Molecular structure calculations without clamping the nuclei

A number of recent papers have considered ways in which molecular structure may be calculated when both the electrons and the nuclei are treated from the outset as quantum particles. This is in contrast to the conventional approach in which the nuclei initially have their positions fixed and so merely provide a potential for electronic motion. The usual approach is generally assumed to be justified by the 1927 work of Born and Oppenheimer. In this paper we discuss what precisely might be anticipated in the way of molecular structure from a mathematical consideration of the spectral properties of the full Coulomb Hamiltonian, to what extent the very idea of molecular structure might be dependent upon treating the nuclei simply as providing a potential and the extent to which the work of Born and Oppenheimer can be used to support this position.     

Chiral ditopic receptors. Application to palladium-catalyzed allylic alkylation

Chiral pyridinooxazoline, quinolinooxazoline, bis(oxazolino)pyridine (pybox), and bisoxazoline (box) derivatives containing crown ether residues were prepared. Some of the ligands were assessed in substrate binding studies and in palladium catalyzed allylic alkylations.     

Syntheses of sulfoxide derivatives in the benzodiazine series. Diazines. Part 37

Syntheses of new sulfinylcinnolines, quinoxalines, quinazolines and phtalazines have been investigated starting from the appropriate halogenobenzodiazine derivatives. The latter were converted in one step to the corresponding sulfanyl benzodiazines which upon oxidation with m-CPBA led to the corresponding sulfoxide derivatives of benzodiazines in moderate to good yields. In parallel to this study, an improved method for the synthesis of 2-methylsulfinylquinoxaline starting from 2-sulfanylquinoxaline is also described and in the quinazoline series a synthetic route has been developed to prepare 2-tert-butyl-5-phenylsulfinylquinazoline with satisfactory yield as well as 2-tert-butyl-5-tert-butylsulfinyl-4(3H)-quinazolinone and 2-tert-butyl-8-tert-butylsulfinyl-4(3H)-quinazolinone.     

Asymmetric hydrogenation of ketones using a ruthenium(II) catalyst containing BINOL-derived monodonor phosphorus-donor ligands

[structure: see text] A series of ruthenium(II) complexes containing BINOL-based monodonor phosphorus ligands have been prepared and applied to the asymmetric catalysis of the hydrogenation of aryl/alkyl ketones. The best ligands for this application are those which contain an aromatic groups with either a methoxide or bromide on the ortho position. Using these ligands, alcohols with ee's of up to 99% are formed.     

Primary amido substituted diborane4 compounds and imidodiborate4 anions

Treatment of the diborane(4) compound B(2)(NMe(2))(4) with aniline or 2,6-dimethylaniline results in the primary amido compounds B(2)(NHR)(4)(R = Ph, 2,6-Me(2)C(6)H(3)); subsequent treatment with n-BuLi in toluene in each case affords the first examples of anionic imidodiborates namely Li(4)(thf)(6)B(2)(NPh)(4) and Li(4)(thf)(4)B(2)(N-2,6-Me(2)C(6)H(3))(4); all complexes have been characterised crystallographically.     

Étude thermodynamique de la complexation de l'argent par la thiourée et certains de ses derives alkylsubstitués

In order to find support for the validity of the hypothesis proposed by one of us regarding the different natures of the bonds in the four successive complexes formed between Ag⁺ and thiourea, the corresponding enthalpies are verified by direct calorimetry.Furthermore, because the influence of substituent groups on the nitrogen atoms may help to characterize the donor atom. the calorimetric enthalpies and potentiometric free energies of complexation between Ag⁺ and N-methylthiourea, N,N′- dimethylthiourea and N,N′-diethylthiourea have also been determined at 25°C, in aqueous medium of ionic strength μ = 0.5 M KNO₃.     

The [4Fe-4S](2+) cluster in reconstituted biotin synthase binds S-adenosyl-L-methionine

The combination of resonance Raman, electron paramagnetic resonance and M?ssbauer spectroscopies has been used to investigate the effect of S-adenosyl-l-methionine (SAM) on the spectroscopic properties of the [4Fe-4S]2+ cluster in biotin synthase. The results indicate that SAM interacts directly at a unique iron site of the [4Fe-4S]2+ cluster in BioB and support the hypothesis of a common inner-sphere mechanism for the reductive cleavage of SAM in the radical SAM family of Fe-S enzymes.     

Far infrared spectra of solid molecular nitrogen

This paper reports the observation of the far infrared absorption spectrum of a single crystal of N₂, measured over absorption paths of 4 cm (lengthwise) and 2 cm (across the crystal). The crystal chamber, with quartz windows, was immersed in a flow of cold helium gas. The spectrum from 20 to 120 cm^–1 was recorded in the liquid phase, the-phase, and over the full temperature range of the-phase (35.6–2.0 K) with a Fourier transform spectrometer. The spectral resolution, which was not instrument-limited, and the large path allowed the observation of more detailed multiphonon-transition structure in the spectrum of the-phase than has previously been observed.     

Energetic and stereochemical effects of the protein environment on substrate: a theoretical study of methylmalonyl-CoA mutase

QM/MM methods were used to study the isomerization step from (2R)-methylmalonyl-CoA to succinyl-CoA. A pathway via a "fragmentation-recombination" mechanism is ruled out on energetic grounds. For the other radicalic pathway, involving an addition recombination step, geometries and vibrational contributions have been determined, and a barrier height of 11.70 kcal/mol was found. The effect of adjacent hydrogen-donating groups was found to reduce the energy barrier by 1-2 kcal/mol each and thus to provide a significant catalytic effect for this reaction. By means of molecular dynamics studies, the stereochemistry of the methylmalonyl-CoA mutase catalyzed reaction was examined. It is shown that TYR89 is essential for maintaining stereoselectivity of the abstraction of a hydrogen in the backreaction. The subsequent selective formation of one isomer of methylmalonyl-CoA is probably due to the presence of a bulky side chain.     

Metallation regioselective en serie pyridinique: Synthese originale d'amino-2 aroyl-3 pyridines

Lithium diisopropylamide reacts with 2-fluoropyridine at low temperature: regioselectivity is excellent and metallation occurs without side reactions such as nucleophilic attack. 2-Fluoro-3-lithiopyridine is formed and with aldehydes it gives the corresponding fluorinated alcohols which are then selectively oxidized. Halogen substitution using amines leads to various 3-oxoalkyl- or 3-aroyl-2-aminopyridines.