Ab Initio SCF MO Calculations Research Articles

Low‐Temperature X‐ray Crystal‐Structure Analysis of the Thermally Unstable Lithiated 2‐Butenyl tert‐Butyl Sulfide: A comparison with model ab initio MO calculations

AbstractSingle crystals of the N,N,N′,N′‐tetramethylethylenediamine (TMEDA) complex 6 of the title compound have been isolated. Compound 6 decomposes in the crystalline state above −20°. From the bond lengths and angles obtained by X‐ray crystal‐structure analysis (data collected at −70°), compound 6 is best described as a (E)‐1‐(tert‐butylthio)‐1‐lithio‐2‐butene with the double bond acting as an additional ligand on lithium (unsymmetrical allylic group). The S‐atom is in a cisoid arrangement in a common plane with the four C‐atoms of the butenyl system. The t‐Bu group and the Li‐atom are located above and below this plane. The structure is discussed with respect to the reactivity of 6 (α/γ reactivity). The gross structure is reproduced surprisingly well by an ab initio SCF MO calculation of the model lithiopropene‐1‐thiol 7(HS instead of t‐BuS, CH2 instead of CHCH3, no solvation of Li). The prominent difference is the symmetry of the allyllic moiety in the calculated structure.

Ab initio molecular orbital and photoelectron spectroscopic study of the pyridine–boron trifluoride electron-donor–acceptor complex

The He(I) and He(II) photoelectron spectra of the adduct C5H5N–BF3 are reported. The structure of this complex is determined by ab initio SCF MO calculations using a 3–21G basis, yielding good agreement with the experimental structure. The valence ionization energies of the complex are calculated using the outer-valence Green's function method resulting in an assignment of the photoelectron spectra reported herein.

Conformational studies of n-propylamine by combined ab initio MO calculations and Raman spectroscopy

The results of ab initio SCF-MO calculations performed with a 3-21G(N*) basis set, for fully optimized geometries of five conformations of n-propylamine, are presented. The calculated relative order of total energies for these conformers is TT≈GG′>TG>GT>GG. At 300 K, the Boltzmann distribution of populations is 18, 37, 20, 19 and 7%, respectively. Raman spectra of n-propylamine and n-propylamine-N- d 2 in the liquid phase exhibit a number of bands whose temperature-dependent intensities clearly suggest the occurrence of different conformers in simultaneous equilibria. Deuteration of the amine group originates pairs of Raman bands at 428 and 440 cm −1 and at 863 and 885 cm −1. The bands at 428 and 885 cm −1 are favoured by reduction of temperature. Normal coordinate calculations permit the assignment of the Raman and i.r. spectra in good agreement with experimental evidence. Among the five possible conformers of n-propylamine, it is possible to detect the presence of at least three conformations in the liquid phase, corresponding to the skeletal trans (TT and GT) and at least one of the skeletal gauche (TG, GG or GG′) forms. In the solid phase, only the bands ascribed to the TT form were observed. The ab initio results for the isolated molecule show that the all- trans conformation, TT, and the conformation GG′ have the smallest energies. On the other hand, the vibrational results for the liquid and solid phases indicate that the all- trans conformation, TT, is the more populated form. In addition, this conformer presents the highest calculated dipole moment, in good agreement with the liquid phase Raman spectroscopic results which point out that this conformation is favoured by polar solvents. Intermolecular interactions operating in the liquid n-propylamine, possibly of the hydrogen bonding type, are responsible for altering the relative order of conformational stability as predicted by the ab initio SCF-MO results for the isolated molecule.

An ab initio SCF-MO study of the aromaticity of some cyclic compounds

The application of three different procedures (PMO Method, the Orbital Energy Method, and the Total Energy Method) to the computation of the π delocalization energies of a series of cyclic compounds (cyclopentadiene, furan, pyrrole, thiophene, and benzene) is described. The study was performed in the framework of ab initio SCF-MO computations at the STO-3G level. It was found that, while the PMO Method and the Orbital Energy Method provide values of the delocalization energy which correlate very well with the experimental resonance energies, the Total Energy Method needs to be “appropriately” corrected. The correction consists of subtracting the energy effect of the corresponding acyclic species from the energy effect of the cyclic compound. It was also found that the use of a quantitative PMO analysis provides information on the role of the various orbital interaction(s) which are responsible for the extent of aromaticity in these compounds.

