September 2, 1938

President James B Conant

Old Lyme, Connecticut

Dear Conant,

I am pleased to learn that you are interested in mentioning resonance in revising your text book of organic chemistry, and I have also been pleased to see from your letter that your understanding of the subject seems to be the same as mine. I would agree completely with the discussion which you gave in your letter of imidazole, urea, and the amides. You are right in saying that a molecule resonating among several valence bond structures retains one equilibrium configuration of the nuclei. Resonance is not identical with tautomerian; I would not say that two or three tautomeric forms of urea differing in the distribution of the hydrogen atoms, were to be combined together in a resonating structure.

Probably one reason for the confusion of resonance and tautomerism in the minds of some chemists results from the fact that often the valence bond structures among which a molecule resonates are similar to the valence bond structure usually written for the individual tautomers. In the case of urea the tautomeric form with no hydrogen on oxygen is usually assigned a valence bond structure with a carbon-oxygen double bond, and the forms in which a hydrogen atom has been transferred from nitrogen to oxygen are assigned structures with a carbon-nitrogen double bond. From the point of view of resonance each of these tautomeric forms is considered to be a normal state to which all of these structures contribute, the greatest contribution in each case being made by the structure which is conventionally assigned to the tautomer. The same resonance occurs when hydrogen is replaced by methyl groups as you mentioned in your letter in

President James B. Conant 2 September 2, 1938

connection with imidazole; the methyl groups probably tend to stabilize the conventional valence bond structure somewhat, however, so that the conventional structure becomes somewhat more important for methylated compounds than for those containing hydrogen. In the case of the carboxyl group, for example, I would estimate about 50-50 percent for the chances of the double bond in the two positions for the ion, about 60-40 for the acid, and about 70-30 for the ester.

My wife and I have been busy this summer with an architect preparing plans for a house which we hope to have built during the fall. We have purchased a tract of 2.1 acres at the foot of Mount Wilson, between Altadena and Sierra Madre. The tract Is located above an arroyo, and has a fine view of the arroyo, the mountains and the valley to the south. It has on it several live oak and eucalyptus trees so that our planting is partially taken care of. It will be a change for me to have to drive to the Institute after having lived across the street for eight years, but the five miles on good roads can be covered in ten or twelve minutes, so that I do not expect to lose much time

With kindest regards to Mrs. Conant,

Sincerely yours,

Linus Pauling

LP/bes