Large perturbations of long-range "J(1H,1H) and "J(1H,19F) by the intramolecular hydrogen bonds in 2-mercaptobenzaldehyde, salicylaldehyde, and some derivatives. Reference structures for intramolecular hydrogen bonds.
著者
和文:
TED SCHAEFER,
RUDY SEBASTIAN,
DAVID M. MCKINNON,
PERRY W. SPEVACK,
KERRY J. COX,
TAKEUCHI CRAIG.
英文:
TED SCHAEFER,
RUDY SEBASTIAN,
DAVID M. MCKINNON,
PERRY W. SPEVACK,
KERRY J. COX,
CRAIG TAKEUCHI.
Precise 1H 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, 5J(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 C=O---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-H---S energies. The experimental data for the CCl4 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 6-31 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.
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