Conformational\nLandscapes of 2,3-, 2,4-, 2,5-, and\n2,6-Difluorobenzaldehyde Unveiled by Rotational Spectroscopy

We report the coexistence of <i>anti</i>-conformers\nand\nenergetically unfavorable <i>syn</i>-conformers of 2,3-,\n2,4-, 2,5-, and 2,6-difluorobenzaldehyde in the gas phase using broadband\nchirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy.\nThe rotational spectra of monosubstituted <sup>13</sup>C isotopologues\nof the <i>anti</i>-conformers have also been assigned in\nnatural abundance, which were used to derive their vibrationally averaged\ngeometries and semi-experimental equilibrium structures. The energy\ndifferences between <i>anti-</i> and <i>syn-</i>conformations are estimated to be 10.9, 11.3, and 12.9 kJ/mol for\n2,3-, 2,4-, and 2,5-difluorobenzaldehyde, respectively, at the theoretical\nlevel of DLPNO−CCSD(T)/def2-TZVP. Despite the steric repulsion\ncaused by the close proximity between the oxygen atom of the aldehyde\ngroup and the <i>ortho</i>-substituted fluorine atom, our\nexperimental results indicate the planarity of the <i>syn</i>-conformations. The frequencies of the large amplitude torsion between\nthe phenyl and aldehyde groups have been estimated by experimental\ninertial defects, which agree with theoretical calculation results.