Direct Identification of the Carbon Skeleton of Organic Compounds using Double Quantum Coherence 13C-NMR Spectroscopy. The INADEQUATE Pulse Sequence

NMR spectroscopy plays an important part in the determination of the structures of organic compounds. The parameters of importance here are the chemical shifts of the ¹H and ¹³C nuclei and the spin-spin interactions both between ¹H nuclei and between ¹H and ¹³C nuclei. Couplings between ¹³C nuclei were almost completely neglected until a few years ago, since they were extremely difficult to observe because of the low natural abundance of ¹³C. However, it is these couplings which afford information directly on the carbon-carbon connectivities in the molecule. It is now possible to use a special NMR pulse sequence to make these couplings more readily visible: the result of using this sequence is a ¹³C-NMR spectrum from which the carbon skeleton concerned can be directly read off. Two-dimensional spectra in particular are very easy to evaluate. The pulse sequence involved, which bears the somewhat puzzling name INADEQUATE, produces double-quantum coherences from which the NMR signals of the coupled carbon nuclei can be obtained. In this article the principle of double-quantum coherence is described and a number of examples for the application of the INADEQUATE pulse sequences to problems in synthetic organic chemistry, biosynthesis and natural products chemistry are presented; in addition, the possibility of applying the INADEQUATE method to other nuclei is considered.     

Absorption Spectra and Termination Kinetics of Transient Free Radicals in Solution Observed by UV./VIS. Spectroscopy with modulated excitation

Free radicals are generated in liquid solutions by harmonically modulated photolysis of suitable substrates. Harmonic analysis of the absorbance as functions of wavelength and modulation frequency yields the optical spectra and the decay kinetics of the transient species. The experimental technique and the analysis are described in detail. Results on t-butyl, 2-propyl and benzyl radicals generated by photolysis of the corresponding dialkyl resp. dibenzyl ketones are reported. They confirm previous spectral assignments and show that the termination reactions are diffusion controlled.     

Positive muons as probes in liquid phase free radical chemistry: Radical structures and reaction rates

Muonium-substituted organic free radicals are observed by muon spin rotation when positive muons are stopped in liquid unsaturated compounds. From muon precession frequencies in high external magnetic fields the isotropic muon-electron hyperfine coupling constants are determined. They lead to radical structure assignments. Results of a variety of projects are described which show thatSR can successfully complement conventional physical methods of free radical chemistry: A study of the temperature dependence of the coupling constants of isotopically substituted ethyl radicals yields information on structure and barriers to internal rotation. Rate constants for several radical reactions are extracted from the damping of theSR signals, in particular for unimolecular rearrangements and cis-trans-isomerizations. The theory for the analysis for the case of reversible site exchange is outlined.All results described here were obtained by E. Roduner, P. Burkhard and W. Strub of this laboratory, and B. Webster, M. Ramos and D. McKenna of the University of Glasgow. We all appreciate support from the Swiss National Foundation for Scientific Research, the National Institut for Scientific Investigations of Portugal, the Carnegie Trust for the Universities of Scotland, the Royal Society London and SIN.     

Synthesis, characterization, and substitution chemistry of [Bu4N]2[W6Cl8(OSO2CF3)6]. A versatile precursor for axially substituted clusters containing the (W6Cl8)4+ core

The new cluster [Bu4N]2[W6Cl8(OSO2CF3)6] (1) has been prepared and structurally characterized. This material is an effective precursor for the generation of cluster ions with the general formula [W6C18L6]n (L = Cl-, Br-, I-, NCS-, NCO-, NCSe-, and O=PPh3; n = 2- or 4+). The last three clusters are new. The products have been characterized by IR spectroscopy, NMR spectroscopy, and FAB mass spectrometry. In addition to 1, the products [Bu4N]2[W6C18(NCS)6] (5) and [Bu4N]2[W6C18(NCO)6] (7) were structurally characterized. Crystal data for 1: space group, P2(1/c) (No. 14); a = 11.116(5) A; b = 27.952(1) A; c = 24.516(1) A; beta = 95.182(9) degrees; V = 7586.3(5) A3; Z = 4. Crystal data for 5: space group, P2(1/n) (No. 14); a = 11.3323(9) A; b = 12.3404(9) A; c = 44.583(3) A; beta = 97.089(1) degrees ; V = 6187.1(7) A3; Z = 4. Crystal data for 7: space group, P1 (No. 2); a = 11.8009(8) A; b = 11.9332(8) A; c = 11.9522(8) A; alpha = 77.904(1) degrees; beta = 95.182(9) degrees; gamma = 62.574(1) degrees V = 1450.5(2) A3; Z = 1.     

Formation of protonic defects in perovskite-type oxides with redox-active acceptors: case study on Fe-doped SrTiO(3)

The thermodynamics of water incorporation into Fe-doped SrTiO(3) was investigated by thermogravimetric measurements. Changes in valence states of redox-active dopant ions (Fe(3+)/Fe(4+)) with water vapor pressure were taken into account in the defect chemical analysis. The proton solubility was significantly enhanced by the presence of the redox centers. The hydration enthalpies and entropies were -60 kJ mol(-1) and -122 J mol(-1) K(-1). The defect chemical model was applied to describe the water vapor dependence of the electrical conductivity in mixed ionic and electronic conducting Fe-doped SrTiO(3) single crystals.     

Bestimmung von Wasser in Dichloressigsäure

Water concentrations in dichloroacetic acid in the range of 0<C <1 % (C = percent by weight) can be determined directly by photometry at 1425 nm. The absorbance A at this maximum is described by the function A=1.267×C^0.93 (cell path d=5 cm, correlation coefficient r=0.997). The variation coefficient for water concentrations of ≈0.06% was found to be V=6.5%.     

Organometallphosphin-substituierte übergangsmetallkomplexe XVII. Tetracarbonyl-di(organometallphosphin)-chrom-, -molybdän- und -wolfram-komplexe

Bicyclo[2.2.1]-2,5-heptadiene complexes of transition metal carbonyls have been treated with organometallic phosphines, to give the disubstituted compounds [(R₃M′)₃P]₂M(CO)₄ [R = CH₃; M′ = Ge, Sn; M = Cr, Mo, W] by exchange of ligands. The chemical and spectroscopic properties of the new compounds are reported.     

On cubature formulae with a minimal number of knots

Summary In this paper an approach is outlined to the two-dimensional analogon of the Gaussian quadrature problem. The main results are necessary and sufficient conditions for the existence of cubature formulae which are exact for all polynomials of degree m and which have a minimal number of 1/2k(k+1) knots,k=[m/2]+1. Ifm is odd, similar results are due to I.P. Mysovskikh ([5, 6]) which will be derived in a new way as a special case of the general characterization given here. Furthermore, it will be shown how this characterization can be used to construct minimal formulae of even degree.     

Anharmonicity in rotational and vibrational excitation of H2 by Li+ collisions

Integral cross sections for pure rotational and vibrational-rotational excitation of H₂(X¹Σ⁺_g) by Li⁺(¹S) impact are computed by close-coupling methods at 0.2, 0.6, and 1.2 eV in the c.m. system using vibrational functions that are numerical solutions of the one-dimensional radial Schrödinger equation for harmonic, Morse, and adiabatically corrected Kolos-Wolniewicz (KW) potential functions. Comparison of results employing KW and Morse functions shows excellent agreement for all transitions studied. Findings using harmonic oscillator functions, however, differ noticeably from KW and Morse values for vibrational (0 → 1) and very large rotational (Δj = 10) transitions, but are satisfactory for lower order (0 → 2, 4, 6, 8) rotational transitions.