Hydrogen Bonds Involving the C-H Link. XI.¹ Effect of Structure on Bonding of Donor and Acceptor Molecules

The CH‥O H-Bond as a Determining Factor in Molecular Structure

“…The sp 2 -hybridized C of the vinyl [41] or phenyl group [42][43][44][45] could also donate a proton, as could other aromatic systems such as imidazole [46], pyrimidine [47] or nucleic acid bases [48][49][50]. Other work concluded that the very simple CH 4 could engage in a HB [51][52][53][54][55][56][57][58][59][60][61], albeit a weak one. An early study of chlorosubstituted methanes [62] indicated that the CH··O HB with water would be progressively strengthened by each exchange of H with Cl.…”

“…Some of our earlier work up through 2006 has been summarized already [71][72][73] so will only be briefly outlined here, with emphasis on major findings. Beginning with the simplest system which might contain a CH··O HB, the binding energy between CH 4 and OH 2 , is very weak [74], on the order of 0.5 kcal/mol or less. But there is a strong substituent effect in that electron withdrawing agents on the proton donor molecule polarize the CH bond, making it a more potent donor.…”

“…But there is a strong substituent effect in that electron withdrawing agents on the proton donor molecule polarize the CH bond, making it a more potent donor. In numerical terms with each replacement of a H atom on CH 4 by F, the HB energy climbs by about 1 kcal/mol, to the point where the F 3 CH··OH 2 HB is very nearly as strong as that in the water dimer. As typical of HBs, the progressive strengthening is also accompanied by a shortening; in this case R(H··O) contracts by about 0.14 Å with each F substitution.…”

“…Also rather constant was the R(C α ··O) distance, 3.31-3.35 Å, and the NMR downfield chemical shift of the C α H proton which varied between − 1.35 and − 1.71 ppm. There was a bit more sensitivity in the contraction of the C-H bond, from 0.3 to 3.1 mÅ; all of the C-H stretching frequencies were to the blue, in the 14-56 cm −1 range, so this pattern fits the idea of blue stretches accompanying sp 3 hybridization In order to expand the scope to charged amino acids, Lys + was modeled [165] by the R sidechain of (CH 2 ) 4 …”

“…A related work [2] was published in 1937, arising from studies of the liquid phase for systems like CHCl 3 and ethers, followed 2 years later by a study [3] that used this concept to explain the high dissociation constant of o-toluic and n-butyric acids. Heat of mixing data for a range of molecules by Marvel et al led this group to suggest [4] CH··X bonds in 1940, and to propose that neighboring groups such as Cl, Br, and phenyl could activate the CH donor. The idea was extended [5] to biological systems such as proteins like collagen shortly thereafter.…”