Hydrogen bonding resonance nature

P. Gilli, V. Ferretti, V. Bertolasi, and G. Gilli, Evidence for resonance-assisted hydrogen bonding. Covalent nature of the strong homonuclear hydrogen bond. Study of the O-H- O system by crystal structure correlation methods, J. Am. Chem. Soc. 116, 909-915 (1994). 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

There are two major experimental techniques that can be used to analyze hydrogen bonding in noncrystalline polymer systems. The first is based on thermodynamic measurements which can be related to molecular properties by using statistical mechanics. The second, and much more powerful, way to elucidate the presence and nature of hydrogen bonds in amorphous polymers is by using spectroscopy (Coleman et al., 1991). From the present repertoire of spectroscopic techniques which includes IR, Raman, electronic absorption, fluorescence, and magnetic resonance spectroscopy, the IR is by far the most sensitive to the presence of hydrogen bonds (Coleman et al., 1991). 

L. Pauung (Pasadena) I should like to object to the use by Dr. Pople of the expression the presence of the structures in the hydrogen bond . I think that one must be careful to use expressions such as 4 the contribution of the structures. . in order that listeners should not be misled as to the nature of the theory of resonance. 

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