Precise 1H and 19F nuclear magnetic resonance chemical shifts and spin-spin coupling constants are reported for 4 mol%
solutions of 2-fluorobenzaldehyde (2FB) and 3-fluorobenzaldehyde (3FB) in CS2/C6DI2/TMS/C6F6 and acetoned6/
TMS/C6F6 solvent mixtures at 300 K. A small amount of the O-cis conformer of 2FB is present even in the nonpolar solvent
mixture, corresponding to a free energy difference of 7.6(3) kJ/mol between the planar 0-cis and 0-trans conformers. In the
polar solvent, this number decreases to 4.5(2) kJ/mol. The 0-cis and 0-trans conformers of 3FB have very similar abundances
in the two solvent mixtures, the former being favored by a free energy difference of 0.38(4) kJ/mol in the nonpolar medium and,
unexpectedly, considering its highly polar nature, by only 0.27(4) kJ/mol in the polar environment. STO-3G MO computations,
with geometry optimization, of the internal rotational potentials of 2FB and 3FB confirm the planarity of the 0-cis conformer of
2FB, that is, that the planar form is more stable than a somewhat twisted conformer. 6-31G MO calculations for the four planar
conformers yield structures of potential use in the fitting of rotational spectra in the vapor. Extrapolation of the 1H nmr data for
2FB implies afree energy difference of 11.5 (0.6) kJ/mol at 300 K in the vapour favoring the 0-trans form, midway between the
two theoretical estimates. The present experiments and computations are compared with the latest rotational, vibrational, and
electronic spectra and with other assessments of the relative conformer stabilities in the vapor and in solution.
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