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).

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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

Nuclear quadrupole coupling constants in molecules depend on the nuclear quadrupole moments and the variation in electrostatic field at the nucleus. It is shown that this variation of electric field is usually simply related to the molecular electronic structure, being primarily dependent on the way in which valence electrons fill the lowest‐energy p‐type orbits. Structural information which can consequently be obtained from known quadrupole coupling constants is discussed. Hybridization of the normal covalent bonds of N, Cl, and As with at least 15 percent s character is clearly shown. The alkali halides appear to be almost purely ionic; the quadrupole coupling data allow no more than 3 percent covalent character. In addition to molecular structure, some nuclear quadrupole moments are approximately evaluated by use of the theory developed here.

Determination of Electronic Structure of Molecules from Nuclear Quadrupole Effects

A relation between the electronegativity difference of two bonded atoms and the ionic character of the bond is obtained for singly bonded diatomic molecules in the gaseous state. The relation is based primarily on the wide variety of nuclear quadrupole coupling constants recently measured by microwave techniques. However, dipole moments of diatomic molecules are also used and discussed. Both quadrupole coupling constants and dipole moments indicate strongly that bonds involving electronegativity differences greater than 2 are almost completely ionic. Certain effects of hybridization, overlap, and polarization on considerations of ionicity are discussed.

The Ionic Character of Diatomic Molecules

Measured nuclear quadrupole effects in solids are discussed, compared with nuclear quadrupole coupling in gases, and correlated to some extent with molecular structure in the solid state. The iodine crystal affords a good example of intermolecular interactions in the solid state. In this crystal, nuclear quadrupole effects combined with crystallographic information show two intermolecular covalent bonds per atom, each of about 9 percent importance.

Nuclear Quadrupole Effects and Electronic Structure of Molecules in the Solid State

Magnetic Shielding and Susceptibility Anisotropies

Dipole moments of eight molecules were determined by the Stark splittings of the rotational transitions in the microwave region. The molecules and their moments in Debye units are: As${\mathrm{F}}_{3}$, 2.815\ifmmode\pm\else\textpm\fi{}0.025; C${\mathrm{H}}_{3}$I, 1.647\ifmmode\pm\else\textpm\fi{}0.014; C${\mathrm{H}}_{3}$Cl, 1.869\ifmmode\pm\else\textpm\fi{}0.010; C${\mathrm{H}}_{3}$Br, 1.797\ifmmode\pm\else\textpm\fi{}0.015; C${\mathrm{H}}_{3}$C${\mathrm{F}}_{3}$, 2.321\ifmmode\pm\else\textpm\fi{}0.034; P${\mathrm{F}}_{3}$, 1.025\ifmmode\pm\else\textpm\fi{}0.009; ${\mathrm{N}}_{2}$O, 0.166\ifmmode\pm\else\textpm\fi{}0.002; and ClCN, 2.802\ifmmode\pm\else\textpm\fi{}0.020. In all cases Stark effects followed very closely theoretical expectations.

Benjamin P. Dailey

Papers

Determination of Electronic Structure of Molecules from Nuclear Quadrupole Effects

The Ionic Character of Diatomic Molecules

Nuclear Quadrupole Effects and Electronic Structure of Molecules in the Solid State

Magnetic Shielding and Susceptibility Anisotropies

Molecular Dipole Moments and Stark Effects. III. Dipole Moment Determinations