshold, arises from excitation of a Si2p electron to an empty MO which is of π symmetry and has appreciable Si3p character. Calculations of vertical and adibatic electron attachment energies establish that the gas-phase anions of all the cyclopolysilanes considered will be unstable with respect to electron detachment and will show electron scattering resonances in the region around 2 eV. We have also measured the total elastic electron scattering cross sections of [Si(CH 3) 2] inn, n=5 and 6, and have indeed found an increase in cross section at an energy of about 2 eV. We have calculated the 29Si NMR shielding of (SiH 2) inn, n=3,4, using the RPA LORG method and have reproduced the large shielding anisotropy recently measured for some [SiR 2] 3 species by solid-state NMR. Calculation of the trace of the square of the anisotropic hyperfine interaction for the Si 3H 6 − anion gives a value reasonably consistent with experiment, but much smaller than that expectred for a single unpaired atomic Si3p electron, due to the delocalization of electron density onto the Hs. Thus, π symmetry and substantial Si3p character are not inconsistent with the small experimental value of the hyperfine anisotropy in such anions, and the apparent inconsistency of ESR and UV data versus ENDOR results is removed.

Calculation of vibrational frequencies of sulphur compounds using semiempirical molecular orbital methods (AM1, PM3, MNDO)

Abstract Vibrational frequencies of 30 sulphur-containing molecules for which detailed experimental data are available were calculated using semiempirical molecular orbital methods (AM1, PM3, MNDO). The agreement between the uncorrected calculated frequencies and the experimental data was found to be satisfactory, the calculated mean errors (AM1, 9.5%; PM3 9.0%; MNDO 12.2%) being comparable with those commonly obtained using ab initio SCF-MO calculations (7–10%). Because of the systematic nature of the errors in the calculated frequencies, simple empirical corrections significantly improve the agreement with experiment (mean errors in the corrected calculated frequencies are: AM1, 3.4%; PM3, 3.3%; MNDO, 3.5%), thus making this kind of calculation of practical value in helping the assignment of vibrational spectra of considerably large sulphur-containing molecules. Thermodynamic quantities evaluated using the semiempirical vibrational frequencies were also calculated, showing very good agreement with the experimentally available values.

Quantitative perturbational molecular orbital analysis of the conformational preference of aniline molecule and its intramolecular force

A new quantitative perturbational molecular orbital treatment, denoted as WSW-Like procedure, is described in detail. It provides, in the framework of the ab initio SCF-MO computations with minimal STO-3G basis set, reliable calculation of the energy effects associated with π-π, σ-σ, π-σ and σ-π orbital interactions. Estimated conformational preference is in agreement with experimental result. The driving force for the out-of-plane twist of the nitrogen frame arises from the π interaction between the π system of the phenyl ring and the lone pair of P electrons on the nitrogen atom. In order to maintain the greatest possible aromaticity of the phenyl ring, the nitrogen frame must be pushed out of the phenyl ring as far as possible.

Ethenethiol and 1-propene-1-thiol: a photoelectron spectroscopic study

Ethenethiol and 1-propene-1-thiol were produced through the thermolysis of thiirane and 2-methylthiirane, respectively, under reduced pressure in a flow system. The photoelectron spectra of the olefin thiols are characterised by the presence of a sharp first band which is well separated from the other bands by more than 2 eV. The spectra are discussed with the aid of AM1 and ab initio SCF MO calculations, and a diagram which correlates the binding energies of the first three orbitals, viz. πCCS∗, πCCS and σCS, with those of the corresponding orbitals of related molecules.

Valence isomers of benzene and their relationship to the isoelectronic isomers of arsenic molecule (As6)

We report the results of geometry-optimized ab initio SCF-MO calculations for As 6 in structural forms corresponding to the five valence isomers of benzene, C 6 H 6 , with which As 6 is valence-isoelectronic. We used Scuseria's 752+P polarized basis set for arsenic. The resulting energy ordering of As 6 isomeric structures is very similar to what we found previously for P 6 isomers but quite different from that known for the C 6 H 6 valence isomers. For As 6 , as for P 6 , prismane and benzvalene structures have the lowest energies while the planar hexagon (benzene) has the highest. The range of energies of the five structures is small, around 45 kcal/mol, somewhat larger than for the P 6 isomers but far smaller than for C 6 H 6

Maximum hardness in P6 isomers

AbstractParr and Chattaraj proposed a principle of maximum hardness for stable molecular structures. Pearson and Palke used ab initio SCF MO calculations for ammonia and ethane to demonstrate the operation of the principle. In this paper, we present ab initio SCF MO results for five isomeric forms of the homoatomic P6 cluster as further support for the principle of maximum hardness. © 1994 John Wiley &amp; Sons, Inc.

FT-IR Characterization and Ab Initio Study of the Surface Species of a Nanosized SiC Powder

ABSTRACTThe v(SiH) stretching absorption band is known to be sensitive to the local environment of the silicon atom. The presence of SiH groups on silicon carbide surface can be taken advantage of to evaluate the oxidation degree of this ceramic powder.The surface analysis of a SiC nanosized powder was performed by FT-IR spectrometry. The evolution of the v(SiH) absorption band, followed in situ under controlled temperatures and oxygen pressures, consists in a frequency shift toward higher wavenumbers when oxidation proceeds. The resolution of the Sill absorption range into four subbands corresponding to the HSiOxC3−x possible surface groups showed the relative evolution of the different species. Ab initio SCF MO calculations on molecular models gave evidence of the charge transfer to the more electronegative oxygen atom, making the silicon atom more positive and seemingly strengthening the SiH bond and thus inducing an upward shift of the v(SiH) frequency.

SCF-MO study of the preferred conformations of nylons 1, n

Semiempirical SCF-MO calculations have been carried out to study the effect of packing interactions on the conformations of nylon 1, n. For this purpose the conformational preferences in vacuo and with the crystal environment of the model molecule bis(acetoamido)methane (BAAM) were investigated. Only two minima with the same energy were found when in-vacuo calculations were performed. These were validated by comparison with ab initio SCF-MO calculations using both 3–21 G and 6–31G ∗ basis sets. To mimic the crystal environment, hydrogen bonding sites of the model molecule were saturated with two molecules of ammonia and two of formaldehyde. The results show a change of approximately 15° in the dihedral angles when the molecule is treated including environmental effects relative to when it is considered isolated, which are in remarkable agreement with the experimental results available. In summary, the BAAM molecule tends to adopt helical conformations with a single direction of hydrogen bonds. These preferences are the consequence of intra-chain interactions, although the conformations are also modified by inter-chain interactions.

Ground state vibrations of N-glycylglycine hydrochloride: an ab initio study

The results of ab initio SCFMO calculations using 3-21G and 4-21G basis sets for N-glycylglycine hydrochloride are presented. The optimized geometries from these calculations are compared with experimental bond lengths and bond angles obtained from X-ray crystallographic data. The vibrational spectrum (frequencies and intensities) for the compound has been calculated using the same basis sets. The predicted spectrum has been compared with the experimental IR spectrum of the molecule in the solid state. The calculated potential energy distribution provides a basis for assigning the spectrum. The calculated IR intensity pattern also shows good agreement with the experimental spectrum.