Senior atoms

Find the shortest path in atoms, irrespective of their nature, from the subunit of the highest seniority to the subunit of the same seniority (Rules 15,16), if present, or of the second highest seniority (Rules 13,14). Where paths of equal length are identified as shortest, the choice depends on the seniority of the remaining subunits and the number and positions of substituents ... 

O is senior to a benzene ring the one-atom path from O to the benzene ring is preferred to the three-atom path.)... 

A. From the senior subunit determined from seniority take the shortest path (smallest number of atoms) to another like or identical unit or to the next most preferred subunit. Thus, for the homo polymer poly(oxymethylene) it is simply going from one oxygen to the next oxygen and recognizing that this is the repeat unit. For a more complex ether this means going on in the shortest direction from the senior unit or atom to the next most senior unit or... 

The seniority of different types of subunits is heterocyclic rings > heteroatoms or acyclic subunits containing heteroatoms > carbocyclic rings > acyclic subunits containing only carbon. The presence of various types of atoms, groups of atoms, or rings that are not part of the main polymer chain but are substituents on the CRU do not affect this order of seniority. 

The simplest kind of formula is a compositional formula or empirical formula, which lists the constituent elements in the atomic proportions in which they are present in the compound. For such a formula to be useful in lists or indexes, an order of citation of symbols (hierarchy) must be agreed. Such hierarchies, often designated seniorities or priorities, are commonly used in nomenclature. For lists and indexes, the order is now generally recommended to be the alphabetical order of symbols, with one very important exception. Because carbon and hydrogen are always present in organic compounds, C is always cited first, H second and then the rest, in alphabetical order. In non-carbon-containing compounds, strict alphabetical order is adhered to. 

The constituent subunit of highest seniority must contain one or more central atoms bridging groups between central atoms in the backbone of the polymer cannot be senior subunits. This is consistent with the principle of coordination nomenclature, in which the emphasis is laid on the coordination centre. 

Alternate to A-41.6 and A-41.7)—Polycyclic compounds containing more than one spiro atom are named in accordance with Rule A-42.1 starting from the senior" end-component irrespective of whether the components are simple or fused rings. 

The methods of theoretical description of many-electron atoms on the basis of tensorial properties of the orbital and spin angular momenta are well established [14, 18] and enable the spectral characteristics of these systems to be effectively found. The relation between the seniority quantum number and quasispin makes it possible to extend the mathematical tools to include the quasispin space and to work out new modifications of the mathematical techniques in the theory of spectra of many-electron atoms that take due account of the tensorial properties of the quasispin operator. 

John Watkin Humberston was an undergraduate at Manchester University and obtained his Ph.D. from University College London. From 1965 to 1966 he taught at Trinity College Dublin and became a Lecturer at UCL in 1966, where he subsequently became Senior Lecturer, Reader and Professor. During this time he had sabbatical leave at the Goddard Space Flight Center, USA, and at York University, Toronto, Canada. Professor Humberston has written numerous research articles, published mainly in Journal of Physics B, as well as several review articles in Advances in Atomic and Molecular Physics and in Physics Reports. 

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