Ab Initio STO-3G Calculations Research Articles

Semi-empirical and ab initio study of pericyclized naphthalene systems containing an eight-membered ring

The conformational properties of peri-bridged naphthalenes in which the C-1 and C-8 atoms of the naphthalene ring are bonded by five atoms in a bridging chain Z [Z=(CH2)5, 1a; (CH2)2O(CH2)2, 1b; (CH2)2S(CH2)2, 1c; CH2OCH2OCH2, 1d; CH2SCH2SCH2, 1e; O(CH2)3O, 1f; S(CH2)3S, 1g] were studied by the Modified Neglect of Diatomic Overlap (MNDO) semi-empirical SCF MO method. Two minimum-energy conformations, namely chair and boat, with similar energies were found for 1a–c and 1e–g, which are separated by fairly low (3.5–6.2kcalmol−1) energy barriers. The barriers for chair-to-chair ring inversion in these compounds are 9.7–12.4kcalmol−1. For compound 1d, only the chair conformation was found to have the lowest energy; the twist and boat conformations are less stable by 1.1 and 1.4kcalmol−1, respectively. Ab initio STO-3G calculations were carried out for 1a.

Nonempirical wave functions for very large molecules. II. The PRDDO/M/FCP method

The authors describe enhancements to the method of partial retention of diatomic differential overlap (PRDDO). The new method, denoted PRDDO/M, employs a basis set of not quite orthogonal atomic orbitals (NQOAOS) and utilizes sparse matrix techniques to greatly increase the computational efficiency for large molecules. Other modifications, including a complete reparametrization of the method against ab initio STO-3G calculations and implementation of integral screening/damping algorithms, are described. The method is an order of magnitude or more faster than are STO 3G single-point calculations using modern ab initio codes, with little loss in accuracy. 15 refs., 5 figs., 6 tabs.

A theoretical study of protonation and tautomerization of N-substituted aminoazobenzenes

Semiempirical AM1 and ab initio SCF STO-3G calculations with full geometry optimization were performed on aminoazobenzene (AAB) and its N-methyl (MAAB), N,N-dimethyl (DMAAB), and N-phenyl (PhAAB) derivatives, as well as their azonium and ammonium conjugated acids. AM1 calculations were also performed on hydrated cationic acids, in order to estimate the effect of amphiprotic solvents on tautomerization. In all the cases studied but DMAAB the AM1 and STO-3G proton affinity of the azo nitrogen was definitely higher than that of the amino nitrogen. For the amino nitrogen the calculated proton affinity was found to increase in the series AAB < MAAB < PhAAB < DMAAB. The calculated proton affinity of the azo nitrogen increased in the same order with the exception of the STO-3G results of PhAAB. The tautomerization energy/enthalpy (i.e. difference of the gas-phase proton affinity of the azo and the amino nitrogen) was found to increase in the series DMAAB < MAAB < AAB, the position of PhAAB in the series depending on whether the AM1 or STO-3G method was used. These results contradict the experimental data regarding aminoazobenzene protonation and tautomerization constants determined in various solvents, which indicates a strong effect of solvation on the protonation and tautomerization equilibria of aminoazobenzenes. AM1 calculations on hydrated cationic acids showed that solvation effects can be satisfactorily accounted for by enthalpy contributions in the case of tautomerization, because the order of tautomerization constants determined in methanol and dioxane–water mixture generally conforms with the order of tautomerization enthalpies with hydration included. However, the estimated proton affinities in water are still ranked in the same order as in vacuo which may indicate that entropy contributions play a much greater role in the case of protonation than in the case of tautomerization phenomena.

Application of resonance theory to fullerene hydride (C60H and C60H2)

Results from ab initio STO-3G calculations on C 60 and C 60 H are presented and compared with recent MNDO-PM3 calculations. These results, and those from recent MNDO-PM3 calculations on C 60 H 2 , are interpreted using resonance theory

Graph theoretical analysis of water clusters

A graph lbooretical representadon of the hydrogen bond network in a water cluster is developed using the concept of directed graphs. Ab initio STO-3G calculations on a large collection of small linear and branched, open chain and monocyclic water clusters are presented. The invariants of the digraph representations are utilized for a statistical analysis of the ab initio results, and this procedure provides physically meaningful factorizations of the hydrogen bond energies. The usefulness of a graphical analysis of this type is further demonstrated by appJying it to (i) the analysis of a previous energy data set (MCY potential/many-body corrections) for highly condensed clusters of water Molecules and (ii) the prediction of the cohesive energy of ice. The graph theory, parameters based on the many-body calculation results give an ice cohesive energy in good agreement with experiment.

On the conformation of the inversion state in the thermal E,Z isomerization of aromatic azomethines

Ab initio STO-3G calculations with full geometry optimization on substituted formaldehyde-, pyrazolone- and hexafluoroacetone-anils indicate different conformations of the linear inversion state in the thermal E,Z isomerization of aromatic azomethines dependent on the extent of push-pull conjugation in the molecular systems; based on this fact, an explanation of Hammett plot deviations from linearity found for special azomethines becomes possible without the assumption of a competition between the inversion and rotation mechanism in the E,Z isomerization.

