Conformational Landscapes of 2,3-, 2,4-, 2,5-, and 2,6-Difluorobenzaldehyde Unveiled by Rotational Spectroscopy

We report the coexistence of anti-conformers
and
energetically unfavorable syn-conformers of 2,3-,
2,4-, 2,5-, and 2,6-difluorobenzaldehyde in the gas phase using broadband
chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy.
The rotational spectra of monosubstituted 13C isotopologues
of the anti-conformers have also been assigned in
natural abundance, which were used to derive their vibrationally averaged
geometries and semi-experimental equilibrium structures. The energy
differences between anti- and syn-conformations are estimated to be 10.9, 11.3, and 12.9 kJ/mol for
2,3-, 2,4-, and 2,5-difluorobenzaldehyde, respectively, at the theoretical
level of DLPNO−CCSD(T)/def2-TZVP. Despite the steric repulsion
caused by the close proximity between the oxygen atom of the aldehyde
group and the ortho-substituted fluorine atom, our
experimental results indicate the planarity of the syn-conformations. The frequencies of the large amplitude torsion between
the phenyl and aldehyde groups have been estimated by experimental
inertial defects, which agree with theoretical calculation results.