Ab Initio STO-3G Research Articles

Experimental and theoretical studies of the effects of hydration on proton exchange equilibria

Theoretical calculations with the ab initio molecular orbital STO-3G basis set on the effect of trihydration on the relative acidities of 3- and 4-substituted phenols, 4-substituted quinuclidinium ions and 4-substituted bicyclo [2.2.2]octylammonium ions are reported. These results are contrasted with calculations on non-hydrated species and compared with results in aqueous solution and in the gas phase. Unlike results for smaller molecules such as substituted acetic acids and methylammonium ions, the solvation effects of three molecules of water are far short of that observed in going from the gas phase to aqueous solution. Reasons for this are discussed in the context of solvent attenuation factors for various molecules.

About the mutagenicity of chlorine-substituted furanones and halopropenals. A QSAR study using molecular orbital indices

Electron affinities, frontier molecular orbital energies and electron densities at individual carbon atoms were calculated for 11 chlorofuranones including the strong mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5 H)-furanone (MX) and for 5 halopropenals by semi-empirical AM1 and ab initio STO-3G methods. Significant correlations were found between Ames TA100 mutagenicity and the following AM1 electronic parameters of chlorofuranones: electron affinity ( r = 0.9556), LUMO energy ( r = 0.9332) and frontier electron density of LUMO at the α-carbon ( r = 0.8882). In halopropenals only LUMO electron density at the β-carbon correlates well with mutagenicity. The observed correlations suggest a reaction mechanism in which chlorofuranones and halopropenals act as electron acceptors in the interaction with DNA.

A molecular-orbital study of the structure-activity relationships of some imidazolidines related to clonidine

Ab initio STO-3G MO calculations were performed for models of twenty imidazolidines related to clonidine. Regression analyses are reported which include electronic factors as well as parameters relating to molecular transport and size. Experimental data for the central hypotensive activity in normotensive rats following intravenous administration were recalculated with corrections for the amounts of basic and protonated forms and for the concentration of drug reaching the brain. The most important electronic parameter in the regressions reported is the LUMO energy which appears in the regressions considered with a positive coefficient. It is suggested that a charge-transfer interaction occurs involving the basic forms of the drug molecules, which leads to a reduction in hypotensive activity. An increase in hypotensive activity appears to follow an ionic interaction of the protonated drug species.

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.

ChemInform Abstract: Determination of the Electronic Structure of Thiophene Oligomers and Extrapolation to Polythiophene

ChemInformVolume 21, Issue 46 Physical Organic Chemistry ChemInform Abstract: Determination of the Electronic Structure of Thiophene Oligomers and Extrapolation to Polythiophene D. JONES, D. JONES ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this authorM. GUERRA, M. GUERRA ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this authorL. FAVARETTO, L. FAVARETTO ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this authorA. MODELLI, A. MODELLI ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this authorM. FABRIZIO, M. FABRIZIO ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this authorG. DISTEFANO, G. DISTEFANO ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this author D. JONES, D. JONES ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this authorM. GUERRA, M. GUERRA ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this authorL. FAVARETTO, L. FAVARETTO ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this authorA. MODELLI, A. MODELLI ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this authorM. FABRIZIO, M. FABRIZIO ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this authorG. DISTEFANO, G. DISTEFANO ICoCEA, Cons. Naz. Ric., 40064 Ozzano Emilia, Bologna, ItalySearch for more papers by this author First published: November 13, 1990 https://doi.org/10.1002/chin.199046056Read the full textAboutPDF 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. Volume21, Issue46November 13, 1990 RelatedInformation

Quantum-mechanical oligomer approach for the calculation of vibrational spectra of polymers

The force-constant matrix of an oligomer, composed of five or more repeat units with appropriate terminal groups, can be used to construct the force-constant matrix of the corresponding planar or helical polymer. The calculations on some typical polymers (polymeric sulfur, polyacetylene, and polyethylene) show that the oligomer approach is accurate enough to duplicate the vibrational frequencies of frozen-phonon calculations. The oligomer approach has much more predictive power if used in conjunction with some form of empirical scaling (scaled quantum-mechanical oligomer force field for polymers). ir and Raman selection rules for helical polymers are also discussed. Vibrational frequencies and intensities of polymeric sulfur and polyethylene have been calculated at the ab initio STO-3G, 4-31G, 6-31G, and 6-31G* basis-set levels. The agreement of frequencies with experiment is excellent. The quality of calculation is limited by the basis set and theoretical model used rather than the oligomer approach.