Ab initio molecular orbital calculation of the HONO torsional potential

Ab initio SCF-MO calculations were performed to obtain a torsional potential function for the HONO molecule. Calculations were made for the cis and trans conformations with the STO-3G, 4-31G, 6-31G, 4-31G*, and 6-31G** basis sets. Complete geometry optimization was carried out within each basis set. It was found that the 4-31G basis set provided values for the geometry and energies closest to the experimental results. The trans-cis isomeric energy difference was calculated to be -0.642 kcal/mol and the energy barrier to rotation from trans to cis was found to 9.68 kcal/mol; the comparable experimental energies are -0.6 and 9.7 kcal/mol, respectively. Completely optimized geometries using the 4-31G basis set were used to calculate a detailed torsional potential. These points were fit with a cosine series.

Ab initio structural and conformational studies of HCSSH, CH 3CSSH and HCSSCH 3

Ab initio SCF-MO calculations were carried out for HCSSH, CH 3CSSH and HCSSCH 3. Structures and energies of several conformations of these molecules were determined by the gradient geometry refinement at the STO-3G and 3-21G levels. Torsional potentials and some conformationally dependent local geometry trends are discussed. The addition of d-polarization functions on the sulphur atoms to the 3-21G basis set was found to be very important to reproduce adequately the experimental CS and CS bond lengths in HCSSH. Molecular atomic charges, dipole moments and ionization potentials are calculated and compared with available experimental values.

Raman Spectra and Electronic Structure of Alkaline Metal Doped Polyacetylene and β-Carotene

Abstract Raman spectra of alkaline metal doped polyacetylene were studied at various levels of doping for (CHMy)x, and (CDMy)x, M=Na and K, with y=0–0.18. Several steps in the doping process can be recognized in changes of the Raman spectra; the initial Raman lines remain for y=0–0.06 but change completely into a new pattern at y=0.13. Raman and ESR spectra of alkaline metal doped β-carotene were measured as a model study of the alkaline metal doping of polyacetylene. The correlation of these spectra together with the ab initio SCF MO calculation on model compounds show that a heavily alkaline metal doped polyacetylene chain does not have a uniform bond length structure but is composed of a charged soliton structure connected by a uniform chain. The new Raman bands were assigned based on this structure, implying that a bond alternation still exists in the metallic state of alkaline doped polyacetylene.

The electronic and molecular structure of cyclopropane, isocyanomethane and isocyanocyclopropane

Ab initio SCF MO calculations have been carried out on cyclopropane, isocyanomethane and isocyanocyclopropane in order to elucidate the relative importance of σ-and π-electronic interactions between the substituent and the cyclopropane ring. Complete geometry optimizations have been carried out. The computed structures are in excellent agreement with experimental structure determinations. Bond lengths, gross charges and a detailed analysis of bond path gradients yield evidence of a conjugative interaction between the isocyano and the cyclopropyl group and a small hyperconjugative interaction in isocyanomethane. In addition, there is a considerable inductive electron flow from the hydrocarbon moiety to the nitrogen atoms as expected from electronegativity.

A comparative ab initio study of methyl formate, methyl thiolformate and methyl thionoformate

Ab initio SCF-MO calculations have been carried out for HCOOCH 3, HC(O)SCH 3 and HC(S)OCH 3. Relative stabilities of s-trans/s-cis conformers are reported and discussed in terms of specific intramolecular interactions. The energy difference between the s-trans and the s-cis form increases in the order methyl thiolformate < methyl thionoformate < methyl formate. The major stabilizing factors of the s-cis forms are the bond dipolar interaction and the mesomeric delocalization through the five member ring involving both the XCYC (X, Y  O, S) skeleton and the out-of-plane hydrogen atoms. These effects are used to explain the trends mentioned. The non-planarity previously proposed for the thionoester is reinvestigated. Our calculations show that this molecule is planar. Molecular atomic charges, dipole moments and ionisation potentials are determined and compared with available experimental values.

