Chalcogen-Nitrogen Bond: Insights into a Key Chemical Motif

Chalcogen-nitrogen chemistry deals with systems in which sulfur, selenium, or tellurium is linked to a nitrogen nucleus. This chemical motif is a key component of different functional structures, ranging from inorganic materials and polymers, to rationally designed catalysts, to bioinspired molecules and enzymes. The formation of a selenium–nitrogen bond, typically occurring upon condensation of an amine and the unstable selenenic acid, often leading to intramolecular cyclizations, and its disruption, mainly promoted by thiols, are rather common events in organic Se-catalyzed processes. In this work, focusing on examples taken from selenium organic chemistry and biochemistry, the selenium–nitrogen bond is described, and its strength and reactivity are quantified using accurate computational methods applied to model molecular systems. The intermediate strength of the Se–N bond, which can be tuned to necessity, gives rise to significant trends when comparing it to the stronger S– and weaker Te–N bonds, reaffirming also in this context the peculiar and valuable role of selenium in chemistry and life.

European and Nuclear Disintegration

The horrific carnage on both sides of the conflict in the Great War of 1914–18 and the harsh postwar treaties transformed the face of Europe. Nuclear physics was also transformed, shortly before Rutherford left Manchester for Cambridge in early 1919, by his discovery of artificial nuclear disintegration, that alpha particles can disintegrate the nitrogen nucleus. He pursued his discovery at the Cavendish with his former Manchester student James Chadwick, who along with Charles Ellis and many others had been interned during the war in former racehorse stables in Ruhleben on the western outskirts of Berlin. Rutherford explained his discovery by assuming that an incident alpha particle expels a proton orbiting about a central core in the nitrogen nucleus, leaving a residual nucleus of lower atomic number.

The topology of the valence shell and the electric field gradient at the nitrogen nucleus in aziridines

The ab initio molecular charge density ρ(r) of substituted aziridines was calculated at the MP2 level for R = -H, -CH3, -CF3, -Cl, -NO2, -CN, -OH, -NO, and -F. The use of the topology of the Laplacian of ρ(r) allowed the analysis of the electric field gradient (EFG) at the nitrogen nucleus directly in terms of its valence shell charge concentration. The EFG changed sign and the orientation of its y- and z-axes with respect to aziridine when R = -Cl, -NO2, -CN, -OH, -NO, and -F. The changes in the EFG in these aziridines upon substitution were correlated with those calculated in the valence charge concentrations along the direction of the N—R bond. The sign of the qzz component and the orientation of the principal axes of the EFG tensor were found to be determined by the relative value of the contributions from the charge concentrations at the points in the valence shell along the N—C and N—R bond directions. Key words: molecular charge distribution, aziridines, Laplacian of the charge density, electric field gradient.

Quadrupole Coupling in 4-Fluoro-benzonitrile. A Microwave Fourier Transform Study

The 14N quadrupole hfs coupling has been studied in Para-fluoro-benzonitrile using the high resolution microwave Fourier transform spectrometer constructed at Kiel University. If interpreted within a simplified MO treatment, the data show that the out-off-plane p-electron density at the Nitrogen nucleus is appearently larger than the in-plane density, contrary to the prediction of a CNDO/2 calculation.

Electrochemical reduction of para-substituted 2-acyl-5-phenylfuranes in dimethylformamide

para Substituted chloro, bromo, and nitro derivatives of 2-acyl-5-phenylfurane are reduced polarographically in a one-electron wave to the corresponding anion radicals, which were studied by the EPR method. The reduction of nitro derivatives, studied by the Kalousek switch, is reversible and leads to a stable anion radical with an unpaired electron center on the nitrogen nucleus; the reduction of the halogen derivatives is only partly reversible and leads to unstable ketyl radicals. The bromo derivatives give polarographic maxima typical for concurrent reactions.

Rutherford

AFTER listening to three admirable lectures by Oliphant, Massey and Feather, there is little that I can add to the story of Rutherford’s remarkable achievements in experimental physics. But I would like to start by saying a few words about how Rutherford influenced my own scientific career. In fact it was the British Admiralty which first started me off as a scientist by sending me, in January 1919, with some 400 other young naval officers to Cambridge to take a six month course to absorb some general culture after four years at sea during the First W orld War. Three weeks later I resigned from the Navy and became an undergraduate. I took Physics Part 2 in 1921 and then became a research student under Rutherford. He gave me a research problem which was ideally suited to my interests and aptitudes. This was to use the beautiful cloud chamber method of C. T. R. Wilson to find out what happened when a fast alpha particle disrupts a nitrogen nucleus. By 1923 this was achieved. Rutherford had set me an almost perfect problem as it was bound to give important results.

Energy Relations in Artificial Disintegration

When certain elements are bombarded by swift α particles, protons or hydrogen nuclei are liberated which are attributed to the disintegration of the nuclei of these elements. It is now generally assumed that in such an artificial disintegration of an atomic nucleus an α particle is captured by the nucleus. Experimental evidence for this capture of an α particle has been obtained by Blackett in the case of the disintegration of the nitrogen nucleus. Blackett photographed eight disintegration collisions of an α particle with a nitrogen nucleus; in each of these the track of the α particle divided into two branches, one of which was due to the residual nucleus set in motion in the collision, and the other was due to the ejected proton. No trace of a third track to correspond to the track of the α particle after the collision was observed. Blackett therefore concluded that the α particle was captured by the nitrogen nucleus.