Determination of the electronic structure of thiophene oligomers and extrapolation to polythiophene

Ionization energies, attachment energies, and electrochemical reduction potentials of thiophene oligomers (n {le} 5) have been determined experimentally (ultraviolet photoelectron and electron transmission spectroscopies and cyclic voltammetry) and theoretically (ionization and attachment energies by MINDO/3). The geometrical parameters of the most stable conformation of 2,2{prime}-bithienyl have been computed at the ab initio STO-3G level with complete relaxation. A short extrapolation of the energy data to the polymer provided accurate and reliable values for important properties of (gas phase) polythiophene, namely, ionization energy (6.9 eV), valence bandwidth (3.2 eV), electron affinity (0.9-1.1 eV), HOMO-LUMO energy gap (5.9 eV), and {lambda}{sub max} (2.7 eV).

Structures of dinitrosobenzenes and 1,2-dinitrosoethylenes

The structures of o-, m-, and p-dinitrosobenzenes have been studied at the ab initio STO-3G and 3-21G basis levels. There is no evidence for any additional effects compared to nitrosobenzene as a model. A study of 1,2-dinitrosoethylenes gave similar results, contrary to a report by earlier workers

Acidities of ortho substituted phenols

Theoretical calculations have been made for the relative acidities of ortho substituted phenols at both the ab-initio STO-3G and 3-21G bases. The agreement with experimental results is similar to that found with meta or para substituted phenols, indicating clearly the usefulness of such calculations in ortho substituted benzenes. Interestingly, the results of calculations over a range of ortho substituents are linear against those for the corresponding para substituents, suggesting that steric effects are unimportant in determining the relative acidities of ortho substituted phenols.

Theoretical description of the conformation and the tautomerism of progabide

A theoretical description of the conformation and tautomerism of the progabide molecule was performed employing the ab initio STO-3G and 3-21G basis sets and a quantum chemical reaction field model for the consideration of the solvent influence. The calculations indicated a conformation with planar orientation of the A ring and considerable rotation of the B ring with respect to the imine bond as the most stable conformation. The consequences for conformation of hydrogen bond formation and breaking were examined. Predominance of the aromatic tautomer in apolar and the quinonoid tautomer in polar solvents is predicted.

The microwave and far-infrared spectra and the geometry of 8-oxabicyclo[3.2.1]octane

The structure of 8-oxabicyclo[3.2.1]octane was calculated by abinitio means at the STO-3G level, and the structural data used to assign the microwave spectrum of the title compound in the range of 26.5 to 40 GHz fitting 144 lines up to J = 48 by adjusting the rotational and three centrifugal distortion constants in the Watson A reduction. Structural parameters were deduced from the rotational constants by allowing all bond lengths and angles to vary in a least-squares manner. These parameters were in good agreement with those calculated at the STO-3G and 3-21G levels. Of many satellite lines observed, only the most intense could be utilized for locating the two lowest fundamental vibrations, estimated to occur at 190 and 240 cm−1. The far-infrared spectrum is reported in the region of 80 to 300 cm−1, exhibiting only one distinct sequence of Q branches at 264 cm−1. On the basis of unsealed abinitio force fields at both levels of the theory the corresponding fundamentals are assigned, respectively, to a twisting (A″) and a bending (A′) of the cycloheptane ring. Keywords: 8-oxabicyclo[3.2.1]octane, microwave and far-infrared spectra, abinitio STO-3G and 3-21G geometries.

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.

