February 9, 1940

Dr. E. Rabinowitch

Research Laboratory of Organic Chemistry

Massachusetts Institute of Technology

Cambridge, Massachusetts

Dear Dr. Rabinowitch:

It is not possible to give a simple answer to the question you ask in your letter of February 7. The nitrogen atom in a porphine or phthalocyanine, whether bonded to carbon by two single bonds or a single bond and a double bond, would tend to form a bend to hydrogen in the median plane, as shown:

C\

N-C

H/

With single bonds to carbon the bond to hydrogen would rise above or fall below the CNC plane and with one CN double bond the NH bond would tend to be in the plane. There is, of course, not room for both hydrogen atoms in the molecule to lie in the plane of the molecule in these median positions, so that if the hydrogen atoms were to lie in the median planes one would be above and below the plane of the molecule, corresponding to your Figure 1.

In order to form hydrogen bonds, as in Figure 2, the hydrogen atoms would have to be pulled somewhat to one side from the expected bond directions. It is improbable that they would be pulled far enough out of line to lie between pairs of nitrogen atoms, because of the strain involved in the bond angles. On the other hand, the attraction of the other nitrogen atoms would, I think, pull the hydrogen atoms over somewhat to one side, giving weak hydrogen bonds. Each proton would be near to one nitrogen atom, at about 1.00 Å, and also would have one other nitrogen neighbor, at about 1.8 Å, I guess. The protons would probably move occasionally from one nitrogen atom to another, but the frequency of this motion would be so low as not to influence the stability of the molecule.

You see that my position on the question is an intermediate one.

With best regards, I am

Sincerely yours,

Linus Pauling

LP/j