Reference conformations for calcium antagonists and agonists of dihydropyridine type

Ab initio STO-3G calculations on model compounds for calcium antagonists and agonists of dihydropyridine type were performed in order to obtain insight into the importance of conformational factors for the structure and activity of these molecules. Employing geometry optimization, structure and energy data obtained in this way may serve as a reference point for the discussion of the problems of planarity/non-planarity of the dihydropyridine ring, ester group orientation in the dihydropyridine ring, orientation of a phenyl group in the 4-position of the dihydropyridine ring, the conformation of ortho substituted 4-aryl-dihydropyridines, and the conformational flexibility of the subsystems of these calcium-channel influencing drugs.

Theoretical study of the dependence of the valence electronic levels on the tacticity and conformation of acrylonitrile model oligomers

With the aim of searching for signatures of conformation and tacticity in electrodeposited polyacrylonitrile valence XPS spectra, exploratory ab initio STO-3G calculations have been performed on four model structures of H—[CH2—CH(CN)]6—H: isotactic and syndiotactic all-trans planar chains and isotactic and syndiotactic (TG)3 helical chains. Contrary to what was assumed in earlier work it is found that the (TG)3 conformation is not a suitable model helix for polyacrylonitrile. Signatures of conformation and tacticity are noted in the ab initio simulated valence XPS spectra; their characteristic features (position on the energy scale, shape and intensity) are within the limits of experimental resolution.

On the zwitterionic nature of strongly twisted push-pull ethylenes: the case of 3-(1,3-diisopropyl-2-imidazolidinylidene)-2,4-pentanedione monohydrate, studied by x-ray diffraction and ab initio calculations

In the crystal structure of the title compound ( 1), a formally double bond 1.475(3) Å long, and a twist angle about it as large as 84.2(2)°, are observed. These geometrical parameters, compared with those reported in the literature for the related molecules 3-(1,3-dimethyl-2-imidazolidinylidene)-2,4-pentanedione trihydrate ( 2), and 3-(2-imidazolidinylidene)-2,4-pentanedione ( 3) indicate a stronger push-pull effect in compound ( 1). Group charge distributions derived from ab initio STO-3G calculations on molecules ( 1) and ( 2), and based on the non-optimized, experimental geometries, reveal the zwitterionic nature of these substituted ethylenes. In molecule ( 1), a negative charge of 0.638 e on the pentanedione fragment, and a positive charge of 0.156 e on the imidazolidine ring are calculated, with the remaining positive charge almost equally distributed on the two isopropyl groups. A formal negative charge of 0.255 e is given to the ethylenic carbon atom belonging to the pentanedione moiety, and a positive charge of 0.363 e to the other. Relevant bond properties derived from a topological analysis of the theoretical electron density ϱ are discussed; both the position of the critical point of the CC bond and the corresponding positive value of the Laplacian of ϱ confirm the highly polar character of the ethylenic fragment.

2-R-2-oxo-1,3,2-dioxaphosphorinan - II - electronic structure of the ground state and acido-basic properties.

We have studied the electronic structure of the ground state and the aci-do-basic properties of 2-R-2-,oxo-1,3,2-dioxaphosphorinans by means of MNDO ab-initio STO-3G and STO-3G calculations. MNDO and STO-3G results are qualitatively equivalent but both fail to describe the structural indices of the P atom, or of the bonds involving this atom. This is mainly due to the fact that these methods give no consideration to the 3d AO. The basic characters of these compounds (through the phosphoryl group) are not negligible but are weak in contrast to their acid character which can be very high when a withdrawing group is bound to the P atom.

ChemInform Abstract: Tautomerism in Alloxan.

ChemInformVolume 18, Issue 46 Physical Organic Chemistry ChemInform Abstract: Tautomerism in Alloxan. S. MILLEFIORI, S. MILLEFIORI Lehrstuhl angew. Phys., Univ. Koeln, D-5000 KoelnSearch for more papers by this authorA. MILLEFIORI, A. MILLEFIORI Lehrstuhl angew. Phys., Univ. Koeln, D-5000 KoelnSearch for more papers by this author S. MILLEFIORI, S. MILLEFIORI Lehrstuhl angew. Phys., Univ. Koeln, D-5000 KoelnSearch for more papers by this authorA. MILLEFIORI, A. MILLEFIORI Lehrstuhl angew. Phys., Univ. Koeln, D-5000 KoelnSearch for more papers by this author First published: November 17, 1987 https://doi.org/10.1002/chin.198746043AboutPDF 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 onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume18, Issue46November 17, 1987 RelatedInformation

Finite hydrocarbon chains incorporating cumulenic structures: prediction by ab initio calculations of their equilibrium geometry and electric polarizability

Using ab initio STO-3G calculations, we compute and analyze the equilibrium geometry and the electric polarizability of five series of polycumulenes H2—[—Ck—]x—H2, k = 2–6, and x = 1–3. The importance of structural relaxations depends on the parity of k, and the average polarizabilities, calculated with the finite-field method, are significantly influenced by these structural changes. The trends in polarizability are briefly discussed in terms of the optical gap and the dipole transition moment.