Stability of substituted pyrylium salts and the corresponding 2- H- and 4- H-pyrans

The stabilities of a series of 4-substituted pyrylium salts have been compared with those of the correspondingly substituted benzenes using the ab initio STO-3G basis set. It is shown that marked field and resonance effects operate in the pyrylium salts covering a range of more than 50 kcal mol −1 compared to the benzenes. The corresponding 4- H- and 2- H-pyrans that would be produced by hydride ion attack have also been studied, the 2- H-pyrans usually being the more stable. Similar substituent effects occur in these equilibria. The relative stability of the parent 2- H- and 4- H- pyrans is discussed, the 4- H-pyran being the preferred structure at the STO-3G, 3-21G and 6- 31G ∗ bases.

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.

Conformation of N-acetyl- l-pro- d-ala- N-methyl tripeptide empirical, semi-empirical MO and AB initio MO calculations

Conformational energies of the title peptide calculated by the empirical program packages GROMOS, CHARMM and DISCOVER (the latter two also with the AMBER force field), semiempirical MO scheme AMI, and the ab initio STO-3G basis set are compared. The lowest energies were obtained for either of the C7C7 (axial and equatorial) conformations except by CHARMM with incorporated AMBER force field, by which the βII conformation is preferred. The relative energy differences of these three conformations are within 3–5 kcal mol −1. Energy minimizations of the peptide surrounded by 121 SPC type water molecules give the lowest energy of hydration for the βII bend. The calculated energies are compared with the results of experimental conformational analysis.

Isotope Effects and Concerted Two-Hydron-Transfer between Formamidine Molecules

With the intention to assist mechanistic studies of two-hydron-transfer reactions using 1H/2H isotope effects we have carried out theoretical studies of the two-hydron-transfer reaction between formamidine molecules. The ab initio STO-3G potential energy surface has been calculated and all stationary points characterized by their force constant matrices. With this basis set the two hydrons are transferred concertedly and synchronously. The transition state (TS) has D2h symmetry. 1H/2H isotope effects were calculated using the Bebovib-IV program. Our results deviated from predictions made from common simple vibrational models. The isotope effects are dominated by the zero-pointenergy contribution. The effect due to the stretching vibrations of the initial state is to a small extent counteracted by stretching of the TS. The bending vibrations appear to play a dominant role in reducing the magnitude of the isotope effect for the two-hydron-transfer reaction to a value expected for a one-hydron-transfer reaction.

Proton affinities of substituted methylamines

Molecular orbital calculations at the ab initio STO-3G, 3-21G, 6-31G and 6-31G ∗ (5D) bases have been performed for the relative proton affinities of an extended series of substituted methylamines, XCH 2NH 2. The minimal basis STO-3G calculations give closest agreement with experiment averaged over all substituents for which comparisons are possible. However, for alkyl and fluoroalkyl substituents, the 6-31G *//6-31G * lead to the lowest mean deviation.

A scale of variable π-electron transfer

The response of ten common substituents to varying π-electron demand has been examined by calculations at the ab initio STO-3G basis. The system used was XCHCH-CH 2 + or XCHCH-CH 2 −, with variation in the length of the C-C single bond. The results allow the quantification of resonance effects for the substituents.

5]Metacyclophane: ab initio STO-3G molecular structure and strain energy

The molecular structure and strain energy of [5]metacyclophane (1) have been calculated with the ab initio STO-3G minimal basis set. The results are compared with previously reported theoretical data of (1) obtained with molecular-mechanics calculations (MM) and the semi-empirical MNDO method, and the X-ray structure determination of 8,11-dichloro[5] metacyclophane (2). Despite quantitative differences the STO-3G results confirm the MNDO results qualitatively. In comparison with the available experimental data the calculated STO-3G structure of the bent benzene ring shows better agreement. The bent benzene ring in the STO-3G structure possesses essentially delocalized aromatic carbon–carbon bond lengths. Although the calculated STO-3G strain energy [Es(tot)] is larger than either the MM or MNDO value, the distribution of Es(tot) over the bent benzene ring [Es(bb)] and the oligomethylene bridge [Es(br)] is similar.