Search results for "Fig"

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 …

Role of the central gold atom in ligand-protected biicosahedral Au 24 and Au25 clusters

2013

The crystal structures of the ligand-protected clusters [Au24(PPh3)10(SC2H4Ph)5Cl2]+ and [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ have been elucidated recently, and they comprise the same biicosahedral structural motif for the Au core. The only difference is the central Au atom joining two icosahedra which is absent in the Au24 cluster. On the basis of density functional simulations, we have evaluated the structural, electronic, optical, and vibrational properties of the clusters in question with a full presentation for the thiolate and phosphine side groups. Our spherical harmonics analysis of the electronic structure shows that the chemical stability of both clusters can be understood based on an 8 …

The Nature of Transannular Interactions in E4N4 and E82+ (E = S, Se)

2015

The electronic structures of tetrachalcogen tetranitrides, E4N4, and octachalcogen dications, E8(2+), and the nature of their intramolecular E···E interactions (E = S, Se) was studied with high-level theoretical methods. The results reveal that the singlet ground states of both systems have a surprisingly large correlation contribution which functions to weaken and therefore lengthen the cross-ring E-E bond. The observed correlation effects are primarily static in E4N4, whereas in E8(2+) the dynamic part largely governs the total correlation contribution. The presented description of bonding is the first that gives an all-inclusive picture of the origin of cross-ring interactions in E4N4 an…

Linear scaling exchange gradients for Hartree–Fock and hybrid density functional theory

2000

Abstract A new method is introduced which allows to form the exchange part of self-consistent field energy gradients in a way which scales linearly with molecular size for molecules with a non-vanishing HOMO–LUMO gap. This is not only important for Hartree–Fock but as well for the popular hybrid density functional theory methods. The method preserves the optimized integral-driven structure of conventional direct methods. Efficiency and scaling behavior of the new method are illustrated for DNA fragments.

Studien zum Vorgang der Wasserstoffübertragung, 57 Einige Beispiele zur diastereoselektiven Hydrierung von Verbindungen mit prochiralen CC-Bindungen

1979

(1R,2S)-N,N-Dimethylephedriniumchlorid, (1S,2S)-N,N-Dimethylpseudoephedriniumchlorid, (1R,2S)-N-Methylephedrin, (1S,2S)-N-Methylpseudoephedrin und [(2S)-2-Hydroxy-2-phenyl-ethyl]trimethylammoniumchlorid werden mit α- und β-Methylzimtsaure zu 1–4 und 7 verestert und an Raney-Nickel, Palladium, Platin und einer Reihe von Edelmetalltragerkatalysatoren hydriert. Die hydrierten Ester werden verseift und die optische Ausbeute der α- bzw. β-Methyldihydrozimtsaure bestimmt. Die optischen Ausbeuten bewegen sich in Abhangigkeit von der optisch aktiven Hilfskomponente zwischen 68 und 4%. Bei optischen Ausbeuten uber 10% wird die absolute Konfiguration der α- oder β-Methyldihydrozimtsaure von der Konfi…

Application of the open-shell size-consistent self-consistent singles and doubles configuration interaction method to multielectronic transitions in …

2000

The size-consistent self-consistent matrix dressing method has been applied on an open-shell single-configuration reference state. Once the reference state is converged, several low-lying roots can be obtained for the dressed configuration interaction (CI) matrices of appropriate symmetry. The CI matrices were built with a complete-active-space singles and doubles CI method in order to deal properly with multiconfiguration excited states. The vertical ionization and ionization–excitation transitions are obtained from the difference to the closed shell ground-state energy of the neutral molecule. The method has been applied to NH+3 and N+2 using atomic natural orbital basis sets and state-av…

Pterocarpol and triterpenes from Daemonorops draco

1981

Struktur-Wirkungs-Beziehungen bei Histaminanaloga, 24. Mitt. Absolute Konfiguration und histaminartige Wirkung der Enantiomere von 4-(2-Aminopropyl)-…

1982

Es werden die Racematspaltung von 4-(2-Aminopropyl)-5-methyl-imidazol (1), die Bestimmung der absoluten Konfiguration sowie die histaminartige Wirksamkeit der Enantiomere am Ileum (H1) und Atrium (H2) des Meerschweinchens beschrieben. Structure-Activity Relationships of Histamine Analogues, XXIV: Absolute Configuration and Histamine-Type Activity of the Enantiomers of 4-(2-Aminopropyl)-5-methylimidazole The resolution of 4-(2-aminopropyl)-5-methylimidazole (1), the determination of the absolute configuration and the histamine-type activity of the enantiomers on the guinea pig ileum (H1) and atrium (H2) are reported.

Synthesis of spongian diterpenes: (−)-spongian-16-oxo-17-al and (−)-acetyldendrillol-1

2001

Abstract An efficient diastereoselective synthesis of the spongian diterpenes (−)-spongian-16-oxo-17-al ( 1 ) and (−)-acetyldendrillol-1 ( 13 ) is described starting from (+)-podocarp-8(14)-13-one ( 6 ) via the ester-dialdehyde 11 as key intermediate. The absolute configuration at C-17 in synthetic compound 13 has conclusively been proved by NOE experiments.

Extension of Walsh's rules to more general systems

1972

It is demonstrated that location of hydrogen atoms within bonding distance of a simple triatomic molecule can alter the order of energy levels relative to that of the parent system without significantly changing the behavior of the orbital energies under geometrical variation. As a result the ground state electronic configurations of H n AB2 systems often differ from those of triatomics with the same number of electrons and this fact is shown to be directly responsible for the existence of such distinctive geometrical structures as those possessed by cyclopropane, cyclopropyl and allyl cations and cyclopropene. Thus the familiar prescription of Walsh's rules which states that isoelectronic …