Large perturbations of long-range ⁿJ(¹H,¹H) and ⁿJ(¹H,¹⁹F) by the intramolecular hydrogen bonds in 2-mercaptobenzaldehyde, salicylaldehyde, and some derivatives. Reference structures for intramolecular hydrogen bonds

Abstract

Can. J. Chem. 71, 960 (1993). Precise 'H nuclear magnetic resonance spectral parameters are reported for salicyladehyde and its 3-fluoro and 5-fluoro derivatives in nonpolar solutions. Such data are also given for the 2-mercapto, 2-methylthio, and 2-methoxy derivatives of benzaldehyde. Comparison of the long-range coupling constants in the various compounds and their conformers shows a large perturbation of their magnitudes by hydrogen bond formation. For the salicylaldehyde system, the perturbation is particularly large for couplings involving the aldehyde proton and protons or fluorine nuclei placed ortho to the hydroxyl group. For example, 'Jl (F, CHO) is reduced by about 50%. The perturbation, as expected, is much smaller for coupling constants of nuclei remote from the site of the hydrogen bond. In 2-mercaptobenzaldehyde the long-range coupling constants are also sensitive to hydrogen bond formation, those involving the sulfhydryl proton markedly so compared to the hydroxyl proton in salicylaldehyde. The strength of the CEO. -. H-S bond is discussed. It is argued that the reference conformer for the mercapto compound in such a discussion is less easily defined than for salicylaldehyde because C=O . . . S are similar to O=C-He . . S energies. The experimental data for the CC1, solutions imply a free energy of formation of the C=O . . . H-S bond of 4.8(5) kJ/mol at 300 K. Molecular orbital computations on the four planar conformers of each salicylaldehyde and 2-mercaptobenzaldehyde with the 6-3 1 G**(5D) basis are reported. For salicylaldehyde, the O=C-H . . -0-H arrangement is taken as the reference conformer, with a computed energy of 25.7 kJ/mol relative to the hydrogen-bonded structure. For 2-mercaptobenzaldehyde, the H-C=O . . S-H and O=C-H . --S-H conformers are calculated to be isoenergetic, at 5.1 kJ/mol relative to the hydrogen-bonded conformer. Hence either arrangement serves as a reference structure in computations of the strength of the hydrogen bond. The computations are consistent with the experimental results for solutions of the molecules under discussion. An appendix gives the computed geometries of the eight planar conformers, as well as some atomic charges, allowing a rationalization of the relative energies of the conformers.

1H chemical shifts in NMR: Part 19. Carbonyl anisotropies and steric effects in aromatic aldehydes and ketones

“…The couplings between the aldehyde hydrogen and H-5 and between the methoxy group and H-3 were observed in this plot and assigned to H-5 and H-3. These couplings had been reported by Schaefer and co-workers 34 and are further evidence that the compound exists as the trans conformer in CDCl 3 . Schaefer and co-workers also noted that CHOÐ Ð ÐH-3 couplings occurred in the cis conformer when it is stabilized due to intramolecular hydrogen bonding.…”

1H nuclear magnetic resonance and molecular orbital studies of 2-formylstyrene. Three significant nonplanar conformers at 300 K

“…The H,,O distance is somewhat less than 2.4 a, very like the computed H,O distances3 in the hydrogen-bonded complexes between water and some fluoromethanes (23). The high energy of A at 0 = 90" is about 4.7 kJ/mol larger than that expected from the complete loss of -rr,-rr conjugation, very likely because of repulsion between the 2p orbital on C-a and the C=O bond In 2-hydroxylbenzaldehyde, a molecule with a strong intramolecular hydrogen bond, perhaps as large as 29 kJlmol (24,25), the microwave spectrum of the vapor corresponds to a H,O distance of 1.76 2 0.01 A (26 Table 1. (reminiscent of the situation in 2-alkylthiobenzaldehydes (27), in which only the 0-trans conformer occurs when the 3p orbital on sulfur is twisted into the molecular plane).…”

“…3 as given by a Boltzmann weighting. The long-range coupling constants over five bonds to the formyl proton are excellent measures of the proportions of A and B (25,28,29).…”

“…(a) The proportions of A and B It has been established (25,28,29) that the ratio of 'J(H-m, CHO), the coupling constant between the formyl proton and a ring proton placed meta to the formyl group, to the sum of the two such coupling constants in a benzaldehyde derivative is a good measure of the conformational bias of the formyl group. Table 2 are only slightly different in the polar and nonpolar solutions, so that their mean has been used above, implying also that these solvents do not alter the proportions of A and B.…”

1H and13C nuclear magnetic resonance and molecular orbital studies of the internal rotational potential and of spin–spin coupling transmission in phenylallene

“…For an evenfold potential, (sin2(8/2)) is 0.5, so that 'J3, is computed as 0.67 Hz, in contrast to the measured value of 0.284(2) Hz. For benzaldehyde and toluene derivatives there is evidence (49,50) that 'J effectively vanishes at 8 = 0" (not an all-trans pathway). An empirical expression for 'J3, can be developed as follows.…”