In contrast to the very large number of special methods applicable to syntheses in the heterocyclic series, relatively few general methods are available. The 1,3-dipolar addition offers a remarkably wide range of utility in the synthesis of five-membered heterocycles. Here the “1,3-dipole”, which can only be represented by zwitterionic octet resonance structures, combines in a cycloaddition with a multiple bond system – the “dipolarophile” – to form an uncharged five-membered ring. Although numerous individual examples of this reaction were known, some even back in the nineteenth century, fruitful development of this synthetic principle has been achieved only in recent years.

1,3-Dipolar Cycloadditions. Past and Future†

Proton and carbon-13 nmr spectra of unsonicated lipid bilayers and biological membranes are generally dominated by strong proton–proton and proton–carbon dipolar interactions. As a result the spectra contain a large number of overlapping resonances and are rather difficult to analyse. Nevertheless, important information on the structure and dynamic behaviour of lipid systems has been provided by these techniques (Wennerstrom & Lindblom, 1977).

/pdf/deuterium-magnetic-resonance-theory-and-application-to-lipid-3a016gcplt.pdf

Deuterium magnetic resonance: theory and application to lipid membranes.

The trans-influence: its measurement and significance

Einer sehr grosen Zahl spezieller Synthesewege in die heterocyclische in die Reihe steht ein nur bescheidener Satz an Aufbaumethoden von allgemeiner Verwendbarkeit gegenuber. Eine erstaunliche Anwendungsbreite bei der Synthese funfgliedriger Heterocyclen kommt der 1.3-Dipolaren Addition zu. Dabei vereinigt sich der nur mit zwitterionischen Oktett-Grenzformeln wiederzugebende „1.3-Dipol” mit einem Mehrfachbindungssystem, dem „Dipolarophil”, in einer Cycloaddition zum ladungsfreien funfgliedrigen Ring. Wenngleich die Kenntnis zahlreicher Einzelbeispiele bis ins vorige Jahrhundert zuruckreicht, gelangte das Syntheseprinzip erst in den letzten Jahren zu fruchtbarer Entfaltung.

1.3‐Dipolare Cycloadditionen Rückschau und Ausblick

Kraitchman has shown that a single isotopic substitution on an atom is sufficient to determine directly the coordinates of that atom with respect to the principal axes of the original molecule. Kraitchman's formulas represent exact solutions of the equations for the equilibrium moments of inertia. However, the effects of the zero‐point vibrations are such that the coordinates obtained by substitution from the ground state moments of inertia I0 are systematically less than r0. These coordinates have here been called r (substitution) or rs, and it is found that rs≃(r0+re)/2, and Is= ∑ imirsi2≃(I0+Ie)/2.In the usual method of solution, the coordinate of one atom is determined from the equation for I0, and therefore the difference I0—Is must be made up by this one coordinate. This introduces a large error in the structures normally determined from ground state constants, and results in variations of 0.01 A in structures determined from different sets of isotopic species. If instead, we obtain the structure on...

A new criterion for the determination of molecular structures from ground state rotational constants

Experimentally determined values of the proton chemical‐shift anisotropy for the methyl halides have been used to test simple phenomenological theories which have been proposed to account for proton magnetic shielding. It is shown that the frequently used magnetic dipole model predicts that the long‐range contribution to the proton shielding anisotropy should largely depend on the average neighbor magnetic susceptibility. Anisotropies calculated from this model are much larger than those measured. Expressions have also been derived relating the proton shielding anisotropy to the electric dipole moment and charge densities of polar substituents. Results of these calculations are not as conclusive as in the magnetic dipole case.

Studies of Chemical Shift Anisotropy in Liquid‐Crystal Solvents. II. Theoretical Calculations for the Methyl Halides

(1970). Proton shielding anisotropy in benzene. Molecular Physics: Vol. 19, No. 2, pp. 285-287.

Proton shielding anisotropy in benzene

Evidence on Nuclear Moments of Stable Ge and Si Isotopes from Microwave Spectra

Fluorine shielding anisotrophy in CFCl3

A pulsed FT NMR spectrometer is used to determine the anisotropy in the carbon‐13 chemical shift tensor and the H–C–H bond angle in methyl alcohol from studies in a nematic liquid crystal solvent. The H–C–H bond angle measured by proton magnetic resonance is 109 ° 20′ ± 4′. The signs of the coupling constants D13CH, DHH, and J13CH and the ordering parameter S33 are established to be all positive. The value of Δσ (13C) obtained from temperature studies of the nematic phase chemical shift is 69 ± 5 ppm. The anisotropy obtained from the nematic—isotropic phase chemical shift difference (at 77 °C) is 63 ± 6 ppm.

Benjamin P. Dailey

Papers

Studies of Chemical Shift Anisotropy in Liquid‐Crystal Solvents. II. Theoretical Calculations for the Methyl Halides

Proton shielding anisotropy in benzene

Evidence on Nuclear Moments of Stable Ge and Si Isotopes from Microwave Spectra

Fluorine shielding anisotrophy in CFCl3

13C chemical shift anisotorpy and the H–C–H bond angle of 13CH3 OH in a nematic liquid crystal solvent