A conformational basis for the description of the thermal E/Z isomerization of aromatic azo and azomethine compounds

Based on ab initio STO-3G calculations with geometry optimization, a general conformational basis for the inversion mechanism of thermal E/Z isomerization of aromatic azo and azomethine compounds is provided. The structure of the E,Z isomers and the inversion transition states is determined. The influence of substituents on conformation and inversion is discussed.

Preliminary theoretical search for possible conformational signatures in the valence X-ray photoelectron spectra of ordered polyethylene surfaces

With the aim of searching for conformational signatures in polymer valence XPS spectra, exploratory ab initio STO-3G calculations have been performed on three conformations of n-C13H28. These structures are intended to model ideal polymer surfaces rich either in folds emerging perpendicularly to the basal plane or in zig-zag planar chains oriented parallel to the surface. The simulated spectra based on these calculations indicate that there is limited hope of relating valence XPS spectral features to conformational changes in saturated hydrocarbons.

On the conformation of bipyridine dications and cation radicals

Ab initio STO-3G calculations with partial geometry optimization were performed to determine the conformational structure of 2,2′- and 4,4′-bipyridine dications and their cation radicals. The results suggest a considerably twisted conformation for the dications, whereas the corresponding radical cations are nearly planar. Reasons for the structure differences are discussed and related to the herbicidal action of these compounds.

A conformational analysis of 4,4'-bipyridinium dication

A conformational analysis of the 4,4'-bipyridinium dication has been investigated using the results from 10 ab initio STO-3G calculations in which the inter-ring bond length was optimized at each torsion angle. The most reliable calculation yields: a lowest energy conformer with an equilibrium torsional angle of 45.3° and inter-ring ( R c-c) bond length of 0.152 nm; a barrier height of 5.86 kJ mol −1. An examination of the fitted Fourier series indicates that half of the theoretical data points effectively reproduced the more extensive surface. Furthermore, a comparison of the calculations and fitted surfaces indicate that a rigid rotor is an excellent approximation for this molecule.

Bond-bond pair calculations of rotational barriers

Ab initio STO-3G calculations of rotation barriers in six single-rotor molecules have shown the inadequacy of the Mulliken approximation in the Surján formula which expresses barriers as a sum of bond-bond pair interactions. Following our early work, expansion to order S 2 followed by pair delocalization is suggested as a possible alternative to semi-empirical advance.

The 9-barbaralyl cation, 1,4-bishomotropylium ion and some related C 9H 9+ carbocations : Electronic structures, stability and reactivity

The electronic structures of the 9-barbaralyl cation( 1), the 1,4-bishomotropylium ion ( 5) and some related C 9H 9 + isomers have been studied by ab initio STO-3G calculations. The stability of the cyclopropylcarbinyl cation 1 and the homoaromatic ion 5 as compared with their allylic counterparts is explained in terms of delocalization and MO interactions between molecular fragments. Both the symmetry of, and the distance between, molecular fragments are of importance for the electronic structure, and even symmetry-allowed interactions are negligible in the absence of favourable geometrical distortions. The non-classical ion 3 with D 3h symmetry presents special problems where other factors, such as strain, are important for the total energy. A choice between proposed mechanisms for the degenerate rearrangements of 1 can now be made with confidence.

A study of electronic structures of some C 9H 9- carboanions

The electronic structures of five C 9H 9 -, carboanions have been studied by ab initio STO-3G calculations, and some general conclusions on related C 9H 9 - and C 9H 9 + structures are presented. Large antibonding interactions in one occupied MO make barbaral-9-yl anion ( 2) unstable as its cationic counterpart ( 8). The proposed D 9h-symmetrical cation and D 3h-symmetrical anion ( 3) do not exist due to Jahn-Teller distortions. A study of the MO correlations confirms that the two tetracyclic anions with C 2v symmetry ( 5 and 6) are the results of the Jahn-Teller distortions of 3. Anion 5 is identified as the proper intermediate of the Cope rearrangement of anion 2.

ChemInform Abstract: OVERTONE SPECTRAL INVESTIGATION OF SUBSTITUENT‐INDUCED BOND‐LENGTH CHANGES IN GAS‐PHASE FLUORINATED BENZENES AND THEIR CORRELATION WITH AB INITIO STO‐3G AND 4‐21G CALCULATIONS

Chemischer InformationsdienstVolume 15, Issue 33 Physical Organic Chemistry ChemInform Abstract: OVERTONE SPECTRAL INVESTIGATION OF SUBSTITUENT-INDUCED BOND-LENGTH CHANGES IN GAS-PHASE FLUORINATED BENZENES AND THEIR CORRELATION WITH AB INITIO STO-3G AND 4-21G CALCULATIONS K. M. GOUGH, K. M. GOUGHSearch for more papers by this authorB. R. HENRY, B. R. HENRYSearch for more papers by this author K. M. GOUGH, K. M. GOUGHSearch for more papers by this authorB. R. HENRY, B. R. HENRYSearch for more papers by this author First published: August 14, 1984 https://doi.org/10.1002/chin.198433045AboutPDF 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. Volume15, Issue33August 14, 1984 RelatedInformation