Ab initioMO study of Fe out-of-plane displacement and Fe-ligand vibration in five-coordinate Fe-porphyrin

Ab initio SCF MO calculations were carried out for fluoro(porphinato)iron(III) in order to elucidate the origin of the out-of-plane displacement of the iron atom and to analyze vibrational modes of the axial ligand bond. Potential curves as functions of the Fe out-of-plane displacement and FeF bond distance were calculated for the ground state. By using these potential curves, the equilibrium iron displacement, FeF bond distance, and normal mode frequency of the FeF stretching vibration were evaluated. The values are in good agreement with experimental values within the differences 0.02 Å, 0.06 Å, and 50 cm−1, respectively. It has been confirmed that the Fe out-of-plane displacement is caused mainly by the Coulomb repulsion between porphine nitrogens and fluorine anion.

Ab initio SCF MO calculation on model compounds of doped polyacetylene

The electronic structure of alkaline metal doped polyacetylene (PA) is studied by the ab initio SCF MO calculation on the model compounds. It is found that two Li atoms coordinate to the polyene molecule and donate electrons to the LUMO of polyene. The results on model compounds, C 12H 14Li 2, showed that the triplet charge transfer state is more stable than the singlet state. The charge density in the HOMO of the polyene is strongly polarized, showing a model of the CDW state. Its polyene chain has a characteristic structure, half has a charged soliton structure and the other half shows little bond alternation. The model provides an atomistic view of the sliding and pinning of the CDW and the origin of paramagnetism in the polaron structure of the heavily doped PA chain.

Raman spectral studies on the alkaline metal doped polyacetylene

Raman spectra of alkaline metal doped polyacetylene are investigated at each stage of doping level for (CHNa y) x with y = 0 ∼ 0.16. Several stages in doping are recognized in the change of Raman spectra. Raman and EPR spectra of alkaline metal doped β-carotene are also studied as a model compound of polyacetylene. The correlation of these spectra together with the ab initio SCF MO calculation on the model compound, C 12H 14Li 2, showed that the structured of the heavily alkaline metal doped polyacetylene chain is composed of the array of the charged soliton type and the uniform type structures alternately in series.

Optimized Structures and Relative Stabilities of the Carboranes from Ab Initio Calculations

AbstractAb initio calculations at the STO‐3G level were performed on almost all of the possible isomers for the entire series of closo‐carboranes, C2Bn‐2Hn, 5 ⩽ n ⩽ 12. Geometry optimizations using the gradient method were also included in all calculations. We report here the relative energies obtained for the various isomers as well as the optimized structures. These calculations confirm our previous predictions of relative stabilities obtained from topological charge stabilization. Comparisons of our structures with those from experimental data provide us with a measure of reliability for bond distances obtained using ab initio SCF MO calculations at the STO‐3G level. Results from the geometry optimization substantiated the experimentally known fluxional behavior of the 8 and 11 atom polyhedra.

Reinvestigation of the molecular structures of some organo-sulfur compounds

The results of ab initio SCF MO calculations at the HF level on HSCH 2SH, CH 3SCH 2SH, ClCH 2CH 2SH and HSCH 2CH 2SH using two kinds of basis sets, 3-21G and 3-21G + d(O.8C + 0.6S + 0.7Cl) are presented. Reasonable geometrical parameters for these molecules were reproduced by the latter set. Valuable information on rotational isomerism of these molecules is proposed. The barrier heights from the gauche to the trans forms in relation to the rotation around the CS and OC bonds are analyzed. The role of hydrogen bonding in governing the stable molecular geometry is discussed for CH 3SCH 2SH and ClCH 2CH 2SH.

Structure and magnetic properties of AH 3 and AMe 3 inorganic radicals and radical ions (AAl −, Si, and P +) by ab initio calculation

Ab initio SCF-MO calculations are reported on the geometry and magnetic properties of some AH 3 and AMe 3 inorganic radicals and radical ions (AAl −, Si, P +). A polarized split-valence procedure was used to determine the optimum geometry in each case and we find little change in the degree of pyramidality upon methylation. The spin density distribution was characterized by calculations employing a basis set of double-zeta quality with polarization functions on all non-hydrogenic atoms. In the hydrides, p-functions were included on the hydrogens. The reassignment of a spectrum originally attributed to AlMe 3 − is supported. Instead we predict for the trimethylalane radical anion A‖ ( 27Al) ∼ 20.5 mT and A⊥ ( 27Al) ∼ 13.5 mT. In an isotropic spectrum a( 27Al)=17.0 ± 0.5 mT is expected. In addition some estimates are made of IR and Raman active vibrational frequencies for this unknown species.

