CH hydrogen bonding: 3. Hammett correlations for phenylacetonitriles and tetrafluorobenzenes with HMPA

Abstract

Understanding the hydrogen bond, or H‐bond, arguably the strongest known intermolecular attractive force, is of great importance. Models of H‐bonding hold that both electrostatic and covalent components are at work. H‐bonding of CH bonds, and thus presumably other AH bonds, is greatly facilitated by polar substituents. We have found that equilibrium constants, K, for CH H‐bonding in series of phenylacetonitriles and tetrafluorobenzenes with HMPA are closely correlated by the Hammett equation. That is, K values vary as substituents of different polarities are introduced at positions remote from the donor CH bond. This result is consistent with a significant electrostatic component in these examples of H‐bonding. It may, however, also be explained by interaction with the σ* orbital of the CH bond, which is involved in the covalent component of the H‐bond. Theoretical models need to be applied to this and other cases. Copyright © 2010 John Wiley & Sons, Ltd.

Substituent Effects in Double‐Helical Hydrogen‐Bonded AAA‐DDD Complexes

“…In all four of these cases, ρ =−1.4 and R 2 ≥0.97 indicating the magnitude of the influence of substitution on the AAA components towards complex stability is nearly identical to that observed upon substitution of the DDD partner 7 . This observation is somewhat surprising considering the closer proximity of the substituents to the interacting atoms in the AAA versus DDD arrays (i.e., three/four bonds distant ( 9 , 13 and 14 ) vs. six ( 6 a, 6 c, 6 g and 6 h )) 21. The negative value of ρ indicates that electron‐donating groups stabilize the developing partial positive charge on the nitrogen atom of the pyridyl rings in 7 , 9 , 13 and 14 as a result of the hydrogen‐bonding interaction with 6 .…”

“…This observation is somewhat surprising considering the closer proximity of the substituents to the interacting atoms in the AAA versus DDD arrays (i.e., three/four bonds distant (9, 13 and 14) vs. six (6 a, 6 c, 6 g and 6 h)). [21] The negative value of 1 indicates that electron-donating groups stabilize the developing partial positive charge on the nitrogen atom of the pyridyl rings in 7, 9, 13 and 14 as a result of the hydrogen-bonding interaction with 6.…”

Experimental Binding Energies in Supramolecular Complexes

“…Similarly, equilibria of para-substituted phenylacetonitriles with the strong acceptor hexamethylphosphoramide (HMPA), and of 3-substituted 1,2,4,5-tetrafluorobenzenes with HMPA exhibit linear correlations with σ p substituent constants (Figure 37). 365 Complexes between a chiral pyridine−crown ether derivative and ammonium salts of α-amino acid methyl esters in chloroform showed discrimination between the enantiomers, which reached a maximum value of −ΔΔG = 5.0 kJ/mol for tryptophan, compared to only −ΔΔG = 1.5 kJ/mol for alanine; the absolute affinities also increased significantly from Ala to Trp. 202 This was explained by interaction between a C−H bond of the crown ether and the aryl residue of the amino acid, which was supported by structural evidence from NMR and crystal structure data.…”

Formation constants in C–H hydrogen bonding. 4. Effects of cyano, nitro, and trifluoromethyl substituents in aromatic compounds

“…K eq 's with DMF were 2 to 3 times smaller for 26 and 28 than with HMPA. 9 since log K and σ of CN and NO 2 are similar. 19 F nmr, then, affords good "ball park" values of K eq in some cases.…”