Comments on the Paper by C. A. CoulsonAuthor(s): W. MoffittSource: Proceedings of the Royal Society of London. Series A, Mathematical and PhysicalSciences, Vol. 207, No. 1088, A Discussion on Bond Energies and Bond Lengths (Jun. 7, 1951),p. 74Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/98677 .

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74 W. Moffitt (Discussion Meeting)

COMMENTS ON THE PAPER BY C. A. COULSON

BY W. MOFFITT

The interesting new definition of electronegativity put forward by Dr Walsh is open to the same sort of criticism as that which may be levelled at that of Pauling. As applied to an atom, the term 'electronegativity', in its original context, was surely a measure of its polar propensities. And both of these empirical definitions appear to preserve only the most tenuous of connexions with this connotation. It is true that A-H bond force constants appear to increase on going from left to right across the periodic table and that they decrease on going down. Deviations from bond addi- tivity exhibit closely related regularities. We may observe that Mulliken's electro- negativities !(I +E) behave in a similar fashion. But it is quite another thing to define electronegativities on the basis of such correlations. Indeed, the work reported by Dr Cottrell and Dr Sutton suggests that both Dr Walsh's and Pauling's data may be satisfactorily explained in terms of a purely homnopolar effect.

Professor Coulson has drawn attention to the very delicate position in which electronegativity theory now finds itself. It may therefore be of interest to the dis- cussion if I may summarize an entirely theoretical approach to these problems which I have developed recently.

It is exceedingly difficult, if not impossible, to formalize the electronegativity concept in terms other than those associated with a rather general atomic orbital approximation. -Further, although electronegativity is supposed to be an atomic property, it is known that the properties of atoms are profoundly influenced by their molecular environments. I assume that the assignment of an electronegativity to an atom is complete when its valence state (or state of hybridization) has been specified. Modern work shows that this specification is necessary. Its sufficiency is considered as fundamental to the whole electronegativity concept.

On the basis of this assumption alone, it is possible to derive a purely theoretical electronegativity scale. I derive a relation which is of the same form as that derived recently, on perhaps rather less appealing grounds, by Mulliken (1950) who used the molecular orbital method. Mulliken uses his relation to justify the beautiful empirical electronegativity scale which he set up in 1934. However, if an appropriate set of atomic functions is available, it seems more logical to use the theoretical rela- tion as a means of calculating absolute electronegativities without using the results of experiments-especially since very few electron affinities are accurately known. (Mulliken's procedure is also open to a rather more fundamental objection.) I have therefore estimated the electronegativities of the hydrogen, carbon, nitrogen and oxygen atoms in this way. It is found that whereas an sp3 hybrid orbital of quadri- valent carbon is less electronegative than hydrogen, the Sp2 and sp hybrid orbitals have progressively greater electronegativities than that of hydrogen. These results will be published in full shortly (Moffitt i 9ob).

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