Subject: ? re competing H-bonds

From: "Hr. Dr. S. Shapiro" <[email-masked]>
Date: Thu, 16 Jan 1997 12:14:55 +0100

Dear Colleagues;

        A question has arisen here about hydrogen bonds for which I haven't
a clear answer.  Perhaps someone knowledgable on this subject who reads this
list can help me.

        Consider an isolated phenolic hydroxy group; depending upon the
other molecules in its vicinity, it can either 

        (a) form a hydrogen bond by donating its hydrogen atom to a nearby
hydrogen acceptor, or

        (b) form a hydrogen bond by donating the electrons on the oxygen
atom to a nearby hydrogen atom, or

        (c) both (a) and (b).

        Will the strength of the hydrogen bond formed between the oxygen
atom of the phenolic hydroxy moiety and a nearby hydrogen atom be weaker
than it might otherwise be if, at the same time, this phenolic hydroxy group
also donates it hydrogen atom in a second hydrogen bond?

        In other words, will the Ar-O --- H-R hydrogen bond in 
                                    |
                                    H

                        Ar-O-H        
                           |            
                           |            
                           |            
                           H-R          

be stronger than ther corresponding Ar-O --- H-R hydrogen bond in
                                       |
                                       H

                        Ar-O-H---O=C-R
                           |
                           |
                           |
                           H-R             ?

        Thanks in advance to all responders.

Sincerely,

S. Shapiro
ZH
[email-masked]