Search results for "MNDO"
showing 10 items of 15 documents
Timolol derivatives. I. X-ray, NMR and theoretical studies of the crystallization of (S)-timolol O,O-diacetyl-l-tartaric acid monoester
1993
Abstract The absolute configurations of (S)-timolol hemihydrate and (S)-timolol O,O-diacetyl-(R,R)-tartaric acid monoester were determined by single crystal X-ray diffraction. An NMR analysis based on the temperature dependence of vicinal coupling constants was carried out to characterize the conformational behaviour of the S,R,R- and R,R,R-forms in solution. The same conformation as in crystalline state was also found in solution, although with a rather low preference over some other conformations. Results of theoretical calculations using MNDO and AMBER force field methods are reported. An infinite chain of hydrogen bonds, along with other favourable inter- and intramolecular forces that …
ChemInform Abstract: Spectroscopic and Theoretical Studies on the Internal Rotational Barrier of 1-Acetylcyclohexene
1987
Abstract MINDO-3 molecular orbital calculations with full geometry optimization have been carried out on 1-acetylcyclohexene (ACH) in order to estimate the rotational barrier for the acetyl group, and to localize the less energetic conformation. MNDO calculations have also been carried out for comparative purposes. Both methods point to the non-delocalized structure with the two π-clouds perpendicular to the less energetic conformation. A decomposition of the barrier energy by means of Fourier analysis is made in order to characterize the contributions to the barrier. The UV spectrum of ACH has been recorded both in vapor phase and in hexane solution. These spectra are related to the one ca…
Theoretical study of degenerate Boulton-Katritzky rearrangements. Semiempirical and ab initio procedures
1998
Abstract A theoretical study of degenerate Boulton–Katritzky rearrangements concerning the anions of the 3-formylamino-1,2,4-oxadiazole, 3-formylmethyl-isoxazole and 3-hydroxy-iminomethyl-1,2,5-oxadiazole has been carried out by using semiempirical MNDO and ab initio Hartree–Fock procedures. Different transition structures and reactive pathways were obtained in the two cases. Semiempirical treatment shows asymmetrical transition states and non-concerted processes via symmetrical intermediates. By contrast, ab initio procedures describe concerted and synchronous processes involving symmetrically-located transition states. Some comments and criticisms on the theoretical treatment of these typ…
The electronic substituent influence on the thermally allowed electrocyclic interconversion cyclobutene/butadiene. A MNDO study
1984
The effects of donor/acceptor substitution on the conrotatory cyclobutene/butadiene interconversion have not yet been experimentally studied. MNDO Synchronous Transit/MINIMAX calculations for all possible combinations of NH2/NH3 + 3 groups at the fissile single bond reveal very large substituent effects on the opening reactions and reverse cyclisations. Diamino(1b/2b), diammonio-(1c/2c) and amino-ammonio-substitutions (1d/2d) change the activation energy of the reference ring opening reaction (1a →2a) from 49.9 kcal/mol to 34.7, 29.8 and 26.0 kcal/mol, respectively. This rate enhancement parallels the monotone shift of the transition state pathcoordinate towards the cyclobutenes. Relative t…
9‐Oxabicyclo[6.1.0]nonine
1989
Semiempirische quantenmechanische Rechnungen (MNDO) und Kraftfeldrechnungen (MM2) zeigen, das fur die drei konstitutionsisomeren cis-9-Oxabicyclo[6.1.0]nonine 1–3 jeweils zwei energiearme, zueinander diastereomere Konformere a und b existieren. Der synthetische Zugang zu den hoch gespannten Verbindungen 1–3 gelingt mit der Selenadiazolmethode oder durch Dehydrobromierung (Schemata 3, 4 und 6). Die NMR-spektroskopische Konformationsanalyse bestatigt bei 2 das vorhergesagte Konformerengleichgewicht; bei 1 gehen selbst bei –60°C die beiden Konformeren durch Umklappen des Achtrings schnell ineinander uber, und bei 3 ist praktisch nur eine Konformation populiert. 9-Oxabicyclo[6.1.0]nonynes Semie…
1‐Oxa‐3‐cyclooctin
1990
1-Oxa-3-cyclooctyne According to an MNDO calculation, 1-oxa-cyclooctyne (13) and 1-oxa-3-cyclooctyne (14) are 8-membered heterocyclic ring systems with high steric energy (ring strain). On the basis of THF (1) and acetoacetic ester (4), a multi-step synthesis is described for 14. The triple bond is introduced in the last step by a thermal fragmentation of the 1,2,3-selenadiazole 12.
Thermische Cycloadditionen von 1,2‐Dihydropentalen
1989
1,2-Dihydropentalen (1) ist ein gekreuzt konjugiertes 6π-Elektronensystem, das in thermischen Cycloadditionen als 2π-, 4π- oder 6π-Komponente fungieren kann. Es werden die Additionen an Tetraphenylcyclopentadienon (1+23), N-Phenylmaleinimid (1+45) und Tropon (1+68 bzw. 10) beschrieben. Auf der Basis von MNDO-Rechnungen werden Peri-, Regio- und Stereoselektivitat anhand von primaren und sekundaren Orbitalwechselwirkungen diskutiert. Thermal Cycloaddition of 1,2-Dihydropentalene 1,2-Dihydropentalene (1) is a cross-conjugated 6π-electron system, that can act in thermal cycloaddition processes as a 2π, 4π, or 6π component. The addition reactions with tetraphenylcyclopentadienone (1+23), N-pheny…
Analyse und Optimierung von Gasphasen‐Reaktionen, 18 1,2) Die Pyrolyse von Methylazid
1981
Die PE-spektroskopisch analysierte Pyrolyse von Methylazid verlauft bei 850 K und liefert als Hauptprodukt Methanimin, welches nach einer MNDO-Singulett-Hyperflache betrachtlich stabiler sein sollte als das tautomere Methylnitren. Analysis and Optimization of Gasphase Reactions, 181,2) The Pyrolysis of Methyl Azide The PE spectroscopically analyzed pyrolysis of methyl azide occurs at 850 K and yields as main product methanimine, which according to a MNDO singlet hypersurface should be considerably stabilized relative to the tautomeric methyl nitrene.
Semi-empirical MNDO study of the structure of some diorganotin (IV) glycylglycinates
1993
The geometrical structures of some diorganotin glycylglycinates have been obtained using the MNDO semi-empirical method. A good agreement with the experimental diffractometric data was found.
Bicyclo[6.1.0]nonine
1988
MNDO-Rechnungen zeigen, das fur die drei konstitutionsisomeren cis-Bicyclo[6.1.0]nonine (1–3) jeweils zwei energiearme, zueinander diastereomere Konformere a und b existieren. Zur experimentellen Uberprufung werden die mit der Selenadiazol-Methode oder durch Dehydrobromierung hergestellten, hoch gespannten Bicyclen mit Hilfe von 1H- und 13C-NMR-Messungen auf die Population der Konformeren und auf deren wechselseitige Umwandlung durch Inversion der Achtringe untersucht. Bicyclo[6.1.0]nonine MNDO calculations reveal that two diastereomeric conformations a and b of low energy exist for each of the three isomeric cis-bicyclo[6.1.0]nonynes (1–3). For the experimental proof, the highly strained b…