ChemInform Abstract: MNDO Semiempirical and 4‐3lG ab Initio SCF‐MO Calculations of Heteroaromatic Compounds.

Chemischer InformationsdienstVolume 17, Issue 19 Physical Organic Chemistry ChemInform Abstract: MNDO Semiempirical and 4-3lG ab Initio SCF-MO Calculations of Heteroaromatic Compounds. E. FOS, E. FOSSearch for more papers by this authorJ. VILARRASA, J. VILARRASASearch for more papers by this authorJ. FERNANDEZ, J. FERNANDEZSearch for more papers by this author E. FOS, E. FOSSearch for more papers by this authorJ. VILARRASA, J. VILARRASASearch for more papers by this authorJ. FERNANDEZ, J. FERNANDEZSearch for more papers by this author First published: May 13, 1986 https://doi.org/10.1002/chin.198619082AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume17, Issue19May 13, 1986 RelatedInformation

Ab initio MO study of the geometries of four-coordinated zinc, copper and nickel acetylacetone complexes

Ab initio SCF MO calculations on the four-coordinated acetylacetone complexes of Zn(II), Cu(II) and Ni(II) have indicated that the conformation of the Zn complex is tetrahedral and those of the Cu and Ni complexes are square-planar which is in agreement with the experimental results. The metal—oxygen bond is predominantly ionic, and the main factor determining the conformation of the complexes is the interaction between the positive holes in the metal d shell and the negative charges on the oxygen ligand atoms.

Stretching force constants of the H 3N ⋯ LiF lithium bonded and H 3N ⋯ HF hydrogen-bonded complexes: Comparison study at the SCF level

Ab initio SCF-MO calculations were carried out on the H 3N ⋯ HF hydrogen-bonded and H 3N ⋯ LiF lithium-bonded complexes to obtain the energy surface, the equilibrium geometry and quadratic, cubic and quartic force constants associated with high and low vibrational modes in the triatomic model X—;Y—Z, where Y = H or Li.

Calorimetric,1H nuclear magnetic resonance, and molecular orbital studies of hydrogen bonding between peroxy acids and oxygen bases. Implications for mono-oxygen donation potential of peroxy acids

The enthalpies of formation of hydrogen-bonded complexes between peroxybenzoic acids and various oxygen bases, ΔH°, were obtained by calorimetry. The 1H n.m.r. shifts of the OOH absorptions, under conditions of complete peroxy acid complexation, were measured. ΔH° and ‘hydrogen-bond shifts’, ΔδOOH =δOOH(base)–δOOH(CCl4), were found to be sensitive to the acidity of the peroxy acid as well as the basicity of the oxygen base. A satisfactory linear correlation between ΔH° and ΔδOOH, expressed by the equation –ΔH°(kJ mol–1)=(7.59 ± 0.92)ΔδOOH +(7.35 ± 1.61), was found. Ab initio MO SCF calculations (STO-4G, 4–31 G*) were carried out for the peroxyformic acid–dimethyl ether complex and peroxyformate ion. The peroxy acid isotope effect for epoxidation of cyclohexene with 4-t-butylperoxybenzoic acid (OH/OD) in NN-dimethylacetamide (kH/kD1.09), together with the results of kinetic studies in other oxygen bases as solvents, suggest that oxygen base is most probably a part of an ‘early’, loose charge-transfer-type transition state for these reactions (‘direct attack mechanism’), and that reduced reactivity of peroxy acids in basic solvents most probably reflects lowered electrophilicity of the peroxy acid due to the forcing of the enlarged negative charge on the terminal oxygen atom after complexation.

Conformational analysis of the iron acetyl complex [(η 5-C 5H 5)Fe(CO)(PPh 3)COCH 3

A conformational analysis for the complex [(ν 5-C 5H 5)Fe(CO)(PPh 3)COCH 3] based on extended Huckel and ab initio SCF MO calculations is described which indicates that the conformational preference for the acyl oxygen is approximately anti periplanar to CO due to steric interactions between the acyl llgand and two of the phenyl groups of the triphenylphosphine.