February 7, 1938

Professor Hugh S. Taylor

Department of Chemistry

Princeton University

Princeton, New Jersey

Dear Professor Taylor:

I am interested by your suggestion that the term conjugation energy be used instead of resonance energy. I think that it would be perfectly satisfactory to use the term conjugation energy for the resonance energy of systems which are usually described as conjugated. I have not been accustomed to thinking of benzene as a conjugated system and since there is a great increase in resonance energy accompanying the completion of the ring, I am not sure whether it is wise to extend the idea of conjugation to include aromatic resonance. Moreover, the idea of resonance has very wide applications. You may recall my discussion of the partial ionic character of single bonds in terns of resonance of the bond type between the extreme ionic and the extreme covalent structures. The extra energy resulting from this resonance could hardly be called conjugation energy.

I myself do not feel that the terms resonance and resonance energy are especially unsatisfactory that were introduced into chemistry in a natural way, and the quantum mechanics resonance phenomenon does have some analogy with the classical resonance phenomenon. It is true that the analogy is no longer very close when resonance of molecules among several electron structures is considered, but I feel that the new ideas themselves must be learned and no choice of name could be made which would simplify this problem appreciably.

I wish again to thank you for your kindness in inviting me to participate in your symposium and in entertaining my wife and me during our visit. I enjoyed the visit immensely and I feel that there is no chemistry laboratory in the country, with the exception of that at Berkeley, which I visit with so much pleasure.

With kind regards to you and to Mrs. Taylor, I am

Sincerely yours,

Linus Pauling

LP/C