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
