Long-range proton coupling constants and molecular orbital calculations as indicators of conformational populations of benzene-1,2- and -1,3-dicarbaldehydes
著者
和文:
TED SCHAEFER,
GLENN H. PENNER,
RUDY SEBASTIAN,
TAKEUCHI CRAIG.
英文:
TED SCHAEFER,
GLENN H. PENNER,
RUDY SEBASTIAN,
CRAIG TAKEUCHI.
The 1H nmr spectra of the benzene-1,2- and -1,3-dicarbaldehydes in carbon tetrachloride, benzene-d6, and acetone-d6
solutions at 300 K are analyzed. The stereospecific long-range couplings over five formal bonds between the sidechain and ring
protons show that the 1,2 isomer exists as an 87: 13 mixture of the cis-trans and trans-trans conformers in carbon tetrachloride.
These populations are insensitive to solvent. Molecular orbital calculations utilizing extensive geometry optimization procedures
imply that the cis-cis form, with proximate C=O bonds, is indeed of negligible significance as assumed in obtaining the
populations of the other forms. Further calculations define a pathway of relatively low energy for interconversion of the two
abundant forms, in agreement with dynamic nrnr studies. For the 1,3 isomer the long-range couplings provide a check of the
conformer populations deduced from dipole moment and "C nmr studies. For example, if the cis-trans form is 70% abundant,
as deduced from the dipole moment in benzene solution, then the long-range couplings imply that the population of the cis-cis
conformer is insignificant.
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